摘要:

Determination of Mercury Compounds in Water Samples by Liquid Chromatography–Inductively Coupled Plasma Mass Spectrometry With an In Situ Nebulizer/Vapor Generator CHIA-CHING WAN, CHIH-SHYUE CHEN AND SHIUH-JEN JIANG* Department of Chemistry, National Sun Yat-Sen University, Kaohsiung, 80424, Taiwan A preliminary study of a cold vapor generation system as the reversed-phase LC with 0.5% m/v L-cysteine solution as the ion pairing reagent and the mobile phase. The various mercury sample introduction device for liquid chromatography– inductively coupled plasma mass spectrometry (LC–ICP-MS) species studied included inorganic mercury (HgII), methylmercury (methyl-Hg) and ethylmercury (ethyl-Hg).Euent from is described. Samples containing ionic mercury compounds were subjected to chromatographic separation before injection the LC column was delivered to the vapor generation system and ICP-MS system for mercury determination. The optimiz- into the cold vapor generator.The species studied include inorganic mercury (HgII ), methylmercury and ethylmercury. ation of the CV generation LC–ICP-MS technique and its analytical figures of merit, and also its application to the The sensitivity, detection limits and repeatability of the LC–ICP-MS system with a cold vapor generator were determination of mercury compounds in NASS-4 open ocean sea-water reference material and a tap water sample collected comparable to or better than those for an LC–ICP-MS system with conventional pneumatic nebulization or other sample from National Sun Yat-Sen University, are described.introduction techniques. The limits of detection for various mercury species were in the range 0.03–0.11 ng ml-1 Hg based EXPERIMENTAL on peak height. The concentrations of mercury compounds in open ocean sea-water reference material NASS-4 and a tap ICP-MS Device and Conditions water sample collected from National Sun Yat-Sen University An ELAN 5000 ICP-MS instrument (Perkin-Elmer SCIEX, were determined.Thornhill, ON, Canada) was used. Samples were introduced Keywords: Inductively coupled plasma mass spectrometry; with an in situ nebulizer/vapor generation sample introduction liquid chromatography; mercury speciation; in situ system. ICP conditions were selected that maximized the nebulizer/vapor generator; water mercury ion signal using a flow injection (FI) method. A simple FI system was used for all the FI work performed in this study.It was assembled from a six-port injection valve In recent years, it has become recognized that trace metal (Type 50, Rheodyne, Cotati, CA, USA) with a 200 ml sample analysis must involve true metal speciation, in addition to loop. A solution of 50 ng ml-1 HgII, methyl-Hg and ethyl-Hg total metal analysis. Biological, biomedical and toxicological in the mobile phase (to be used for subsequent chromatoproperties depend on the specific form in which the metal is graphic separations) was loaded in the injection loop and present, and combinations of metals have dierent eects on injected into the mobile phase, which worked as the carrier of the environment depending on the nature of the mixture.the FI system. The cold mercury vapor generated was then Information about the various species in a sample can be transported to the ICP-MS system for mercury determination. obtained by a newer form of chromatographic separation with The sensitivity of the instrument may vary slightly from day element-selective/specific final detection.to day. The ICP-MS operating conditions used are summarized Environmental pollution caused by mercury is almost in Table 1. entirely due to industrial application in the production of The data acquisition parameters used are given in Table 1. pesticides, electrical apparatus, paints and dental applications, and the toxicity of these species is well documented. A knowledge of speciation is important when assessing the mobility of Table 1 ICP-MS equipment and operating conditions mercury in the environment.1,2 Several methods of liquid chromatography (LC) and gas ICP-MS instrument Perkin-Elmer SCIEX ELAN 5000 chromatography (GC) coupled with element-specific detection Plasma conditions— for mercury speciation have appeared, including electrochemi- Rf power 1100W cal detection (EC),3 cold vapor atomic absorption spectrometry Plasma gas flow rate 15 l min-1 Intermediate gas flow rate 0.74 l min-1 (CVAAS),4–7 atomic emission spectrometry (AES)8,9 and induc- Aerosol gas flow rate 0.98 l min-1 tively coupled plasma mass spectrometry (ICP-MS).10–14 The Mass spectrometer settings— cold vapor (CV) generation sample introduction technique has Bessel box lens +15.34 V been applied in several LC–atomic spectroscopic applications Bessel box plate lens -79.10 V for mercury speciation.4–7,9,10 The use of the cold mercury Photon stop lens -10.05 V vapor generation technique increases the signal of mercury Einzel lenses 1 and 3 -0.04 V Resolution Normal significantly.In this work, a simple in situ nebulizer/ Dwell time 120 ms vapor generator system was employed as a sample intro- Sweeps per reading 5 duction device in LC–ICP-MS for mercury speciation Points per spectral peak 1 determination.15–17 Isotope monitored 202Hg Ionic compounds containing mercury were separated by Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 (683–687) 683The element-selected chromatograms were recorded in real HgII, methyl-Hg and ethyl-Hg in the LC mobile phase were prepared. These stock solutions were then loaded into the time and stored on the hard disk with Graphic software. The dwell time, sweeps per reading and points per spectral peak injection loop and injected into the CV generation system. Several operating parameters aected the eciency of CV parameters were set so that each data point could be obtained in less than 1 s.formation. The concentration of sodium tetrahydroborate (NaBH4), concentration of acid and the volume of the mixing coil were studied to obtain the optimum conditions. Chromatographic Apparatus and Conditions A Hitachi Model L-7100 LC pump, injector (Rheodyne Model Reagents 7225i) and reversed-phase column (Spherisorb ODS-2, 5 mm Analytical-reagent grade chemicals were used without further diameter particles, 150×4.6 mm id) comprised the LC system.purification. Sodium tetrahydroborate was obtained from Samples were loaded with a syringe into a 100 ml sample loop. Janssen Chemical (Geel, Belgium), L-Cysteine from TCI All separations were performed at room temperature under Chemical (Tokyo, Japan), mercury nitrate and methylmercury isocratic conditions. Separations were attempted with several chloride from Merck (Darmstadt, Germany) and ethylmercury combinations of column, organic modifier concentration, Lchloride from TCI Chemical.Standards containing 200 mg l-1 cysteine concentration and pH. The conditions given in Table 2 (as element) of each individual species in 2% v/v H2SO4 were are those which yielded the best chromatographic resolution prepared. These standards were combined and diluted with for the various sets tested. The column outlet was connected the LC mobile phase and analyzed by CV–ICP-MS. To prepare to the vapor generation device with Teflon tubing (Fig. 1).the solutions to be used as the mobile phase, a suitable amount of L-cysteine was dissolved in pure water to the desired Cold Vapor Generation System and Conditions concentration. A simple in situ nebulizer/vapor generation sample introduction system was coupled with LC–ICP-MS for mercury speci- Sample Preparation ation determination. A schematic diagram of the LC The applicability of the method to real samples was demon- nebulizer/vapor generator system is shown in Fig. 1. With this strated by the analysis of National Research Council of Canada sample introduction system, the entire injected sample was (NRCC) NASS-4 (open ocean sea-water reference material for nebulized.The nebulization process, in which the liquid is trace metals). The reference sample was diluted twofold with shattered into fine droplets in an argon stream, is a very the LC mobile phase, then a 100 ml portion of the sample eective way to purge Hg vapor from the liquid, probably solution was injected into the LC–CV–ICP-MS system.A tap more so than bubbling argon through a static reservoir of water sample collected from National Sun Yat-Sen Univer- bulk liquid, as in a conventional gas–liquid separator. Cold sity was treated with same procedure and analyzed by vapor generated from the vapor generation system was LC–ICP-MS. delivered to ICP-MS system for mercury determination. The operating conditions for cold mercury vapor generation were optimized using an FI method.The LC pump and column RESULTS AND DISCUSSION were removed from the system during these studies. Since HgII, Selection of LC Operating Conditions methyl-Hg and ethyl-Hg show dierent behaviors and dierent sensitivities in the CV generation process, dierent mercury The eect of an organic solvent on the plasma is generally to species were studied successively to obtain compromised reduce significantly its excitation properties. In this study, no operating conditions for the vapor generation system.Stock organic solvent was added to the LC mobile phase. The eects standard solutions of various mercury species at 50 ng ml-1 of the concentration of L-cysteine and flow rate of the mobile phase on the liquid chromatogram were studied to obtain the best LC separation. Table 2 Liquid chromatography and hydride generation conditions Fig. 2 shows the eect of the concentration of L-cysteine on the chromatogram. Each mercury species was present at L C conditions— Pump Hitachi Model L-7100 50 ng ml-1.As shown in Fig. 2(a), the concentration of Column Spherisorb ODS-2, 5 mm, L-cysteine did not aect the chromatogram significantly. On 150×4.6 mm id the other hand, as shown in Fig. 2(b), the concentration of Mobile phase 0.5% m/v L-cysteine (pH 5) L-cysteine did aect the signal-to-background value of the Mobile phase flow rate 1.6 ml min-1 mercury ion signal and an optimum concentration of Sample loop 100 ml 0.5% L-cysteine was obtained.Hydride generation conditions— Fig. 3 shows the eect of the LC mobile phase flow rate on NaBH4 solution 0.1% m/v in 0.02 mol l-1 NaOH the chromatogram. The retention times decreased with increase NaBH4 solution flow rate 1.0 ml min-1 in the mobile phase flow rate. In another experiment, we found that the pH of the mobile phase did not aect the retention times of the mercury species studied significantly. For the best LC resolution, a solution containing 0.5% m/v L-cysteine (pH 5) at a flow rate of 1.6 ml min-1 was adopted in subsequent experiments. Selection of Hydride Generation Conditions Since HgII and organomercury show dierent behaviors and dierent sensitivities in the CV generation process,4,6,9 dierent mercury species were studied successively to obtain compromise operating conditions of the CV generation system.The concentration of NaBH4 is critical in the determination of Fig. 1 Schematic diagram of LC–CV system. mercury by CV generation.We therefore investigated the eect 684 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12Fig. 4 Eect of NaBH4 concentration on Hg ion signal. All the data points were relative to the signal obtained with conventional pneumatic nebulization (concentration of NaBH4 0%). The concentration of HNO3 was 0.05 mol l-1 at a flow rate of 0.5 ml min-1. system. The enhancement factors for mercury were dierent for these three Hg species.This may be attributed to the variation of the vapor generation eciency of the various mercury species. Fig. 5 shows the peak height of the FI peaks as a function of the concentration of HNO3. Since the LC mobile phase already contained 0.5% m/v L-cysteine, the concentration of HNO3 did not aect the mercury signal significantly. In fact, the ion signals of all three mercury species decreased when extra HNO3 was added. In the following experiments, no extra Fig. 2 Eect of L-cysteine concentration on (a) retention time and HNO3 was used in the CV generation system.(b) signal-to-background ratio of various mercury species. The mobile Although not illustrated here, in another experiment we phase flow rate was 1.0 ml min-1. Each mercury species was present found that the volume of the mixing coils (0–2 ml) did not at 50 ng ml-1. aect the mercury signal significantly. In fact, the mercury ion signals decreased slightly when an extra mixing coil was used.In the following experiments, no extra mixing coil was used, except for the necessary connecting tubing (40 cm×0.5 mm id). A summary of the optimum operating conditions of the vapor generation system is given in Table 2. Mercury Speciation A typical chromatogram (ICP-MS detection) for a solution containing HgII, methyl-Hg and ethyl-Hg is shown in Fig. 6. Fig. 3 Eect of mobile phase flow rate on liquid chromatogram. Each mercury species was present at 50 ng ml-1.Other LC conditions are given in Table 2. of NaBH4 concentration on the generation of mercury vapor. The results are shown in Fig. 4. As the NaBH4 concentration increased, the peak heights of various mercury species increased rapidly and reached the maximum when the NaBH4 concentration was about 0.1%. In order to avoid any possible matrix interference, the concentration of NaBH4 used was as low as possible. In the following experiments, 0.1% m/v NaBH4 was used. Compared with the conventional nebulization (0% Fig. 5 Eect of HNO3 concentration on Hg ion signal. All the signals NaBH4), as shown in Fig. 4, the mercury ion signals increased were relative to the first point. For concentrations and flow rates of other reagents, see Fig. 1. 8–36-fold when 0.1% NaBH4 was used in the CV generation Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 685Table 4 Calibration parameters (0.1–20 ng ml-1) for the mercury species Sensitivity/ Correlation Detection Compound counts s-1 ng-1 ml coecient limit/ng ml-1 HgII 590 0.9989 0.11 Methyl-Hg 2140 0.9994 0.03 Ethyl-Hg 1440 0.9995 0.04 Table 5 Mercury detection limits (ng ml-1) Method HgII Methyl-Hg Ethyl-Hg LC–CV–ICP-MS* 0.11 0.03 0.04 LC–CV–ICP-MS† 1.2 0.6 1.2 LC–PN–ICP-MS‡ 7 16 16 LC–USN–ICP-MS§ 0.12 0.22 0.26 LC–DIN–ICP-MS¶ 4.0 4.0 4.0 PC–LC–ICP-MSd 0.017 0.016 — LC–CV–MIP-AES** 0.15 0.35 — * This work, 100 ml sample loop.† Ref. 10, 100 ml sample loop. ‡ Ref. 10, 100 ml sample loop.PN=pneumatic nebulizer. Fig. 6 Typical Hg-selective chromatogram for A, HgII, B, methyl-Hg § Ref. 11, 200 ml sample loop. USN=ultrasonic nebulizer. and C, ethyl-Hg. Each mercury species was present at 5 ng ml-1 (as ¶ Ref. 13, 2 ml sample loop. DIN=direct injection nebulizer. Hg). For LC and CV conditions, see Table 2. d Ref. 14, 100 ml sample loop and preconcentration of 1.0 l of sample solution. ** Ref. 9, 100 ml sample loop. As shown, all three species studied were fully resolved and the separation was complete in less than 6 min.The background was used as a standard reference. A 100 ml injection of the at m/z 202 increased when CV generation sample introduction sample solution was analyzed for mercury using the CV was used, which could be due to the trace mercury contamigeneration system. The chromatogram obtained for this deter- nation of the reagents used for LC separation and CV genermination is shown in Fig. 7. As can be seen, both inorganic ation and to the better analyte transport eciency with CV mercury and methylmercury were present in this sample.A generation sample introduction or the memory of mercury small dip in the background in front of the HgII peak was from the spray chamber. Peak area measurements indicated found which was due to the matrix of the sea-water sample. that the response for mercury was dierent for these three Apparently, the chromatographic separation has isolated the mercury species.This may be attributed to variations in the mercury from the matrix. As shown in Table 6, the recoveries CV generation eciency of the various mercury species menof various mercury species from the sea-water sample were in tioned earlier. Similar results were observed when the analyte the range 93–97%. The amount of mercury present in this was determined in the FI mode. water sample was quantified by the external calibration method Repeatability was determined using five injections of a test and the results are given in Table 6.The concentration of mixture containing 5 ng ml-1 HgII, methyl-Hg and ethyl-Hg. mercury determined by LC–CV–ICP-MS is higher than the As shown in Table 3, the RSD of the peak heights was less reference value given in Ref. 21. It should be mentioned that than 4% for all the species, which is similar to the precision the reference value for mercury in the sample is given for obtained in previous ICP-MS experiments with LC separations total mercury.using other types of sample introduction device.11,16,18–20 A tap water collected from National Sun Yat-Sen University Calibration curves based on peak heights were linear for each was also analyzed for mercury. As shown in Table 6, no mercury compound in the range tested. The detection limits mercury was detected in this sample, possibly because the were calculated from these calibration curves and based on the amount (or concentration) necessary to yield a net signal equal to three times the standard deviation of the background.The absolute detection limits were 3–11 pg, which corresponds to relative values of 0.03–0.11 ng ml-1 (see Table 4). The use of more purified reagent should lower the detection limit. As shown in Table 5, the detection limits obtained in this work are comparable to or better than previous results with similar techniques.9–11,13,14 Determination of Mercury inWater Samples In order to prove that the system works for practical analysis, an open ocean sea-water reference material (NRCC NASS-4) Table 3 Repeatability of retention time and peak height of the LC elution peaks (n=5) Retention time Repeatability of Compound ±s/s peak height (RSD) (%) HgII 84±1 1.6 Fig. 7 Typical Hg-selective chromatogram of open ocean sea-water Methyl-Hg 143±1 1.5 NASS-4. The concentrations of HgII and methyl-Hg in the injected Ethyl-Hg 326±2 3.3 solution are 0.75 and 0.09 ng ml-1, respectively. 686 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12Table 6 Recoveries and concentrations of mercury in water samples as measured by LC–CV–ICP–MS. Values are means±standard deviations for three determinations Concentration Reference Sample Compound Recovery (%) found/ng ml-1 value/ng ml-1 NASS-4 sea-water HgII 93±6 1.50±0.12 0.9† Methyl-Hg 97±4 0.18±0.03 Ethyl-Hg 93±4 ND* Tap water HgII 102±7 ND Methyl-Hg 100±1 ND Ethyl-Hg 101±3 ND * ND=not detectable.† Ref. 21. 6 Aizpun, B., Fernandez, M. L., Blanco, E., and Sanz-Medel, A., concentrations of the mercury species in this sample were J. Anal. At. Spectrom., 1994, 9, 1279. below the detection limits of the LC–CV–ICP-MS system. 7 Schickling, C., and Broekaert, J. A. C., Appl. Organomet. Chem., Recoveries of various Hg species in a spiked tap water sample 1995, 9, 29. were in the range 100–102%. 8 Bulska, E., Emteborg, H., Baxter, D. C., Frech, W., Ellingsen, D., and Thomassen, Y., Analyst, 1992, 117, 657. 9 Costa-Fernandez, J. M., Lunzer, F., Pereiro-Garcia, R., Sanz- CONCLUSION Medel, A., and Bordel-Garcia, N., J. Anal. At. Spectrom., 1995, 10, 1019. The merits of coupling LC and ICP-MS with the CV generation 10 Bushee, D. S., Analyst, 1988, 113, 1167. technique for mercury speciation have been demonstrated. The 11 Huang, C.-W., and Jiang, S.-J., J. Anal. At. Spectrom., 1993, 8, 681. 12 Bushee, D. S., Moody, J. R., and May, J. C., J. Anal. At. Spectrom., detection limits of various mercury species obtained with this 1989, 4, 773.system are low enough for the mercury speciation of many 13 Shum, S. C. K., Pang, H.-M., and Houk, R. S., Anal. Chem., 1992, real samples without complicated sample pre-treatment. The 64, 2444. use of more purified reagents should lower the detection limit. 14 Bloxham, M. J., Gachanja, A., Hill, S. J., and Worsfold, P. J., Other applications of this CV generation ICP-MS system are J. Anal. At. Spectrom., 1996, 11, 145. under investigation. 15 Hwang, J. D., Huxley, H. P., Diomiguardi, J. P., and Vaughn, W. J., Appl. Spectrosc., 1990, 44, 491. 16 Hwang, C.-J., and Jiang, S.-J., Anal. Chim. Acta, 1994, 289, 205. This research was supported by a grant from the National 17 Huang, M.-F., Jiang, S.-J., and Hwang, C.-J., J. Anal. At. Science Council of the Republic of China. Spectrom., 1995, 10, 31. 18 Jiang, S.-J., and Houk, R. S., Spectrochim. Acta, Part B, 1988, 43, 405. REFERENCES 19 Yang, H.-J., Jiang, S.-J., Yang, Y.-J., and Hwang, C.-J., Anal. Chim. Acta, 1995, 312, 141. 1 Schroeder, W. H., T rends Anal. Chem., 1989, 8, 339. 20 Yang, H.-J., and Jiang, S.-J., J. Anal. At. Spectrom., 1995, 10, 963. 2 Harrison, R. M., and Papsomanikis, S., Environmental Analysis 21 Debrah, E., Denoyer, E. R., and Tyson, J. F., J. Anal. At. Using Chromatography Interfaced with Atomic Spectroscopy, Ellis Spectrom., 1996, 11, 127. Horwood, New York, 1989, ch. 10. 3 Evans, O., and McKee, G. D., Analyst, 1988, 113, 243. Paper 6/05765I 4 Fujita, M., and Takabatake, E., Anal. Chem., 1983, 55, 454. Received August 19, 1996 5 Lupsina, V., Horvat, M., Jeran, Z., and Stegnar, P., Analyst, 1992, 117, 673. Accepted February 24, 1997 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 687

ISSN:0267-9477    

DOI:10.1039/a605765i

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Speciation of Arsenic Compounds in Drinking Water by Capillary Electrophoresis with Hydrodynamically Modified Electroosmotic Flow Detected Through Hydride Generation Inductively Coupled Plasma Mass Spectrometry With a Membrane Gas–Liquid Separator MATTHEW L. MAGNUSON*a , JOHN T. CREEDb AND CAROL A. BROCKHOFFb aUnited States Environmental Protection Agency, National Risk Management Research L aboratory, Water Supply and Water Resources Division, T reatment T echnologies Evaluation Branch, 26 W.Martin L uther King Drive, Cincinnati, OH 45268, USA bUnited States Environmental Protection Agency, National Exposure Research L aboratory, Human Exposure Research Division, Chemical Exposure Research Branch, 26 W.Martin L uther King Drive, Cincinnati, OH 45268, USA Capillary electrophoresis (CE) was used to speciate four depending on the distribution of the arsenic species.Therefore, environmentally significant, toxic forms of arsenic: arsenite, determining total arsenic can be misleading while quantifying arsenate, monomethylarsonic acid and dimethylarsinic acid.individual arsenic species provides essential information for Hydride generation (HG) was used to convert the species into exposure assessment. Four of the more toxic arsenic comtheir respective hydrides. The hydride species were detected pounds are arsenite (AsIII), arsenate (AsV), monomethylarsonic with inductively coupled plasma mass spectrometry. The HG acid (MMA) and dimethylarsinic acid (DMA).The organounit utilized a microporous PTFE tube as a gas–liquid arsenicals, MMA and DMA, are important metabolites of separator. The injection mode for CE was electrokinetic in arsenic in humans and are not typically found in drinking conjunction with the novel use of hydrodynamically modified water. For drinking water analysis, the more important forms electroosmotic flow (HMEOF). In HMEOF, the are inorganic arsenic species, AsIII and AsV. AsIII and AsV are electroosmotic flow is modified by applying hydrodynamic likely to be found in water because of release of arsenic pressure opposite to the direction of the electroosmotic flow.compounds from natural mineral deposits.1,2 Inorganic arsenic HMEOF provides the capability of injecting increased may also enter the water supply through industrial release.1,2 quantities of analyte by osetting the electroosmotic flow, The risk associated with the natural or industrial release of which limits conventional electrokinetic injection.In order to arsenic is best characterized via speciation. Speciation can also correct for imprecisions in the electrokinetic injection in aid in studies of remediation of arsenic in drinking water matrices of dierent ionic strength, the use of a surrogate for because AsV is typically easier to remove from water than AsIII.3 the injection of arsenic species was investigated. Germanium Given the diversity in toxicity and the need to determine was investigated because it forms a hydride and has a low separate species, the detection scheme must be extremely natural occurrence. The separation also utilized HMEOF, sensitive.Sensitivity in the detection of arsenic species can be which allowed for greater freedom in buer choice. The greatly increased through the use of hydride generation detection limits in distilled, de-ionized water were 25, 6, 9 and (HG).4–12 Recent advances in ICP-MS interface designs com- 58 ppt for the four species listed above, respectively. The bined with the increased transport eciency of arsenic hydride detection limit was calculated from 3.14sn-1 of seven replicate species make the coupled HG–ICP-MS technique one of the injections and represents the precision of measuring the ratio most sensitive approaches to the determination of arsenic of the area of the arsenic peaks to the area of a germanium species.A popular approach to speciation of arsenic comsurrogate peak.Standard addition was used to determine pounds is to use high-performance liquid chromatography arsenate in drinking water samples. Recoveries of arsenite and (HPLC), often in the form of ion chromatography arsenate from drinking water samples are reported using (IC).4,7–9,13–29 Magnuson et al.4 have recently developed an IC germanium as a surrogate to correct for sampling bias of the separation for the four arsenic species listed above in conjuncelectrokinetic injection.tion with an HG system and ICP-MS detection. Capillary electrophoresis (CE) has recently been interfaced Keywords: Speciation; arsenic compounds; capillary to UV,30–32 indirect UV,33 conductivity,30 and ICP-MS34,35 electrophoresis; electrokinetic injection; modified detectors to speciate arsenic compounds. The interfaces electroosmotic flow; hydride generation; microporous between CE and ICP-MS, based on nebulization, have allowed membrane gas–liquid separator; inductively coupled plasma for lowered detection limits.34,35 The detection limits in these mass spectrometry systems are determined by the eciency of the nebulizer, while the CE buer salt can cause build-up on the sampling interface, which can influence long-term stability.A CE interface which The chemical form of arsenic greatly influences its toxicity.1,2 incorporates HG could circumvent both of these nebulization When arsenic is present in the AsIII valence state, it is several based problems.The gaseous hydride is nearly quantitatively orders of magnitude more toxic than organoarsenic species. If introduced into the plasma without the high dissolved solids the risk of environmental exposure to arsenic is based on the commonly associated with CE buers. These advantages are total amount of the element present, the risk of the exposure can be significantly underestimated or overestimated demonstrated in this paper by interfacing a CE unit to a Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 (689–695) 689microporous membrane as a gas–liquid separator for the EXPERIMENTAL detection of arsenicals via ICP-MS.Instrumentation Arsenic speciation with CE–HG–ICP-MS presents two chal- Important parameters for the CE–HG–ICP-MS system are lenges: (1) design of a suitable interface between the CE and summarized in Table 1. The ICP-MS instrument was a the HG–ICP-MS systems and (2) increasing the amount of Hewlett-Packard (Avondale, PA, USA) Model 4500 (HP 4500) analyte injected into the capillary (an inherent CE limitation) benchtop system.Optimized system parameters for the HP to decrease the detection limits. The CE–HG–ICP-MS 4500 with HGwere similar to solution nebulization parameters. interface developed in this paper utilizes the HG–ICP-MS The standard HP 4500 utilizes a nickel sampler cone (1.0 mm system developed earlier.4 The use of this system provided orifice) and a nickel skimmer cone (0.4 mm orifice). The excellent chromatographic peak shapes, picogram detection instrument was tuned by adding 2 ppb of arsenic to the HCl limits and good long-term reproducibility.4 The chloride interused for HG, allowing optimization on a continuous arsenic ference at m/z 75 common to HG systems was eliminated by using a microporous membrane as the gas–liquid sep- signal.The rf power was set at 1200 W, the plasma gas flow rate at 15 l min-1 and the auxiliary gas flow rate at 1.0 l min-1.arator. The membrane eciently transports the gaseous hydride, leaving the chloride interference in the liquid A sampling depth of 5.2 mm was used with the torch position slightly (-0.1 mm horizontally, 0.8 mm vertically, set in the phase.4,5,8,10,11,36–44 To address the analyte injection challenge, electrokinetic instrument software) o the axis of the sampling orifice to reduce the noise while maintaining the signal. injection was utilized.In electrokinetic injection, charged species migrate into the CE capillary under the influence of an Electrophoretic data were collected in the time resolved analysis (TRA) mode, and peak areas were measured with the applied high voltage electric field at rates governed by their electrophoretic mobilities. This well known technique34,35,45–49 chromatographic integration software provided with the instrument. Although CE–HG–ICP-MS produces unique peak is limited because the high voltages of injection produce an electroosmotic flow within the capillary, which causes broaden- shapes, the software is suitable for integrating electrophoretic peak shapes.For consistency, the peak area was integrated ing of the analyte zone as the injection time (injected amount) is increased. This limitation is eliminated by using a small using the average level of the background to define the beginning and end of each peak. hydrodynamic pressure applied opposite to the direction of the electroosmotic flow.The flow induced by this applied The CE unit was a Dionex (Sunnyvale, CA, USA) Model CES-1. The polyimide-coated, fused-silica capillary was pressure is referred to as hydrodynamically modified electroosmotic flow (HMEOF). Once the electroosmotic flow is obtained from Polymicro Technologies (Phoenix, AZ, USA). To apply pressure for the HMEOF, the pressure regulator ‘balanced’ with the HMEOF, the injection time becomes arbitrary, resulting in an increase in the amount of analyte built into the CE unit was replaced with an external pressure source, a sub-miniature pressure regulator (McMaster Carr, injected, and the detection limits can be lowered by increasing the injection time.This is particularly useful for analytes with Chicago, IL, USA; Part No. 41795K3) equipped with a suitable pressure gauge (McMaster Carr; Part No. 3842K5). This low electrophoretic mobility, such as AsIII. The eciency of electrokinetic injection is aected by the ionic strength of the allowed control of the pressure applied to the sample vial in increments of 0.1 psi (689.5 Pa), compared with the 0.5 psi matrix, so germanium (GeIV) was investigated as a surrogate for quantification to correct for sampling bias of the electro- (3447.5 Pa) increments with the pressure regulator standard with the CE unit.kinetic injection. Germanium was chosen because of its hydride forming ability and its low natural occurrence.HMEOF can Fig. 1 is a schematic diagram of the CE–HG–ICP-MS system. Because of the low flow from the capillary column (nl also be used during separation to control the bulk flow characteristics, allowing for a wider choice of buers. The use min-1), a make-up buer, supplied by a peristaltic pump (Minipuls, Gilson, Middletown, WI, USA), acts as a liquid of hydrodynamic pressure during CE is unusual, and is avoided for many CE applications because it induces a laminar flow, carrier for the capillary euent.The capillary ground is completed through this make-up buer. The platinum ground which may distort the ‘flat’ electroosmotic flow profile.48 For HG–ICP-MS detection, the benefits of HMEOF seem to wire and the make-up buer enter through a Teflon tee (Omnifit, Toms River, NJ, USA). The remaining port is connec- outweigh the eects of the laminar flow. The CE–HG–ICP-MS system was used to analyze several drinking water samples.ted via an inert tube to a Teflon cross (Omnifit). One port of Table 1 CE–HG–ICP-MS experimental parameters Capillary electrophoresis— Fused silica capillary column 85 cm×75 mm id Buer (20 mM potassium hydrogen phthalate–20 mM boric acid) pH 9.03 Analyte injection method Electrokinetic Electro–osmotic flow modifier Hydrodynamic pressure CE–HG interface (make-up buer) peristaltic pump flow rate 0.32 ml min-1 Hydride generation system— HCl (10% m/m) flow rate 1.13 ml min-1 NaBH4 (1% m/m) flow rate 0.59 ml min-1 Carrier gas (argon) flow rate 0.13 l min-1 Make-up gas (argon) flow rate 1.22 l min-1 ICP-MS— Acquisition mode Time resolved analysis Ge mass monitored m/z 74.0 Integration time for Ge per electropherogram point 0.1 s As mass monitored m/z 75.0 Integration time for As per electropherogram point 2.0 s 690 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12water used was de-ionized to 18 MV using a Milli-Q system (Millipore, Milford, MA, USA).Solutions were prepared in fresh Nalgene polyethylene bottles. The CE sample vials were purchased from Dionex. The HCl (Fisher, Fair Lawn, NJ, USA; ACS+ certified) was used after determining that the arsenic concentration within this acid was lower than a ‘high purity’ acid. Dilutions were made by mass with distilled water. The NaBH4 (97+% pure, Alfa AESAR, Johnson Matthey, Ward Hill, MA, USA) was made up on a mass basis. The NaBH4 was stabilized by adding 7.5 ml of 50% m/m NaOH (Fisher) per liter of solution, and fresh solutions were prepared daily.The arsenic solutions were prepared on an arsenic mass basis. Arsenite was derived from solid arsenic(III ) trioxide (SPEX Industries, Edison, NJ, USA) made up to 1000 ppm in 1% nitric acid, and arsenate was prepared from a 1000 ppm standard of orthoarsenic acid in 2% nitric acid (SPEX Industries). Monomethylarsonic acid and dimethylarsinic acid (both 98% pure) were obtained from Chem Service Chemicals (West Chester, PA, USA).Working solutions of the arsenic species were prepared by dilution from a 1 ppm (in arsenic) standard in distilled water. The concentration of these 1 ppm standards in distilled water was confirmed by inductively coupled plasma atomic emission spectrometry (ICP–AES). Germanium solutions were prepared from a 1000 ppm standard solution (SPEX Industries) of (NH4)2Ge(C2O4)3 4H2O. Working solutions were prepared by dilution from a 1 ppm (in germanium) standard in distilled water.The salts used in the buer solution for the CE were chosen because they contained lower arsenic and germanium backgrounds than other salts available in our laboratory. For the phthalate–borate system chosen, the source of borate ion was Fig. 1 Schematic diagram of CE–HG–ICP-MS unit. boric acid (99%, Fisher). Phthalate ion was derived from potassium hydrogen phthalate (primary standard, Fisher). the cross has a short open ended tube which is positioned at These sources served to minimize the arsenic and germanium the same height as the sampling end of the capillary.This backgrounds in our buers. pH was adjusted with 10% m/m prevents siphoning and relieves any pressure imbalance inside ammonia solution (Optima, Fisher). The pH meter used was the cross, which would induce a flow in the capillary. A an Orion Research (Boston, MA, USA) Model 620 pH meter Minipuls peristaltic pump isolates the CE from the HG system.equipped with a temperature compensated Orion 6165 probe. This is necessary because the HG reaction produces large The pH adjustment prior to analysis did not cause crossvolumes of hydrogen, which induces a back-pressure on the contamination of the samples. column exit. This back-pressure would tend to cause a large, undesirable flow in the capillary. The capillary euent along with the make-up buer is delivered via an isolating peristaltic pump into a three-way PTFE manifold where it mixes with RESULTS HCl.The HCl (1.13 ml min-1) is delivered using the built-in Optimization of CE–HG–ICP-MS System peristaltic pump of the HP 4500. This mixture flows into a second three-way manifold where NaBH4 (0.59 ml min-1) is The optimization of the CE–HG–ICP-MS system can be delivered using a peristaltic pump. After passing through a broken into two parts: (1) setting the flow rate of the peristaltic 27 cm glass bead mixing coil (not shown in Fig. 1), the gas– pump isolating the CE from the rest of the system (see Fig. 1) liquid mixture proceeds through the membrane gas–liquid and (2) optimizing the HG conditions using this flow rate. separator (MGLS) shown at the bottom of Fig. 1. The analyte The optimization of the isolating peristaltic pump flow rate gases and the excess H2 migrate across the microporous involves two concerns, the peak shape and the analytical membrane of the MGLS into a stream of argon carrier gas response. An AsIII solution was placed in a CE sample vial which is introduced directly into the central channel of the and continuously introduced into the make-up stream by ICP torch via a short length of Tygon tubing.The construction pressurizing the sample vial. Fig. 2 shows the decrease in the of the membrane based gas–liquid separator has been described analytical response from a 10 ppb solution of AsIII as the previously.4 The membrane used in the gas–liquid separator isolating flow rate is increased. The decrease in signal is was a ‘high density’ (0.9 g ml-1) expanded PTFE material, possibly due to dilution of the HG reagents, which in turn 0.7 mm id, available from International Polymer Engineering decreases the hydride generating eciency.This increase in (Tempe, AZ, USA). Because of the relatively high flow rates signal intensity induced by low peristaltic pump flow rates through the HG system, attempts to decrease the volume of must be balanced with CE peak broadening which occurs at the system did not dramatically aect the residence time and, low flow rates.The lower the flow rate, the longer is the in turn, peak shape. residence time of the analyte in the tubing, and the longer the residence time, the greater is the diusional broadening. Pharmed tubing with id 0.015 in provided for a suciently Reagents short residence time to avoid excessive peak broadening at a compromise flow rate of 0.32 ml min-1. Smaller diameter All reagents and solutions were handled and prepared in a Class 100 clean air hood to avoid contamination.The distilled tubing did not significantly change the peak width, possibly Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 691Fig. 2 Response changes of 10 ppb of AsIII as a function of the flow rate of the peristaltic pump which isolates the CE unit from the Fig. 4 Response changes of the four arsenic species and germanium HG system. as a function of HCl concentration for 1% m/m NaBH4.Capillary Electrophoresis because any peak narrowing is overshadowed by the greater pH of sample and buer considerations broadening which occurs in the rest of the HG system. The concentrations of the HG reagents are related to the Speciation of arsenic compounds by CE has attracted attention amounts of the arsines and germane produced. All four arsenic recently.30–33,48,49 Owing to the low direct injection volumes species and germanium have diering responses to the HG (nl), sample stacking with solvent plug removal has been used conditions.Fig. 3 is a plot of response of these five species as to increase the amount of analyte injected.32 Another method a function of NaBH4 concentration at a 10% m/m HCl to increase injection volumes is electrokinetic injection,45–49 in concentration. Analyte solutions were placed in the CE sample which the charged species migrates into the capillary under vials and continuously introduced into the make-up stream by the influence of an applied electric field.The eciency with pressurizing the sample vials, resulting in an approximately which a charged species can be electrokinetically injected is 3.1 ml min-1 flow rate, determined by the amount of time related to its electrophoretic mobility. The arsenic species required to empty the capillary filled with the analyte into the studied here are acids, so their electrophoretic mobilities along HG system.The count rates in Fig. 3 have been normalized with their net charge are dependent on the pH of the sample to 10 ppb of each analyte being added to the make-up stream. solution. In general, the higher the degree of deprotonation, The responses of the five analytes increase up to 1% NaBH4. the higher is the electrophoretic mobility. The eects of pH on Between 1 and 2%, the responses for AsIII and MMA decrease the electrophoretic mobility of AsIII, AsV, MMA and DMA and those for AsV, DMA and Ge remain fairly constant.It have been documented.30–33 AsIII has its only pKa at 9.3,31 so has been suggested that this response behavior represents a to allow for ecient electrokinetic injection, the pH of the competition between the actual reduction of the species by the sample solutions was increased to 10.00 with ultra-pure NaBH4 and a decrease in ionization eciency of the plasma ammonia solution. The pH of normal drinking water is around resulting from an increase in the amount of H2 produced by 8.The use of a carbonate ion chromatographic buer at the HG reaction.4 pH 10.3 resulted in total conversion of AsIII into AsV in the Fig. 4 shows plots of the responses of the five analytes as a time for the chromatographic analysis (minutes).25 In this function of HCl concentration using a 1% NaBH4 concen- work, no conversion of AsIII to AsV was observed on the day tration. The responses of the four arsenic species are similar the sample was adjusted to pH 10.00 with ammonia solution.to those reported previously.4 The response for Ge drops Over the course of several days, however, AsIII was observed dramatically from 35000 to 5000 between 2 and 10% HCl. to convert into AsV. Therefore, the samples were analyzed These response dierences most likely represent a dierence in immediately after adjusting the pH. mechanism for the formation of the respective hydrides. The pH of the CE buer aects the separation characteristic Optimum conditions of 10% HCl and 1% NaBH4 were based by aecting the electrophoretic mobility of the arsenic on the relatively low rate of change in sensitivity for Ge, AsIII, anions30–33 as they migrate through the buer under the MMA and AsV. influence of the applied electric field.For the 20 mM borate– phthalate conductive system used, the buer pH was empirically adjusted to 9.03, measured accurately, to resolve AsV and DMA. Although the buer pH (9.03) required for the separation was dierent to that of the sample solution (10.00), no serious eect on the CE separation was observed.Hydrodynamically modified electroosmotic flow (HMEOF) In the CE–HG–ICP-MS system, two CE related goals were set: to maximize the injection volume and minimize the analysis time. In practice, both of these could be achieved through controlling the bulk flow characteristics of the liquid through the capillary. The flow in a non-siphoning, internally uncoated, fused silica capillary is conventionally governed by the electroosmotic flow generated during high voltage CE operations.It is possible to modify the bulk flow by pressurizing the sample vial during high voltage CE operations, which generates Fig. 3 Response changes of the four arsenic species and germanium as a function of NaBH4 concentration for 10% m/m HCl. another flow (opposite to the bulk flow) within the CE capillary. 692 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12Because in the internally uncoated fused silica capillary the decreasing the separation time. The increase in the amount of analyte is demonstrated in Fig. 6. In Fig. 6, the total peak area electroosmotic flow generated by a negative voltage (relative to the injection side of the capillary) is toward the injection (and therefore the amount injected) is given as a function of time for electrokinetic injection with HMEOF. The increase side, pressurizing the sample vial has the eect of counteracting the electroosmotic flow.This HMEOF can be utilized for both in peak area for the analytes is fairly linear. The dashed vertical line around 0.5 min represents the limit that can be achieved the electrokinetic injection and the CE separation of anions. The HMEOF experiment is illustrated in Fig. 5. First, a with electrokinetic injection without the use of HMEOF. This time limit exists because the analytes are flushed back (via short plug of distilled, de-ionized water is injected into the capillary [Fig. 5(a)], which has beneficial eects for electroki- electroosmotic flow) into the sample vial and are unavailable for detection. The use of HMEOF osets the electroosmotic netic injection by avoiding a bias caused by the change in electric field characteristic between the sample and the buer.47 flow, thereby allowing an arbitrary choice of electrokinetic injection times which, in turn, lowers the detection limits.Next, the sample vessel is pressurized to the injection pressure [P(i)#2.0 psi (13.79 kPa)], and the high voltage (-22000 V) The eect of HMEOF on analysis times is illustrated in Fig. 7. In Fig. 7, the approximately 16 min electropherogram is applied for electrokinetic injection. The high voltage [Fig. 5(b)] produces a bulk flow which is a combination of the (light trace) is of the four arsenic species and the Ge surrogate with about 2.2 psi (15.2 kPa) of applied pressure during electroosmotic flow of the water and the electroosmotic flow of the buer. This bulk flow pushes the water plug out the separation, P(s) [see Fig. 5(c)]. The solution which was electrokinetically injected for 2 min (with HMEOF) contained 1 ppb injection side of the capillary. The bulk flow slows as the water plug is removed because the contribution to the electroosmotic of each arsenic species and 50 ppb of germanium. The dierence in intensities between the species is related to their response flow from the water is greater than that from the buer.With the use of HMEOF, the final plug length is related to the to the HG process (Figs. 3 and 4) and the integration times used for each mass (Table 1). The order of elution of the hydrodynamic pressure [P(i)], which alters the electroosmotic flow, causing the water plug length to remain constant, as anions is the same as that predicted by the electrophoretic mobility.31 The retention times are 7.2 and 14.2 min for AsV illustrated in Fig. 5(b). The sample anions electromigrate through the water plug and stack up at the interface between and AsIII, respectively. If the applied pressure is increased to P(s)#2.9 psi (20.0 kPa), then the retention times (Fig. 7, left the water plug and the buer. In practice, the ionic strength of the sample must be suciently weak, or separation will side, bold trace) decrease to 3.6 and 4.5 min for AsV and AsIII, respectively.The peaks corresponding to these retention times occur while electrokinetic injection takes place. For most water samples, the ionic strength is suciently low to avoid simul- have been oset from the baseline and scaled for graphical purposes. The shorter analysis precludes separate observation taneous separation and injection. The length of the water plug is important for separation. for MMA and DMA, but these compounds are not usually present in drinking water, so the faster separation has utility The water plug must be suciently short or the voltage drop across it will prevent separation from occurring.For 20 mM for AsIII and AsV, which are the common arsenic species in drinking water. phthalate–borate solution at pH 9.03, P(i)#2.0 psi (13.79 kPa) provided a reasonable plug length. For separation, the capillary The application of hydrodynamic pressure during high voltis moved to the buer solution and the separation occurs at -22000 V as illustrated in Fig. 5(c). The high negative voltage induces an electroosmotic flow away from the detector, but the application of the HMEOF separation pressure [P(s)] causes bulk flow toward the detector. The retention time of the analytes is then dependent on the HMEOF induced bulk flow rate and the electrophoretic mobilities of the analytes [Fig. 5(c)]. The use of HMEOF to direct the bulk flow has two benefits: increasing the amount of analyte injected and potentially Fig. 6 Eect of the amount of analytes injected through the use of HMEOF in comparison with electrokinetic injection without HMEOF.Fig. 7 Two electropherograms of arsenic species and germanium surrogate illustrating the influence of applied pressure P(s), which Fig. 5 Schematic diagram of HMEOF experiment. creates HMEOF during separation. Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 693age operations has been applied to electrochromatography.50 additions, the use of an external calibration for quantification was investigated.For external calibration, it is necessary to For CE, hydrodynamic pressure induces laminar flows and alters the well known ‘flat’ electroosmotic flow profile.34 Thus, correct for changes in the eciency of the electrokinetic injection caused by the ionic strength of the matrix, e.g., applying hydrodynamic pressure is generally avoided for CE applications. HMEOF with relatively low pressures does not sampling bias. For this reason, germanium (GeIV) was investigated as a surrogate to correct for matrix eects because it seem to produce excessively laminar peak shape in our HG–ICP-MS detection system.The peak shapes in Fig. 7 are forms a hydride and is present in low natural abundance. The RSDs of the ratio of retention times (Table 2) of the symmetrical and do not appear to have been aected by the application of low hydrodynamic pressures [<3 psi arsenic peaks versus the germanium peak are about 5% for AsIII, MMA and DMA and about 10% for AsV.This indicates (20.7 kPa)]. Unsymmetrical laminar peak shapes were observed for high pressure HMEOF experiments [12 psi that retention times are stable with the HMEOF technique. Table 2 also shows that the imprecision of measuring the ratio (82.7 kPa), not shown], so laminar broadening probably increases with pressure. of the Ge to As peak areas is generally less than 5%. Using this ratio, recoveries of two drinking water samples fortified Table 2 shows that there is a small dierence in the detection limits measured under these two P(s) conditions.Specifically, with 1 ppb of AsIII showed recoveries of 91 and 92% with RSDs of 0.4 and 2.7%, respectively. Germanium thus appears the detection limit of AsV increases from 6 to 17 ppt, probably owing to some small change in the peak shape resulting in to be a good surrogate match for quantification of AsIII, in terms of correcting for electrokinetic sampling bias. For AsV, increased variation by the integration software in determining the beginning and end of an eluting peak.The detection limit the recoveries were 60–70% with similar RSDs, which indicates that germanium is not as good a match for the determination of AsIII actually decreases from 25 to 17 ppt, most likely because its shorter residence time in the capillary limits thermal of AsV. This dierence may be explained by dierences in the electrophoretic mobilities of Ge and AsV.The dierence in broadening (produced by Joule heating within the capillary), thereby producing less variation in the integration. In general, the electrophoretic mobilities reflects the small dierence in the retention times of AsIII and Ge and the large dierence the CE–HG–ICP-MS peaks are 50–100% broader than in other reports of CE separation of arsenic species detected with between AsV and Ge. Investigations into alternate surrogates for AsV are at this stage preliminary.zero dead volume detectors, such as a UV detector.30–33 The dierence in peak width is not surprising given the dead volume of the hydride generator and membrane gas–liquid CONCLUSIONS separator. In spite of the dead volume, the peak shapes from the CE–HG–ICP-MS system are fairly symmetrical. The CE–HG–ICP-MS system described here is capable of speciating four of the environmentally significant, toxic arsenic compounds and can speciate AsIII and AsV in real water DrinkingWater Samples samples in less than 5 min.The use of HMEOF during Two water samples were analyzed with the CE–HG–ICP-MS electrokinetic injection improves the detection limits by increas- system using the method of standard additions. No AsIII was ing the amount of analyte injected. The use of HMEOF during found in the samples. This is not surprising since the drinking separation does not appear to aect the peak shapes adversely waters had been disinfected by an oxidative process, and it through laminar asymmetries and shortens the analysis time.was expected that any AsIII would be converted into AsV in The use of Ge as a surrogate allows the direct determination the disinfection process. AsV was present in both samples, one of AsIII, but Ge is not appropriate for AsV, possibly because at 1.1 ppb and the other at 0.2 ppb. of electrophoretic mobility dierences. Because of the additional time required to perform standard This work was performed while Matthew L.Magnuson held a National Research Council–US EPA Associateship in the Table 2 Method detection limits (MDL) and HMEOF results for CE–HG–ICP-MS of arsenic species in distilled de-ionized water National Exposure Research Laboratory, Cincinnati, OH. AsV DMA MMA AsIII REFERENCES MDL*,† for four 6 58 9 25 species (ppt) 1 US Public Health Service, T oxicological Profile for Arsenic, 1989, ATSDR/TP-88/02, US Public Health Service, Washington, DC, MDL*,‡ for two 17 — — 17 species (ppt) 1989. 2 US Environmental Protection Agency, Special Report on Ingested As/Ge retention 0.54±0.04 0.69±0.03 0.62±0.04 0.94±0.01 time ratio§ Inorganic Arsenic: Skin Cancer; Nutritional Essentiality, EPA/625/3–87/013, US EPA, Washington, DC, 1988. As/Ge peak 1.9±0.1 1.07±0.05 2.8±0.1 2.15±0.09 area ratio§ 3 US Environmental Protection Agency, T reatment and Occurrence—Arsenic in Potable Water Supplies, Contract No. 68-CO-0062, US EPA, Washington, DC, 1993.* Calculated from 3.14sn-1 of seven replicate injections.51 Thus, the MDL represents the precision of measuring the ratio of the area of 4 Magnuson, M. L., Creed, J. T., and Brockho, C. A., J. Anal. At. Spectrom., 1996, 11, 893. the arsenic peaks to the area of a 50 ppb germanium surrogate peak. The solution was adjusted to pH 10.00. 5 Creed, J. T., Magnuson, M. L., Brockho, C. A., Chamberlain, I., and Sivaganesan, M., J. Anal. At. Spectrom., 1996, 11, 505. † Based on a 2.0 min electrokinetic injection with HMEOF.The hydrodynamic pressure applied during separation was 2.25 psi 6 Hwang, C.-J., and Jiang, S.-J., Anal. Chim. Acta, 1994, 289, 205. 7 Le, X.-C., Cullen, W. R., and Reimer, K. J., T alanta, 1994, 41, 495. (15.51 kPa), 0.25 psi (1.72 kPa) greater than the hydrodynamic pressure applied during electrokinetic injection. 8 Story, W. C., Caruso, J. A., Heitkemper, D. T., and Perkins, L., J. Chromatogr. Sci., 1992, 30, 427, and related personal ‡ Based on a 2.0 min electrokinetic injection with HMEOF.The hydrodynamic pressure applied during separation was 2.90 psi communications. 9 Roehl, R., Alforque, M. M., and Riviello, J., paper presented at (20.00 kPa), 0.90 psi (6.21 kPa) greater than the hydrodynamic pressure applied during electrokinetic injection. the 1992 Winter Conference on Plasma Spectrochemistry, January 6–11, 1992. § The concentrations of the arsenic species were 1 ppb and that of the germanium surrogate was 50 ppb.The solution was adjusted to 10 Branch, S., Corns, W. T., Ebdon, L., Hill, S., and O’Neill, P., J. Anal. At. Spectrom., 1991, 6, 155. pH 10.00 with dilute ammonia solution. The buer was 20 mM borate– phthalate, adjusted to pH 9.03. Values and sn-1 were based on seven 11 Wang, X., Viczian, M., Lasztity, A., and Barnes, R. M., J. Anal. At. Spectrom., 1988, 3, 821. replicate injections.51 694 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 1212 Thompson, M., and Walsh, J. N., A Handbook of Inductively 32 Li, K., and Li, S. F. Y., Analyst, 1995, 120, 361. 33 Lin, L., Wang, J., and Caruso, J., J. Chromatogr. Sci., 1995, 33, 177. 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ISSN:0267-9477    

DOI:10.1039/a607730g

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Langmuir Probe Potential Measurements for Reduced-pressure Inductively Coupled Plasma Mass Spectrometry XIAOMEI YAN*†a , BENLI HUANGa , TOMOKAZU TANAKAb and HIROSHI KAWAGUCHIb aDepartment of Chemistry, Xiamen University, Xiamen FJ 361005, China bDepartment of Materials Science and Engineering, Nagoya University, Nagoya 464-01, Japan A floating Langmuir probe was used to measure the apparent frequency (rf ) power with the plasma and result in a violent secondary discharge at the sampling orifice.6 This phenomenon dc oset potential of a reduced-pressure inductively coupled plasma near a substitute sampling orifice of a mass was particularly serious at lower pressure, where strong copper signals originating from the sputtering of the sampling orifice spectrometer.The experimental results demonstrate that the dc oset potential causes the secondary discharge at the were observed in mass spectra.1 Further investigation showed that the employment of an electrostatically shielded water- sampling orifice. The plasma potential is in the range +3.5 to +20 V and varies with the plasma operating conditions.The cooled (ESWC) torch configuration could be an eective way to eliminate the secondary discharge, and the capacitive coup- manner by which a water-cooled torch is shielded has a substantial eect on the plasma potential, then the secondary ling eect can be controlled by a carbon-film resistor inserted in the shielding circuit.1 discharge.The measured values of the potential give a good explanation for the enhanced capacitive coupling eect in Generally, it is recognized that the secondary discharge occurring at the sampling orifice is correlated with the rf and reduced-pressure ICP-MS reported previously. then the dc plasma potential originating from the capacitive Keywords: Reduced-pressure plasma; inductively coupled coupling eect.6–8 Therefore, a higher dc oset potential could plasma mass spectrometry; plasma potential; secondary be assumed for reduced-pressure ICP-MS, especially at low discharge; L angmuir probe pressure.Usually, the plasma potential can be estimated directly with a Langmuir probe measurement8–10 or indirectly via ion kinetic energy determination.8 It should be noted that, Recently, a number of investigations on reduced-pressure inductively coupled plasma mass spectrometry (ICP-MS)1–4 since the mobility of electrons in the plasma is much higher than that of positive ions, electrons will move preferentially to have shown its potential in solving the problems encountered in conventional atmospheric pressure ICP-MS, such as numer- the probe, leaving a positively charged plasma.This process continues until the potential between the plasma and probe is ous polyatomic interferences, ionization diculties for nonmetals and high gas and power consumption. Evans and sucient to keep the plasma neutral. Under this condition, the probe is at floating potential.The plasma potential is Caruso2 reported easy plasma generation for a number of gases, such as He, O2, N2 and air. The detection limits obtained always slightly positive with respect to the floating potential (plasma-sheath property). In reality, the floating potential is for halogenated gas chromatographic euents using a helium low-pressure ICP-MS system are comparable to those found the quantity that is being measured. However, the Langmuir probe can provide useful information, although approximate, with helium MIP-MS.3 The capability of adjusting the degree of fragmentation of organic components was also demonstrated about the ICP potential, because the error introduced is less than 1 V,8,11 which is much lower than the plasma potential with the employment of very low incident power (15–50 W).4 In our fundamental study on the characteristics of reduced- reported in the literature.6–10 Hence the floating potential is taken as the plasma potential in the present paper.pressure ICP-MS,1 it was found that the ionization eciency of non-metallic elements could be much improved owing to Here, a special Pyrex discharge chamber was constructed to be incorporated with the original extended torch used for the obvious deviation from local thermal equilibrium (LTE) and the energy transfer from high kinetic energy electrons. reduced-pressure ICP-MS.1,5 The dc oset plasma potential was measured by a floating Langmuir probe inserted into the This was confirmed later through alkali metal halide determination using tungsten filament high-capacity condenser dis- reduced-pressure ICP and the eects of plasma parameters (e.g., power, pressure and gas flow rate) and the shielding plate charge ETV as a quantitative sample introduction method.5 With the same sustaining plasma gas of argon, order of grounding mode are discussed.magnitude lower detection limits for Cl and Br were achieved at reduced pressure (17 Torr) compared with those obtained EXPERIMENTAL at atmospheric pressure.Among the cases cited above, plasma Discharge Chamber gas consumption was reduced to a very low level. As shown previously,5 a lower coolant gas flow rate (e.g., The details of the plasma potential measurement system for 50 ml min-1) is to be preferred for non-metallic element analy- reduced-pressure ICP-MS are shown in Fig. 1; additional sis, while keeping the forward power as high as possible (e.g., information about the ICP generator, matching box and 500W).Therefore, an ecient water cooling system is needed extended ESWC torch can be found in a previous paper.1 For for the reduced pressure ICP torch to avoid overheating. the convenience of inserting the Langmuir probe into the However, the presence of water between the load coil and the plasma, the mass spectrometer sampling interface was replaced plasma would enhance the capacitive coupling eect of radio- with a specially designed Pyrex interface chamber (Glass Machine Shop, University of Nagoya, Nagoya, Japan).Because the dc oset potential of the plasma was thought † Present address: Department of Chemistry, University of Florida, Gainesville, FL 32611, USA. to originate from the contact of the ICP with the grounded Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 (697–701) 697Fig. 1 Schematic diagram of the plasma potential measurement system for reduced-pressure ICP-MS.sampling cone,8 a substitute sampling cone was fixed inside forming an appreciable oxide coating. Very little etching of the probe has been observed over extended periods of use. the interface chamber at the same distance from the load coil as the original sampling cone (17 mm).1 The substitute sam- To investigate the cause of the secondary discharge, the position of the probe tip was set right in front of the sampling pling cone was constructed from a 10 mm thick copper plate with a 1.0 mm diameter central hole drilled through it; the orifice and the measuring procedure was similar to that reported previously.6 The rf wave picked up by the probe was front of the hole can be used to observe the secondary discharge approximating the real one with the original sampling cone.monitored with an oscilloscope (Iwatsu SS-5711D, 50 MHz) through a 1051 signal attenuator. The peak-to-peak (Vpp) and The substitute sampling cone was grounded through a 1.0 mm diameter copper rod inserted into the central hole from the dc oset (Vav) voltages of the rf wave were measured from the oscilloscope traces.Vav, i.e., the arithmetic mean of the positive end side. In order to make the most of the pumping eciency, six 5 mm diameter holes were drilled around the fringe of the and negative peak voltages, is considered to be the plasma potential and it is positively biased normally. Because the ICP sampling cone.It could be noted that for convenience of chamber fabrication and experimental manipulation, the is in contact with the grounded sampler, when the plasma potential is driven positive, ions attempt to reduce the potential 10 mm thick flat plate is dierent from the thin, pointed, sampling cone in MS. The vacuum system is also slightly by carrying current to the sampler. If the plasma is driven negative, electrons of much higher mobility reduce the potential dierent, which may result in dierent flow rates to those under normal MS conditions.Although the absolute values by carrying current to the sampler. A rectification eect occurs, and the average plasma dc potential usually becomes positive may not reflect the actual MS interface, measurements were carried out to investigate the trend of the plasma potential with respect to the ground.8 In order to examine the interference from the ICP’s high rf with variations in plasma conditions, which may help us explore in more detail the secondary discharge occurring in power environment when using a Langmuir probe, a 100– 500 W rf power was applied to the load coil without igniting reduced-pressure ICP-MS.As shown in Fig. 1, the Pyrex interface chamber was water the plasma. The peak-to-peak rf voltages picked up by the probe were less than 1 V and the averaged dc potentials were cooled. The end tube of the chamber was stopped by a siliconerubber septum through which the copper rod lead wire pro- barely measurable, which suggests that the potential picked up by the probe was all due to the plasma potential itself and truded for grounding.The side tube was connected to a rotary vacuum pump (372 l min-1). A Varian vacuum valve (4 mm the contribution from the induction eect of the probe can be ignored. id) was inserted into the vacuum line before the rotary pump; thus the pumping speed of this system could be adjusted over a wide range.The vacuum gauge was the same as used Reagent previously.1 Ultra-high purity argon gas (99.9995%) served as the plasma gas to reduce the interference from gas species under the Langmuir Probe plasma conditions. The Langmuir probe consists of a tungsten wire (0.4 mm diameter) sealed into a demountable tapered Pyrex plug. This RESULTS AND DISCUSSION plug is fitted in a socket opened on the wall of the interface General Observations chamber. The plug and the socket are ground fit and can be sealed by vacuum grease. The plug containing the probe can The assembly shown in Fig. 1 permits a stable ICP to be sustained at a power level between 30 and 800 W and plasma be changed very easily if needed. The tip of the tungsten wire rod is sheathed with an alumina sleeve so that only a short gas flow rates from less than 1 ml min-1 to several liters per minute. The ease of plasma generation at reduced pressure length of 1 mm is exposed to the plasma. External cooling of the probe is unnecessary because of the comparatively lower and self-ignited phenomena were confirmed again when the pressure was lower than 10 Torr.1 forward power (usually 300W) and lower gas flow rate.The argon environment also prevents the tungsten probe from As the interface chamber is made of glass, the appearance 698 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12of the reduced-pressure ICP can be clearly observed. It was conditions. The above results confirm and clarify that the dc oset potential originates from the existence of the grounded found that the shape and color of the plasma varied with the change of pressure and incident power.At a low pressure sampling cone and it is this dc oset potential which causes the secondary discharge.7 (several Torr), the ICP was diuse, with its tail reaching about 10 mm behind the substitute sampling cone with a bluish to purplish color. On the other hand, when the plasma was Eect of Forward Power generated under higher pressure and forward power, its appearance resembled an atmospheric pressure ICP, condensed and The dependences of the dc oset voltages (Vav) measured at whitish.However, the inner gas flow can always punch the 7.5 and 17 Torr on the rf forward power are presented in plasma, leaving a clearly discernible ‘donut’ structure. Usually, Fig. 2(a) and (b), respectively, for dierent torch shielding a higher gas flow rate requires a corresponding higher forward modes.It can be seen from Fig. 2 that Vav varies from several power to support. Judging from the appearance of the tungsten volts to 20 V and is identical with what has been reported for probe surface, it seems that the gas temperature increased with atmospheric pressure ICP-MS,6 although considerably lower increase in pressure while keeping the forward power constant. power was used here. Another important phenomenon pertinent to the present Apparently, Vav continues to increase with increase in forexperiments was that when the sampling cone was grounded ward power in any case.This can be explained by the enhanced through the copper lead wire, an obvious discharge could be capacitive coupling between the load coil and the plasma with observed in the vicinity of the central hole at low pressure. increase in rf power and, generally, a higher Vav corresponds However, this discharge disappeared as soon as the lead wire to a higher Vpp.became floating. Simultaneous plasma potential measurement At the same forward power, Vav is highest with the WC indicated that if the sampling cone was kept floating, the torch and lowest with the grounded ESWC torch over the average dc oset potential always equaled zero, although the whole range of rf power examined. The grounding mode of peak-to-peak potential varied with the plasma operating the shielding plate plays an important role in the dc oset potential.ESWC.G or even ESWC.10 V can reduce Vav, and thus the secondary discharge, eciently, which gives a good explanation for the phenomena reported previously.1 Eect of the Shielding Circuit Resistance It has been reported previously that the capacitive coupling eect of rf power between the load coil and the plasma can be controlled by the adjustment of the shielding circuit resistance.1 Here, the dc oset potentials were measured by inserting 10, 20 and 100 V carbon-film resistors into the shielding circuit with the other operating parameters (20 Torr, 300W) maintained the same as before.The results are shown in Fig. 3, and the variation of the copper ion signal (63Cu+) obtained previously1 is also shown for comparison. In Fig. 3, 0 V represents the shielding plate being grounded directly, and both Vav and the copper ion intensity are normalized to those for the floating mode. Clearly, the Vav tendency tallies with the behavior of Cu+.With the decrease in resistance, Vav decreases abruptly between 2 (floating) and 20 V, then levels out. It should be mentioned that there are still several volts of Vav left even though the shielding plate was grounded directly. However, provided that this voltage is close to, or less than, the breakdown value Fig. 2 Variation of Vav as a function of the forward power at (a) 7.5 and (b) 17 Torr for dierent torch shielding modes. The outer, intermediate and carrier gas flow rates are (a) 45, 0 and 15 and (b) 50, 0 and 180 ml min-1, respectively.WC, water-cooled torch; ESWC.F, Fig. 3 Variation of Vav and Cu+ as a function of the resistance ESWC.G and ESWC.10 V, the shielding plate of the ESWC torch was floating, grounded or grounded through a 10 V resistor, respectively. between the shielding plate and ground. Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 699needed for the sputtering of sampling cone material, there will curves tally with our previous report that the secondary discharge eect was much more intense at lower pressure.1 be few or no copper ions to be observed.The resistance eect can be explained by the existence of a shielding circuit allowing However, it should be mentioned that these two curves intersect in the lower carrier gas flow rate region, which suggests that some part of the rf current to flow to the ground through the shielding plate, which results in a lower rf power coupled to in addition to plasma pressure, Vav must be influenced by some other factors at the same time.the plasma. Because the shielding circuit resistance aects Vav directly, the secondary discharge can be controlled by adjusting Fig. 4(b) shows the dependence of Vav on the outer gas flow rate while keeping the carrier gas flow rate (CG) at 0.2 and the resistance. The similarity between the Vav and Cu+ curves confirms again that the dc oset potential of plasma is the 0.8 l min-1.When the carrier gas flow rate is kept at 0.8 l min-1 (lower curve), Vav increases with decrease in the outer gas flow main cause of the secondary discharge. rate, because the pressure decreases at the same time. At a carrier gas flow rate of 0.2 l min-1 (upper curve), however, Vav Eect of Gas Flow Rates decreases with decrease in the outer gas flow rate, although very slowly. This indicates again that in addition to the The eect of gas flow rates on Vav was investigated with an pressure, some other factors exist that aect Vav.ESWC torch, keeping the shielding plate floating. The carrier and outer gas flow rate eects are shown in Fig. 4(a) and (b), respectively, the corresponding pressure values being given Eect of Pressure adjacent to each point. As shown in Fig. 4(a), with a change in carrier gas flow rate In order to study the eect of the plasma pressure alone on Vav, the sum of the carrier gas and outer gas flow rates was from 0.2 to 0.8 l min-1 while holding the outer gas flow rate (OG) at 0.8 l min-1, an increase in plasma pressure from 87 kept constant (at 0.8+0.2 and 0.2+0.8 l min-1), and the pressure was changed by changing the pumping speed with a to 130 Torr results in a decrease in Vav from 16.5 to 11.5 V.When the outer gas flow rate was lowered to 0.2 l min-1, Vav needle valve. The results are shown in Fig. 5. As expected, Vav increases with decrease in the plasma also followed the same trend but with a smaller slope.Both pressure when holding the gas flow rates constant. This phenomenon might be explained as follows: the mobility dierence between the electrons and ions in the plasma was increased with increase in the mean free path at lower pressure, and electrons will move much more preferentially to the probe, leaving a much higher positively charged plasma. However, this pressure eect reminds us that the enhanced ion signals of halogens obtained at reduced pressure in the previous work may partly be contributed to by the intense secondary discharge since a WC torch was used.5 From the two curves in Fig. 5, it can be seen that the increase in the proportion of carrier gas in the total gas flow rate will cause a decrease in Vav when the overall gas flow rate is kept constant, which is dierent from the result reported for atmospheric pressure ICP-MS.6,7 The reason for these results could not be clarified in the present experiments.CONCLUSION The present work not only demonstrates the feasibility of Langmuir probe measurement of the plasma potential in reduced-pressure ICP-MS but also clarifies some ideas about the secondary discharge. The measured values of Vav tally well Fig. 4 Dependence of Vav on the gas flow rates for an ESWC torch Fig. 5 Eect of plasma pressure on Vav for an ESWC torch with the shielding plate floated. OG, IG and CG represent the outer, intermedi- with the shielding plate floated.(a) Carrier gas; (b) outer gas. Pressure (in Torr) is indicated adjacent to each point. ate and carrier gas flow, respectively. 700 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 123 Castillano, T. M., Giglio, J. J., Evans, E. H., and Caruso, J. A., with the intensity of secondary discharge at the sampling J. Anal. At. Spectrom., 1994, 9, 1335. orifice, which indicates that Vav is the main reason for the 4 Evans, E. H., Pretorius, W., Ebdon, L., and Rowland, S., Anal. discharge. The electrostatically shielded torch configuration Chem., 1994, 66, 3400. can greatly attenuate the secondary discharge via the decrease 5 Yan, X., Tanaka, T., and Kawaguchi, H., Spectrochim. Acta, Part in Vav. The positive dc oset potential increases with increase B, 1996, 51, 1345. 6 Tanaka, T., Yonemura, K., Tanabe, M., and Kawaguchi, H., Anal. in the forward power and decreases as the plasma pressure Sci., 1991, 7, 537. increases. The gas flow rate, especially the carrier gas flow 7 Gray, A. L., Houk, R. S., and Williams, J. G., J. Anal. At. rate, also plays an important role in Vav and usually a higher Spectrom., 1987, 2, 13. flow rate leads to a lower potential. 8 Douglas, D. J., in Inductively Coupled Plasma in Analytical Atomic Spectrometry, ed. Montaser, A., and Golightly, D. W., VCH, New York, 2nd edn., 1992, ch. 13. The authors thank Mr. T. Tanabe of Nagoya University for 9 Lim, H. B., Houk, R. S., and Crain, J. S., Spectrochim. Acta, Part fabricating the Pyrex discharge chamber and Langmuir probe. B, 1989, 44, 989. 10 Lim, H. B., and Houk, R. S., Spectrochim. Acta, Part B, 1990, 45, 453. 11 Johnson, E. O., and Malter, L., Phys. Rev., 1950, 80, 58. REFERENCES Paper 6/06934G 1 Yan, X., Tanaka, T., and Kawaguchi, H., Appl. Spectrosc., 1996, Received October 9, 1996 50, 182. 2 Evans, E. H., and Caruso, J. A., J. Anal. At. Spectrom., 1993, 8, 427. Accepted February 26, 1997 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 701

ISSN:0267-9477    

DOI:10.1039/a606934g

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Chromatographic Isolation of Molybdenum From Human Blood Plasma and Determination by Inductively Coupled Plasma Mass Spectrometry With Isotope Dilution† ELISE T. LUONGa , R. S. HOUKa AND ROBERT E. SERFASS*b aAmes L aboratory, US Department of Energy, and Department of Chemistry, Iowa State University, Ames, IA 50011, USA bCenter for Designing Foods to Improve Nutrition, Iowa State University, Ames, IA 50011, USA A method was developed for the determination of Mo in steps and contamination.Dilution to a typical total solute level (0.1%) reduces the concentration, which is not tolerable human blood plasma by ion-exchange chromatography and for elements such as Mo that are already at concentrations of inductively coupled plasma mass spectrometry (ICP-MS). <~1 ng g-1. Even if the Mo signal remains detectable, spec- Molybdenum was isolated from the plasma matrix by tral interferences become problematic, and it is dicult to microwave digestion and anion exchange on an alumina obtain the desired precision for isotope ratio measurements if column.Recoveries were 88±10% (±1 standard deviation, the signal level is too low. n=4) from a solution that contained 5 ppb Mo and 92 ppm For these reasons, this paper describes a chromatographic phosphate, and 82±5% (n=5) for a solution containing isolation and preconcentration procedure for Mo. Although 0.5 ppb Mo and 92 ppm phosphate. The RSD of the count rate the objective of the determination of Mo concentration by for 98Mo+ at 0.5 ppb was 5–10% and the 94Mo/98Mo ratio isotope dilution is emphasized here, the same procedure could had an RSD of 0.5–2.0%.The detection limit for Mo was be readily used for isotope tracer studies. In the latter appli- 1 ng l-1. The average Mo concentration in reference bovine cation, the samples come from clinical feeding protocols and serum determined by the proposed method was are very valuable; isolation of the analyte helps ensure the 10.2±0.4 ng g-1, compared with the certified value of success of the analysis and is well worth the extra time and 11.5±1.1 ng g-1 (95% confidence interval ).The Mo eort. In this paper, we first describe procedures for control of concentration in one pool of human blood plasma from two contamination and loss of Mo during sample preparation. healthy male donors was 0.5±0.1 ng g-1. This area has been neglected in the recent literature but is Keywords: Molybdenum; isotope dilution; inductively coupled often the limiting factor in accuracy of analysis by ICP-MS.A plasma mass spectrometry; human blood plasma; ultratrace procedure for the isolation of Mo from human blood plasma analysis and the use of isotope dilution for quantification of Mo are then described. Finally, results for the determination of Mo concentration in a standard reference material and in pooled Molybdenum is an essential trace element for plants, animals human blood plasma are presented.and microorganisms. The concentration of Mo in the diet and tissues of humans is generally below 1 mg g-1.1 Mo contributes EXPERIMENTAL to the function of 11 known enzymes. The first tracer study of Mo metabolism in humans was reported in 19642 using the Instrumentation only available radioisotope 99Mo, which has a radioactive half- The ICP-MS instrument was a TS Sola (Finnigan, Hemel life that is too short (69 h) for metabolic studies. Hence the Hempstead, UK).The ultrasonic nebulizer with desolvating study of Mo metabolism in humans has progressed slowly.3 A system (Model U-5000AT) was purchased from CETAC method for determining Mo concentration and isotopes in Technologies (Omaha, NE, USA). Instrumental operating con- human blood plasma is needed for further progress in studies ditions are given in Table 1. A Milestone microwave digestion of the metabolism, functions and distribution of this element. apparatus with ten 100 ml capacity fluoropolymer vessels Atomic absorption spectrometry is the main method used (Model MLS 1200 MEGA) was obtained from Buck Scientific to determine trace metals in biological samples.4 Neutron (Norwalk, CT, USA).A class 100 clean room with a class 10 activation analysis is a valuable confirmatory method. For polypropylene fume cabinet was installed by Controlled isotope tracer studies, thermal ionization mass spectrometry Environmental Services (Mansfield, MA, USA).The laser (TI-MS) has been used to some extent. The precision of TI-MS particle counter (Model 1506) used to monitor the particulate is very good, typically±0.01% RSD or better. Unfortunately, level in the clean room was obtained from Met One (Grants TI-MS requires at least 1 mg of Mo, which corresponds to at Pass, OR, USA). Barnstead Thermolyne (Dubuque, IA, USA) least 1 l of human blood.5 furnished the clean room with two analytical grade ultrapure There are two general strategies for trace element analysis cartridges and one organic-free cartridge (Nanopure II), which of biological fluids by ICP-MS.In some cases, the elements of supplied 18MV de-ionized water. Empty polyethylene chroma- interest can be determined by simply diluting the sample tographic microcolumns with frits (1 ml capacity) were without removing the matrix.6 The advantages of such a direct obtained from J. T. Baker (Phillipsburg, NJ, USA). procedure are speed and minimization of sample handling Materials and Reagents † Journal Paper No.J-17173 of the Iowa Agriculture and Home Ultrapure reagents (concentrated HNO3, 30% H2O2) contain- Economics Experiment Station, Ames, IA, USA. Project No. 3150, and supported by Hatch Act and State of Iowa funds. ing less than 0.005 ng ml-1 Mo and 30% ammonia solution Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 (703–708) 703Table 2 Alumina column background Table 1 Standard ICP-MS operating conditions Radiofrequency 27.18 MHz Mo concentration (ppb) using Forward power 1.3 kW Reflected power <2W 98Mo+ 94Mo+ Pressure 2.3×10-5–2.5×10-5 Torr 0.015 0.011 Argon flow rate: 0.009 0.006 Outer gas 15 l min-1 0.013 0.005 Intermediate gas 1.0 l min-1 0.011 0.006 Nebulizer gas 1.0 l min-1* 0.010 0.007 Solution uptake rate 1.04 ml min-1 0.010 0.008 Desolvating system 140°C 0.014 0.009 Condenser 0°C 0.008 0.005 Sampling position 14 mm from load coil Peak hopping parameters Three points per peak, 8 channels, 0.011±0.002* 0.007±0.002* 16 ms dwell time, 60 passes per scan, 15 scans * Mean±SD.* Selected on a daily basis to maximize the signal for 98Mo+. with ICP-MS. The cleaned quartz crucibles are stored in d-H2O in the clean room. Polypropylene bottles are soaked in 3 mol l-1 HCl for 7 d, containing <0.02 ng ml-1 Mo (indicated on the certificates) then rinsed copiously with d-H2O. Bottles are filled with were purchased from Fisher Scientific (Pittsburgh, PA, USA) 1 mol l-1 HNO3 (ultrapure acid), and leached for a further and J.T. Baker and were used without further purification. 2 d. In the final rinse, the bottles are filled with d-H2O, capped Trace metal grade HNO3 used for cleaning plasticware was and stored inside the clean room. Polypropylene vials and also obtained from Fisher Scientific. Molybdenum standard centrifuge tubes are also acid washed in the same manner. stock solutions (1.000 g l-1) were supplied by Fisher Scientific.Pipet tips and polyethylene transfer pipets are rinsed up and Enriched stable isotopic 94Mo (Lot 134191, Oak Ridge down twice, first with 1 mol l-1 HNO3 (ultrapure acid), then National Laboratory, Oak Ridge, TN, USA) was used to with d-H2O, rinsing immediately before use. Molybdenum-free prepare a stock standard solution in which the 94Mo concen- alumina powders are prepared by soaking in 3.75 mol l-1 tration was 10.8±0.1 mg g-1. Fused quartz crucibles were NH4OH (ultrapure base) for 7 d with three changes per day.purchased from Fisher Scientific. Activated alumina powder These powders are then washed thoroughly and stored in column packing material (Activity Grade Super I, type WA-4) 18 MV d-H2O. The cleaned alumina powders are checked by and ammonium heparin (porcine) were obtained from Sigma analyzing column blanks by the procedure described under (St. Louis, MO, USA). Bovine serum Standard Reference Ion-exchange chromatography. Results are presented in Table 2 Material 1598 containing Mo certified at 11.5±1.1 ng g-1 and explained under the section on background investigation.(95% confidence interval) was purchased from the National Institute of Standards and Technology (NIST) (Gaithersburg, MD, USA). Human blood samples were provided by two Sample Preparation healthy male donors. Microwave digestion Blood from the donor is drawn into a polypropylene syringe Contamination Control with a stainless-steel needle that contains 5–10 ml of ammonium heparin stock solution (24 mg ml-1) in 0.9% aqueous NaCl, Except for centrifugation and microwave digestion, the samples transferred and pooled into a 50 ml polypropylene centrifuge were prepared in the clean room described earlier.In ultratrace tube. These samples are spun at 1100 g for 25 min. Plasma is analyses, control of contamination and loss remains a major separated from the formed elements and divided into ~2 ml challenge, especially when the sample is subjected to several aliquots in quartz crucibles.Each weighed aliquot of plasma purification stages. Therefore, the procedures for providing is spiked with a known mass of 94Mo working solution. The Mo-free containers and utensils are described here. quartz crucibles are warmed at 50–60°C on a hot-plate until Closed microwave digestion vessels with polytetrafluoroemost of the liquid has evaporated. The samples are cooled to theimide liners that contain 5 ml of ultrapure concentrated room temperature and pre-digested with 1 ml of concentrated HNO3 are heated at 230°C for 30 min (step 1), rinsed with a HNO3 for 45 min.A booster of 1 ml of concentrated HNO3 large volume of 18MV de-ionized water (d-H2O for short), plus 40 ml of 30% H2O2 is added to each sample (called B). replaced with 80 ml of d-H2O, heated again under the same To each empty microwave vessel (called C), 5 ml of concen- conditions (step 2), followed by rinsing with a large volume of trated HNO3 are added.Sample B is added to C, then d-H2O. A 2 ml volume of 0.15 mol l-1 HNO3 (ultrapure acid) microwave digestion is performed in a closed system under a is added to each vessel, which is heated again under the same pressure of 4–5 bar, with temperature programming from 90 conditions as in step 1. The vessels are cooled to room to 230°C in 20 min, then digested for a further 25 min at temperature, then the solutions are analyzed by ICP-MS.If 230°C (45 min total). The resultant solutions in the crucibles the Mo concentration of the 0.15 mol l-1 HNO3 measured at are evaporated to dryness at 50–60°C on a hot-plate and m/z 94 and 98 is greater than 0.025 ng ml-1, steps 1 and 2 are reconstituted in 1 ml of 0.01 mol l-1 HNO3 (called D). repeated. When the blank is below this value, each Mo-free vessel is filled with d-H2O, covered and stored in the clean room. Ion-exchange chromatography Fused quartz crucibles are soaked in hot concentrated HNO3 (trace metal grade) in an acid washed polypropylene The use of alumina to isolate Mo from potential interferences such as Na+, Cl- and H2PO4- was adapted from McLeod container for 24 h, rinsed copiously with d-H2O, then boiled in a large volume of d-H2O, rinsed again with d-H2O and et al.,7 who used alumina columns to separate phosphorus in steel. Human blood plasma, standard bovine serum or blank soaked in hot concentrated HNO3 (ultrapure acid) for another 24 h.The sequence is repeated until the background Mo is samples were all processed by the same anion-exchange procedure as follows: below 0.010 ng ml-1 when measured in 0.15 mol l-1 HNO3 704 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 121. Prepare Mo-free alumina microcolumns 0.5 cm long, one viously in blood serum,6 but our experience has been that matrix components suppress Mo and In intensities to dierent column per sample. 2. Rinse each column once with 1 ml of 3.75 mol l-1 extents, aecting the accuracy of analysis. As shown in Fig. 2, the Mo+ signal drifts substantially with time, but the isotope ammonia solution and collect the euent. 3. Rinse each column twice with 1 ml of 18MV d-H2O, ratio drifts only slightly. In addition, moderate losses of analyte during the chromatographic isolation step do not aect the twice with 1 ml of 0.01 mol l-1 HNO3, then add sample digest (D). accuracy of the measured concentration.However, isotope dilution does not compensate for spectral overlap interference 4. Discard the initial column euent containing cations Na+, etc. or contamination. Therefore, keeping the background at m/z 94 and 98 as low as possible is critical. The cleaning procedures 5. Wash with 1 ml of 0.01 mol l-1 HNO3 and discard the euent containing weakly bound anions such as Cl- and I-. described under Experimental were helpful in this respect. 6. Elute Mo from the columns as MoO42- with 1 ml of 3.75 mol l-1 ammonia solution.Collect into 2 ml polypropylene vials. Eect of solvent 7. Evaporate to dryness by heating at 90°C in a heating The solvent used for the analytical solutions is also important. block. Fig. 3 shows that the count rate for 98Mo+ does not increase 8. Dissolve the solid in 2 ml of 0.15 mol l-1 HNO3 for linearly with increase in Mo concentration when the solvent analysis. is d-H2O. A similar curve shape is seen in electrothermal 9.Discard the column and packing after use. vaporization ICP-MS when the sample is dilute and no carrier is used to provide condensation nuclei for proper transport of the analyte to the plasma.9 Molybdate probably suers trans- RESULTS AND DISCUSSION port losses in the desolvation system, as further indicated by the following result. After 1 ppb of Mo in d-H2O was nebulized, Isotope Dilution the Mo signal rinsed out to 500 counts s-1 in 3 min. Molybdenum has seven stable isotopes of nominal masses 92, Nebulization of Mo-free, aqueous HNO3 at 0.15 mol l-1 94, 95, 96, 97, 98 and 100.The natural abundances are 14.84, produced an immediate rise in the 98Mo+ signal to 9.25, 15.92, 16.68, 9.55, 24.13 and 9.63%, respectively. Fig. 1 20000 counts s-1. This Mo came from somewhere in the shows a low resolution mass spectrum of a 0.25 ppb Mo sample introduction system. Under neutral or basic conditions, standard solution. The most abundant isotope (98Mo, 24.13%) MoO42- should be the main molybdenum species.10 If Cl- is was chosen as the reference isotope and 94Mo as the spike present, however, Mo can form various compounds with the isotope. 100Mo might be preferred over 94Mo, but we plan to analyze samples from a metabolic study in which 100Mo is used as a label, so 94Mo and 98Mo are the only appropriate Mo isotopes at natural abundance in the samples. If we assume the isotope distribution of Mo in blood is natural, the concentration (Cx nmol g-1) of Mo in the sample can be calculated from the isotope dilution equation:8 Cx=(CsWs/Wx) [(As-RmBs)/(RmBx-Ax)] where Cs = Mo concentration in spike (nmol g-1); Wx = mass of sample (g); Ws = mass of spike (g); Ax = atom fraction of 94Mo in sample (0.0925); Bx = atom fraction of 98Mo in sample (0.2413); As = atom fraction of 94Mo in spike (0.93808); Bs = atom fraction of 98Mo in spike (0.00764); Fig. 2 Instrument stability. Count rate for 98Mo+ and 94Mo598Mo Rm = altered 94Mo/98Mo abundance ratio, measured by ratio for a 0.50 ppb Mo solution monitored over 15 h of continuous operation.The value 1.00 for the normalized ratio corresponds to an ICP-MS. actual measured ratio of 0.3957. Isotope dilution is based on ratio measurements, which can compensate for many sources of error such as signal drift and matrix eects. Indium internal standard has been used pre- Fig. 1 Low-resolution mass spectrum of 0.25 ppb molybdenum standard solution showing six stable Mo isotopes. 98Mo was chosen as the reference isotope and 94Mo as the spike isotope. The natural abundance 94Mo598Mo ratio is 0.38334. Conditions: eight channels Fig. 3 Calibration curve of 98Mo+ count rate versus Mo concen- per m/z value, dwell time 16 ms per channel, 14 passes, full-scale= 50000 counts s-1. tration in H2O. Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 705Table 3 Composite background Mo concentration (ppb) 94Mo598Mo ratio RSD (%) 0.045±0.002 0.6700 4.92 0.036±0.001 0.4425 4.95 0.031±0.001 0.5712 3.52 0.028±0.001 0.5585 4.95 0.029±0.001 0.6308 4.79 0.052±0.004 0.4316 3.42 0.029±0.003 0.4460 5.01 0.045±0.003 0.4444 3.30 0.022±0.003 0.4826 5.97 0.036±0.002 0.4526 4.68 0.045±0.002 0.4588 3.10 0.024±0.002 0.4278 3.34 0.036±0.002 0.4728 3.29 0.029±0.002 0.4591 7.89 0.090±0.004 0.4090 3.50 0.038±0.017* 0.4905±0.0790* 16.10 Fig. 4 Calibration curve of 98Mo+ count rate versus Mo concentration in 0.15 mol l-1 HNO3.* Mean±SD. Table 4 Influence of H3PO4 or H2SO4 on measured 94Mo598Mo ratios for a 5.0 ppb Mo standard solution in 0.7% HNO3 [H2SO4] or Change in 94Mo598Mo ratio (%) [H3PO4]/mmol l-1 Influence of Influence of H3PO4 H2SO4 0 0 0 0.234* -0.90 -0.013 0.937† -0.93 -0.18 3.75 -1.50 -0.74 * Typical sulfate level in human serum. † Typical phosphate level in human serum. background from 0.15 mol l-1 HNO3) is~0.001 ppb. Column blanks are 0.011±0.002 ppb Mo (Table 2). This level is very Fig. 5 Count rate for a 0.5 ppb Mo standard solution as a function close to the Mo background from the 0.15 mol l-1 HNO3, of HNO3 concentration (mol l-1). which indicates that contamination of Mo from the alumina column is insignificant. The composite background (everything but sample) shows 0.038±0.017 ppb Mo, which is 15 times general formula MoOxCly (x=1–3 and y=2–5). For example, MoOCl4, MoO2Cl2 and Mo2O3Cl5 species are highly volatile11 below the typical analyte concentration in blood (~0.5 ppb).Table 3 shows the composite background measurement for and could sublime in the desolvation system, which causes transport losses. The extent of the loss is related to the amount the 94Mo598Mo ratio. The mean ratio value is 0.4905 (RSD= 16.10%, n=15). This is higher than the natural abundance of complex formed. It is related to the Mo5Cl- ratio, other anions present and especially pH. What is the source of Cl-? ratio of 94Mo to 98Mo (0.3833).This background ratio (0.4905) increases the measured value of 94Mo598Mo ratios in the The 1000 ppm Mo stock solution is in dilute aqua regia, a solvent mixture containing three parts of HCl for each part of samples, which causes low concentration values. Unfortunately, correction for this problem is dicult because dierent losses HNO3. When a 1 ppb Mo solution is prepared by 106 dilution of the 1000 ppm stock Mo with d-H2O, the Mo5Cl-5NO3- or contamination of analyte can occur in dierent stages of sample preparation.The current composite background level ratio is unchanged. However, if a 1 ppb Mo solution is prepared by diluting the same stock solution with 0.15 mol l-1 HNO3, causes~5% error in the analysis of a sample that contains 0.5 ppb of Mo. This error becomes smaller as the Mo concen- the Cl-5NO3- ratio changes substantially, and this solution becomes much more acidic. These conditions keep Mo as tration in the sample increases. [Mo7O24]6- or [Mo6O21]6- species,10,12 rather than the MoOxCly compounds. Thus, the calibration curve becomes Recovery of Mo from Alumina Column linear and the sensitivity increases by a factor of 10 when the samples are reconstituted in aqueous HNO3 (Fig. 4). Major inorganic species in blood plasma such as sodium The dependence of the Mo+ signal on HNO3 concentration (3.1–3.4 mg ml-1), potassium (0.16–0.20 mg ml-1) and chlorwas also investigated. Fig. 5 shows that the optimum count ide (3.6–3.8 mg ml-1) are removed by the anion-exchange rate for the reference isotope (98Mo) occurs between 0.07 and alumina column, as described under Experimental.Our cur- 0.33 mol l-1 HNO3. For this reason, all samples were dissolved rent work is focused on the separation of phosphate in 0.15 mol l-1 HNO3 before ICP-MS analysis. (78–132 mg ml-1) and sulfate (25–50 mg ml-1) from Mo because these two anions are the fourth and fifth major inorganic constituents in human blood.13 This amount of Background phosphate alters the 94Mo598Mo ratio measurements slightly (Table 4).The source of interference is not clear. The measured The average background measured at m/z 98 from 0.15 mol l-1 HNO3 varies from day to day between 0.004 and 0.012 ppb 94Mo598Mo ratio decreases as the anion concentration increases, which is the direction expected from a matrix eect. total Mo. The detection limit for Mo with our instrument (defined as the Mo concentration necessary to provide a net Alternatively, spectral interferences from 1H31P18O16O3+ and/ or 34S16O4+ are possible. signal equal to three times the standard deviation of the 706 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12Solutions of dierent Mo concentrations with and without the natural abundance ratio (0.3833) divided by the measured ratio from a solution of known Mo concentration. The meas- phosphate or sulfate were tested for Mo recovery. The percentage recoveries and 94Mo598Mo isotope ratio measurements ured 94Mo598Mo ratio from the spiked samples is then multiplied by this K factor to yield a new 94Mo598Mo ratio (called are summarized in Table 5.The percentage recovery is defined as (98Mo+ signal for the solution containing phosphate, sulfate Rm¾). Rm¾ is used in place of Rm for the calculation of the Mo concentration in the original samples from the isotope or both that has passed through the column)/(98Mo+ signal for the solution containing Mo alone that has not passed dilution equation.Table 6 shows measured results for Mo concentration in through the column)×100%. Molybdenum recoveries vary between 68 and 100%, which is attributed to the variation of nine replicate aliquots of bovine serum Standard Reference Material obtained using the proposed method. The measured the flow rate of the euent. In spite of these dierences in recoveries, the 94Mo598Mo ratios are very close to the natural Mo concentration is 10.2±0.4 ng g-1 (one SD) of serum; the NIST value is 11.5±1.1 ng g-1.Our result is slightly lower abundance ratio (0.3833). Analysis of the mass spectra of eluates at m/z 31 revealed that 86% of the P was removed; the than the NIST value but is still within the 95% confidence limit. A plausible explanation verified by spectral scans of remaining P has no eect on the measurement of the 94Mo598Mo ratio (Table 5). blanks (Fig. 6) is that some contamination with 94Zr occurs from the walls of the fluoropolymer vessels during microwave The separation of S from Mo cannot be examined by analysis of the mass spectrum of eluates at m/z 32 because of digestion, even though the sample was isolated from the fluoropolymer vessel wall by the quartz crucible (94Zr+ is most the background ion O2+.Detection of sulfate in column eluates is positive upon addition of a barium chloride solution. likely responsible for the elevated composite background However, analysis of a standard solution of 5.0 ppb Mo containing 0.234 mmol l-1 (23 ppm) sulfates shows that the 94Mo598Mo ratio measurement shifts negatively by only 0.013% (Table 4), which is insignificant in comparison with other sources of error.Results from Standard Reference Material and Human Blood Plasma The measured 94Mo598Mo ratio for a known concentration of Mo standard solution is slightly dierent from the natural abundance ratio. This comes from two causes: small and Fig. 6 Spectral scan of a blank microwave digestion vessel. The signal variable amounts of Zr and Mo in the nitric acid background at m/z 94 is mainly from 94Zr+ and partly from 94Mo+. Conditions: and the mass bias of the ICP-MS instrument. A normalization eight channels per m/z value, 16 ms dwell time per channel, 15 passes, full-scale=10000 counts s-1. factor K is used to correct for this dierence; K is defined as Table 5 Recovery of molybdenum from alumina column Concentration of Condition Mo (ppb) Recovery (%) 94Mo598Mo ratio RSD (%) 92 ppm 5.00 96±4 0.3886 1.68 H3PO4 85±4 0.3804 2.47 97±3 0.3849 2.00 75±3 0.3855 3.05 92 ppm 0.50 82±2 0.3808 1.45 H3PO4 73±3 0.3838 1.22 87±3 —* —* 84±3 —* —* 85±4 —* —* 92 ppm 0.50 97±4 0.3883 0.81 H3PO4 100±4 0.3930 0.60 23 ppm 0.25 68±4 —* —* H2SO4 68±4 —* —* 100±2 —* —* * Scan only.Table 6 Results for NIST SRM 1598 using ICP-MS (K=0.961) Sample 98Mo+ signal/ 94Mo598Mo ratio Concentration of No. counts s-1 RSD (%) (Rm¾) RSD (%) Mo/ng g-1* 1 237000 12.5 1.0594 1.29 9.50±0.09 2 295000 3.47 1.0489 1.25 9.70±0.1 3 292000 3.72 1.0398 1.27 9.90±0.1 4 256000 1.47 1.0087 1.29 10.5±0.1 5 257000 2.58 1.0212 1.37 10.2±0.1 6 245000 2.43 1.0192 1.28 10.4±0.1 7 256000 5.35 0.9896 1.78 10.8±0.1 8 243000 5.41 0.9901 2.19 10.6±0.1 9 301000 3.34 1.0075 1.39 10.4±0.1 Mean: 10.2±0.4 * NIST value: 11.5±1.1 ng g-1.Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 707Table 7 Results for blood pooled from two male donors (K=0.928) Sample 98Mo+ signal/ Spike/ 94Mo598Mo ratio Concentration of No.counts s-1 RSD (%) pmol (Rm¾) RSD (%) Mo/ng g-1 1 36900 4.11 2.4141 1.2850 1.20 0.488±0.007 2 29700 2.10 2.4569 1.2800 1.09 0.501±0.007 3 32100 1.62 2.5447 1.5315 0.92 0.406±0.005 4 30500 5.61 4.9566 1.6852 0.73 0.696±0.007 5 35200 2.32 4.9543 1.9699 1.14 0.581±0.008 6 34200 2.30 4.9408 2.5000 0.95 0.424±0.006 Mean: 0.5±0.1 94Mo598Mo ratio, as discussed earlier). Nonetheless, replace- Francisco, CA, provided the isotopically enriched 94Mo and William Keyes of that laboratory confirmed its isotopic abun- ment of these vessels with quartz vessels should eliminate this problem.The precision of the measured ratios is still good dances by thermal ionization mass spectrometry. Special thanks go to Alexandre Smirnov for extensive discussions (~1.4% RSD) even though the Mo concentration is only 10 ppb in the original sample. during the course of this study.This research was supported by a Carver Trust Grant from Iowa State University. One of The Mo concentration in pooled blood plasma from two healthy male donors is 0.5±0.1 ng g-1 (n=6) (Table 7). The the authors (R.S.H.) is supported by the US Department of Energy, Oce of Basic Energy Sciences, through the Ames RSDs of the 94Mo598Mo ratio measurements range from 0.7 to 1.2%, whereas the RSDs expected from counting statistics Laboratory (contract No. W-7405-Eng-82).are ~0.7%. The RSDs of the determined concentrations are ~1.4%; much of the fluctuation in excess of that contributed by counting statistic is attributed to fluctuations of the analyt- REFERENCES ical balance as the spike isotope is weighed in the clean room. 1 Rajagopalan, K. V., Ann. Rev. Nutr., 1988, 8, 401. This problem can be solved by putting the balance on a marble 2 Roso, B., and Spencer, H., Nature (L ondon), 1964, 202, 410. weighing table. 3 Turnlund, J. R., Keyes, W. R., and Peier, G.L., Am. J. Clin. Nutr., 1995, 62, 790. 4 Versieck, J., and Cornelis, R., Anal. Chim. Acta, 1980, 116, 217. CONCLUSION 5 Turnlund, J. R., Keyes, W. R., and Peier, G. L., Anal. Chem., 1993, 65, 1717. Subnanogram amounts of Mo in human blood plasma are 6 Vanhoe, H., Vandecasteele, C., Versieck, J., and Dams, R., Anal. determined quantitatively by combining anion-exchange chro- Chem., 1989, 61, 1851. matography with isotope dilution ICP-MS. Currently, our 7 McLeod, C. W., Cook, I. G., Worsfold, P. J., Davies, J. E., and method can be applied to the analysis of blood from adults Queay, J., Spectrochim. Acta, Part B, 1985, 40, 57. only, because 12 ml of blood are required for triplicate measure- 8 Fassett, J. D., and Paulsen, P. J., Anal. Chem., 1989, 61, 634A. ments. Improvements are expected to reduce the sample 9 Ediger, R. D., and Beres, S. A., Spectrochim. Acta, Part B, 1992, 47, 907. volume required to 1 ml per replicate by recycling the unnebul- 10 Busev, A. I., Analytical Chemistry of Molybdenum, Israel Program ized sample from the spray chamber of the ultrasonic nebulizer, for Scientific Translations, Jerusalem, 1964. by using a microscale nebulizer or by changing software 11 Weast, R. C., and Astle, M. J., CRC Handbook of Chemistry and sequences. Other areas we would like to improve are lowering Physics, CRC Press, Boca Raton, FL, 1982–83, p. B-122. the composite background at m/z 94 and 98 and modifying 12 Emeleus, H. J., and Anderson, J. S., Modern Aspects of Inorganic the microwave digestion vessels. These improvements would Chemistry, Van Nostrand, New York, 1973, p. 215. 13 Thiele, V. F., Clinical Nutrition, C. V. Mosby, St. Louis, MO, lower the detection limit. 1976, p. 194. The authors express appreciation to Richard Kniseley and Kaustuv Das for their technical assistance in this work. Judith Paper 6/08265C Received December 9, 1996 Turnlund of US Department of Agriculture (USDA), Western Human Nutrition Research Laboratory, Presidio, San Accepted February 19, 1997 708 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12

ISSN:0267-9477    

DOI:10.1039/a608265c

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Effects of Bias Voltage and Easily-ionized Elements on the Spatial Distribution of Analytes in Furnace Atomization Plasma Emission Spectrometry VICTOR PAVSKI†a , RALPH E. STURGEON* b AND CHUNI L. CHAKRABARTIa aOttawa-Carleton Chemistry Institute, Centre for Analytical and Environmental Chemistry, Department of Chemistry, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario, Canada K1S 5B6 bInstitute for NationalMeasurement Standards, National Research Council of Canada, Ottawa, Ontario, Canada K1A 0R9 The eect of bias voltage and the presence of easily-ionized ical sources such as FAPES and its low-pressure dc glow elements (EIEs) on the spatial distribution of excited-state discharge counterparts, hollow cathode (HC)21 and hollow atoms and ions of Cu, Ag, Cs and Ca in furnace atomization anode (HA)22,23 furnace atomization non-thermal excitation plasma emission spectrometry is presented.The dc bias of the spectrometry (FANES), is the potential to decouple atomizcentre electrode significantly aects the spatial distribution of ation and excitation phenomena, thereby potentially minimiz- He I, Cu I, Ag I, Cs I, and Ca II emission in the absence of ing matrix eects.Additionally, the collision-rich environment EIEs. A reasonably uniform distribution of excited-state provided by the plasma was hoped to be suciently robust to analyte atoms over the central cross-section of the tube occurs dissociate or be insensitive to any matrix components that when the centre electrode is self-biased during the course of an were co-volatilized with the analyte.Unfortunately, complete atomization transient. A depleted area of Cs I emission around decoupling of excitation and atomization processes in FAPES the centre electrode coupled with enhanced Ca II emission in cannot be realized as the high temperature of the graphite the same region reveals that ionization of analytes is most tube influences the production of thermionic electrons which pronounced in this region.With positive dc bias, concentric can alter plasma processes.8,16 Additionally, the rf power rings of enhanced emission occur between the centre electrode influences the temperature of the centre electrode which, in and the tube wall for analyte atoms and the He I plasma gas, turn, determines the extent to which it acts as a site for primary although the overall breadth of analyte emission distribution is condensation and re-desorption of analytes.5,9,17,20 Furtherdecreased.With NaCl, NaNO3 and CsCl serving as EIEs, more, the fact that quantitative analysis of ‘real’ samples conanalyte emission from Ag I, Cu I and Ca II in the region ducted with combined ‘non-thermal’ excitation sources have between the centre electrode and the tube wall is strongly thus far employed the method of standard additions12 suggests suppressed with self-bias. The degree of the suppression that matrix eects play a significant role in altering the depends on the extent of vapour cloud overlap between analyte excitation characteristics of these plasmas. and EIE.In general, equimolar amounts of NaNO3 and CsCl A frequent interference from the sample matrix reported in suppress analyte emission similarly and both produce a greater plasma emission is that resulting from the presence of Group suppression than NaCl. Equal amounts of Fe, added as an I and II elements, commonly referred to as easily ionized interfering matrix, produces a suppression of analyte emission elements (EIEs).Electrons derived from the ionization of EIEs similar to that of EIEs, suggesting that the primary cause of will alter the discharge potential in capacitively coupled suppression is the loss of energy from the plasma (as photons) plasmas, possibly producing substantial voltage drops which due to excitation and ionization of matrix vapour. Control of attenuate the energetics of the discharge, thus decreasing the dc bias enhances the radial distribution of excited analyte analyte excitation.Less common, but also possible, are atoms in the presence of EIEs and Fe, but only at low enhancements in analyte emission as the result of the EIE (2 mg) interferent loadings. acting as a ‘buer’ for analyte ionization. Falk24 has developed general equations which permit esti- Keywords: Furnace atomization plasma emission spectrometry; mation of the concentration of sample matrix that would imaging; charge-coupled device camera; easily-ionized dissipate a given fraction of the input power of the plasma via elements; dc bias excitation losses.The maximum tolerable Na concentration which would bring about a 10% loss of discharge power is Furnace atomization plasma emission spectrometry (FAPES), 0.01% for FAPES, 0.005% for low-pressure FANES sources is a combined source for spectrochemical analysis which has and 0.05% for inductively-coupled plasma (ICP) emission been the subject of growing attention.1–20 In FAPES, a 1 mm sources.24 These calculations clearly show that matrix eects diameter (typically graphite) electrode is co-axially centred can be severe for combined ‘non-thermal’ excitation sources within a graphite furnace atomizer and coupled capacitively even at relatively low matrix concentrations, with the severity to a radiofrequency (rf ) power supply.An atmospheric pressure of the interference dependent upon the degree of temporal (glow discharge) plasma is sustained within the graphite tube overlap between the analyte and interferent.As an example, and serves as an excitation medium for analytes thermally emission from Cu was suppressed 20% in the presence of 2 mg desorbed from the graphite tube surface during the conven- of NaCl in HC FANES.24 For HA-FANES, which is geometri- tional high-temperature volatilization step. One of the primary cally identical to FAPES, Riby and Harnly25 reported that motivating factors behind the development of combined analyt- significant perturbations in discharge voltage occurred in He at 400 Torr (1 Torr=133.322 Pa) and 130 mA with only 0.25 mg NaCl. Operation at elevated pressure (600 Torr) and †Present address: Ames Laboratory USDOE and Department of Chemistry, Iowa State University, Ames, Iowa 50011-3020, USA.higher current (200 mA) was found to significantly reduce Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 (709–723) 709voltage perturbations in the presence of up to 2.5 mg NaCl. plasma and to determine whether control of the dc bias changes excitation conditions suciently to ameliorate eects Smith et al.3 found that emission from Ag in FAPES was initially enhanced in the presence of NaCl, but that it decreased of EIEs. Sodium chloride, NaNO3, CsCl and Fe were selected as matrix species with Ag, Cu, Ca and Cs as test analytes as beyond 1.2 mg of the salt.The initial enhancement was ascribed to suppression of Ag ionization by NaCl (‘buering’ eect), they exhibit mutual temporal overlaps in the vapor phase. whereas the subsequent intensity decrease at higher salt masses was believed to result from a decrease in plasma excitation EXPERIMENTAL characteristics. Hettipathirana and Blades7 subsequently found Reagents that even small masses (162 ng) of NaCl and NaNO3 were sucient to alter plasma excitation characteristics and the Liquid chromatographic grade He (Air Products Canada, extent of molecular dissociation for Pb and Ag in a FAPES Nepean, ON, Canada) was used as the plasma gas.It was first source operated at 14–40W and a low heating rate of passed through a molecular sieve before entering the graphite 360 K s-1. Significant suppression was observed for Pb emis- furnace via the internal purge gas port at a flow rate of sion signals in the presence of 1–3 mg NaCl, although high 200 ml min-1.High purity Ar (99.995%; Canadian Oxygen, input powers could decrease its severity. Using a commercial Mississauga, ON, Canada) served as the external purge gas FAPES source based on an integrated contact cuvette, Gilchrist and was maintained at a flow rate of 1 l min-1. Stock solutions and Liang10 found that the emission intensity of Tl was (1000 or 10000 mg ml-1) were prepared from dissolution of suppressed by the presence of only 0.08 mg NaCl. the high-purity metals (Cu, Ag and Fe) or their salts (NaCl, Easily-ionized elements may aect analyte excitation in NaNO3 and CsCl).Working standards were prepared fresh FAPES by various means. Analyte emission intensity may be daily by dilution of the stocks with high-purity (18.3MV cm decreased by physical expulsion of analyte from the excitation/ resistivity) distilled, de-ionized water and acidified to 1% v/v observation volume as the result of: co-vaporization with the with HNO3 (Ultrex grade; J.T. Baker Canada, Toronto, EIE matrix; chemical interference due to analyte molecule ON, Canada). formation; decreased plasma power available for analyte excitation as the result of photon emission by excited EIE matrix Instrumentation species; de-tuning, or loss of plasma power coupling eciency because of changes in the load impedance of the plasma A detailed description of the imaging spectrometer, atomizer workhead, rf generator and dc bias control unit used has brought about by the EIE; and alterations in the self-bias voltage resulting from changes in electron density in the already been provided in a previous publication.20 For the present study, a Perkin-Elmer (Norwalk, CT, USA) model presence of the EIE.Enhancements in analyte emission intensity in the presence of EIEs may occur as a result of an HGA-500 graphite furnace power supply/controller was used in place of the HGA-76B power supply employed previously. increased gas density of the diusion medium as a significant amount of He is replaced by matrix vapour and shifts in The HGA-500 supply has a maximum power heating rate of ~2000°C s-1, compared with the ~1250°C s-1 achievable analyte ionization equilibrium due to ionization of matrix elements.Finally, analyte excitation may either be enhanced with the HGA-76B controller. This is advantageous in terms of analytical sensitivity. Furthermore, the HGA-500 is a micro- or decreased by an alteration of the electron energy distribution function (EEDF) due to the release of electrons from the EIE processor-driven unit, permitting acquisition of emission transients in increments of 1 s before or after the actual through ionization, or by attenuation of plasma electron energy from excitation or molecular dissociation of matrix species, commencement of the high-temperature volatilization step.Because time-resolved imaging of analyte emission transients depending on the analyte excitation energy relative to the original and altered EEDF.Imai and Sturgeon18 have shown was desired, the charge-coupled device (CCD) camera (Photometrics, Tucson, AZ, USA) used to image the central that suppression of analyte excitation in the presence of EIEs is most likely due to radiative power losses as a result of cross-section of the graphite furnace interior was operated in frame-transfer mode. The CCD chip was binned 4×4 to obtain excitation of EIE matrix species and alterations in the EEDF, rather than by analyte molecule formation, gas-phase expulsion a nominal image array size of 62×62.A 5 ms exposure time was used resulting in an acquisition rate of 50 ms per image. and de-tuning of the plasma. All prior investigations into the eect of EIEs on FANES To obtain the greater spatial resolution required for imaging He I emission from the plasma, the full resolution of the CCD and FAPES response have sampled one region of the plasma exclusively (typically immediate to the centre electrode).chip was used (i.e., 1×1 binning), whereas all other image acquisition steps remained unchanged. As a consequence of Previous investigations4,8,20 have revealed that FAPES is a highly non-homogenous discharge with regions of emission the larger image array size that is produced when the chip is not binned, the time required for the CCD system to read out that vary widely in intensity and that the spatial structure of the plasma is strongly aected by the dc bias of the centre the contents of the image array increased.Thus, for imaging of emission from excited state He I, the acquisition rate was electrode.20 Sturgeon et al.16 found that control of this dc bias decreased fluctuations in excitation temperatures during the 400 ms per image, with an exposure time of 100 ms. Optical resolution in the imaging system is achieved through course of an atomization transient. Since EIEs alter excitation temperatures in non-thermal excitation sources,25 bias control the use of interference filters.Although this results in a bandpass of 10 nm or more, the CCD allows the background may aord a more robust plasma to minimize their eects. Several studies of ICP discharges26,27,28 have revealed that to be stored and subtracted from the analytical run for accurate correction of analytical transients. The spectral transitions EIE eects are highly spatially-dependent. Since spatial eects are observed for EIEs in such a comparatively symmetric monitored and interference filters used are listed in Table 1.The wide spectral bandpass of the system means that, in the plasma, it is not unreasonable to assume that the spatial response of analytes to the introduction of EIEs would be case of Cu and Ca II, the images obtained were composites of emission from at least two spectral transitions. Imaging of Ag even more pronounced for FAPES. This study was undertaken in an eort to determine whether changes in excitation con- in the presence and absence of NaCl and NaNO3 was accomplished using a 330 nm central maximum (10 nm ditions of the plasma (induced by the dc bias of the centre electrode) lead to significant changes in the spatial distribution FWHM) interference filter to isolate the Ag I 328.07 nm resonance line.Although Na I transitions at 330.23 nm and of analyte emission, to ascertain whether the introduction of EIEs significantly alters the excitation characteristics of the 330.30 nm are passed by this filter, emission from Na I was 710 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12Table 1 Spectral features monitored and interference filters used for changes in the spatial response of the plasma and analytes detection of Ag I, Cu I, He I, Ca II and Cs I occur in the absence of matrices during atomization. In particular, the spatial distribution of excited state He I is important, Spectral Excitation Central wavelength FWHM of as this will serve as a ‘template’ of plasma excitation against transition energy/eV of interference interference which all analyte emission is convoluted.There is no practical filter/nm filter/nm way of masking the intense blackbody radiation from the Ag I 328.33 nm 3.45 330 10 heated tube walls using the imaging system. At long wave- Cu I 324.75 nm 3.82 326 10 lengths and high temperatures, blackbody emission over- Cu I 327.40 nm 3.78 326 10 whelms the He I signal, giving the illusion that the plasma has He I 388.86 nm 23.00 390 10 ‘collapsed’.Thus, imaging of He I under high temperature Ca II 393.37 nm 9.26 395 10 conditions must be performed in the ultraviolet and is restricted Ca II 396.85 nm 9.24 395 10 Cs I 852.11 nm 1.45 850 25 to the 388.86 nm line. This is unfortunate as it is spectrally isolated with the imaging system using a 10 nm FWHM, 390 nm central maximum interference filter. Studies by not detected by the CCD as a consequence of its insensitivity.Sturgeon et al.11 and Hettipathirana and Blades4 have revealed Before recording emission images from analytes in the presence that strong second positive and N2+ first negative system of interfering species, images from the interfering species alone bands are excited in the FAPES discharge in the 385–395 nm were recorded. In all cases, no emission from the interfering wavelength region. Additionally, CN violet system bands species was detected.arising from reaction of entrained air with volatilized carbon The temperature–time characteristics of the furnace were cannot be ignored. Because the Massmann furnace used in recorded following the output of a calibrated Ircon series 1100 this study operates in an ‘open’ environment, it is not possible automatic optical pyrometer (Ircon, Niles, IL, USA) focused to completely remove the contribution of the nitrogenous onto the graphite tube wall through the dosing hole.background species, making unambiguous determination of He I emission structure at high temperature impossible. Procedures Nevertheless, the spatial distribution of excited N2 species in the FAPES source has been shown to be very similar to that Images of FAPES transients were obtained as follows. of He I.4,8,20 Furthermore, although mechanisms of excitation Solutions (5–40 ml) of the analyte and/or interferent were of other gas species may be dierent, it is reasonable to assume manually pipetted into the graphite furnace using fixed-volume Eppendorf pipettes.The samples were then dried at 120°C for that all excitation mechanisms are ultimately powered by 60 s and ‘charred’ at 400°C for 60 s. Temperature readings electrons24,31 so that the spatial distribution of such species refer to those read directly from the HGA-500 power supply. should still be provided by the excited state He excitation The lengthy drying and ‘charring’ times were used to minimize ‘template’.the eect of entrained water vapour which has been shown to False-coloured images (where white represents the highest have deleterious eects on similar ‘non-thermal’ discharges.29,30 intensity and black the lowest) obtained for the He I 388.86 nm The graphite tube was then allowed to cool to room tempera- line in a dry, unloaded furnace are presented in Fig. 1. A ture, after which the plasma was ignited. The tube temperature forward rf power of 50W was applied and the centre electrode was then ramped to 400°C for 5 s (to function as a plasma was allowed to self-bias while the tube wall temperature was ‘pre-stabilization period’). Using maximum power heating con- ramped to 2600°C using maximum power heating. Under such ditions, atomization at the desired temperature was then conditions the dc self-bias exhibits its greatest change from initiated for 5 s.For Cu and Ca II, this was set to 2600°C, -10 to -125 V and it is in this ‘free-running’ mode that whereas for Ag this was set to 1700°C.Cesium and sodium FAPES sources are most commonly operated. Before pro- were atomized at both 2600°C and 1700°C to reflect the gressing to a discussion of the image analysis it must be relative temporal overlap of these elements with Cu, Ca and mentioned that each image was scaled for optimal viewing Ag analytes. In the case of an analyte in a clean matrix, the and, hence, no absolute intensity intercomparisons can be background contribution to the emission intensity was assessed drawn from the series of false-coloured images.The imaging by atomizing a blank consisting of 1% HNO3 (under the same software associated with the CCD permits no direct detailed conditions as the analyte). In the case of analyte/interferent image analysis (e.g., no spatial integration of emission intensity) combinations, the blank consisted of the interferent alone in and only the highest pixel intensity in the array is readily 1% HNO3, which was again atomized under conditions ident- displayed.As an aid to interpretation of the false-coloured ical to that of the analyte. Emission from He I and plasma images presented in Fig. 1, selected three-dimensional response background species were imaged under transient conditions surfaces of emission intensity are plotted in Fig. 2 and a plot by heating an empty tube from 400°C to 2600°C. The blank of maximum pixel intensity values versus time is given in Fig. 3. for such a system consisted of heating the empty tube in the The intensity values plotted in Fig. 3 do not necessarily absence of a plasma. In all cases, the blank series of images correspond to the same pixel in each image frame of Fig. 1, was subtracted from those of the analyte to yield background- but simply represent the maximum intensity recorded, which corrected images of analyte distributions. In most cases, rf in all cases corresponds to a position proximate to the centre forward power was 50W (as read on the port of the rf tuner), electrode.In general, the overall shape of the curve in Fig. 3 although for other experiments the rf power was varied from is similar to that obtained by the PMT-based system used by a minimum of 20W to a maximum of 100 W. Reflected powers Sturgeon et al.16 were tuned to 1W in all cases (‘room temperature’ value). The initial image in the series is typical of the room The dc bias on the centre electrode was varied from -50 to temperature He I images previously shown for the He I +40 V using a series of 12 V motorcycle batteries, as described 667.82 nm line (which is devoid of spectral interference) and previously.16,20 features the centre electrode and weak outer ring plasmas already noted.20 Thus, it may be tentatively concluded that RESULTS AND DISCUSSION this series of high-temperature images should provide a good Eect of Dc Bias on Excited State of Helium Distribution assessment of the actual excited state He I spatial distribution.During an Atomization Transient From 0.8 s to 1.6 s, over which the furnace reaches a steadystate temperature of 2600°C, He I emission intensity gradually Before obtaining images of the spatial response of analytes in the presence of EIEs, it was first necessary to determine if any increases. This arises despite a 3–4-fold lower neutral He Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 711Fig. 1 False-coloured images for plasma gas species, including He I, N2, N2+ and CN obtained using a 390 nm peak maximum, 10 nm FWHM interference filter for an empty tube ramped to 2600°C under maximum-power heating conditions in a 50 W plasma with a self-biased centre electrode. number density in the graphite tube. Furthermore, the centre electrode plasma increases in thickness, commensurate with the increased mean free path of electrons and increased sheath thickness at the lower He gas density.An additional factor accounting for this enhanced centre electrode plasma intensity is the high negative dc self-bias developed on the centre electrode. From an initial value of ~-10 V at room temperature, this reaches a maximum negative value of -125 V at 2.0 s, causing the centre electrode to behave increasingly as the cathode in the system during a given half-cycle and enhancing the intensity of the plasma around the centre electrode.20 Comparison of the image at 1.6 s relative to that at 0.0 s reveals a decreased thickness of the outer ring plasma, which eventually collapses the 90 mm ‘dark space’.This may arise as a result of liberation of thermionic electrons from the tube wall at this temperature (2600°C), since dark space thickness decreases as secondary electron production increases.32 By 2.0 s into the transient, maximum emission intensity is attained and a dramatic decrease in the thickness of the plasma around the centre electrode occurs.This is the time when the dc selfbias reverses polarity, eventually resuming its ~-10 V ‘room Fig. 2 Three-dimensional response surfaces for plasma gas species, temperature’ value near 3.2 s. The flux of thermionic electrons including He I, N2, N2+ and CN obtained using a 390 nm, 10 FWHM from the centre electrode surface is responsible for the interference filter for an empty tube ramped to 2600°C under maxi- decreased centre electrode sheath, analogous to the situation mum-power heating conditions in a 50 W plasma with a self-biased described above for the tube wall.Near the end of the transient, centre electrode. from 3.6 to 4.0 s, there is a general ‘levelling out’ of the spatial emission structure such that it begins to resemble that obtained prior to heating the tube. These factors are probably a consequence of the equalization of temperatures of the centre electrode and the tube wall, resulting in the resumption of a small and constant dc self-bias.Emission transients obtained under negative bias conditions were not significantly dierent from those arising from self-bias runs; consequently, no specific data is presented for these conditions. Figs. 4 and 5 show the false coloured images and selected three-dimensional response surfaces, respectively, for He I 388.86 nm emission intensity during an atomization transient in which the dc bias of the centre electrode is held at +40 V.A plot of maximum pixel intensity versus time is presented in Fig. 6. This graph does not correlate well with the He I line Fig. 3 Emission intensity at 390±10 nm versus time for a 50 W forward power plasma with a self-biased centre electrode. intensities at +38 V bias obtained by Sturgeon et al.16 In the 712 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12Fig. 4 False-coloured images for plasma gas species, including He I, N2, N2+ and CN obtained using a 390 nm peak maximum, 10 nm FWHM interference filter for an empty tube ramped to 2600°C under maximum-power heating conditions in a 50 W plasma with a +40 V dc bias imposed on the centre electrode.latter study, intensities for a number of He lines first increased to 2 s and then exhibited a precipitous drop, possibly as a result of the formation of a mini-arc which resulted in a loss of the plasma image on the entrance slit of the monochromator. 16 In the present study, He I intensity progressively increases to 3.2 s, after which a smooth decline occurs.The overall He I intensity is lower than that for the self-bias case, consistent with the intensity of the centre electrode plasma decreasing with increasing positive bias.20 In terms of spatial features, the most striking characteristic is that of expanded radial emission occurring at 0.4 s, in agreement with earlier observations.20 ‘Ring-like’ structures arise which persist, in general form, throughout the heating transient. The origin of these structures is, presently, unknown and they are detected only when a 0 V or positive bias is imposed on the centre electrode.Their intensity increases with increasing positive bias whereas their diameter correspondingly decreases. They may simply be the result of the positive dc bias repelling He+ into a region intermediate between the electrode and the wall where they are ‘held’ as a consequence of the reversal of the rf field.The Fig. 5 Three-dimensional response surfaces for plasma gas species, formation of a ring of alternating positive and negative space including He I, N2, N2+ and CN obtained using a 390 nm, 10 FWHM charge in the annular space of concentric cylindrical corona interference filter for an empty tube ramped to 2600°C under maxi- discharges under an ac field has also been noted.33 Additionally, mum-power heating conditions in a 50 W plasma with a +40 V dc it may be that the positive bias of the centre electrode induces bias imposed on the centre electrode. a resonance interaction between the electrons in the plasma and the magnetic field of the rf antenna (centre electrode), which propagates circularly around the circumference of the graphite tube,34 thus creating an environment for the formation of ring-like regions of emission.Other salient features of the images merit discussion. At a dc bias of +40 V, the outer ring plasma merges towards the wall, producing an intense wall glow, as noted previously.20 From 0.8–1.6 s the tube wall is ramped to its steady-state temperature.Beyond 2 s the center electrode has radiationally heated suciently to commence significant thermionic emission from its surface. The plasma near the center electrode increases in intensity (Fig. 6) and breadth (Figs. 4 and 5) in response to the enhanced electron density. Beyond approximately 3 s a decrease in plasma intensity occurs, likely as a consequence of Fig. 6 Emission intensity obtained at 390±10 nm versus time for a enhanced reflected power losses.16 50 W forward power plasma with a +40 V bias imposed on the centre electrode. Several general conclusions can be drawn from Figs. 1 to 6. Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 713At no time does the plasma collapse as a consequence of Fig. 9 shows the atomization sequence for 10 ng of CsCl at 2600°C with a forward rf power of 50 W and a self-biased injected thermoelectrons, even for prolonged heating at 2600°C.This is also supported by monochromator-PMT- centre electrode. Emission from Cs I commences from the tube wall at approximately 0.05 s and gradually fills the internal based measurements of He I line intensities and excitation temperatures during atomization transients.16 Recently, tube volume relatively uniformly, with the exception of the area proximate to the centre electrode.A dark gap remains in LeBlanc and Blades8 reported that the plasma in their FAPES source extinguished at tube wall temperatures in excess of this region throughout the duration of the atomization transient. This gap is probably the consequence of ionization of Cs 1330°C and suggested that this arose as a result of thermionic electron emission changing the impedance of the source beyond within the intense centre electrode plasma. The apparent ‘intensity’ at the tip of the centre electrode is an artifact of the range of, or faster than the response time of, their matching network.Since the rf generator and tuner used by these authors thermal and plasma-induced heating of the centre electrode, which becomes so intense at ~850 nm that complete back- is identical to that used in this study, it is evident that the plasma quenching described by LeBlanc and Blades8 is unique ground correction is problematic. The ‘rings’ of excitation observed for He I in Fig. 3 and Cu I in Fig. 8 are absent in to their system, rather than being emblematic of a general limitation of FAPES. Possibly, the commercial graphite furnace this self-biased system. As the forward rf power to the plasma is increased to 100 W the depletion region around the centre recently employed by these authors,8 with its very high maximum power heating rate10 of 3500 K s-1, is part of the electrode grows, consistent with plasma-induced ionization of Cs in the (expanded) centre electrode plasma.This plasma- problem. The data additionally reveal that no emission ‘plume’ from induced ionization process was verified in a subsequent experiment in which 100 mg CsCl was atomized and excited in the the center electrode to the sample injection hole is present, despite the increased flux of He through it during the 0–1.6 s absence of a plasma at 2600°C (the higher analyte mass being required to obtain an emission signal in the absence of a period when rapid heating of the graphite tube causes expansion of the internal gas out of the injection hole.The absence plasma). Fig. 10 clearly shows that excited-state Cs fills the tube volume reasonably uniformly before dissipating. of a ‘plume’-like structure also suggests that the contribution of N2 and CN background species to the image is small, as an Furthermore, the entire event is delayed more than 1 s compared to that in the presence of a plasma, indicating the key emission plume has previously been observed from one or both of these sources when the tube wall was heated.20 Perhaps role the plasma plays in excitation and ionization of Cs.Only at very weak plasma power (20W) could a condition be most importantly, the images presented in Figs. 1 and 4 suggest that a relatively small change in the positive dc bias can realised in which the ‘depletion zone’ due to ionization around the centre electrode was eliminated, owing to the decreased produce very significant changes in the He emission ‘template’, with the formation of an annulus or ‘rings’ of enhanced production of Cs+ in this region.Similar distributions, in terms of ionization depletion and power dependencies, were excitation around the centre electrode plasma. Recent experiments have revealed that a helium FAPES plasma operated at also observed for Na (whether injected as the chloride or nitrate salt). The ‘depletion zone’ detected for Na was smaller 40.08 MHz is sustained with a peak-to-peak rf voltage of only ~75 V.35 It is not unreasonable to assume that an rf voltage in diameter than that for Cs, as a consequence of its higher ionization potential. of similar magnitude exists in the 13.56 MHz system used in this work. This is particularly significant since the measured rf Experiments were also undertaken to determine whether bias control had any eect on the ionization ‘depletion zone’ voltage is more than an order of magnitude less than that previously assumed to exist in FAPES.4 These small rf voltages around the centre electrode.Fig. 11 shows the results obtained for 10 ng of CsCl atomized as before into a 50 W rf power suggest that the relatively small dc bias potentials applied to the centre electrode may influence the voltage gradients present plasma with a +25 V bias on the centre electrode. Fig. 12 displays the results obtained for a -47 V bias. At +25 V bias in the source and the resulting emission patterns.The implications of changes in the centre electrode dc bias on the the emission rings again form (this is particularly evident when images obtained at 0.15 and 0.20 s in Fig. 11 are compared atomization of analyte transients will now be discussed. with those at the same time intervals as in Fig. 9) and the size of the ‘depletion zone’ is enhanced relative to the self-bias run Eect of Dc Bias on the Spatial Distribution of Excited-state shown in Fig. 9.For the -47 V bias situation the results are Atoms during an Atomization Transient reversed and Cs I emission encroaches much more closely around the centre electrode than for a self-bias run at the same Figs. 7 and 8 show the data obtained for atomization of 200 ng of Cu at 2600°C with a forward rf power of 50 W. The centre rf input power (although not as closely as the results obtained for 20W or in the absence of a plasma). Initially, this would electrode was allowed to self-bias (Fig. 7) or was maintained at +25 V (Fig. 8). In the self-bias transient, significant Cu I appear to be counter-intuitive, since the results presented earlier20 show that the intensity of the centre electrode plasma emission begins to appear at 0.45 s and generally fills the tube volume from 0.75 to 1.25 s before dissipating beyond 1.35 s. decreases with positive bias, implying that positive bias should be more favourable for decreasing the extent of Cs ionization. The ‘crescent’ at the left side of images from 0.75–1.15 s is a (recent) artifact of the CCD chip which becomes most evident However, positive bias also has the eect of enhancing both the radial emission intensity of the plasma and creating rela- when the emission intensity recorded by the chip is high.Positive bias produces enhanced Cu emission directly around tively intense annular structures surrounding the centre electrode. Since the ‘emission event’ for Cs ends at ~0.4 s, the bias the centre electrode, which is particularly evident at 0.85 s and beyond.Bias control (primarily 0 V bias) increases the eective potential at the centre electrode never exceeds ~-20 V for the images obtained in a self-biased run.16 Thus, a more likely length of the atomization event, possibly by providing a more consistent excitation environment. This was also observed for explanation for the dependence of the Cs I emission distribution on the dc bias can be obtained by realizing that a Fe and Pt by Sturgeon et al.16 Silver exhibits similar overall excitation characteristics to that for Cu, with the exception positive bias extends the radial excitation capacity of the plasma such that significant ionization of Cs occurs further that its distribution more uniformly extends to the walls, consistent with the higher anity of Cu for graphite surfaces.36 away from the centre electrode (i.e., proximate to the ‘excitation annulus’ developed at positive bias). A negative bias of -47 V, For comparison with the atomization characteristics of transition elements, the atomization of an easily ionized on the other hand, is more negative than the ‘indigenous’ value obtained in a self-bias run and has the eect of decreasing the element, Cs, was examined as bias and power were varied. 714 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12Fig. 7 False-coloured images for 200 ng Cu atomized from the tube wall for a forward rf power 50 W, with the centre electrode allowed to self-bias. Fig. 8 False-coloured images for 200 ng Cu atomized from the tube wall for a forward rf power 50 W, with +25 V dc bias imposed on the centre electrode. intensity of the radial emission region near the centre electrode conductive sample deposit is observed at 1.00 s and Ca II emission begins at 1.30 s, progressing radially around the plasma. Thus, despite an increase in the centre plasma intensity, a decrease in the radial excitation capability of the plasma centre electrode from 1.55 s onward.Intense Ca II emission occurs around the centre electrode between 1.6 and 2.0 s. Thus, permits a closer encroachment of Cs I to the centre electrode. This eect may have more practical uses in future applications Ca II emission is concentrated primarily around the centre electrode, providing a ‘mirror image’ to the depletion in of FAPES (e.g., as an ion source for mass spectrometry). Complementary information regarding ionization in FAPES emission noted for Cs in this region.This relatively localized Ca II emission pattern reflects the high excitation energy of was obtained by directly imaging an excited-state ion population. Calcium was selected for study and emission from the the Ca II emission lines and is consistent with that obtained for other high-lying transitions (e.g., CO+ and N2+).20 Biasing Ca II 393.37 nm and Ca II 396.85 nm lines was isolated using a 10 nm FWHM, 395 nm peak maximum interference filter the centre electrode to 0 V or positive voltage results in more enhanced wall emission, in accordance with an expansion of (see Table 1).Fig. 13 displays the results obtained for atomization of 100 ng Ca at 2600°C into a 50 W plasma with a self- the plasma (0 V bias) and the formation of a wall plasma at positive biases. biased centre electrode. Initial arcing of the plasma to the Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 715Fig. 9 False-coloured images for 10 ng CsCl atomized from the tube wall for a forward rf power 50 W, with the centre electrode allowed to self-bias. Fig. 10 False-coloured images for 100 mg CsCl atomized from the tube wall at 2600°C in the absence of a plasma. Eect of Easily-ionized Elements on the Spatial Distribution of the same pattern of excitation suppression as Cu, although the duration of the eect is longer. This pattern of decreased Excited-state Analyte Atoms During an Atomization Transient excitation suggests that it is a consequence of reduced plasma electron energy, or a lowered electron energy distribution The eect of 10 mg NaCl on the emission from Cu atomized at 2600°C into a 50W plasma with a self-biased electrode is function arising from excitation and/or ionization of Na.Beyond 0.90 s greater radial Cu emission is evident, approxi- presented in Fig. 14. In contrast to the Cu emission transient obtained in the absence of NaCl (Fig. 7), there is significant mating the distribution found in the absence of NaCl. Dissipation begins at 1.30 s. This implies that the most signifi- suppression of Cu excitation in the annular region between the centre electrode and the graphite tube wall. This is particu- cant vapour cloud overlap between Cu and Na occurs between 0.40 and 0.80 s, which was confirmed by direct CCD imaging larly evident from 0.45–0.90 s. Indeed, at these times Cu I emission is generally confined to a region around the centre of emission from the Na I 589.00 nm and Na I 589.59 nm lines (figures not shown).In any graphite furnace source the degree electrode and the tube wall. This is not surprising, considering that these regions contain the centre electrode plasma and the of vapour cloud overlap between interferent and analyte will determine the extent of the interference eect. As a consequence, outer electrode plasma, which are the most intense regions of excitation.Silver, atomized in the presence of EIEs, produces one would expect analytes of volatility lower than that of Cu 716 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12Fig. 11 False-coloured images for 10 ng CsCl atomized from the tube wall for a forward rf power 50 W, with a +25 V dc bias imposed on the centre electrode. Fig. 12 False-coloured images for 10 ng CsCl atomized from the tube wall for a forward rf power 50 W, with a -47 V dc bias imposed on the centre electrode.to exhibit a decreased interference eect, whereas more volatile complete suppression occurring at 20 mg NaCl (compared with 100 mg NaCl in the HGA-500 system). This is the consequence analytes would be more severely aected by the presence of NaCl. For Cu, the above ‘quenching’ of analyte emission begins of the lower maximum power heating rate of the HGA-76B supply relative to the HGA-500 power supply (~1250°C s-1 in the presence of 2.5 mg NaCl. This is in accordance with the results of Falk24 who calculated that a concentration of 0.01% versus ~2000°C s-1) and is consistent with the work of Hettipathirana and Blades7 who observed significant suppres- NaCl (equivalent to 2 mg in a 20 ml aliquot) would be sucient to bring about a 10% power loss in FAPES.With 100 mg NaCl, sion of emission for Pb at NaCl masses of only 162 ng with a heating rate of 90°C s-1. In contrast, Imai and Sturgeon18 only weak Cu emission around the centre electrode is evident.Previous Cu atomization experiments conducted with an using a nominal 1600°C s-1 heating rate and an identical ICC FAPES source, observed a 50% enhancement in Pb emission HGA-76B power supply revealed that the interference eect manifested itself at much lower NaCl concentrations, with signals in the presence of 500 ng of NaCl, which they ascribed to the early volatilization and subsequent plasma-induced substantial suppression occurring at 5 mg NaCl and essentially Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 717Fig. 13 False-coloured images for 100 ng Ca atomized from the tube wall for a forward rf power 50 W, with the centre electrode allowed to self-bias. Fig. 14 False-coloured images for 200 ng Cu atomized from the tube wall in the presence of 10 mg NaCl for a forward rf power 50 W, with the centre electrode allowed to self-bias. dissociation of PbCl. Suppression of the Pb emission signal arc. Formation of the arc must therefore be related to ionization and changes in plasma conductivity.It is interesting that the only manifested itself at NaCl masses of 10 mg or more. These data serve to point out the critical importance of the heating arc occurs exclusively in the direction of the dosing hole and not to other points along the outer electrode (tube wall) rate of the furnace in aording a ‘plug’ of atomic vapour to the plasma, readily available for excitation. surface.This may be related to the convective flow of He gas inside the tube, or it may simply be an edge eect due to An additional salient feature of Fig. 14, which is common to all the EIE matrices examined, is the development of an voltage fields near the sample injection hole. No arc occurs when the dosing hole is plugged with a graphite rod during emission ‘arc’ from 0.50–0.80 s between the sample injection hole and centre electrode. This arc forms readily when analytes atomization. Note that because the graphite plug protruded into the graphite tube somewhat, it is conceivable that an are atomized in the presence of an EIE matrix or when suciently large (e.g. 0.2 mg) amounts of EIEs themselves are ‘edge eect’ may still arise due to perturbations of voltage fields around it. Similarly, when Ca is atomized and emission atomized. It is not significant for atomization of other elements or in non-EIE matrices. For example, atomization of Cu in from the ionic state is monitored, no arc forms to the dosing hole.Ionic calcium emission commences at 1.30 s (Fig. 13), the presence of only 5 mg NaCl was sucient to produce the arc, but atomization in the presence of 40 mg Fe yielded no after the furnace has reached a steady-state temperature and 718 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12rapid expulsion of He out of the dosing hole has ceased. The prior to the appearance of Cu at ~0.45 s. This is not observed lack of arc formation under these experimental conditions, in when an equimolar mass of NaNO3 is used, suggestive of early addition to when the dosing hole is plugged, suggest that it is transfer of molecular NaCl to the centre electrode, which then related to the flow of He and its rapid expulsion from the enables the arc to form more readily.As well, images obtained dosing hole as the furnace reaches a steady-state temperature for equimolar amounts of NaCl and NaNO3 at high charring (at ~1 s under maximum power atomization conditions).temperatures are more intense for the latter salt, suggesting Imaging of Na transients has shown that, although arc forma- more ecient removal of NaCl at these charring temperatures. tion accompanies it as well, it has ceased by the time of Cu On the other hand, NaNO3 is reduced to metallic sodium on arc formation and Na is simply present within the volume of the graphite surface. Hence, the extent of EIE eects for Na the graphite tube (cf.Cs, Fig. 9, at 0.15 s). Thus, Cu is not (and for other EIEs as well) may depend upon the chemical co-vaporized in an arc along with the Na matrix. Arcing to form in which the EIE is present as well as its thermochemical the sample injection hole for Cu is a consequence of the properties. If the EIE is present as a less volatile salt, it will presence of EIE vapour which forms a more conductive path be more eciently retained within the graphite tube, producing from the centre electrode to the tube wall.Imaging these a more pronounced interference eect. The beneficial eects of analyte–interferent systems in an enclosed integrated contact thermal pretreatment (charring) for removal of volatile forms cuvette (ICC) furnace without a flow of gas would prove useful of EIEs will be more restricted in FAPES than graphite furnace in further elucidating the reason(s) underlying arc formation AAS as a consequence of the cooler centre electrode acting as in EIE matrices.an ecient condensation site for matrix vapour. The eect of 7.3 mg NaNO3 (equivalent to 5 mg NaCl) on In the presence of 14.4 mg CsCl (equivalent to 5 mg NaCl), the emission from 200 ng Cu atomized at 2600°C into a 50W Ca ion emission for 100 ng of Ca atomized at 2600°C into a plasma with a self-biased electrode is shown in Fig. 15. Despite 50 W self-bias plasma is suppressed everywhere except around the lower mass of EIE, suppression of Cu emission is much the centre electrode (Fig. not shown).The eect of CsCl on more extensive for NaNO3 than for NaCl. This enhanced Ca II emission is similar to that obtained for NaCl, but more suppression from NaNO3 was a general trend observed for all pronounced due to its lower ionization potential. The suppress- analytes examined (Ag, Cu, Ca). Indeed, for all images through- ive eects of equimolar amounts of CsCl and NaNO3 are, out the Cu transient, emission is confined to a small annulus however, comparable, suggesting that ionization of Cs does around the centre electrode with only very weak emission near not contribute as significantly to the loss of excitation capa- the tube walls and radial emission being almost negligible.The bility as photons radiated during the excitation of Cs and Na. reason for this may be related to dierences in the thermal In addition, attenuation of plasma electron energy from exci- properties of NaCl and NaNO3.Hettipathirana and Blades7 tation and dissociation of molecular matrix species may occur. measured Na emission arising from the vaporization of NaCl In general, NaCl, NaNO3 and CsCl produce the same overall and NaNO3, and concluded that NaCl vaporized earlier from pattern of suppression of excitation for all analytes studied the tube wall, consistent with a mechanism for formation of (Cu, Ag and Ca). The extent of the suppression is dependent Na on the graphite surface by carbon reduction of Na2O.37 upon the degree of vapour cloud overlap between analyte and When NaNO3 is used: EIE.Cesium should, however, have a greater vapour cloud overlap with analytes at higher temperatures due to the greater 4 NaNO3(s)�2 Na2O(s)+4 NO2(g)+O2(g) involatility of Cs vis a ` vis Na. It should be emphasized that Na2O(s)+C(s)�2 Na(s)+CO(g) the CCD imaging software does not permit calculation of integrated emission intensities over any region of the image, When a charring temperature of 1300°C is employed for the so no conclusion concerning the eect of EIEs on spatially vaporization of Cu in the presence of NaCl, an early arc (at ~0.10 s) arises from the centre electrode to the dosing hole integrated signals can be made.However, it is reasonable to Fig. 15 False-coloured images for 200 ng Cu atomized from the tube wall in the presence of 7.3 mg NaNO3 (equivalent to 5 mg NaCl) for a forward rf power 50 W, with the centre electrode allowed to self-bias.Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 719assume that the suppressed ‘emission patterns’ observed are imposed on the centre electrode. Clear in these images (and also strongly evident for a 0 V bias on the centre electrode) is reasonable representations of results that would be obtained in monochromator/PMT-based detection systems, from the the expansion of the centre electrode plasma, as was observed for He and analytes with no matrices.Emission in this region simple consideration that the analysis volume imaged in such systems consists of a thin slice of the plasma near the centre is enhanced relative to the self-bias case and the arcing of Ag to the sample injection hole, lasting up to 1.0 s under self-bias, electrode from the top to the bottom of the graphite tube. Any suppression in radial emission in that slice would thus be is significantly reduced for the zero or positive bias condition.Applied negative biases produce very pronounced arcs to the manifest as a loss of integrated emission detected by the PMT. The fact that suppressions in integrated emission signals have dosing hole throughout the atomization transient. The lack of arc formation at 0 V or slightly positive biases (i.e., 12–25V) been observed in PMT-based detection systems7,18 for similar analytes and the same masses and types of EIEs used in this compared to self-bias or large negative bias may derive from a reduced potential dierence between the centre electrode and study supports this.the outer tube wall during a given half-cycle under these conditions. At 0 V dc bias, the centre electrode and tube wall Eect of Dc Bias on the Spatial Distribution of Excited-state serve as cathode and anode for equal periods of time during Analyte Atoms in the Presence of Easily-ionized Elements a given rf half-cycle, while for a +12 to+25 V bias, the tube wall is the anode for only a slightly greater period of time Dc bias has been shown to significantly influence the spatial than the centre electrode during a given rf half cycle.Because distribution of analyte emission in clean matrices and its an arc forms most readily when this potential dierence is control aords robustness of excitation temperatures during accentuated (i.e., in the self-bias or large negative bias case), an atomization transient.16 Thus, the influence of dc bias in its formation may be somewhat attenuated under 0 V or small controlling the eects of the ‘excitation suppression pattern’ positive bias.Copper and Ca exhibit similar eects. observed for EIEs in self-bias systems was investigated. Unfortunately, study of this radial enhancement was restricted Fig. 16 shows the images obtained during atomization of to low interferent loadings as application of a positive bias 50 ng Ag at 1700°C into a 50W self-bias plasma, whereas with interferent masses in excess of 0.5 mg NaCl for Ag or Fig. 17 shows those for the atomization of the same mass of 2.5 mg for Cu and Ca produced a suciently conductive sample Ag with 125 ng NaCl under the same operating conditions. deposit to encourage arcing of the plasma to the site of sample Clearly, extensive suppression of Ag emission throughout the deposition. course of the transient occurs at even these low interferent loadings and despite a larger eective plasma power (reflected power is 1–3 W at 1700°C compared to 33–35 W at 2600°C). Eect of Dc Bias on the Spatial Distribution of Excited-state The primary reason for this is that, unlike Cu, for which Na Analyte Atoms in the Presence of an Iron Matrix vapour overlap occurred for ~0.3 s into the 1.0 s transient, CCD imaging of Na I reveals that it persists throughout the The similarity in suppression eects observed for CsCl and NaNO3 implies that attenuation of plasma excitation power entire Ag transient at 1700°C.Early (0.10–0.20 s) emission of Ag is completely absent, arcing is prevalent and, unlike Ag is primarily a consequence of loss of photons from the excitation of the EIE. To support this, Fe was used as an interferent without a matrix, a complete cross-sectional filling of the graphite tube with emission from the Ag I 328.07 nm line is since excitation of its large number of available spectral transitions may bring about a similar suppression in analyte never realized.As with the other analytes and, for reasons already discussed, the magnitude of this eect increases when intensity as a result of photon losses.24 The use of Fe is also fortuitous in that its atomic mass is close to the molecular equimolar amounts of NaNO3 or CsCl are present. Fig. 18 shows the results obtained for the same interferent mass of NaCl, thus permitting ready comparison of these two interferents at approximately the same mass loadings. and operating conditions as above but with a +25 V bias Fig. 16 False-coloured images for 50 ng Ag atomized from the tube wall for a forward rf power 50 W, with the centre electrode allowed to self-bias. 720 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12Fig. 17 False-coloured images for 50 ng Ag atomized from the tube wall in the presence of 0.125 mg NaCl for a forward rf power 50 W, with the centre electrode allowed to self-bias. Fig. 18 False-coloured images for 50 ng Ag atomized from the tube wall in the presence of 0.125 mg NaCl for a forward rf power 50 W, with +25 V dc bias imposed on the centre electrode.Fig. 19 shows the images obtained during the atomization being expelled from the dosing hole during the first 1.6 s in both cases. This further implicates the increased conductivity of 50 ng Ag in the presence of 20 mg Fe at 2600°C into a 50W self-bias plasma. Unlike Ag vaporizing with no matrix, for imparted to the plasma by the sample matrix (i.e., EIE) in being responsible for the formation of the arc to the sample which Ag I emission commences at 0.10 s, the onset of Ag I is delayed until 0.35 s in the presence of 20 mg Fe, presumably injection hole.A larger mass of Fe than NaCl is required to produce an equivalent matrix eect. For example, whereas because Ag is embedded in the less volatile Fe matrix. More significantly, however, the radial distribution of Ag is altered only 0.125 mg NaCl produces substantial suppression of radial emission from Ag I, significant suppressions by Fe only begin throughout the entire transient such that the only regions of high Ag intensity are those around the centre electrode and at interferent loadings of 5 mg.Suppression eects for Cu and ionic Ca in the presence of Fe are generally similar to the the tube wall. This situation is similar to the radial ‘excitation suppression’ already noted for Ag in the presence of those observed for Ag, except that a larger mass of Fe (20 mg) is required to produce the onset of quenching of the radial 0.125 mg NaCl, with the exception that there is essentially no arc formation in the presence of Fe, despite the fact that He is distribution.Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 721Fig. 19 False-coloured images for 50 ng Ag atomized from the tube wall in the presence of 20 mg Fe for a forward rf pow0 W, with the centre electrode allowed to self-bias.Fig. 20 illustrates the eect of application of a +22 V bias their influence was not addressed in studies involving interfering matrices. Biasing eects for ionic Ca and Cu were similar on the centre electrode during the atomization of Ag in the presence of the 20 mg Fe. Greater radial emission is evident in to those observed for Ag. The general trend of suppression of excitation by Fe for all analytes studied and the similarity of all images as the development of the radial ‘excitation annulus’ occurs.The general emission pattern is analogous to that the spatial response of dierent analytes in the presence of this interfering element to the dc bias suggest that the major obtained for similar bias conditions for Ag and 0.125 mg NaCl, with the exception that formation of excited-state Ag atoms is mechanism underlying the suppressive eects of EIEs on analyte distributions is associated with the loss of photons due delayed due to the Fe matrix and the radial excitation pattern is wider in the case of Fe.As with NaCl (or NaNO3 or CsCl), to the excitation of the matrix (Fe or EIE) and the less energetic plasma which ensues. Although ionization eects negative biases were unusable as these resulted in significant arcing to the site of sample deposition. Application of a positive likely play some role in analyte suppression, it is probable that this mechanism does not cause a significant energy loss in the bias up to +38 V decreased the volume of the excitation annulus somewhat (as with the EIEs) and, as positive biases plasma, since equimolar amounts of Cs (as CsCl) and Na (as NaNO3) produce similar quenching eects.in excess of 38 to 40 V tend to cause arcing in clean systems, Fig. 20 False-coloured images for 50 ng Ag atomized from the tube wall in the presence of 20 mg Fe for a forward rf power 50 W, with +22 V dc bias imposed on the centre electrode. 722 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 1212 Sturgeon, R. E., Willie, S. N., Luong, V. T., and Berman, S. S., CONCLUSIONS J. Anal. At. Spectrom., 1990, 5, 635. The dc bias of the centre electrode plays a significant role in 13 Sturgeon, R. E., Willie, S. N., Luong, V. T., and Berman, S. S., J. Anal. At. Spectrom., 1991, 6, 19. determining the spatial distribution of analyte emission in the 14 Sturgeon, R. E., Willie, S. N., Luong, V. T., and Dunn, J. G., FAPES plasma for both clean samples and when interfering Appl.Spectrosc., 1991, 45, 1413. matrices are present. As a relatively small voltage (~75 V 15 Sturgeon, R. E., and Willie, S. N., J. Anal. At. Spectrom., 1992, peak-to-peak) sustains the FAPES plasma,35 application of a 7, 339. relatively small dc bias voltage to the centre electrode signifi- 16 Sturgeon, R. E., Luong, V. T., Willie, S. N., and Marcus, R. K., Spectrochim. Acta, Part B, 1993, 48, 893. cantly alters the voltage gradients present in the source and, 17 Imai, S., and Sturgeon, R.E., J. Anal. At. Spectrom., 1994, 9, 493. by extension, the spatial distribution of analyte emission. The 18 Imai, S., and Sturgeon, R. E., J. Anal. At. Spectrom., 1994, 9, 765. utility of bias control is limited to low interferent loadings 19 Imai, S., Sturgeon, R. E., and Willie, S. N., J. Anal. At. Spectrom., (2 mg, generally), otherwise arcs and instabilities begin to 1994, 9, 759. manifest themselves. It should be stressed, however, that the 20 Pavski, V., Chakrabarti, C. L., and Sturgeon, R. E., J. Anal. At. Spectrom., 1994, 9, 1399. current CCD imaging system did not permit emission intensit- 21 Falk, H., Homann, E., and Ludke, Ch., Prog. Anal. Spectrosc., ies to be spatially integrated over any given cross-section of 1988, 11, 417. the tube. As such further investigation will be required (e.g., 22 Ballou, N. E., Styris, D. L., and Harnly, J. M., J. Anal. At. with conventional PMT detection) to ascertain the full extent Spectrom., 1988, 3, 1141. of some of the suppressions and enhancements observed. 23 Harnly, J. M., Styris, D. L., and Ballou, N. E., J. Anal. At. Spectrom., 1990, 5, 139. 24 Falk, H., J. Anal. At. Spectrom., 1991, 6, 631. 25 Riby, P. G., and Harnly, J. M., J. Anal. At. Spectrom., 1993, 8, 945. REFERENCES 26 Tripcovic�, M. R., and Holclajtner-Antunovic�, I. D., J. Anal. At. Spectrom., 1993, 8, 349. 1 Liang, D. C., and Blades, M. W., Spectrochim. Acta, Part B, 1989, 27 Galley, P. J., Glick, M., and Hieftje, G. M., Spectrochim. Acta, 45, 1059. Part B, 1993, 48, 769. 2 Sturgeon, R. E., Willie, S. N., Luong, V. T., Berman, S. S., and 28 Kitagawa, K., and Horlick, G., J. Anal. At. Spectrom., 1992, 7, 1207. Dunn, J. G., J. Anal. At. Spectrom., 1989, 4, 669. 29 Larkins, P. L., Spectrochim. Acta, Part B, 1991, 46, 291. 3 Smith, D. L., Liang, D. C., and Blades, M. W., Spectrochim. Acta, 30 Ratli, P. H., and Harrison, W. W., Spectrochim. Acta, Part B, Part B, 1990, 45, 493. 1994, 49, 1747. 4 Hettipathirana, T. D., and Blades, M. W., Spectrochim. Acta, Part 31 Hieftje, G. M., Spectrochim. Acta, Part B, 1992, 47, 3. B, 1992, 47, 493. 32 Von Engel, A., Ionized Gases, Oxford University Press, London, 5 Hettipathirana, T. D., and Blades, M. W., J. Anal. At. Spectrom., 2nd edn., 1965. 1992, 7, 1039. 33 Cobine, J. D., Gaseous Conductors: T heory and Engineering 6 Banks, P. R., Liang, D. C., and Blades, M. W., Spectroscopy, Applications, Dover Publications, Inc., New York, 1958. 1992, 7, 36. 34 Gilmour, Jr., A. S., Microwave T ubes, Artech House, Inc., 7 Hettipathirana, T. D., and Blades, M. W., J. Anal. At. Spectrom., Dedham, MA, 1986. 1993, 8, 955. 35 Sturgeon, R. E., unpublished data. 8 LeBlanc, C. W., and Blades, M. W., Spectrochim. Acta, Part B, 36 McNally, J., and Holcombe, J. A., Anal. Chem., 1987, 59, 1105. 1995, 50, 1395. 37 Campbell, W. C., and Ottaway, J. M., T alanta, 1974, 21, 837. 9 Gilchrist, G. F. R., Celliers, P. M., Yang, H., Yu, C., and Liang, D. C., J. Anal. At. Spectrom., 1993, 8, 809. Paper 7/00843K 10 Gilchrist, G. F. R., and Liang, D. C., Am. L ab., 1993, 25, 34U. Received February 5, 1997 11 Sturgeon, R. E., Willie, S. N., Luong, V. T., and Berman, S. S., Anal. Chem., 1990, 62, 2370. Accepted April 15, 1997 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12

ISSN:0267-9477    

DOI:10.1039/a700843k

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Axial Viewing and Modified Cup Design for Direct Sample Insertion Inductively Coupled Plasma Atomic Emission Spectrometry CAMERON D. SKINNER AND ERIC D. SALIN* Department of Chemistry, McGill University,Montreal, Quebec, Canada H3A 2K6 Axial viewing generally provides an advantage over lateral optical design since the plasma tail is directed at the optical viewing of the plasma for direct sample insertion. The signal system. Typically, some form of sampling cone or sheathing change is element-specific and has produced detection limit gas is used to protect the optic that is exposed to the plasma.5 improvements as high as a factor of 10 in the set studied.In To enhance the optical design of commercial instruments some the axial viewing mode the incandescent cup does not manufacturers are abandoning the traditional mirror-based contribute to an increased background; however, the optical systems in favor of lens systems.6 Since the central background and subsequent background noise levels are channel of the plasma is cylindrical, a cylindrical achromatic increased for most elements. Carrier gases were introduced lens may be used to focus the circular region in the central into the cup via a hollow stem resulting in a reduction in the channel into a line image on the entrance slit to maximize the background signal, notably in the longer wavelength region.light throughput. As a result of the higher throughput, smaller When 1000 ppm Freon-12 in argon is used as a carrier slit-widths may be used to increase the resolution and improve through the center of the cup, the normally problematic the signal-to-background ratio. The combination of these refractory elements are vaporized easily.Modifications and modifications in turn reduces the detection limits and improvements to the design of the direct sample insertion minimizes interferences. device are also discussed. The second liability of ICP-AES is the poor sample introduction eciency of pneumatic nebulizers.ETAAS overcomes this Keywords: Axial viewing; direct sample insertion; inductively disadvantage by producing a transient signal that has a high coupled plasma; Freon; atomic emission spectrometry; sample analyte concentration in the sample cell. This high analyte introduction concentration gives rise to increased sensitivity and lower detection limits. A similar increase in sensitivity can also be Inductively coupled plasma atomic emission spectrometry observed in ICP-AES when sample transport eciency is (ICP-AES) has been the workhorse of modern analytical increased with the use of devices such as ultrasonic7 and frit8 elemental determinations for many years because of its multi- nebulizers.In addition to these more traditional liquid sample element capability, large linear dynamic range, relative freedom introduction techniques, alternative sample introduction from non-spectral interferences and low detection limits.methods such as electrothermal vaporization (ETV)9 and direct However, for samples that require great sensitivity and lower sample insertion (DSI)10 may be used. With ETV, the sample detection limits, electrothermal atomic absorption spec- vapor is swept out of the furnace and introduced into a trometry (ETAAS) and more recently ICP mass spectrometry standard ICP. In the DSI technique, a sample is introduced (MS) have been the techniques of choice.ETAAS is primarily into the plasma on a sample carrying probe. Our laboratory a single element technique and suers from interferences and has primarily focused on the use of graphite cups as probes a shorter linear dynamic range than ICP-AES. These draw- for DSI experiments but metal probes have also been used.11 backs are unfortunate because of the low detection limits and With DSI the sample is vaporized within the plasma, and the relatively low cost of the technique.On the other hand, ICP-MS technique can be considered to be a 100% ecient sample has very low detection limits and is capable of determining introduction method. multiple elements by rapidly scanning through the masses of Axial viewing in combination with a sample introduction the elements of interest; however, the instrumentation is expens- technique that is 100% ecient would appear to have the ive to purchase and operate, and is more prone to interferences potential to reduce detection limits of ICP-AES significantly.than ICP-AES.1 The ideal solution would seem to be an To this end we have attached a DSI device to an ICP extension of the working range of ICP-AES to levels that are configured for axial viewing. We also speculated that DSI may competitive with those of ETAAS and ICP-MS. gain an additional advantage in that the background intensity In an attempt to augment the capabilities of ICP-AES, many might be considerably lower with axial viewing as the DSI instrument manufacturers are switching from lateral to axial cup would appear to be black.Given the significantly higher viewing of the plasma. This method oers several potential light levels anticipated from axial viewing, one could expect a advantages of which the most important are increased optical dramatic improvement in signal-to-background ratio and throughput and higher line-to-background ratios.2 The consequently detection limits.5,6 resulting configuration provides lower detection limits that Even though DSI-ICP-AES with a thin-walled cup is rela- may allow ICP-AES to replace ETAAS for a variety of tively free from memory eects, refractory compounds and applications.However, axial viewing of the plasma is not elements can be problematic because of their low volatility. In without its disadvantages. There is an increased risk of inter- some cases, incomplete vaporization may result in tailing that ferences by viewing through the plasma tail.3 To help counter does not return to the baseline during a 10–15 s insertion.this problem the torch can be extended, which some manufac- Carbide-forming elements are especially troublesome since turers claim maintains the traditional freedom of the ICP from most carbides vaporize at temperatures greater than those that chemical interferences.4 Viewing the plasma axially presents some diculties in the can be obtained in a conventional DSI cup in an argon plasma.Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 (725–732) 725Vaporization of these compounds can be improved by increas- deep into the base of the electrode for an adapter that mates the hollow glass shaft to the cup. Next, a 5.16 mm (13/64 in) ing the temperature using oxygen in the plasma (outer) gas12 or by the introduction of halogenating agents.13 Increasing the hole 9.7 mm deep is bored into the face of the electrode to form the cup.The wall of the cup is produced by removing forward power that is applied to the plasma eectively increases the temperature; however, it is often not sucient to obtain carbon from the electrode with the cutting tool. The wall requires several light passes. If too much carbon is removed complete vaporization. The elements that form refractory oxides and carbides all have halide forms that vaporize at in one pass, the cup tends to deform and crack. When the underside of the cup is being cut, the tool must significantly lower temperature.This property has been exploited by using solid halide salts.14 We have modified the be oriented such that only the point of the tool is in contact with the base of the cup (see Fig. 1). If the heel of the tool is DSI so that gaseous halogenating agents (usually Freon-12) may be introduced directly into the cup during the insertion. allowed to rub the base of the cup, the friction and the resulting torque inevitably breaks the stem.The base of the cup and the Note that others have directly introduced Freon into the plasma gases.15 Freon breaks down to produce fluorine and stem are all cut in one pass since any pressure on the thin stem usually results in a broken cup. Deformation and cracking chlorine radicals which react rapidly to form volatile halides.16 Gases injected through the cup establish a central channel are not a problem when machining the base of the cup because of the high structural strength of the base.The dimensions of that reduces the background and also increases the analyte signal intensity by entraining the analyte vapors up through the cup are given in Fig. 2. After the cup has been machined it must be cleaned prior the viewing zone. The addition of low volumes of halogenating gases to the plasma gases does not establish a central channel to use. The cup is cleaned by inserting it into the plasma so that the top of the cup is approximately 5–10 mm above the but does enhance the volatility.The purpose of this study was the evaluation of axial viewing top of the load coil (ATOLC). An insertion to this depth ensures that contaminants in the base and stem do not vaporize for DSI-ICP-AES and a study of direct injection of gases through DSI cups. during routine use. Typical analytical insertion depths are to 0 mm ATOLC. The primary contaminants that have been observed in the plasma during the cleaning step are sodium EXPERIMENTAL (from handling) and iron from the cutting tool. The 0.7 mm thick stem is not the thinnest stem that has The DSI cups fabricated for this investigation are made from been produced.Stems as fine as 0.5 mm can be easily produced standard carbon electrodes, Table 1. The cups are an improvebut have been found to be too delicate for practical purposes. ment over those that are traditionally used because the walls Cups with such fine stems tend to break on handling and do and the base of the cup are especially thin to minimize the not survive the vibrations of the DSI device during insertions mass introduced into the plasma.This allows the plasma to and retractions. The survival rate of the 0.5 mm stem cups heat the cup and sample rapidly. We have found from experidepends on the individual cup. Some cups have survived ence that one of the most important parameters in cup design hundreds of insertions whereas some break on the first run.is the thickness of the cup base.17 If the base or wall is too This large inter-cup variability is not found with the thicker thick then the analytes are slow to vaporize and the signal tends to tail.17–19 Memory eects are only observed on carbideforming and highly refractory elements. In the past, the cups were machined on a lathe (Table 1) that rotated at 1180 rev min-1; however, we have recently found that operating the lathe at its maximum speed of 2880 rev min-1, allows a cup of finer dimensions to be produced.The cutting tool is a simple high speed steel cutter with a slightly rounded triangular point. The sharpness of the cutting edge is critical to the success of cutting the cup. The cutter must be resharpened after every 3–5 cups are machined due to the abrasive nature of the graphite. Fig. 1 Cutting of DSI cup stem. The first step in cup production is the drilling out of the base. A 3.18 mm (1/8 in) twist drill is used to bore a hole 9 mm Table 1 Equipment supplies and manufacturers Carbon cup blanks Bay Carbon Electrode: #S-8 high density Bay City, MI, USA Boiler caps: #BC-1 Spectrometer modifications TruLogic, and upgrade Mississauga, Ontario, Canada Plasma Unit, Model 2500 Plasma Therm, Kresson, NJ, USA Ball valve F.C.ProValve, Pointe Claire, Quebec, Canada 1000 ppm Freon-12 in argon Matheson Ville St. Laurent, Quebec, Canada Lathe Norvik Moore, Emco Compact 5 Ville St. Laurent, Quebec, Canada Fig. 2 DSI cup dimensions. 726 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12stemmed cups and so the 0.7 mm thick stem has been used mirror can sit on top of the torch box. The chimney is also removed from the torch box cover and a 90° folding mirror is throughout this work. There was no appreciable dierence in the performance of the two cup types. The lifetime of the cup placed directly above the plasma. With this arrangement the magnification of the system remains constant, see Fig. 3. with the 0.7 mm stem has been found to be in excess of 200 insertions, and probe demise is usually due to a handling The folding mirror directly above the plasma is protected from the plasma tail by a gas cut-o that is directed across accident. The cups that are designed to introduce gases into the the tail of the plasma. The gas cut-o is created by passing a 14 l min-1 nitrogen gas stream through a 2 in Perkin-Elmer plasma through their bases are machined similarly; however, after the cup has been drilled out with the 5.16 mm drill bit, flame atomic absorption slot burner head.The DSI device (DSID) that was used for this work has the center of the electrode is drilled out with a 1.58 mm (1/16 in) or 0.88 mm (0.035 in) drill bit. The full depth of the undergone extensive modifications from the original design of Sing and Salin.21 The modifications were necessary in order bit is used to drill through the entire probe in one pass.The method of cutting the stem is the same except that the diameter to allow greater ease in set-up and alignment as well as the introduction of gases into the plasma through the cup. An is increased to 2.28 mm to accommodate the hollow stem (Fig. 2). There was no significant dierence in analytical per- explanation of the new design follows as well as a schematic diagram (Fig. 4). formance between the two cups, so the cup with the 1/16 in hollow diameter shaft was used throughout.The shaft on which the DSI cup is placed is 1/4 in medium wall glass tubing. The shaft is mounted near the center of a The adapter plug that mates the DSI cup to the glass shaft is cut from graphite and is designed to fit into the hole in the brass block by a 1/4 in to NPT PTFE Swagelok fitting. The shaft extends to about 5 cm below the block. The Swagelok base of the cup 3.18 mm (1/8 in) and into the glass shaft (approximately 2.5 mm) (Fig. 2). A second adapter with a hole fitting was drilled out to allow the shaft to pass through the fitting.This extension allows a gas fitting to be attached bored out to 1.58 mm was made so that gases could be introduced into the DSI cup. directly to the end of the glass shaft which can then be used to introduce gases into the cup via the hollow stem cup and A boiler cap is a commercially available lid that fits snugly over the graphite electrodes that the cups are made from. The shaft adapter (Fig. 5).The brass drive block has a 12.7 mm (1/2 in) hole drilled DSI cup that was used for the boiler cap experiment had a thicker stem of 1.4 mm and slightly thicker walls of 0.4 mm through it to accept a 1/2 in od aluminium guide shaft that extends the length of the drive section of the DSID. On the with an exterior diameter of 6.17 mm (0.243 in) so that the boiler caps would fit the cup. Three boiler caps were drilled opposite face of the drive block, a 2 mm thick plate of aluminium is mounted with six Allen screws.The two ends of the taut out to orifice diameters of 0.40 mm (1/64 in), 3.18 mm (1/8 in) and 8.73 mm (11/32 in). Two of the three caps were made from drive belt are held in position by tightening the plate down over the belt with the screws (Fig. 5). After these experiments standard Bay Carbon boiler caps which have an orifice of 0.97 mm (0.038 in); the third was made from carbon rod had been performed an additional guide shaft was installed on the system so that the DSI shaft is bracketed by the guide stock, Table 1.The spectrometer used for this series of experiments was a shafts. This addition provides further insertion precision and prevents the guide block from rotating while in motion. Jarrell-Ash 750 direct reader originally designed for spark emission. The spectrometer has been modified for rapid back- At the bottom of the DSID, the drive belt passes over a sprocket that is attached to the stepper motor.An idler ground correction with the installation of a galvanically driven quartz refractor plate and high speed electronics (Table 1).20 sprocket is located at the upper end of the DSID. An optical interrupter switch is mounted on the body of the DSID and Throughout this work data acquisition software (SF20) written by G. Le�ge`re specifically for transient signal acquisition was is connected to the stepper motor controller. The flag which triggers the optical interrupter is mounted onto the drive block used.A commcial version of this hardware is available from TruLogic Systems (Table 1). so that the lower limit of travel can be detected and the stepper motor controller has a zero reference position. The spectrometer input optical system uses an all-mirror design to minimize chromatic aberrations (Fig. 3). A 0.75 m focal length o-axis concave mirror is used to image the plasma onto the entrance slit. A 90° folding mirror is placed between the entrance slit and the focusing mirror to facilitate experimental set-up.In the lateral viewing mode the plasma, mirrors and the spectrometer all lie on the same optical plane. In the axial viewing mode the torch box (Table 1) is lowered below the optical plane of the spectrometer so that the focusing Fig. 4 Schematic diagram of direct sample insertion device. Fig. 3 Overhead view of axial viewing configuration. Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 727reverse of the normal optical path.23 The beam can be followed from the entrance slit and the mirrors adjusted as needed to ensure that the beam focuses at the appropriate position directly above the DSI cup.Sample introduction into the cup is straightforward. The plasma is extinguished and the cup is raised to approximately 5 cm ATOLC and a volume of sample is pipetted into the cup. In this series of experiments 10 ml of standard were used. The cup is then retracted to the center of the load coil and dried inductively. The cup is then withdrawn until the DSID triggers the optical interrupter.The stepper motor controller then drives the cup up to the base of the plasma. It briefly halts (2 s) at about 5 mm below the base of the plasma to allow the plasma to stabilize. As the DSI shaft is driven up towards the plasma, the plasma can be seen to jitter slightly. The cup is then inserted into the plasma. Since the sample is pre-dried there is no need for a drying step beneath the plasma.14,21,24 Transient analyte emission is observed with the spectrometer.Fig. 5 Diagram of drive block. The on-line as well as the background intensities are measured in rapid succession. After the signal has been recorded, the cup is withdrawn from the plasma and allowed to cool before the Above the drive block and in-line with the glass shaft is a Teflon plate 1 cm thick with a hole for the shaft. This plate next sample is introduced.serves two functions: it guides the shaft up the middle of the torch and it provides a gas-tight seal between the plasma and RESULTS AND DISCUSSION the external atmosphere while the ball valve is open. In the future a sample deposition box will be added to the DSID on In order to compare the relative merits of axial viewing with top of the drive section. The cup can then be withdrawn from lateral viewing for DSI, the dependence of the transient signal the plasma into the box and the sample can be sprayed into intensity on several experimental parameters was investigated. the cup and dried while the plasma is still operating.Spray Table 2 lists the experimental parameters used for this series deposition yields better detection limits and good reproduc- of measurements in both the axial and lateral viewing modes. ibility and is preferable since it is easily automated.22 During In the axial viewing mode the gas cut-o height was varied sample deposition and drying the ball valve can be closed to first.Three dierent cut-o heights were investigated. The prevent the plasma from settling on the inner torch tube. The lowest was approximately 25 mm ATOLC and visibly sheared ball valve (Table 1) can be disassembled in situ, so that the the conical top of the plasma o. The highest was approxitorch, Swagelok fitting and part of the ball valve can be mately 70 mm ATOLC so that the luminous region of the removed as a unit for cleaning or replacement without remov- plasma was unaected by the cut-o gas.The middle height ing the DSID or losing the alignment of the system. was the median of the upper and lower heights. The plasma Additionally, the torch can be safely removed so that when was only slightly aected although the top of the discharge the DSID is removed from the system there is no danger of would occasionally be disrupted by turbulence from the gas torch breakage. cut-o. Fig. 6 indicates that the position of the gas cut-o does In the past, alignment of the DSID within the torch was a not have a dramatic eect on the signal.Since the highest gas problem. In this configuration, the bottom of the torch box cut-o height visually disturbed the plasma the least it was has been cut out and replaced with a removable torch box used for subsequent experiments. This strategy may not be plate that can be changed for each type of sample introduction advisable if samples of a complex nature are analyzed since device.The problem of alignment has been overcome by recombination takes place in the tail of the plasma and may attaching the torch and the DSID to the removable torch box lead to interferences and loss of linearity.6 plate. The torch is held at the bottom by a 1/2 in PTFE The insertion depth parameter was varied next. Fig. 7 shows Swagelok fitting. This fitting is attached to a pneumatically the optimization curve with traditional lateral viewing at a driven 1/2 in poly(vinyl chloride) (PVC) ball valve.The ball viewing height of 20 mm ATOLC. Fig. 8 shows how signal valve, pneumatic drive and torch assembly are all mounted onto a plate. Between the torch/ball valve plate and the match Table 2 Instrumental operating parameters box plate is a large (3 cm diameter) O-ring. The torch/ball valve plate is attached to the torch box plate via three Allen Plasma forward power 1 kW screws. To align the torch in the center of the load coil the Reflected power #5 W three screws are adjusted.The O-ring accommodates the tilt Plasma gas flow rate 14 l min-1 of the plate while providing a gas-tight seal. Auxiliary gas flow rate 0.8 l min-1 DSI torch purge gas flow rate #0.3 l min-1 In a similar fashion, the drive section of the DSID is (only before ignition of plasma) mounted to the underside of the match box plate with three Axial cut-o gas flow rate #14 l min-1 Allen screws and a large O-ring.This allows the cup to be Sample volume for calibration 10 ml 1–100 ppb centered in the load coil when the drive shaft is extended. Sample volume for hollow stem 10 ml at 0.5 ppm At the point where the ball valve attaches to its mounting Insertion depth 0 mm ATOLC* plate a small (1/8 in Swagelok) gas fitting is used to attach the Viewing height 20 mm ATOLC Exposure time 20 ms per position gas flow that is normally used for the nebulizer to the DSID. Number of on-line and o-line This ‘nebulizer’ gas is used to purge the ball valve when the exposures per trace 200 instrument is first set up but during routine use it is usually Galvanometer settle time 10 ms turned o.Background oset #0.8 nm Alignment of the optical system is relatively simple since the Gas flow through cup 290 ml min-1 installation of a diode laser in the zero order light trap of the spectrometer. When the laser is on, the beam follows the * ATOLC: Above top of load coil. 728 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12the case similar behavior between Zn and other volatile elements would be expected. Axial viewing in combination with DSI allows a wide range of viewing heights to be used without loss of sensitivity. There are two reasons for this; first, when DSI is employed the plasma does not have widely dierent thermal zones such as those with liquid nebulization because there is no cooling/ desolvation process.26 Second, the optical arrangement tends to average the signal intensity over the length of the plasma which ‘homogenizes’ the signal.To verify this, the cup was inserted to 0 mm ATOLC and the viewing height of the system was adjusted by moving the focusing mirror slightly. The viewing height of the system was measured as the distance from the focal point of the alignment laser to the top of the load coil (ATOLC, as is the insertion depth). Fig. 9 shows that when the plasma is viewed axially the signal is nearly indepen- Fig. 6 Intensity of emission in axial viewing mode as a function of the position of the gas cut-o. (&, Sn; +, Zn; $, Pb; X, Cu). dent of the viewing height although the signal drops close to zero as the viewing height approaches the interior of the cup. This indicates that, operating under compromise conditions, viewing height should not be a problem. The signal intensities that are observed with lateral viewing DSI show a greater dependence on the viewing height but there is no strong optimum.For comparison, both systems were subsequently tested with the same viewing height and insertion depth. The method was calibrated in both the lateral and axial viewing modes using the operating conditions listed in Table 2. Those elements for which calibration graphs were acquired are listed in Table 3 with an asterisk. In general, axial viewing does provide an advantage over lateral viewing. Net emission intensities are higher; however, background noise is also increased over that of lateral viewing.The overall result is an increase in the signal-to-background noise ratio and improved Fig. 7 Lateral viewing signal intensity versus cup insertion depth. detection limits (Table 4). The detection limits for peak height (X, Sn; $, Zn; +, Pb; &, Cu). measurements were calculated based on three times the noise of the background divided by the slope of the calibration Fig. 8 Axial viewing intensity versus cup insertion depth.(X, Sn; $, Zn; +, Pb; &, Cu). Fig. 9 Peak area in the axial viewing mode as a function of the viewing height. The viewing height is the focal point of the spectrometer, the top of the DSI cup is inserted 0 mm ATOLC. (X, Cu; peak height varies with insertion depth when axial viewing is $, Zn; +, Pb; &, Cd; ,, Mg). used. Note that Fig. 7 only covers the 0–10 mm insertion depth range whereas with axial viewing (Fig. 8) the range was from -10 to 10 mm.The reason for the smaller range was an Table 3 Elemental lines studied oversight while the instrument was set up in the lateral viewing Element Wavelength/nm mode. Zinc is the most sensitive element with respect to insertion depth. At the -10 mm insertion depth the base of C I 193.1 Zn I 213.9 Calibration graph determined the cup is sitting in the edge of the plasma base. At this Pb II 220.4 Calibration graph determined position the emission intensity was sucient to saturate the Cd I 228.8 Calibration graph determined detector.It is unclear why Zn shows such a strong dependence Sn I 235.5 Calibration graph determined on insertion position. Initially we believed that it might have Fe II 259.9 been a question of hard/soft line behavior; however, both Pb Mg II 279.6 and Sn are also hard lines and yet do not display this strong Al I 309.3 Cu I 324.8 dependence on insertion position.25 In fact Cu, Pb and Sn Co I 345.3 Spark line behaved essentially in the same way even though the Cu line Mg I 383.2 Background emission only (Fig. 11) is a soft line. This anomalous behavior has also been observed Ca II 393.4 by Umemoto and Kubota, who claim that the line intensity is Pb I 405.7 Background emission only (Fig. 11) governed by the rate of vaporization;26 however, if this were Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 729Table 4 Detection limit (pg) comparison between axial and lateral viewing (10 ml sample size).To convert to the concentration detection limit divide by 10 and the units will be in ppb Lateral viewing Axial viewing Improvement Element Height Area Height Area Height Area Sn 1800 7000 390 1500 4.6 4.6 Zn 8 33 46 130 0.17 0.25 Pb 60 270 21 220 2.8 1.3 Cu 6 20 1.2 1.8 4.9 11 Fig. 11 Background emission of the plasma with a DSI inserted to 0 mm ATOLC versus wavelength and power for the two viewing modes. Data taken from o-line measurements. graph.The detection limits for the peak area were calculated by multiplying three times the noise of the background by the time that the element normally takes to vaporize divided by apparent blackbody emission curve the cup was inserted into the slope of the calibration graph. As is traditional for transient the plasma and allowed to reach its equilibrium temperature. analysis techniques, we have expressed detection limits as mass. The data acquisition system was triggered on the spectrometer In these experiments, a 10 ml sample volume was used; hence, and, after 3 s, the plasma was extinguished by stopping the for example, a detection limit of 20 pg corresponds to a plasma gas flow and tripping the rf interlock.The gas flow concentration of 2 ppb. The sample cup can actually hold over was cut o so that the cup would cool slowly and allow the 100 ml; however, one should keep in mind when predicting spectrometer to follow the decay in light emission. The data detection limits that our preferred technique for sample depos- showed that the intensity dropped close to the dark level ition is ‘aerosol deposition’, in which approximately 1 ml of within 0.5 s whereas the cup was still visibly incandescent for solution is deposited.27 at least 10 s.This indicates that the background is due to the With the range of concentrations used to prepare the plasma and not the DSI cup. The high background intensity calibration graphs (1–100 ppb) the calibration graphs were observed is similar to the increased background that has been linear in contrast to what has been observed with liquid found with liquid nebulization axial viewing.3,6 The presence nebulization and axial viewing.This is because of the limited of the DSI cup in the lower region of the plasma, where there concentration range and the minimal amount of sample that is intense continuum emission, partially masks the background is actually introduced into the plasma when compared with but the plasma ingresses into the viewing zone above the cup liquid nebulization.and produces an increased background. Calibration graphs were also determined at 1.2 and 1.5 kW. In an attempt to force the analyte plume from the cup into In all cases the slope of the calibration graph increased with the viewing zone, three dierent graphite boiler caps were power while calibration linearity was maintained. placed on a DSI cup. When the emission intensities of the Fig. 10 shows three superimposed Cu 324.8 nm traces that three caps were compared using the same analyte concentration have not been corrected for background. The plasma back- there was only a 16% increase in analyte emission intensity ground is elevated with increased power prior to insertion of (peak area) as the diameter was decreased. This minimal the cup at 1.7 s but rapidly drops to nearly the same level once improvement suggests that the analyte cloud is being rapidly the cup is inserted into the plasma.This indicates that better mixed and dispersed in the plasma as soon as it emerges from detection limits should be possible with increased power since the DSI cup. the signal-to-background ratio is increased. In the Introduction we suggested that axial viewing DSI In the lateral viewing mode, the spectrometer cannot view might provide significant improvements in the signal-to- the cup unless the cup is inserted to extreme depths (+20 mm background ratio because of the increased light levels and low ATOLC).In the axial viewing mode the DSI cup lies along background due to the presence of the cup. These experiments the optical path of the spectrometer, and the cup is clearly demonstrate that our optical arrangement does not provide incandescent while it is within the plasma. This suggested that this result. When a DSI cup is used without a central gas flow part of the increased background observed in the axial viewing the plasma ingresses into the region above the cup resulting mode may have been due to the incandescence from the DSI in a higher background, especially at longer wavelengths. The cup.Fig. 11 shows the o-line intensities measured at various flow-through cups were not tested in the axial viewing arrange- wavelengths and powers. The data in Fig. 11 are uncorrected ment, because significant increases in signal intensity are not for photomultiplier tube response and filter losses; however, observed unless Freon is used, and exposing the optics to the the shape of the curve is strikingly similar to that of a halogens that the Freon produces seemed imprudent.blackbody emission curve. To determine the source of this The relatively large diameter of the DSI cup, 5.16 mm (Fig. 2), gives rise to a diuse plume of analyte that is approximately twice the cup diameter at a viewing height of 20 mm ATOLC. One of the primary reasons for modifying the DSI was to allow gases to be introduced into the cup because it was believed that the carrier gas would form a central channel constraining the analyte in a narrow axial region thereby increasing the sensitivity.Fig. 12 shows that the width of the emission zone is largely independent of the presence of the carrier gas flow, suggesting that strong dispersion forces operate in the central zone of the 27 MHz plasma.We also expected that the central gas flow would reduce background intensity and this was found to be the case.Fig. 13 shows a plot of the ratio of the background intensities (measured from the o-line data) when Freon is used as a central gas to when no gas is Fig. 10 Cu 324.8 nm traces at 1.0, 1.2 and 1.5 kW obtained with axial used. A similar reduction in background intensity is also viewing and not corrected for background emission. The DSI is inserted into the plasma at 1.7 s. observed when argon is used as the carrier gas.The three 730 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12elements chosen for the plot cover the wavelength range indicated in Table 3. These data as well as the background data of the other elements show that the intensity of the background is significantly reduced in the longer wavelength regions. This is similar to the central channel that is observed with liquid nebulization. However, with the introduction of argon as the central gas, the analyte signal intensity is also reduced as a consequence of local plasma cooling.For the elements studied, the emission intensity was approximately 35–55% of the intensity observed when no argon was used in the cup. The exceptions to this are Ca, Al and Co. The Al and Co signals are comparable and the Ca signal is 2.6 times greater than those obtained when no carrier gas is used, Table 5. It appears that the signals from these three elements Fig. 12 Iron peak area as a function of radial distance and type of are enhanced because the argon sweeps the analyte out of the carrier gas introduced through the DSI cup obtained in the lateral cup as it vaporizes, reducing the probability of forming a viewing mode.Carrier gas flow rate was 290 ml min-1. X, Argon; $, argon+1000 ppm Freon-12; &, no gas. refractory compound (e.g., CaC, CoC, Al2O3). The signal does not return rapidly to the baseline with argon as the carrier gas, indicating incomplete vaporization.Calcium is by far the most refractory of the elements studied (Fig. 14), but both Al and Co show similar tailing. Despite the general loss of sensitivity in using a central injector gas, there is an advantage to using argon as a carrier gas because the noise of the background is reduced by an average of 60% of the value observed when no gas is used, Table 5. This is advantageous for the elements that are not highly volatile (Ca, Al and Co) as well as for Cu, Mg and Fe.However, for the volatile hard-line elements (Zn, Pb and Cd), a carrier gas is a disadvantage as can be seen from the reduction in the signal-to-background noise ratio (Table 5) and poorer detection limits. When Freon is used as a carrier gas, dramatic improvements Fig. 13 Ratio of the background intensities for cups with gas to cups in vaporization rate are observed for some elements, especially without gas at three dierent wavelengths. The carrier gas introduced those that are refractory.When compared with running with through the DSI cup was argon with 1000 ppm Freon-12 at argon, all of the elements displayed an increased peak height; 290 ml min-1. $, Zn 213 nm; ,, Mg 382 nm; &, Pb 405 nm. however, for the volatile elements (Pb and Cd), the increased Table 5 Figures of merit for carrier gas DSI. Determined from 25 ml of 0.5 ppm multi-element solution Carrier gas* Element sb† Peak height S/B‡ S/N§ Detection limit¶/pg Argon Zn 213.9 9.5 9880 19.7 1040 14 Freon 8.9 37300 74.7 4210 3.6 No gas 15 22500 42.2 1490 10 Argon Cd 228.8 14 13800 27.9 960 16 Freon 15 32700 65.4 2180 7 No gas 3.5 38000 75.6 10900 1.4 Argon Pb 220.4 2.3 450 0.94 199 75 Freon 3.6 800 1.65 218 69 No gas 3.4 1100 2.29 320 47 Argon Fe 259.9 16 1450 2.88 93 160 Freon 21 17700 34.1 838 18 No gas 24 2720 4.78 114 130 Argon Mg 279.6 0.36 1250 204 3420 4 Freon 0.35 2800 456 8100 2 No gas 2.1 2340 292 1110 13 Argon Al 309.3 23 870 1.30 37.5 400 Freon 21 15500 23.9 746 20 No gas 31 820 1.09 26.5 570 Argon Cu 324.8 2 6780 84.2 3420 4 Freon 3 19900 224 6560 2 No gas 3 12800 108 3880 4 Argon Co 345.3 34 4520 6.67 134 110 Freon 26 16600 22.8 630 24 No gas 45 3580 3.40 80 190 Argon Ca 393.4 2.4 1771 19.6 754 20 Freon 14 43000 1230 3030 5 No gas 2.7 690 12.4 258 58 * The carrier gas flow rate was 290 ml min-1.Freon indicates 1000 ppm Freon-12 in argon. † sb is the noise measured on the background signal while the cup is in the plasma.‡ S/B is the ratio of the peak height to the average background intensity while the cup is in the plasma. § S/N is the ratio of the peak height to the noise of the background. ¶ Detection limit determined from three times the noise of the background divided by the slope of the calibration graph. Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 731has shown that axial viewing DSI provides higher light levels than lateral viewing. In addition, axial viewing DSI usually provides better signal-to-background ratios than lateral viewing.This suggests that there are further advantages to be obtained by using axial viewing DSI with higher resolution spectrometers such as those now used commercially. The signals observed did not depend strongly on the height at which the plasma tail was cut o or the observation height but were dependent on the insertion depth, most notably with Zn. The background that was observed was not from the incandescence of the DSI cup but rather from the plasma.Constraining the analyte plume that emerges from the cup to a narrow region by using a boiler cap produces very small Fig. 14 Comparison of Ca emission with carrier gases flowing through the DSI cup. When argon is used the signal is increased but increases in intensity, suggesting that dispersion forces are vaporization is incomplete; the use of 1000 ppm Freon-12 in argon significant in the center of the plasma just above the DSI cup.saturates the system. The carrier gas flow rate through the cup was The use of hollow stem cups allows the introduction of gases 290 ml min-1. through the cup and forms a central channel. For some elements, the gas reduces analyte signal due to plasma cooling. For the refractory elements even argon increases the signal but signal is still lower than that obtained when no gas is used. tailing and incomplete vaporization remain problematic. When Fig. 15 shows that the analyte peak signal, both height and Freon-enriched argon is used, dramatic improvements in signal area, for Pb is slightly decreased when Freon-enriched argon quality are observed for refractory elements.Peak tailing is is used as the central gas. In general, lower peak heights and reduced or eliminated. Detection limits are always improved areas are observed for the volatile elements; however, Freon when compared with those obtained with argon alone, but are does significantly enhance the vaporization of refractory and not always better than those obtained when no gas is used.carbide-forming elements. The Freon-enriched argon that was used in this series of REFERENCES experiments was fixed at 1000 ppm Freon-12 in argon because it was purchased as a premixed gas. For this series of experi- 1 Shao, Y., and Horlick, G., Appl. Spectrosc., 1991, 45, 143. ments this level of Freon was sucient to promote the vaporiz- 2 Danielsson, A., ICP Inf.Newsl., 1978, 4, 147. ation. The optimum concentration of Freon was not 3 Ivaldi, J. C., and Tyson, J. F., Spectrochim. Acta, Part B, 1995, 50, 1207. determined and remains to be investigated. Concerns have 4 Fisons advertising brochure for the Maxim system. Document been raised as to the use of Freon because of its detrimental No. VGE/SM/PM/023, September 1993. eect on the atmosphere; however, in this application, the 5 Faires, L. M., Bieniewski, T. M., Apel, C. T., and Niemczyk, Freon is completely destroyed in the plasma and poses no T.M., Appl. Spectrosc., 1985, 39, 5. threat to the upper atmosphere. 6 Demers, D. R., Appl. Spectrosc., 1979, 33, 584. 7 Castillano, T. M., Vela, N. P., Caruso, J. A., and Story, W. C., J. Anal. At. Spectrom., 1992, 7, 807. CONCLUSIONS 8 Cliord, R. H., Montaser, A., Sinex, S. A., and Capar, S. G., Anal. Chem., 1989, 61, 2777. One of the primary advantages of axial viewing of the ICP is 9 Hall, G. E. M., Pelchat, J.-C., Boomer, D. W., and Powell, M., an increase in the optical throughput of the system. This allows J. Anal. At. Spectrom., 1988, 3, 791. 10 Karanassios, V., and Horlick, G., Spectrochim. Acta Rev., 1990, manufacturers to employ techniques which provide higher 13, 89. resolution (e.g., smaller slits, higher orders) without becoming 11 Karanassios, V., and Horlick, G., Spectrochim. Acta, Part B, 1989, quantum noise limited. Our work with a conventional system 44, 1361. 12 Liu, X. R., and Horlick, G., J. Anal. At. Spectrom., 1994, 9, 833. 13 Karanassios, V., Abdullah, M., and Horlick, G., Spectrochim. Acta, Part B, 1990, 45, 119. 14 Blain, L., and Salin, E. D., Spectrochim. Acta, Part B, 1992, 47, 399. 15 Fujimoto, K., Okano, T., and Matsumura, Y., Anal. Sci., 1991, 7, 549. 16 Kantor, T., Hanak-Juhai, E., and Pungor, E., Spectrochim. Acta, Part B, 1980, 35, 401. 17 Rattray, R., PhD Thesis, McGill University, 1995. 18 Fujimoto, K., Okano, T., and Matsumura, Y., Bunseki Kagaku, 1992, 41, 609. 19 Blain, L., Salin, E. D., and Boomer, D. W., J. Anal. At. Spectrom., 1989, 4, 721. 20 Le�ge`re, G., and Burgener, P., ICP Inf. Newsl., 1982, 13, 521. 21 Sing, R. L. A., and Salin, E. D., Anal. Chem., 1989, 61, 163. 22 Rattray, R., Min�oso, J., and Salin, E. D., J. Anal. At. Spectrom., 1993, 8, 1033. 23 Ren, J. M., Legere, G., and Salin, E. D., Appl. Spectrosc., 1993, 47, 1953. 24 Mohammad, A., Keiichiro, F., and Hiroki, H., Appl. Spectrosc., 1987, 41, 715. 25 Inductively Coupled Plasma Emission Spectroscopy. Part 1: Methodology, Instrumentation and Performance, ed. Boumans, P. W. J. M., Wiley, New York, 1987, p. 201. 26 Umemoto, M., and Kubota, M., Spectrochim. Acta, Part B, 1991, 46, 1275. 27 Rattray, R., and Salin, E. D., J. Anal. At. Spectrom., 1995, 10, 829. Fig. 15 Emission of Pb with dierent carrier gas flows. When no gas is used, the volatile elements exhibit maximum sensitivity. When argon Paper 6/07322K is the carrier gas, sensitivity is reduced but the addition of 1000 ppm Received October 28, 1996 Freon-12 restores some of the sensitivity. The carrier gas flow rate was set at 290 ml min-1. Accepted April 2, 1997 732 Journal of Analytical Atomic Spectrometry, July 1997, Vol

ISSN:0267-9477    

DOI:10.1039/a607322k

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Some Observations on the Effect of Molecular Structure Upon the Determination of the Selenium to Carbon Ratios in Various Organoselenium Compounds Using Gas Chromatography With Atomic Emission Spectrometric Detection RICHARD BOS AND NEIL W. BARNETT* School of Biological and Chemical Sciences, Deakin University, Geelong, V ictoria, 3217, Australia The performance of a commercially available gas early 1970s,7,8 was the ability of the technique to provide interelement ratios and therefore empirical formula information.chromatograph employing an atomic emission detector has been evaluated with respect to analytical figures of merit and The numerical ratio of any particular element to carbon in an unknown compound can be calculated from the element the determination of selenium to carbon ratios using a series of asymmetric and symmetric diorganyl diselenides and selective chromatograms of both the unknown and a reference compound, by employing the following expression:3,9,10 selenides.Three selenium atomic emission lines, at 196.09, 203.98 and 216.42 nm, were employed during the study. A temporally resolved secondary emission phenomenon was N¾E N¾C = R¾E R¾C × RC RE × NE NC (1) observed near the 196.09 nm line which was tentatively attributed to molecular emission from SeO. This emission where NC and N¾C are the numbers of carbon atoms in the impaired the analytical performance of the 196.09 nm atomic reference and unknown compounds, respectively; NE and N¾E emission line, with the best overall figures of merit being are the numbers of analyte atoms in the reference and unknown achieved at 203.98 nm.The accuracy of the selenium to carbon compounds, respectively; RC and R¾C are the carbon responses ratios was found to be adversely eected when the for the reference and unknown compounds, respectively; RE concentration of the analyte varied significantly from that of and R¾E are the analyte responses for the reference and the reference compound, especially when the Se 196 nm unknown compounds, respectively. emission line was used.Under optimal conditions, it was found The above expression assumes the absence of random instruthat the accuracy of Se5C ratios could be determined to within mental error as well as operation within the linear calibration ±10% in the majority of cases. The instrumentation was range of the analyte elements in question. Errors associated successfully used to separate and identify the products from with empirical formulae calculated for organosulfur9 and two separate series of organometallic exchange reactions, the organochlorine11 compounds using GC–MIP-AES have been first involving the eight symmetrical diselenides with found to be within ±5%, and for organotin compounds,12 themselves, and secondly these same diselenides with diphenyl ±10%.However, van Dalen et al.13 observed that the elemenditelluride. tal response in MIP-AES could be dependent upon molecular structure of the analyte species. Subsequent investigations12–19 Keywords: Gas chromatography; microwave-induced plasma; have revealed empirical formulae determined by GC–MIP- atomic emission spectrometric detection; organoselenium; AES may also be dependent upon molecular structure; organotellurium compounds additionally, carbon to hydrogen ratios determined in this way show sensitivity to concentration.11,14,20 More than three decades ago, McCormack et al.1 together In the present paper the evaluation of the performance of a with Bache and Lisk2 first demonstrated the feasibility of commercially available gas chromatograph using atomic emis- coupling a gas chromatograph to an argon microwave-induced sion detection21,22 (AED) is reported for the determination of plasma (MIP) in order to achieve element selective detection selenium to carbon ratios for various classes of organoselenium of the eluents via atomic emission spectrometry (AES).The compounds. high eciency of capillary gas chromatography (GC) facilitates the introduction of small bands of pure compounds into such a detector. This combination yields a technique with the EXPERIMENTAL separating power of chromatography enhanced by the selec- Instrumentation and Procedures tivity and sensitivity of AES.3 Advances in the development of microwave cavities, particularly the work of Beenakker4 who The GC–AED instrumentation was configured from an HP 5890 Series II gas chromatograph equipped with electronic designed a cavity which could sustain a helium plasma at atmospheric pressure, greatly facilitated the use of MIP-AES pressure control and an HP 7673 automatic sample injector.The chromatograph was interfaced to an HP 5921A atomic for element selective detection in GC. The excitation energy associated with the helium plasma is sucient to produce emission detector. Control and operation of the system was achieved using an HP 35920A pascal chemstation with atomic emission from all elements in the Periodic Table.5 The analytical applications of MIP-AES as an element selective GC–AED software.Instrumental operating parameters are summarised in Table 1. The chromatographic column and detector have been recently reviewed by Uden.6 Another advantageous feature of GC–MIP-AES, first highlighted in the conditions used with the GC–electron impact ionisation mass Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 (733–741) 733Table 1 Instrumental operating conditions for the GC-AED both peak shape and resolution. Initial pressures higher than 225 kPa resulted in very high flow rates, which caused the Injection port temperature 250°C dimethyl diselenide to be unresolved from the solvent front. Cavity temperature 250°C Water temperature 66°C Transfer line temperature 250°C Reagents and Standards Injection volume, mode 0.5 ml, splitless The homologous series of symmetrical dialkyl diselenides Column HP1 25 m×0.32 mm with 0.17 mm film thickness (dimethyl to dihexyl), dibutyl selenide, 1,3-dihydrobenzo- Electronic pressure program 225 kPa, (giving a flow rate of 110 ml [c]selenophene and 2,1,3-benzoselenadiazole, were synthesized min-1), 0.05 min, then decreasing according to established literature methods25–28 and were at 680 kPa min-1 to 65 kPa, purified by either double distillation using a GKR-1 and set to constant carrier flow of Kugelrohr micro distillation apparatus (Buchi Laboratoriums- 1.91 ml min-1 Technik AG, Flawil, Switzerland) at reduced pressure, or in Spectrometer purge 2 l min-1 N2 Solvent vent on, o 1.1 min, 2.2 min the case of 2,1,3-benzoselenadiazole, triple sublimation was Reagent gas Hydrogen used.The six asymmetric aromatic selenides (compounds A–F) and diphenyl ditelluride were available from colleagues within the School, while the diphenyl diselenide was commercially available (Fluka Chimica, Buchs, Switzerland).The spectrometer (HP 5972) were the same as those used for molecular structures of the organoselenium compounds GC–AED (Hewlett-Packard Australia, Blackburn, Victoria, employed in this study have been shown in Fig. 1. [N.b. It Australia). Nuclear magnetic resonance (NMR) spectra (77Se) must be remembered that working with inorganic and organic were obtained with a JNM-GX 270 MHz Fourier Transform selenium compounds may entail a serious risk of poisoning if NMR spectrometer (Jeol, Tokyo, Japan).they are handled without due care.29 In particular, if an excess Reagent and make-up gas conditions were as per manufac- of sodium borohydride is added during the synthesis of selen- turers recommendations. The purity of gases used were: nitro- ides, then the highly toxic gas hydrogen selenide could be gen 99.99, hydrogen 99.999, and helium 99.999% (Linde Gas, produced. The more volatile members of the dialkyl selenides Sydney, Australia), the helium was further purified using a and dialkyl diselenides are also exceedingly malodourous.] getter, Model PS2GC50R (SAES, Milano, Italy).The oxygen purity was 99.999% (BOC Gases Australia, Chatswood, Australia). The reagent gas added to the plasma during the RESULTS AND DISCUSSION simultaneous determination of carbon and selenium was Analytical Standards hydrogen. The oven temperature program for the determination of the The purity of the each of the selenium compounds used in the present study was estimated from five replicate C 193 chroma- analytical performance was 70°C initially (held for 0.3 min), then increasing at 25°C min-1, with the final temperatures tograms run for each of the individual analytes.The percentage purity of each compound was calculated by summing the being dependent upon the particular analyte compound. For the separation of diselenide exchange reaction products the average areas of all extraneous peaks obtained in the C 193 chromatograms and calculating the ratio of the analyte peak program was 70°C initially (held for 0.3 min), then increasing at 3°C min-1, to a final temperature of 250°C. For the area to the total area of all peaks present.With the exception of 1,3-dihydrobenzo[c]selenophene being at 95% m/v purity, separation of diselenide–ditelluride exchange reaction products the program was 70°C initially (held for 0.3 min), then increas- all other compounds used in this study exhibited purities better than 98% m/v.ing at 3°C min-1 up to a temperature of 150°C, then increasing at 6°C min-1, up to a final temperature of 300°C. The Five replicate injections of each of 11 standard solutions were used to construct individual calibration graphs for the remaining determinations were carried out at 70°C initially (held for 0.3 min), then increasing at 10°C min-1, with the compounds listed in Table 2. Owing to the availability of only small amounts of the compounds 1,3-dihydrobenzo[c]seleno- final temperature in each case being dependent upon the particular analyte compound.All solutions used for GC–AED phene, 3-phenylbenzo[b]selenophene, C, D, E and F (see Fig. 1), it was not possible to ascertain the analytical figures were made up in n-hexane unless otherwise specified; those for NMR experiments were prepared in deuterated chloroform. of merit for these species. Given the structural similarities between the selenophenes and between the compounds A–F, The atomic emission line wavelengths23 selected for this investigation were as follows: C I 193.0905, Se I 196.09, Se I it was reasonable to assume that their respective elemental responses and hence their analytical figures of merit would 203.98, Se I 216.42 and Te I 200.002 nm.For simplicity these atomic emission lines will be hereafter abbreviated to C 193, reflect this. The first ten analyte standards were made up in the concentration range 0.70–3000 mg l-1 of Se.The Se 196, Se 204, Se 216 and Te 200, respectively. Hydrogen was deliberately omitted as preliminary results indicated that 3-methylbenzo[b]selenophene and compounds A and B were made up in solutions of n-hexane and dichloromethane (1+1) hydrogen was found to be concentration dependent, and thus not reliable. Additionally, the determination of hydrogen (at in the concentration range 0.70–1500 mg l-1 of Se; the inclusion of dichloromethane greatly facilitated the solubility 486.13 nm) would have required an additional injection, thus doubling an already large data set, resulting in little gain.of these analytes. The slopes, linear correlation coecients (r2) and detection As this study employed the use of organoselenium compounds known to be thermally labile, pressure programming limits (DL), calculated for each of the resultant log–log calibration graphs have been listed in Table 2.Some typical using the electronic pressure control (EPC) was employed to reduce thermal decomposition during injection. The method log–log calibrations for each of the selenium atomic emission lines used are shown in Fig. 2. The DLs were determined from used was directly analogous to that recently documented by Vincenti et al.24 It was found that utilising splitless injection the individual calibration functions using a response equal to a signal-to-noise ratio of 351.The linearity, as measured by in conjunction with EPC resulted in a pressure split injection with an eective split ratio of 6.5:1. Using diphenyl diselenide the respective slopes of the calibrations (see Table 2) was clearly superior for the Se 204 line compared with the other and dimethyl diselenide (the analytes with the highest and lowest boiling points, respectively), it was found that an initial two emission lines. It should be borne in mind that the slopes and linear correlation coecients listed in Table 2 are those pressure of 225 kPa gave the best chromatograms, in terms of 734 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12Fig. 1 Molecular structure of the various organoselenium compounds used in the present study. Table 2 Analytical figures of merit for selected organoselenium compounds. The slopes and correlation coecients (r2) were calculated from linear regression carried out on the individual log–log calibration graphs.The detection limits (DL, in mg l-1 of Se injected) were determined from each calibration function using a response equivalent to a signal-to-noise ratio of 351 Se 196 Se 204 Se 216 Analyte Slope r2 DL Slope r2 DL Slope r2 DL Dimethyl diselenide 0.80 0.972 0.06 0.99 0.996 0.1 0.89 0.998 1 Diethyl diselenide 0.77 0.979 0.05 0.93 0.997 0.08 0.78 0.999 0.9 Diisopropyl diselenide 0.80 0.977 0.05 0.95 0.997 0.1 0.82 0.998 0.9 Dipropyl diselenide 0.85 0.965 0.08 1.02 0.993 0.1 0.86 0.998 1 Dibutyl diselenide 0.81 0.977 0.06 0.94 0.995 0.1 0.77 0.999 0.9 Dipentyl diselenide 0.84 0.964 0.08 0.98 0.990 0.1 0.78 0.996 0.9 Dihexyl diselenide 0.85 0.969 0.08 0.98 0.992 0.1 0.81 0.980 1 Diphenyl diselenide 0.87 0.921 0.2 1.01 0.985 0.4 0.85 0.994 2 Dibutyl selenide 0.78 0.978 0.03 0.93 0.999 0.05 0.81 0.999 0.6 2,1,3-Benzoselenadiazole 0.85 0.952 0.1 1.02 0.984 0.1 0.85 0.993 1 3-Methylbenzo[b]selenophene 1.02 0.988 0.5 0.98 0.998 0.8 0.80 0.996 10 Compound A 0.97 0.987 0.5 1.02 0.993 0.8 0.85 0.998 10 Compound B 1.13 0.918 0.6 0.94 0.996 0.8 0.84 0.996 10 calculated from the line of best fit.The dimethyl diselenide dierence in the correlation coecients obtained for the calibration functions with either the Se 204 or Se 216 lines, those calibration plots in Fig. 2 clearly indicate some degree of curvature for all three atomic emission lines, with the Se 196 calculated from the Se 196 calibration functions were found to be considerably worse and covered a much broader range.line being most severely aected. However, there also appears to be some compound dependence superimposed upon the This result reflects the curvature exhibited by these calibrations (see Fig. 2). results in Table 2. For example, diethyl diselenide and dibutyl selenide produced some of the largest deviations from linearity The generally poorer linearity observed with the Se 196 line arises from a temporally resolved (secondary) emission, which on all three emission lines.Some considerable improvements in the linearity of calibrations using the Se 196 line were could be due to the presence of SeO in the plasma. This secondary emission became evident with all analytes at concen- observed with 3-methylbenzo[b]selenophene and compounds A and B. The latter observations reflect the narrower cali- trations between 500 and 700 mg l-1 of Se, with the eect being most noticeable on the Se 196 line (see Fig. 3).The bration range employed and the slightly inferior DLs achieved with these three analytes. While there is little or no significant secondary peak shown in Fig. 3 was not a closely eluting Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 735Fig. 4 Linear plot of peak area versus concentration for dibutyl diselenide using the Se 196 line which illustrates the dramatic eect of Fig. 2 Log–log calibration plots for dimethyl diselenide at each of the secondary emission upon the calibration function.the three selenium atomic emission lines. 50928 cm-1 (196.35 nm), with less intense bands occurring at 49128 (203.55) and 46068 cm-1 (217.07 nm).31 These three molecular emission band-heads are, respectively,+0.26, -0.55 and +0.65 nm from the Se 196, Se 204 and Se 216 nm atomic emission lines. The more pronounced secondary emission observed at the Se 196 line (see Fig. 3) is consistent with both the relative intensities of the SeO band-heads31 and the dispersion of the spectrometer employed in the GC–AED instrument (0.2 nm per pixel in the vacuum UV).22 The DLs listed in Table 2, for each atomic emission line, show no significant dierences within the homologous series of dialkyl diselenides.However, somewhat inferior detectability was realized with diphenyl diselenide, 3-methylbenzo[ b]selenophene, plus compounds A and B. These dierences in elemental response may reflect the various molecular environments of the selenium (see Fig. 1). For example, the varying carbon–selenium bond strengths as well as carbon Fig. 3 Typical example of the temporally resolved secondary emission to selenium ratios could aect the eciency of atomization observed while monitoring the Se 196 line, in this case the analyte was and excitation within the plasma. Carbon to element ratios dihexyl diselenide (#3000 mg l-1 of Se). have previously been found to be sensitive to samples of high relative molecular mass.15,32 The marginally better DLs achieved with the Se 196 line organoselenium impurity since no such peak was seen in any of the GC–EI-MS experiments.This finding was confirmed compared with the Se 204 line have also been previously reported by Timmins,33 who employed a tantalum electrother- using the wavelength snapshot facility on the GC–AED instrument, which indicated only background levels of carbon from mal vaporiser to introduce samples into a helium MIP. To the best of our knowledge, the analytical figures of merit for the the C 193 line at the appearance time of the secondary peak.The curvature observed with the Se 196 calibrations occurs Se 216 line reported here are presented for the first time. The DLs achieved in the present study are generally compar- due to the integration of both the primary and secondary peaks together at concentrations below (500–700 mg l-1 of Se) able to those obtained by earlier workers employing GC–MIPAES instrumention30,34–37 (see Table 3). While it is clear that and separate integrations above this range, thus causing deviation of the calibration function towards the concentration units of DL given in Table 2 (mg l-1 of Se injected) have far more relevance to the practising analyst, the conversions to pg axis (see Fig. 2) owing eectively to decreasing the response per unit of analyte concentration. This eect is more dramati- or pg s-1 in Table 3 were necessary in order to compare the present work with earlier studies.30,34–37 With the exception of cally demonstrated using a linear–linear calibration plot where this artifact of the integration caused by the secondary emission the work of Tsunoda et al.36 there are only minor dierences in the reported DLs for similar compounds at each of the two produces two separate calibration functions (see Fig. 4). The temporal resolution could result from the chemisorption of commonly used atomic emission lines. It should be noted that the analytical figures of merit reported in the present study selenium onto the internal walls of the silica discharge tube, followed by subsequent desorption of a selenium–oxygen com- were routinely achieved over several months with the commercially available instrumentation by adhering to the manufac- pound into the plasma.Estes et al.30 have reported similiar secondary emission problems with the determination of turers recommended operating parameters. An evaluation of the instrumental precision was carried organoboron compounds using GC–MIP-AES.They, likewise, postulated the adsorption of the analyte onto the discharge out using two organoselenium compounds, diphenyl diselenide and 2,1,3-benzoselenadiazole, at concentrations of 100 and tube prior to the desorption of a boron–oxygen species.30 The molecular emission spectrum of SeO has been studied in detail 120 mg l-1 of Se, respectively, for each of the three simultaneously monitored selenium atomic emission lines.Twenty- by Reddy and Azam31 using a microwave supported discharge, and they qualitatively reported a very strong band at five replicates of each standard solution were sequentially 736 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12Table 3 Comparison of analytical figures of merit achieved for several organoselenium compounds using GC–MIP-AES Atomic DL Cavity type and emission plasma gas Analyte line/nm pg s-1 pg Reference Raytheon 7097–1001 2,1,3-Benzoselenadiazole 203.98 —* 40 34 G1 and 7097–500 G1 Argon Beenakker TM010 Diethyl selenide 203.98 5.3 62 30 Helium Hewlett Packard TM010 —* 196.09 4.0 —* 35 Helium —* Dimethyl selenide 203.98 —* 500 36 Hewlett Packard TM010 Diphenylselenide 196.09 —* 10 37 Helium Hewlett Packard TM010 Dibutyl selenide 196.09 8 15 Present Helium 203.98 12 25 study 216.42 100 200 2,1,3-Benzoselenadiazole 196.09 15 50 203.98 15 50 216.42 200 500 * Information not available.injected (0.5 ml). Using these two standard solutions relative range 1.0–1.2, whereas those for 3-phenylbenzo[b]selenophene standard deviations (RSD) could be determined at concen- were in the range 0.8–1.0 (with one exception). This result trations which varied from 50 times to 1200 times the achiev- suggests some type of systematic eect upon the calculation able DL (see Table 2). The results of the precision which may reflect the dierences in molecular size or structure determinations are summarised in Table 4.The slightly between the two analytes. However, no such systematic deviimproved precision attained with the Se 204 line compared ation was observed with the determination of the selenium with the Se 196 line could reflect the irreproducible nature of content of compounds A–F using four reference compounds the secondary emission. The inferior precision attained with (see Table 6). In this more extensive evaluation the Se 204 line the Se 216 line was consistent with the poorer detectability at exhibited a consistently more accurate performance compared this atomic emission line (see Fig. 2). Overall, the achievable with the Se 196 and Se 216 lines. The poorest estimation of precision at all three atomic emission lines was acceptable for the selenium content of compounds A–F was achieved using inter-element ratio determinations. the Se 196 line and 2,1,3-benzoselenadiazole as a reference. A detailed examination of the chromatograms employed for these determinations revealed the presence of a small but significant Determination of Selenium to Carbon Ratios secondary emission peak which was included in the peak The chromatographic experiments used to evaluate the ability integration.This explains the high selenium results for these of GC–AED to calculate elemental ratios were set up with compounds. The overall performance of GC–AED (with some selected compounds compared with a series of internal refer- exceptions) for the determination of selenium content was ence compounds.These combinations were made on the basis fairly good with no obvious relationship emerging between of chromatographic resolution and molecular diversity in order accuracy and the molecular structure of either analytes or to ascertain how the molecular structures of both the analyte reference compounds. and reference compounds might aect the calculation of The eight symmetrical diselenides (see Fig. 1) could be elemental ratios. The results calculated for selenium were readily separated from each other, and the chromatogram normalised to the number of carbon atoms in the analyte shown in Fig. 5 was obtained from the analysis of a mixed molecule, consequently only the calculated numbers of sel- diselenide standard solution plus 2,1,3-benzoselenadiazole enium atoms per molecule appear in the following tables. The (which was present as the reference compound) injected calculations were performed using eqn.(1) with the tabularised immediately after preparation. Given the ease with which the results being the average of five individual determinations for members of this homologous series could be separated from each analyte and each reference combination. each other, and the selected reference compound, a more The results listed in Table 5 show no obvious dependence detailed experiment was designed. Mixed diselenide stan- upon either concentration or the choice of emission line.dard solutions were prepared at various concentrations of Overall, the estimation of selenium content was, at worst, from 1 up to 2000 mg l-1 of Se each containing 2,1,3-benzo- ±20%, with the majority being within ±10% of the actual selenadiazole at approximately the same concentration as value. Interestingly, the choice of reference compound, based the diselenides. These standard solutions were then analysed upon a similar molecular environment of the heteroatom, does five times each with a view to determining the selenium not appear to be essential, since both the dibutyl selenide and content in a similiar manner to the compounds in Tables dihexyl diselenide performed, on average, equally as well as 4 and 5.As the suite of standard solutions took several hours did the 1,3-dihydrobenzo[c]selenophene. It was noteworthy to prepare, owing to the care required for the handling of that the results for 3-methylbenzo[b]selenophene were in the volatile and toxic compounds, the analyses were performed overnight using the programmable autosampler.Examination Table 4 RSDs (%) for 2,1,3-benzoselenadiazole (120 mg l-1 of Se) of the considerable number of resultant chromatograms from and diphenyl diselenide (100 mg l-1 of Se) obtained simultaneously at the C 193, Se 196, Se 204 and Se 216 atomic emission lines each of the three selenium atomic emission lines revealed that there were many more peaks present than there were analytes in the original standard solutions (see Fig. 6). Compound Se 196 Se 204 Se 216 Also, those peaks at the retention times previously determined 2,1,3-Benzoselenadiazole 1.3 0.8 3.6 for the individual diselenides, when integrated, gave area values Diphenyl diselenide 1.3 1.1 2.8 which translated to concentrations well below what was Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 737Table 5 Calculated selenium to carbon ratios for 3-methylbenzo[b]selenophene and 3-phenylbenzo[b]selenophene normalised to the number of carbon atoms in each of the two analyte molecules using various reference compounds at two concentrations of selenium Reference compound 1,3-Dihydrobenzo- Analyte compound Wavelength/nm Dibutyl selenide [c]selenophene Dihexyl diselenide 3-Methylbenzo[b]selenophene Concentration of analyte and reference compounds, 100 mg l-1 of Se — Se 196 Se1.2 Se1.0 Se1.1 Se 204 Se1.2 Se1.0 Se1.1 Se 216 Se1.2 Se1.0 Se1.1 Concentration of analyte and reference compounds, 500 mg l-1 of Se — Se 196 Se1.1 Se1.2 Se1.1 Se 204 Se1.1 Se1.0 Se1.0 Se 216 Se1.1 Se1.0 Se1.0 3-Phenylbenzo[b]selenophene Concentration of analyte and reference compounds, 100 mg l-1 of Se — Se 196 Se1.0 Se0.9 Se0.9 Se 204 Se0.9 Se0.8 Se0.9 Se 216 Se1.0 Se0.8 Se0.9 Concentration of analyte and reference compounds, 500 mg l-1 of Se — Se 196 Se1.0 Se1.1 Se1.0 Se 204 Se0.9 Se0.8 Se0.8 Se 216 Se1.0 Se0.9 Se0.9 Table 6 Calculated selenium to carbon ratios for compounds A–F (normalised to the number of carbon atoms in each of the six analyte molecules) using various reference compounds with both analytes and reference compounds at 500 mg l-1 of Se Reference compound used Analyte Dibutyl 2,1,3-Benzo- 3-Methylbenzo- Diphenyl Compound selenide selenadiazole [b]selenophene diselenide Se 196 — A Se1.0 Se1.5 Se0.9 Se1.2 B Se0.8 Se1.2 Se0.7 Se0.9 C Se0.9 Se1.3 Se0.8 Se1.1 D Se1.0 Se1.5 Se0.9 Se1.2 E Se0.9 Se1.4 Se0.8 Se1.1 F Se0.8 Se1.3 Se0.8 Se1.0 Se 204 — A Se1.0 Se1.0 Se1.0 Se1.2 Fig. 5 An Se 196 chromatogram obtained from a mixed diselenide B Se1.1 Se1.1 Se1.0 Se1.2 standard solution injected immediately after preparation: 1, dimethyl; C Se0.9 Se0.9 Se0.9 Se1.0 2, diethyl; 3, dipropyl; 4, 2,1,3-benzoselenadiazole; 5, dibutyl; 6, dipen- D Se0.9 Se0.9 Se0.9 Se1.0 tyl; 7, dihexyl; and 8, diphenyl. E Se0.9 Se0.9 Se0.8 Se1.0 F Se0.8 Se0.8 Se0.8 Se0.9 Se 216 — equilibrium exchange reactions of the type A Se1.2 Se1.2 Se1.1 Se1.3 R-Se-Se-R+R¾-Se-Se-R¾=2R-Se-Se-R¾ (2) B Se1.2 Se1.2 Se1.1 Se1.3 C Se0.8 Se0.9 Se0.8 Se1.0 As the mixed standard solutions contained eight symmetrical D Se1.0 Se1.0 Se0.9 Se1.1 diselenides, the maximum number of asymmetric exchange E Se0.9 Se0.9 Se0.8 Se1.0 products possible was 28, thus giving a total of 36 compounds.F Se0.8 Se0.8 Se0.7 Se0.9 The characterisation of these exchange products was obtained by analysing a series of nine mixed diselenide standard solutions.The first standard contained all eight symmetrical dis- expected from earlier calibration functions. However, the peak area for the 2,1,3-benzoselenadiazole was not diminished in elenides, whilst in the remaining eight standards each had a dierent diselenide omitted from the mixture. Therefore, by any of the selenium chromatograms. As the carbon and selenium chromatograms were all identical with regard to the comparing each of the resultant chromatograms from the latter eight standards with that containing all the symmetrical dis- number of peaks and their retention times, the additional compounds observed were, therefore, all organoselenium elenides, a process of elimination clearly identified the presence and molecular formula of the 36 exchange products (see Fig. 7). species. Decomposition of the individual analytes upon standing or during the chromatographic process could be ruled out These assignments were subsequently confirmed using GC–EI-MS.The random distribution of the relative peak on the basis of previous calibration studies. Nevertheless, all the diselenides were redistilled and re-assayed to confirm their areas in Fig. 7 probably reflects the complexity of the 36 compounds attempting to equilibrate with each other via a purity. Using these re-purified compounds another suite of mixed diselenide standard solutions were prepared and ana- myriad of exchange reactions.The characterisation of the exchange products resulting from lysed overnight with the same result as shown in Fig. 6. An examination of the organometallic literature38 revealed that the symmetrical diselenide mixture allowed the calculation of the selenium to carbon ratios for an extra 28 compounds. As the symmetrical diselenides exhibit a propensity to undergo 738 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12Table 7 Calculated selenium to carbon ratios for the seven symmetrical diselenides plus the 28 asymmetrical diselenides resulting from the exchange reactions of the eight parent diselenides, with dipropyl diselenide arbitrarily selected as the reference compound C5Se Compound ratio Se 196 Se 204 Se 216 Me-Se-Se-Me C1Se Se0.6 Se0.8 Se0.7 Et-Se-Se-Me C3Se2 Se1.0 Se1.7 Se1.4 Pri-Se-Se-Me C2Se Se1.0 Se1.0 Se1.0 Et-Se-Se-Et C2Se Se0.8 Se0.9 Se0.7 Me-Se-Se-Pr C2Se Se0.6 Se1.0 Se0.9 Et-Se-Se-Pri C5Se2 Se2.2 Se2.1 Se2.1 Et-Se-Se-Pr C5Se2 Se1.8 Se1.9 Se1.8 Pri-Se-Se-Pri C3Se Se0.7 Se0.9 Se0.8 Me-Se-Se-Bu C5Se2 Se1.8 Se1.8 Se1.8 Pri-Se-Se-Pr C3Se Se1.2 Se1.0 Se1.1 Pr-Se-Se-Pr C3Se (Used as reference compound) Bu-Se-Se-Et C3Se Se1.0 Se1.0 Se0.9 Me-Se-Se-Pe C3Se Se0.9 Se1.0 Se0.9 Bu-Se-Se-Pri C7Se2 Se2.5 Se2.0 Se2.3 Bu-Se-Se-Pr C7Se2 Se2.2 Se2.0 Se2.1 Et-Se-Se-Pe C7Se2 Se2.4 Se2.3 Se2.2 Hx-Se-Se-Me C7Se2 Se1.7 Se1.9 Se1.8 Pe-Se-Se-Pri C4Se Se1.2 Se1.1 Se1.2 Me-Se-Se-Ph C7Se2 Se2.6 Se2.2 Se2.3 Fig. 6 An Se 196 chromatogram obtained from a mixed diselenide Bu-Se-Se-Bu C4Se Se1.1 Se1.0 Se1.0 standard solution which had been allowed to stand for several hours Pe-Se-Se-Pr C4Se Se1.1 Se1.0 Se1.0 prior to injection.Et-Se-Se-Hx C4Se Se0.9 Se1.0 Se0.9 Et-Se-Se-Ph C4Se Se1.3 Se1.2 Se1.3 Hx-Se-Se-Pri C9Se2 Se2.4 Se2.1 Se2.3 with the earlier determinations the selenium content was Bu-Se-Se-Pe C9Se2 Se2.4 Se2.1 Se2.4 derived from eqn. (1) using the simultaneous responses from Hx-Se-Se-Pr C9Se2 Se2.1 Se1.9 Se2.0 Pri-Se-Se-Ph C9Se2 Se2.5 Se2.2 —* the simultaneously monitored C 193, Se 196, Se 204 and Se Ph-Se-Se-Pr C9Se2 Se2.5 Se2.3 Se2.3 216 lines.Dipropyl diselenide was arbitrarily chosen as the Pe-Se-Se-Pe C5Se Se1.2 Se1.1 Se1.1 reference compound and all results are listed in order of elution Bu-Se-Se-Hx C5Se Se1.2 Se1.0 Se1.1 in Table 7. Unlike the results shown in Tables 5 and 6, in these Bu-Se-Se-Ph C5Se Se1.3 Se1.0 —* experiments the Se 204 atomic emission line clearly out per- Hx-Se-Se-Pe C11Se2 Se2.4 Se1.9 Se1.8 formed the other two lines with all the determinations being Pe-Se-Se-Ph C11Se2 Se1.9 Se1.8 —* Hx-Se-Se-Hx C6Se Se0.9 Se1.0 Se0.8 within ±20%, and 29 of the 35 within ±10% of the true Hx-Se-Se-Ph C6Se Se1.4 Se1.1 Se0.7 value. This represents fairly reasonable performance consider- Ph-Se-Se-Ph C6Se Se1.4 Se1.2 Se1.0 ing the significant variations in the concentrations of the reference and analyte compounds based upon peak areas (see * Below the DL, where: Me is methyl, Et is ethyl, Pr is propyl, Pri Fig. 7). The Se 196 results exhibited the greatest deviations is isopropyl, Bu is butyl, Pe is pentyl, Hx is hexyl and Ph is phenyl. from the actual selenium values, with variations from -50 to +40%. This relatively poor performance most probably arises from the secondary emission eect exacerbated by the large Fig. 7 An Se 196 chromatogram showing all 36 resultant peaks (two co-eluting) from the exchange reactions between the eight symmetrical diselenides, where: 1, dimethyl; 2, ethylmethyl; 3, isopropylmethyl; 4, diethyl; 5, methylpropyl; 6, ethylisopropyl; 7, ethylpropyl; 8, diisopropyl; 9, butylmethyl; 10, isopropylpropyl; 11, dipropyl; 12, ethylbutyl; 13, methylpentyl; 14, butylisopropyl; 15, butylpropyl; 16, ethylpentyl; 17, hexylmethyl; 18, isopropylpentyl; 19, methylphenyl; 20, dibutyl; 21, pentylpropyl; 22, ethylhexyl; 23, ethylphenyl; 24, hexylisopropyl; 25, butylpentyl; 26, hexylpropyl; 27, isopropylphenyl; 28, phenylpropyl; 29, dipentyl; 30, butylhexyl; 31, butylphenyl; 32, hexylpentyl; 33, phenylpentyl; 34, dihexyl; 35, hexylphenyl; and 36, diphenyl.Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 739Fig. 8 An 77Se NMR spectrum of an equimolar mixture of dimethyl diselenide and diphenyl diselenide (collected 16 h after mixing) showing the two resonances arising from the exchange product at 291 and 440 ppm. dierences in concentration between some analytes and the reference compound. Fig. 9 A Te 200 chromatogram showing seven organylselenenyltellur- As noted previously, a mixed diselenide standard, if prepared ides which result from the reactions between the seven dialkyldiselen- and analysed rapidly, would give a chromatogram which ides and diphenyl ditelluride: 1, methyl; 2, ethyl; 3, isopropyl; 4, propyl; contained the same number of peaks as there were compounds 5, butyl; 6, pentyl; 7, hexyl; and 8, diphenyl ditelluride.in the solution. If, however, the mixed standard solution was allowed to stand for ca. 2 h prior to injection (especially on a warm day) the onset of the diselenide exchange reactions was chromatographic determination. The eight symmetrical dis- clearly evident with many small peaks appearing between the elenides used earlier were mixed with diphenyl ditelluride at symmetrical diselenides. Most of the previous investigations approximately equal concentrations (2000 mg l-1 as Se or Te) into diselenide exchange reactions have employed 77Se NMR and allowed to stand for 24 h at room temperature (#25°C).spectroscopy. A 77Se NMR experiment was performed in order A further eight solutions, each containing diphenyl ditelluride to establish the time taken for dimethyl diselenide and diphenyl plus seven of the diselenides, were prepared at the same time. diselenide to reach equilibrium with methylphenyl diselenide. Each of these exchange solutions was then analysed using the These two compounds were chosen because of the considerable GC–AED which monitored the Te 200 atomic emission line.dierence in their 77Se chemical shifts, with dimethyl diselenide The resultant Te 200 chromatogram from the solution contain- at 268 ppm and diphenyl diselenide at 463 ppm, with the ing the diphenyl ditelluride and each of the eight symmetrical resultant two resonances from the asymmetrical exchange diselenides is shown in Fig. 9. By comparing this chromatog- product being at 291 and 440 ppm. The experiment was ram with the other eight (each of which had one of the performed with approximately equimolar amounts (6.8×10-3 symmetrical diselenides missing) the identity of the peaks in mol) of each diselenide at room temperature (#25°C). The Fig. 9 could be assigned in an analogous manner to the peaks first spectrum (collected after 2 h) showed no extra peaks in Fig. 7. These assignments were subsequently confirmed using characteristic of the presence of the exchange product.Some GC–EI-MS. Interestingly, whilst all the symmetrical dialkyl 16 h after mixing, a subsequent spectrum showed that exchange diselenides exchange products with the diphenyl ditelluride had clearly started (see Fig. 8). Based upon further spectral could be readily detected, the exchange product with diphenyl investigations, the time taken for these two diselenides to reach diselenide was not found. The exchange mixtures were allowed equilibrium was approximately 36 h.Given the greater sensito stand for a further 14 d, after which time they were tivity and speed of analysis oered by GC–AED compared re-analysed with no diphenylselenenyltelluride being detected. with 77Se NMR, the former technique may prove to be a useful This particular exchange reaction has been studied by 125Te tool for studying such reactions, particularly those having and 77Se NMR spectroscopy and the formation of the exchange more than three species under investigation.product diphenylselenenyltelluride (Ph-Se-Te-Ph) is known to The present study is not the first to employ GC to examine form,45 a result which was confirmed using the same technique. the diselenide exchange reactions. Evans and Johnson,39 The non-appearance of this exchange product in the tellur- attempting to characterise organoselenium compounds of bioium chromatogram may be due to the lability of the logical importance using GC with electron capture detection, seleniumMtellurium bond in this particular exchange product, observed the asymmetrical exchange products of dimethyl, emphasising the requirement that analytes must be able to diethyl and dipropyl diselenides.More recently, Cai and withstand the chromatographic conditions employed. co-workers40–43 have reported the detection of exchange prod- However, the power of GC–AED for the investigation of ucts arising from the interaction of diselenides with disulfides, certain organometallic reactions at relatively low concen- these workers employed GC–AED instrumentation similar to trations has been ably demonstrated.that used in the current study. With a view to evaluating the applicability of the GC–AED further for following such exchange reactions a preliminary CONCLUSION investigation into the reaction between the symmetrical diselenides and diphenyl ditelluride was conducted. MacFarlane and The GC–AED method has particular applicability in the area of synthetic chemistry where the products and/or reactants MacFarlane44 employed 125Te NMR to study the exhange products of diselenides and ditellurides, and reported the contain hetero atoms and are suitable for GC analysis.The progress of reactions can be monitored along with the elemen- presence of various dichalcogenides, such as dimethylselenenyltelluride (Me-Se-Te-Me) and methylpropylselenenyltelluride tal ratios of the products.This technique is ideally suited to microscale synthesis, requiring less than a microlitre of dilute (Me-Se-Te-Pr). Diphenyl ditelluride was selected for the study on the basis of its ready availability, purity and suitability for solution per determination. 740 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 1220 Wylie, P. L., Sullivan, J. J., and Quimby, B. D., J. High Resolut. The authors express their sincere gratitude to Jim Watson, Chromatogr., 1990, 13, 499.Peter Harrison and Rod Minett (Hewlett-Packard Australia) 21 Quimby, B. D., and Sullivan, J. J., Anal. Chem., 1990, 62, 1027. for arranging the loan of, and supporting the GC–AED 22 Sullivan, J. J., and Quimby, B. D., Anal. Chem., 1990, 62, 1034. instrumentation, and for the use of the GC–EI-MS equipment. 23 CRC Handbook of Chemistry and Physics, ed. Lide, D. R., CRC Our thanks go also to Carl Scheisser for provision of the Press, Boca Raton, FL, 77th edn., 1996, pp. 10–1 to 10–127. 24 Vincenti, M., Minero, C., Sega, M., and Rovida, C., J. High selenium compounds A–F, and to Jenny O’Connell for the Resolut. Chromatogr., 1995, 18, 490. provision of the diphenyl ditelluride. 25 Gunther, W. H. H., J. Org. Chem., 1966, 31, 1202. 26 Syper, L., and Mlochowski, J., Synthesis, 1984, 5, 439. 27 Klayman, D. L., and Grin, T. S., J. Am. Chem. Soc., 1973, 197. 28 Parker, C. A., and Harvey, L. G., Analyst, 1962, 87, 558. REFERENCES 29 Hazards in the Chemical L aboratory, ed.Muir, G. D., The 1 McCormack, A. J., Tong, S. C., and Cooke, W. D., Anal. Chem., Chemical Society, London, 2nd edn., 1977. 1965, 37, 1470. 30 Estes, S. C., Uden., P. C., and Barnes, R. M., Anal. Chem., 1981, 53, 1829. 2 Bache, C. A., and Lisk, D. J., Anal. Chem., 1965, 37, 1477. 31 Reddy, S. P., and Azam, M., J. Mol. Spectrosc., 1974, 49, 461. 3 Element Specific Chromatographic Detection by Atomic Emission 32 Uden, P. C., Yoo, Y., Wang, T., and Cheng, Z., J.Chromatogr., Spectroscopy, ed. Uden, P. C., American Chemical Society 1989, 486, 319. Symposium Series 479, ACS, Washington, DC, 1992. 33 Timmins, K. J., J. Anal. At. Spectrom., 1987, 2, 251. 4 Beenakker, C. I. M., Spectrochim. Acta, Part B, 1976, 31, 483. 34 Talmi, Y., and Andren, A. W., Anal. Chem., 1974, 46, 2122. 5 Brandl, P. G., and Carnahan, J. W., Spectrochim. Acta, Part B, 35 Sullivan, J. J., and Quimby, B. D., J. High Resolut. Chromatogr., 1994, 49, 105. 1991, 14, 110. 6 Uden, P. C., J. Chromatogr., A., 1995, 703, 393. 36 Tsunoda, A., Matsumoto, K., Haraguchi, H., and Fuwa, K., Anal. 7 Dagnall, R. M., West, T. S., and Whitehead, P., Anal. Chem., Sci., 1986, 2, 99. 1972, 44, 2074. 37 De La Calle Guntin�as, M. B., Lobinski, R., and Adams F. C., 8 McLean, W. R., Stanton, D. L., and Penketh, G. E., Analyst, J. Anal. At. Spectrom., 1995, 10, 111. 1973, 98, 432. 38 Luthra, N. P., and Odom, J. D., in T he Chemistry of Organic 9 Evans, J. C., Olsen, K. B., and Sklarew, D. S., Anal. Chim. Acta, Selenium and T ellurium Compounds, ed., Patai, S., and Rappoport, 1987, 194, 247. Z., John Wiley, New York, 1986, vol. 1, ch. 6. 10 Valente, A. L. P., and Uden, P. C., Analyst, 1990, 115, 525. 39 Evans, C. S., and Johnson, C. M., J. Chromatogr., 1966, 21, 202. 11 Uden, P. C., Slatkavitz, K. J., and Barnes, R. M., Anal. Chim. 40 Cai, X.-J., Uden, P. C., Block, E., Zhang, X., Quimby, B. D., and Acta, 1986, 180, 401. Sullivan, J. J., J. Agric. Food Chem., 1994, 42, 2084. 12 Lobinski, R., Dirkx, W. M. R., Ceulemans, M., and Adams, F. C., 41 Cai, X.-J., Block, E., Uden, P. C., Quimby, B. D., and Sullivan, Anal. Chem., 1992, 64, 159. J. J., J. Agric. Food Chem., 1995, 43, 1751. 13 van Dalen, J. P. J., de Lezenne Coulander, P. A., and de Galan, L., 42 Cai, X.-J., Block, E., Uden, P. C., Zhang, X., Quimby, B. D., and Anal. Chim. Acta, 1977, 94, 1. Sullivan, J. J., J. Agric. Food Chem., 1995, 43, 1754. 14 Pedersen-Bjergaard, S., Asp, T. N., and Greibrokk, T., J. High 43 Cai, X.-J., Block, E., Uden, P. C., Zhang, X., Quimby, B. D., and Resolut. Chromatogr., 1992, 15, 89. Sullivan, J. J., Pure Appl. Chem., 1996, 68, 937. 15 Dingjan, H. A., and de Jong, H. J., Spectrochim. Acta, Part B, 44 McFarlane, H. C. E., and McFarlane, W., J. Chem. Soc., Dalton 1983, 38, 777. T rans., 1973, 2416. 16 Slatkavitz, K. J., Uden, P. C., Hoey, L. D., and Barnes, R. M., 45 Dean, P. A. W., and Vittal, J. J., unpublished work, University of J. Chromatogr., 1984, 194, 247. Western Ontario, Canada. 17 Jelink, J. Th., and Venema, A., J. High Resolut. Chromatogr., 1990, 13, 448. Paper 6/08372B 18 Huang, Y.-R., Ou, Q.-Y., and Yu, W.-L., J. Chromatogr. Sci., Received December 13, 1996 1990, 28, 584. 19 Huang, Y., Ou, Q., and Yu, W., J. Anal. At. Spectrom., 1990, 5, 115. Accepted March 18, 1997 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 7

ISSN:0267-9477    

DOI:10.1039/a608372b

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Rapid Determination of Inorganic Mercury and Methylmercury in Biological Reference Materials by Hydride Generation, Cryofocusing, Atomic Absorption Spectrometry After Open Focused Microwave-assisted Alkaline Digestion CHUN MAO TSENG, ALBERTO DE DIEGO†, FABIENNE M. MARTIN, DAVID AMOUROUX AND OLIVIER F. X. DONARD* L aboratoire de Chimie Bio-Inorganique et Environnement, EP CNRS 132, Universite� de Pau, He� lioparc, 64000, Pau, France. E-mail: Olivier.Donard@univ-pau.f r A rapid and simple microwave-assisted digestion method with environment, it is formed by biotic and abiotic methylation of inorganic mercury and it accumulates in the tissue of fish and alkaline solution (tetramethylammonium hydroxide or methanolic KOH solution) for speciation analysis of inorganic other biota.2,3 Mercury as methylmercury usually represents more than 85% of total mercury present in fish.4,5 Mercury mercury and methylmercury (MeHg+) in biological tissues was developed.Extracts with quantitative recoveries of poisonings are mainly caused by consumption of contaminated fish through MeHg+ accumulation in the food chain, such as mercury species after microwave-assisted alkaline dissolution of the sample were directly analysed by an automated on-line in the case of Minamata Bay.6 The development of analytical methods for the determination of low levels of mercury species hyphenated system incorporating aqueous hydride generation, cryogenic trapping, gas chromatography and detection by in various complex matrices is still a challenge.The quality of the results is mostly associated with sample pre-treatment atomic absorption spectrometry. Optimum conditions allow sample throughput to be controlled by the instrumental stages, in spite of significant improvements in instrumentation. 7,8 Thus, monitoring of mercury levels to avoid the analysis time (#10 min per sample) and not by the sample preparation step. At a power of 40–60 W, sample preparation ecotoxicological risk and to understand the biogeochemical cycling of mercury compounds in the environment demands time is only 2–4 min, which is much faster than previous literature methods.The proposed method was validated by the the development of accurate and sensitive analytical methods for sample preparation and determination techniques.9 analysis of three biological certified reference materials, CRM 463, DORM-1 and TORT-1, and one BCR sample from an For extraction and separation of methylmercury in biological tissues, most methods are still based on the conventional interlaboratory study, Tuna Fish 2. The detection limit of the overall procedure was found to be 3 ng per gram of pulverised Westo�o� technique,10,11 e.g., the use of potassium bromide and sulfuric acid saturated with copper(II ) sulfate in a first extrac- dried biological tissue, for both labile Hg2+ and MeHg+.After alkaline extraction, a mean recovery of 102% with a tion step to increase the extraction eciency of MeHg+ from relative standard deviation of 7% was obtained for fish tissues.12 The extraction procedures are usually followed methylmercury concentrations ranging from 0.128 to by sample acidification (hydrochloric, hydrobromic or hydri- 3.464 mg g-1 as Hg in the four reference fish tissues mentioned odic acid), successive extractions and back-extractions with above.Total mercury, calculated as the sum of determined organic solvents (toluene or benzene) and an aqueous cominorganic mercury and methylmercury, was also in agreement plexing reagent (cysteine or thiosulfate) prior to GC-ECD with the certified values.The methylmercury extraction determination. Only methylmercury is finally extracted by this eciency of five extractants, viz., nitric, hydrochloric and procedure, not inorganic mercury. New isolation methods, acetic acid, and tetramethylammonium hydroxide and such as KOH–methanol extraction, distillation, extraction with methanolic KOH solution, the temperature profile of these dichloromethane, have been developed.Bloom13 described the extractants under microwave irradiation and the behaviour and use of methanolic KOH solutions for the extraction of MeHg+ stability of methylmercury in a focused microwave field were from fish tissues. This technique has been successfully improved also investigated. by the use of an ultrasonic bath to decrease the extraction time14 or by further extraction steps to eliminate matrix Keywords: Mercury speciation; microwave-assisted extraction; interferences.15 Steam distillation has also been attempted for biological tissue; hydride generation; cryogenic trapping; gas mercury speciation.16 The technique was modified by Horvat chromatography; atomic absorption spectrometry and co-workers17,18 to release MeHg+ from biological samples. Special care must be taken, however, to avoid risks of inter- Mercury is a well known toxic element, especially in the form ferences in the commonly used distillation method.19 Such of methylmercury (MeHg+) compounds, which are consider- sample preparation methods all suer from some common ably more toxic than inorganic mercury (Hg2+).1 disadvantages, such as laborious procedures, solvent- and time- Methylmercury may be directly released into the environment consuming problems and lack of eciency and reliability.For from various anthropogenic and natural sources.In the example, in the classicWesto�o� method using solvent extraction, recoveries of MeHg+ are generally poor, yielding about 60–80%.20,21 Alkaline digestion (25% KOH in methanol) and † On leave from the Department of Analytical Chemistry, University of the Basque Country, 644 P. K., 48080 Bilbao, Spain. distillation can quantitatively extract MeHg+ from biological Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 (743–750) 743tissues but it can take 1–24 h to complete sample preparation, extraction eciency of methylmercury under a microwave field were investigated. The power applied versus exposure time depending on the heating source used for extraction.13–18,22 Moreover, further sources of error are introduced because of relationship was also optimised in terms of quantitative MeHg+ recovery and minimum time required for sample the complexity of the methods and multiplicity of the analytical steps.A relatively fast method, supercritical fluid extraction, preparation. The results obtained confirm that alkaline solutions such as TMAH are optimum extractants for simultaneous has recently been developed for organotin23 and mercury24 speciation analysis. The expensive equipment required and quantitative recovery of Hg2+ and MeHg+ from biological tissues after microwave-assisted extraction under mild con- increases the cost of the analysis and the extraction steps still require 20–50 min.Moreover, if ionic mercury compounds are ditions. In addition, the proposed method shows better performance than conventional procedures in terms of accuracy, to be extracted, special attention must be paid to overcome both potential limitations of the technique and sample matrix analysis time, reaction kinetics and cost. eects.25–27 Most extraction procedures are usually followed by an in EXPERIMENTAL situ derivatization reaction with NaBEt4 solution to form Apparatus volatile ethylated species, which are later separated by GC and detected by cold vapour atomic fluorescence or atomic absorp- Methylmercury was extracted from biological samples in a tion spectrometry.Filipelli et al.28 first developed an alternative 50 ml round-bottomed open borosilicate vessel (150 mm, procedure based on derivatization by hydride generation with 35 mm id) with a Microdigest Model A301 (2.45 GHz, maxi- NaBH4 instead of ethylation with NaBEt4, to generate volatile mum power 200W) microwave digestor (Prolabo) equipped mercury species in environmental samples after acid or alkaline with a TX32 programmer which allows the applied energy to digestion.It was later used by Quevauviller et al.29 and Puk be selected from 10 to 200 W in increments of 10 W. The time andWeber.30 Derivatization with NaBH4 considerably reduces of exposure, up to 99 min, can be set in steps of 1 mreaction time (5 min instead of 20 min by ethylation), the focused single-mode microwave digestor is an open system cost of the analysis (about 5–20 times less expensive) and with respect to atmospheric pressure equilibration.39–41 The slightly improves the overall accuracy of the method.microwave energy of the magnetron can be delivered with Moreover, unknown peaks in the blanks have revealed poor great reproducibility and is focused on the sample with maxi- purity in some commercial batches of NaBEt414,18 and mum intensity by the waveguide.A refluxing condenser unit decomposition of MeHg+ via dismutation processes has been on the top of the sample holder prevents possible losses of the observed during the analysis of pure methylmercury standards analytes by volatilisation. Furthermore, the temperature of the by ethylation.14 sample can be continuously measured in real time with a Microwave-assisted extraction methods for sample prep- Megal 500 thermometer (Prolabo) connected to a microcom- aration have been widely evaluated in various environmental puter, which allows the determination of the temperature applications, e.g., total digestion for element analysis,31–34 profile of the sample solution during microwave irradiation.extraction of selected organic compounds35–37 and mercury The determination of recovered mercury species was per- speciation analysis38 from dierent environmental matrices, formed with an automated on-line hyphenated HG-CT- such as soils, sediments and biological tissues. This technique GC–ETAAS system.45–47 All the analytical steps are controlled overcomes the disadvantages of conventional extraction tech- through an electronic panel, which is programmed by a niques in terms of time, eciency and solvent consumption.computer equipped with BORWIN software (JMBS Microwave-assisted extraction for organometallic speciation Developments). The system consists of a peristaltic pump analysis was shown to accelerate and enhance leaching of (Ismatec), which quantitatively pumps the NaBH4 solution organotin species from solid samples, e.g., sediments and into the reaction vessel, a 250 ml reaction vessel in which the biomaterials.39–41 The outcome was to reduce dramatically the derivatization and purging steps take place, two electronic analysis time by a factor of 20–100 when compared with Teflon-valves (Asti), which by-pass the flow of purging and conventional methods.Moreover, extension of the developed stripping gas (He, 99.995%), a U-shaped Pyrex column (45 cm technique has led to flow-through sample extraction schemes length×5 mm id) packed with 2.5 g of Chromosorb W HP for on-line determination and it has facilitated automated (60–80 mesh) coated with 10% SP2100 (Supelco) and silanized methods.42,43 Microwave-assisted extraction has recently been with hexamethyldisilazane (Fluka) prior to use and a Dewar confirmed as a new tool for rapid preparation of solid samples bath of liquid N2 lifted by a pneumatic pump (Joucomatic). for organometallic speciation analysis.44 However, the stability The column is wrapped with 0.5 mm diameter Nichrome wire, of target compounds in a microwave field must be carefully which can be heated by an adjustable dc power supply evaluated and essential parameters, such as the extraction (Hemitechnic).A flow meter (Platon) controls the He gas flow. medium, power applied and exposure time, must be fully Atomisation of mercury species occurs in a T-shaped quartz optimised.39,40 furnace48 (light pathlength 20 cm, 1 cm id) thermally controlled In this work, a simple, rapid and accurate protocol for by an MHS-20 unit (Perkin-Elmer).The furnace is mounted sample preparation and simultaneous determination of on an atomic absorption spectrometer (Model 5100, Perkin- inorganic mercury and methylmercury in fish tissues was Elmer), which is operated at 253.7 nm with a 0.7 nm slit-width.developed. A microwave-assisted extraction technique was Data acquisition is undertaken by the chromatographic used in combination with an automated on-line interface of software running on a personal computer. Further information hydride generation–cryogenic trapping–gas chromatography– about the system can be found elsewhere.45–47,49,50 electrothermal atomic absorption spectrometric detection (HG-CT-GC–ETAAS). All the techniques combined provide Reagents fast and reliable results because the number of analytical steps and potential sources of error are considerably reduced.All chemicals used were of analytical-reagent grade unless stated otherwise and Milli-Q quality water (Millipore) was The extraction eciency of three acids, HNO3, HCl and CH3COOH, and of two bases, tetramethylammonium hydrox- used throughout. KOH pellets, methanol and 100% acetic acid were obtained from Merck. NaBH4 and TMAH (25% in ide (TMAH) and methanolic KOH solution, was compared by analysis of three certified reference materials, CRM 463, water) were purchased from Fluka.The concentrated nitric (67%) and hydrochloric (37%) acids, specific for trace mercury DORM-1 and TORT-1, and a reference material used in an interlaboratory study, Tuna Fish 2, and the behaviour and analysis, were supplied by Prolabo. Mercury chloride and 744 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12methylmercury chloride were purchased from Strem Chemicals.times: Hg0, 1.36±0.03 min; CH3HgH, 1.95±0.04 min. A typical chromatogram corresponding to a mixture of 5 ng each of An approximately 4% m/v solution of NaBH4 was prepared daily by dissolving the reagent in water. All glassware used for Hg2+ and MeHg+ dissolved in water is provided in Fig. 1. The linear calibration range extends from 0.5 to 20 ng as Hg dilution and storage was cleaned with RBS 50 detergent (Fluka), thoroughly rinsed with tap water, soaked in a 10% for both MeHg+ and Hg2+ with a sample volume of 50 ml.The relative standard deviation of ten replicates at a concen- HNO3 solution overnight and finally rinsed with Milli-Q water before use. tration of 5 ng as Hg is less than 10% for each mercury species. The detection limits, calculated as three times the standard deviation of ten blank measurements, are 50 pg for labile Hg2+ Calibrants and Biological Reference Materials and MeHg+. Calibrant stock solutions (1000 mg ml-1) of HgII and methylmercury were prepared by dissolving mercury(II ) chloride in Procedure 1% HNO3 and methylmercury chloride in methanol, respectively.All stock solutions were stored in a refrigerator and A sample of 0.1–0.5 g of pulverised freeze-dried tissue and 5 ml of extractant solution were placed in an extraction tube and protected from light. Working calibrant solutions were prepared by appropriate dilution of stock solutions with water exposed to microwave irradiation at a fixed power in the range of 20–80W for 1–4 min.After extraction, the sample solution and stored for a maximum of 1 week. Two certified reference materials, DORM-1 (Dogfish was cooled to room temperature and then diluted with 5 ml of methanol, shaken to ensure homogeneity, transferred into a Muscle) and TORT-1 (Lobster Hepatopancreas), obtained from the National Research Council of Canada, and one 22 ml Pyrex vial with a Teflon cap and finally stored in a refrigerator until analysis.A clean-up procedure was not certified reference material, CRM 463 (Tuna Fish Muscle), obtained from the Community Bureau of Reference (BCR), necessary before analysis of the extracts by the hydride generation method. Aliquots of 50–300 ml of the extract were were used to validate the proposed method. Additionally, the methylmercury content of Tuna Fish 2, tuna fish muscle analysed by means of the hyphenated HG-CT-GC–ETAAS system. The generated elemental mercury and methylmercury material used in an interlaboratory study organised by the BCR,51 was also analysed. Analytical Operating Conditions The analytical performance of the determination technique using hydride generation with NaBH4 as derivatization step was investigated using mercury chloride and methylmercury chloride calibrants in Milli-Q water.Optimum working conditions of the system are listed in Table 1. Target analytes react in the following way with NaBH4 to aord the corresponding volatile mercury species:30 Hg2++2NaBH4+6H2O�Hg0+7H2+2H3BO3+2Na+(1) MeHg++NaBH4+3H2O�MeHgH+3H2+H3BO3+Na+(2) The conversion and stability of both volatile compounds (Hg0 and MeHgH) has been previously confirmed.28,52,53 Mercury Fig. 1 Typical chromatogram of a mixture of Hg2+ and MeHg+ species eluted from the column by thermal desorption in order (5 ng as Hg) in Milli-Q water obtained after aqueous hydride generation. of increasing molecular weight with the following retention Table 1 Optimum parameters of the automated on-line HG-CT-GC–ETAAS system for mercury speciation analysis Hydride generation— Derivatization 5 ml of 4% m/v NaBH4 Acidification 0.15 ml of 12 mol l-1 HCl Reaction time 0.5 min Cryogenic trapping— GC column U-shaped glass tube, 45 cm×5 mm id GC phase 10% SP-2100 on Chromosorb W-HP 60/80 mesh size Carrier gas Helium (99.995%) Pre-cooling duration 1.5 min Purging duration 4.5 min Purging flow rate 150 ml min-1 Desorption— Stripping gas Helium (99.995%) Stripping flow rate 150 ml min-1 Desorption voltage 30 V Data acquisition— Instrument Perkin-Elmer AAS 5100 Wavelength 253.7 nm Quartz furnace temperature 800°C Acquisition duration 4 min Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 745hydride were purged from the reaction vessel and trapped in concentrated HCl, 100% CH3COOH, methanolic KOH and 25% TMAH were successively spiked with 25 ng of MeHg+ the column, which was immersed in liquid N2 (-196°C).Subsequently, the column was gradually warmed and the and exposed to 40 W power for various heating times. After irradiation, the sample solution was diluted with 5 ml of volatile mercury species successively eluted on the basis of increasing boiling-point. Atomisation and detection took place methanol and the methylmercury content was determined as described above. Fig. 3(a) shows the recovery of MeHg+ after in the quartz furnace heated to 800°C.The calibration was performed by three-point standard additions in the same dierent heating times for each solvent studied. The analytical signal obtained is strongly dependent on the reaction medium. extract used for sample analysis to overcome possible matrix interferences and each sub-sample was subjected to triplicate A 100% MeHg+ recovery was achieved after 4 min of heating for CH3COOH, TMAH and methanolic KOH solutions. For analysis. Blanks were run after each triplicate analysis to check for possible memory eects.a 6 min heating time, quantitative recovery was also observed for TMAH, but only 85–90% of the initial signal was obtained in CH3COOH and methanolic KOH. The lower recovery of RESULTS AND DISCUSSION MeHg+ might be caused by volatile losses due to long heating times, especially for methanolic KOH medium, in which the Behaviour of Dierent Extractants Under Microwave maximum temperature in the temperature profile (Fig. 2) Irradiation exceeds the normal boiling-point of the solution (64°C) by The ability to absorb and propagate microwave energy varies 20°C.Additionally, the signal for MeHg+ rapidly decreases with extraction media owing to dierent dielectric proper- with heating time for concentrated HNO3 and HCl solutions, ties.54,55 In consequence, the behaviour of methylmercury in a suggesting rapid breakdown of MeHg+. The analysis of the microwave field should be aected by the physico-chemical corresponding chromatograms [Fig. 3(b)] demonstrates that properties and concentration of the extractant used in the inorganic mercury is the most important degradation product. leaching process. The temperature profiles of five solutions, Comparison of the chromatograms allows one to conclude concentrated HNO3 and HCl, 100% CH3COOH, 25% TMAH that cleavage of the mercury–carbon bond is more probable and methanolic KOH solution, were monitored during micro- in HCl than in HNO3.In addition, evaporation losses of wave exposure at 40W for 6 min (Fig. 2). The absorption of MeHg+, probably related to the formation of Hg0, with microwave energy implies a rise in the temperature of the generated acid fumes due to extensive superheating, are mainly extraction medium. The temperature of each solution rapidly responsible for the signal reduction in HNO3.57 increases during the first minute of heating. In this stage, the eciency of microwave absorption for HNO3, CH3COOH, HCl, methanolic KOH and TMAH, respectively, decreases as follows: 1.5, 1.3, 1.2, 0.9 and 0.8°C s-1.The maximum temperatures achieved at the plateau after vigorous refluxing within 3 min are 130, 122 and 85°C for HNO3, CH3COOH and methanolic KOH solution, respectively. For HCl and TMAH, the temperature gradually increases with heating time, reaching a maximum, viz., 110 and 113°C, respectively, after 6 min. The maximum temperatures achieved for all the solutions, except for HCl, are 5–20°C above their normal boiling-points, showing that a rapid heating rate results in superheating of the sample solution during microwave irradiation, as has been suggested previously.56 Consequently, a reflux condenser should be used in extraction procedures based on open microwave digestion of the sample at low power with long heating times to prevent evaporation losses of extractants and target analytes.Stability of Methylmercury Under Microwave Irradiation To investigate the stability of MeHg+ in simple solutions under microwave irradiation, 5 ml each of concentrated HNO3, Fig. 3 (a) Stability of 25 ng of methylmercury in dierent media after 6 min of microwave irradiation at 40 W. MeHg+ content normalised to that of a solution not exposed to microwave irradiation. Error bars represent standard deviation of three measurements. (b) Chromatograms corresponding to solutions of A, HNO3; B, HCl; and Fig. 2 Temperature profiles of dierent solvents during microwave C, TMAH (2.5 ng of MeHg+ as Hg) exposed to microwave irradiation (40 W; 6 min). 1, Hg2+; 2, MeHg+. irradiation at 40 W. 746 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12Evaluation of Dierent Extractants for Quantitative MeHg+ to degradation or volatile losses of methylmercury, resulting in a higher concentration of inorganic mercury. Additionally, Recovery from Biological Materials: Potential Chemical Pathways lower yields of both methylmercury and inorganic mercury obtained from CH3COOH tissue extracts are due to incomplete HNO3, HCl, CH3COOH, TMAH and methanolic KOH solu- dissolution of the biological tissue.tion are the reagents most commonly used in acid/alkaline Mercury in biological tissues is mainly attached to depro- digestion methods for biological materials.5,13–15,29,30,58,59 tonated forms of thiol ligands, nucleobases or long-chain fatty Their MeHg+ extraction eciency was investigated using two acids.60 Both acid and alkaline hydrolysis of these bonds yield dierent biological certified reference materials, CRM 463 and analogous mercury compounds: DORM-1, and a laboratory reference material, Tuna Fish 2.Acid digestion: For quantitative recovery of methylmercury, complete dissolution of the biological tissue should be achieved, assuring full cleavage of the bond between methylmercury and organic Hg(XR)2+H2O CA HA HgAn(n-2)-+2RXH (3) groups, such as thiol group-containing lipids and proteins, and reduction of organic matrix interferences.Complete dissolution MeHg(XR)+H2O CA HA MeHgAn(n-1)-+RXH (4) of biological tissue for organotin speciation was obtained for optimized conditions of 40–60W and 3–4 min with 5 ml of X5S-II, O-II, N-III A5Ac-, NO3-, Cl- 25% TMAH solution in an open focused microwave field.41,44 Fig. 4(a) shows that the recovery of methylmercury after 4 min Alkaline digestion: of irradiation at 40W varies significantly from one extraction medium to another.The best recoveries were obtained with Hg(XR)2+H2O CA BOH Hg(OH)n(n-2)-+2RXB (5) 25% TMAH or methanolic KOH solution, resulting in concentrations which lie within the confidence intervals of the certified value for each biocal tissue. Under these conditions, no MeHg(XR)+H2O CA BOH MeHg(OH)n(n-1)-+RXB (6) degradation of methylmercury is observed and recoveries are always between 95 and 105%. The use of concentrated HNO3 X5S-II, O-II, N-III B5K+, TMA+ and HCl and 100% CH3COOH systematically leads to lower The stability of the mercury complexes obtained depends recoveries due to methylmercury degradation by concentrated on both the pH and the nature of the A or B groups of the HCl, degradation and volatile losses of methylmercury (or acidic or basic extractant.The complexing and redox properties formation of Hg0) by concentrated HNO3 and insucient of these groups may lead to degradation of the mercury– dissolution of the biological tissue by 100% CH3COOH.The methyl bond via redox or dismutation processes: phenomenon leading to methylmercury degradation is illustrated in Fig. 4(b). The average recovery of inorganic mercury Hg2+ CA red/oxCH3Hg+ CA red Hg0 (7) using 5 ml of concentrated HNO3 or HCl is 2–4 times higher than that using 5 ml of 25% TMAH or methanolic KOH solution. It can be concluded that acid digestion in a focused Hg2+ BC dism CH3Hg+ CA dism Hg0 (8) microwave field with concentrated HNO3 or HCl readily leads Chloride forms very stable complexes with inorganic mercury which may favour dismutation processes (degradation to Hg2+ and evaporation losses as Hg0).Nevertheless, only degradation to Hg2+ was observed after leaching with HCl. Nitrate provides an oxidizing medium which may induce the cleavage of the mercury–methyl bond in an oxidation process. Evaporation losses with generated NO2- fumes are also probable due to the particularly large superheating eect after microwave irradiation of HNO3.Acetate is not suciently strong to catalyse the hydrolysis of the Hg–XR bond completely. The fairly high pH provided by KOH and TMAH stabilises both MeHg+ and Hg2+ in the corresponding hydroxide forms, preventing the occurrence of destructive redox processes. However, a complete understanding of these chemical pathways would need further investigation. Summarising, TMAH or methanolic KOH solution should be considered as the most appropriate tissue solubilizers, as they are able to dissolve the sample completely and rapidly under mild microwave conditions, avoiding problems due to methylmercury decomposition. Thus, microwave assisted alkaline digestion for mercury speciation in biological tissues provides quantitative recoveries in a few minutes, allowing the sample throughput to be controlled by the duration of the analysis, instead of the sample preparation step.Concerning the acid digestion method, the concentration of acid should be carefully optimised to avoid degradation and losses of MeHg+ during microwave extraction.Owing to higher extraction Fig. 4 Recovery of (a) methylmercury and (b) inorganic mercury from eciency and lower solution volatility compared with meth- CRM 463, DORM-1 and Tuna Fish 2 tissues using dierent extract- anolic KOH solution, microwave-assisted digestion using ants: power, 40 W; irradiation time, 4 min; volume, 5 ml. Relative ratio TMAH solution as extractant, which has not been reported of Hg2+ normalised to a mean value of Hg2+ in 25% TMAH and previously to our knowledge, is recommended for the determi- methanolic KOH solutions.Error bars represent standard deviation of three measurements. nation of methylmercury in fish tissues. Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 747Optimisation of Microwave-assisted Alkaline Digestion for the superheating eect of the extractant by an ecient focused microwave energy.56 In addition, quantitative MeHg+ recover- MeHg+ Determination ies were also obtained with 5 ml of 10% TMAH solution at Two main factors to be taken into account when analysing 40 W power after 4 min.Under the same conditions, only 65% the stability of MeHg+ in an open-focused microwave field recovery was achieved using 5% TMAH. TMAH solution are decomposition by microwave attack and evaporation due oers ecient extraction and quantitative methylmercury to extreme heating for long periods of time or to high power recovery from fish samples, even at low solvent concentration settings.Studies on the stability of MeHg+ in 10 ml of 2 (10%). Nevertheless, the use of 25% TMAH is advisable to mol l-1 HNO3 solution50 show that MeHg+ losses due to assure a better precision and reproducibility at moderate evaporation start to occur after 8, 6 and 4 min at 80, 120 and microwave conditions. Furthermore, sample heating for 160 W, respectively.In this work, the stability of 25 ng of 2–4 min with 25% TMAH solution at a power of 40–60W is MeHg+ spiked into 5 ml of 25% TMAH solution during recommended as the optimum conditions for the microwave- microwave irradiation at various microwave power settings assisted alkaline digestion procedure in the simultaneous and exposure times was investigated. No loss of MeHg+ was determination of inorganic mercury and methylmercury in observed during microwave irradiation at applied powers in biological materials.the range 20–100W and for exposure times of 1–6 min, except for a power of 100 W and heating times of 5–6 min [Fig. 5(a)]. In this case, only 20–75% of the initial signal and poor Analytical Figures of Merit precision were obtained, possibly related to evaporation of The simple procedure described here reduces sample prep- MeHg+ connected with extractant losses. However, good aration time and the number of handling steps.After micro- stability of MeHg+ was achieved for low microwave powers wave digestion, extracts can be directly analysed without any (20–80 W) and short exposure times (1–4 min). clean-up procedure, minimising the potential source of analyt- The eect of dierent applied powers and exposure times ical error. The performance of the proposed microwave-assisted (20–80 W/1–4 min) was also studied using CRM 463. Results alkaline digestion procedure for biological tissues was critically showed that MeHg+ recoveries consistently lay in the interval evaluated by the analysis of the biological reference material of the certified value within a ±95% confidence level, except CRM 463.Results showed that a quantitative, simultaneous when a power of 20W and a heating time of 1 min were and rapid determination of labile Hg2+ and MeHg+ is achieved applied [Fig. 5(b)]. Under such conditions, a recovery of only and that the obtained contents are in good agreement with 86% was achieved, owing to incomplete solubilization of the the certified values, as shown in Table 2.Measured MeHg+ biological tissue. This confirms the high rate and eciency of and Hg2+ contents in DORM-1 and TORT-1 are also provided methylmercury extraction by microwave-assisted alkaline in Table 2. Biological reference materials in Table 2 are not digestion. The time for sample preparation is shortened to certified for Hg2+. The ‘certified’ concentration presented was 1–4 min, depending on the applied power (80–20W). Rapid calculated by subtracting the MeHg+ content from the certified dissolution of the tissue can be attributed to the extensive value for total mercury. The corresponding precisions, obtained reaction between the sample matrix and the extractant, due to by propagation of random errors, showed very high values. Analysis of CRM 463 and DORM-1 after application of the proposed alkaline digestion procedure gave Hg2+ concentrations within the confidence level of the certified value, but close to the high limit.This eect was not observed for the TORT-1 material, in which the MeHg+/Hg2+ ratio is not so high. This suggests that further purification of reagents to reduce the Hg2+ content in the blank should be attempted when determining mercury species in fish tissues with very low Hg2+ concentration by hydride generation. A precision of 4–10% (RSD) for the determination of mercury compounds was obtained from three independent replicates of each sample of CRM 463.The detection limits for both Hg2+ and MeHg+ were calculated to be 50 ng g-1 for 0.2 g of pulverised dry sample and 0.05 ml of extract. Additionally, it was also observed that quantitative MeHg+ recoveries can be obtained with 5 ml of 25% TMAH solution for up to 1 g of biological tissue (dry mass). The analytical method was validated by the analysis of three biological reference materials, CRM 463, DORM-1 and TORT-1.Typical chromatograms are shown in Fig. 6. CONCLUSIONS A simple, fast and accurate method for sample preparation and mercury speciation in biological tissues has been developed in which, after microwave-assisted digestion with TMAH (or, alternatively, methanolic KOH) solution, inorganic mercury and methylmercury concentrations are directly determined by an automated on-line hyphenated HG-CT-GC–ETAAS Fig. 5 Stability of methylmercury (a) in 25% TMAH solution (25 ng system. The advantages of the microwave-assisted alkaline of MeHg+ as Hg) and (b) recovery of methylmercury from CRM 463 digestion technique compared with the analogous acid pro- reference tissue.Yield normalised to that of a solution not exposed to cedure have also been shown in terms of extraction eciency microwave irradiation for (a) and to that of an extract exposed to and discussed in terms of potential chemical pathways. microwave irradiation at 40W for 4 min heating for (b).Extractant, 5 ml of 25% TMAH solution. Optimum conditions for microwave-assisted alkaline digestion 748 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12Table 2 Results for the determination of inorganic mercury and methylmercury in certified reference biological tissues Content/mg g-1 as Hg* Determined† Certified TMAH‡ KOH‡ Material Hg2+ MeHg+ Total Hg§ Hg2+ MeHg+ Total Hg§ Hg2+¶ MeHg+ Total Hg CRM 463 0.235±0.030 2.735±0.106 2.970±0.110 0.218±0.028 2.718±0.140 2.936±0.143 0.02±0.22 2.83±0.15 2.85±0.16 DORM-1 0.120±0.035 0.728±0.028 0.848±0.045 0.127±0.037 0.713±0.042 0.840±0.056 0.067±0.095 0.731±0.060 0.798±0.074 TORT-1 0.184±0.024 0.142±0.017 0.326±0.029 0.178±0.032 0.149±0.015 0.327±0.035 0.202±0.062 0.128±0.014 0.33±0.06 * Calculated for dry mass.† Three independent experiments. ‡ Extracted with a power setting of 60 W and a heating time of 2 min. § Calculated as Hg2++MeHg+. ¶ Calculated as total Hg-MeHg+.quantitative recovery of the analytes and prevents decomposition of the organomercury species. The authors thank D. Mathe� (Prolabo) for allowing the use of a prototype microwave digestor and R. Lobinski, J. Szpunar and V. O. Schmitt for fruitful contributions to the microwaveassisted speciation-related research. C.M.T. acknowledges the Taiwan government for his PhD grant. A. de D. is grateful to the Spanish Government for his post-doctoral fellowship. REFERENCES 1 Hempel, M., Chau, Y.K., Dutka, B. J., McInnis, R., Kwan, K. K., and Liu, D., Analyst, 1995, 120, 721. 2 Craig, P. J., in Organometallic Compounds in the Environment, Principles and Reactions, ed. Craig, P. J., Longman, Essex, 1986, pp. 65–101. 3 Moore, J. W., and Ramamoorthy, S., in Heavy Metals in Natural Waters, Applied Monitoring and Impact Assessment, ed. Moore, J. W., and Ramamoorthy, S., Springer-Verlag, New York, 1984, Fig. 6 Typical chromatograms, obtained by HG-CT-GC–ETAAS, of pp. 125–160. A, 50 ml; B, 200 ml; and C, 300 ml of extract after microwave-assisted 4 Bloom, N. S., Can. J. Fish. Aquat. 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O. F. X., Analyst, 1995, 120, 2665. 60 Kaim, W., and Schwederski, B., Bioinorganic Chemistry: Inorganic 41 Szpunar, J., Schmitt, V. O., Lobinski, R., and Monod, J.-L., Elements in the Chemistry of L ife, Wiley, Chichester, 1994, J. Anal. At. Spectrom., 1996, 11, 193. pp. 338–343. 42 Tsalev, D. L., Sperling, M., and Welz, B., Analyst, 1992, 117, 1729. 43 Welz, B., Tsalev, D. L., and Sperling, M., Anal. Chim. Acta, 1992, Paper 7/00956I 261, 91. Received February 11, 1997 44 Szpunar, J., Schmitt, V. O., Donard, O. F. X., and Lobinski, R., T rends Anal. Chem., 1996, 15, 181. Accepted April 1, 1997 750 Journal of Analytical Atomic Spectrometry, July 1997, Vol

ISSN:0267-9477    

DOI:10.1039/a700956i

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Computer Simulation of an Analytical Direct Current Glow Discharge in Argon: Influence of the Cell Dimensions on the Plasma Quantities ANNEMIE BOGAERTS* AND RENAAT GIJBELS Department of Chemistry, University of Antwerp, Universiteitsplein 1, B-2610 Wilrijk-Antwerp, Belgium A set of three-dimensional mathematical models was developed DESCRIPTION OF THE MODELS for describing the behavior of the dierent plasma species in a A number of separate models have been developed for the direct current glow discharge in argon used as an analytical dierent plasma species in a direct current glow discharge in ion source for mass spectrometry.The models were applied to argon with a copper cathode, and these models were combined cylindrical cells (flat cathode and hollow anode) with various to obtain an overall picture of the glow discharge. Table 1 dimensions to study the eect of the dimensions on the gives an overview of the dierent species assumed to be present calculated plasma quantities.The results show that the cell in the plasma and the models used to describe these species. dimensions have no significant influence on the qualitative Fluid models are utilized for plasma species that are more or behavior of the plasma quantities, but they do aect the less in equilibrium with the electric field in the discharge (i.e., absolute values, at least for cell dimensions ranging from 0.5 the energy gained by the electric field is more or less balanced to 2 cm.For larger cells, the absolute values also remain more by the energy lost due to collisions), so that they can be or less constant. The results suggest that, for the selected considered as a fluid and described with continuity and trans- discharge conditions of 1000 V, 1 Torr and about 2 mA and a port equations. Monte Carlo simulations are employed for copper cathode, a cell with both length and radius equal to plasma species that are far from equilibrium, so that their 2 cm is a good choice for analytical mass spectrometry.This behavior has to be described explicitly (i.e., trajectory calcu- paper demonstrates that the models are in principle able to lated with Newton’s laws and collisions treated statistically by predict trends in plasma behavior and performance in random numbers). A short explanation of the dierent models analytical applications and that they can therefore be useful in (i.e., the relevant processes considered in each model) is also developing new cells.included in Table 1. The models are combined and solved iteratively until final convergence is reached. More information Keywords: Glow discharge; modeling; cell design about these models can be found elsewhere.3–12 The models are applied to the cell geometry represented in Fig. 1. We chose a very simple and general cell configuration, so that the results of this study can most easily be applied to Glow discharges are used for a range of applications: as various, more specific types of cell geometries. The length and spectroscopic sources for mass spectrometry or optical spectro- radius of the cell are each varied independently between 0.5 metric techniques,1 for deposition of thin films and for plasma and 4 cm.In the initial ‘standard cell’, the length and radius etching and modification of surfaces in the semiconductor are each taken as 2 cm. A metallic disk of 0.25 cm radius acts industry,2 as plasma displays, as metal vapor ion lasers and as the cathode and is represented by the black rectangle at l= also in the lighting industry. It is to be expected that the 0.All the other cell walls (i.e., the cylindrical portion and the results in the dierent application fields will depend strongly cylinder ends) are at anode potential. The cathode and anode on the cell configuration. One possibility for optimization is are separated by an insulating ring (0.1 cm wide).The fluid to build dierent cells and to investigate which configuration models are developed in two dimensions: due to the cylindrical and which dimensions yield the best results. However, this is symmetry of the cell, the three dimensions could, indeed, be sometimes based on trial and error, and it can be a time- reduced to two dimensions. The Monte Carlo models, however, consuming and expensive approach. It would be much cheaper are completely constructed in three dimensions.to predict the optimum cell configuration by computer simulations prior to building the cells. We have developed a set of mathematical models for describ- RESULTS AND DISCUSSION ing the behavior of the dierent species present in a direct Discharge Current as a Function of Voltage and Pressure current glow discharge in argon used as an ion source for mass spectrometry. These models were first developed in one The calculations were all performed at a 1000 V discharge dimension3–8 and later extended to three dimensions and voltage and 1 Torr argon gas pressure.When the pressure, applied to the standard cell used for analyzing flat samples in voltage and gas temperature are given, the models allow the a VG9000 glow discharge mass spectrometer.9–12 Reasonable self-consistent calculation of the electrical current flowing agreement with experimental results (e.g., based on laser- through the cell. Comparison of the calculated currents with induced atomic fluorescence measurements) could be experimental values can then be used to test whether the achieved.13–15 These models can in principle be used to predict models present a realistic picture of the glow discharge.the optimum cell design.16 To illustrate this in the present Assuming a gas temperature of 450 K, the calculated currents paper, the models were applied to a simple, cylindrically for the dierent cell dimensions investigated are presented in symmetrical glow discharge cell with a flat cathode.The cell Table 2. They range from 0.7 to 2.6 mA for all cell dimensions configuration was kept constant, but the length and the radius under study. Hence the discharge conditions investigated (1000 of the cell were varied to investigate their influence on the V, 1 Torr and 0.7–2.6 mA) are typical discharge conditions for glow discharge mass spectrometry (GDMS). calculated quantities. Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 (751–759) 751Table 1 Overview of the plasma species assumed to be present in the glow discharge plasma, the models used to describe these species, the relevant processes considered in these models and references giving more information about these models Plasma species Model Relevant processes Ref. Ar atoms at rest Not explicitly calculated Atoms assumed to be uniformly distributed throughout the cell — Fast electrons (i.e. energy Monte Carlo model Elastic collisions with Ar atoms; electron impact ionization of Ar atoms in the ground state 3,4,9 high enough for and in the metastable level, and of sputtered Cu atoms; electron impact excitation of Ar inelastic collisions) atoms in the ground and metastable states Thermalized electrons Fluid model Continuity and transport equations (transport by diusion and migration in the electric field); 4,9 equations coupled to Poisson equation, to obtain self-consistent electric field Ar+ ions Fluid model idem 4,9 Ar+ ions in the CDS* Monte Carlo model Symmetric charge transfer; elastic collisions with Ar atoms; ion impact ionization and 3,5,9 excitation of Ar atoms Ar fast atoms in the CDS Monte Carlo model Elastic collisions with Ar atoms; atom impact ionization and excitation of Ar atoms 3,5,9 Ar metastable atoms Fluid model Balance equation with dierent production terms (electron, ion and atom impact excitation to 6,11 the metastable levels and electron–ion radiative recombination), and loss terms (electron impact ionization and excitation from the metastable levels, electron collisional transfer to the nearby levels, metastable atom–metastable atom collisions, Penning ionization of sputtered Cu atoms, two- and three-body collisions with Ar atoms); moreover, transport is diusion controlled, and subsequent de-excitation at the walls is an additional loss process Cu atoms Monte Carlo model Thermalization immediately after sputtering, due to collisions with Ar gas atoms 10 Cu atoms and Cu+ ions Fluid model Further transport of Cu atoms (diusion controlled), ionization of Cu atoms (by Penning 7,11 ionization by Ar metastable atoms, asymmetric charge transfer by Ar ions and electron impact ionization) and transport of the Cu ions (by diusion and migration in the electric field) Cu+ ions in the CDS Monte Carlo model Elastic collisions with Ar atoms 7,11 * CDS=Cathode Dark Space.Table 2 Calculated quantities for the dierent cell dimensions investigated (at 1000 V and 1 Torr, argon discharge with copper cathode) l=2 cm l=0.5 cm l=1 cm l=3 cm l=4 cm l=2 cm l=2 cm l=2 cm l=2 cm Parameter r=2 cm r=2 cm r=2 cm r=2 cm r=2 cm r=0.5 cm r=1 cm r=3 cm r=4 cm Electric current/mA 2.4 0.65 1.9 2.45 2.5 0.87 1.3 2.5 2.55 Length of the CDS/cm 0.15 0.23 0.16 0.15 0.145 0.20 0.17 0.15 0.15 Max.value of the plasma potential/V 1.9 4.6 2.2 1.9 1.9 9.8 3.8 1.9 1.9 Max. axial electric field strength at cathode/kV cm-1 -20 -15 -19 -20 -20 -15 -17 -20 -20 Max.axial electric field strength at anode 13 240 36 7 4 70 27 12 12 end-plate/V cm-1 Max. radial electric field strength at anode side-walls/ 5 2 3 5 4.5 413 32 1.6 0.8 V cm-1 Max. value of Ar ion and thermalized electron 1.9×1012 1.6×1011 1.3×1012 1.9×1012 2.0×1012 5.7×1011 8.4×1011 2.0×1012 2.1×1012 density/cm-3 Max. value of fast electron density/cm-3 9.6×107 3.4×107 8.7×107 9.9×107 1.1×108 5.7×107 5.8×107 9.9×107 9.9×107 Max.value of sputtered Cu atom density/cm-3 2.1×1013 4.6×1012 1.7×1013 2.1×1013 2.1×1013 2.0×1012 3.5×1012 2.1×1013 2.1×1013 Max. value of Cu ion density/cm-3 4.6×1010 2.5×108 1.8×1010 4.7×1010 5.5×1010 1.4×109 4.1×109 5.7×1010 5.9×1010 Ratio of Cu ion to Ar ion density (%) 2.4 0.16 1.4 2.5 2.8 0.25 0.50 2.9 2.8 Degree of ionization of Cu (%) 2.1 0.009 0.57 2.4 2.7 0.033 0.23 4.1 4.2 752 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12Fig. 1 Schematic diagram of the cylindrical glow discharge cell with various dimensions to which the models were applied (flat cathode and hollow anode).The cell with length (l ) and radius (r) equal to 2 cm is taken as the ‘standard cell’. The lengths and radii were each varied independently from 0.5 cm to 4 cm. It appears that for lengths and radii smaller than 2 cm, the current increases clearly with increasing length and radius. The fast electrons can travel longer in the plasma before they reach the walls and can hence give rise to more ionization collisions, more electron multiplication and hence higher currents.Moreover, the ions and (slow) electrons will not arrive so rapidly at the walls, where they would be neutralized by electron–ion recombination, so that their density in the plasma is higher and they can carry more current. At lengths and radii larger than 2 cm, the calculated current does not appear to Fig. 2 Calculated potential distribution for the standard cell (l=r= increase further.This indicates that at the selected discharge 2 cm) at 1000 V, 1 Torr and 2.4 mA in an argon discharge with copper cathode. conditions the range in which fast electrons can produce ionization collisions is more or less limited to about 2 cm from the cathode (see below). the thin plasma sheath in front of the walls stays behind with a positive space charge. This leads to a potential increase in Potential and Electric Field Distributions the plasma with respect to the walls, which gives rise to a positive plasma potential, i.e., the plasma is the most positive Fig. 2 presents the potential distribution throughout the discharge for the cell of length and radius 2 cm. The cathode is body in the discharge. It can be understood that in a small discharge cell, the perturbation by the walls is more significant, represented by the black rectangle at z=0. The potential is -1000 V at the cathode, and increases rapidly immediately in and the plasma has to react more to oppose the potential change at the walls, yielding a higher (more positive) plasma front of the cathode.It goes through zero at about 0.15 cm from the cathode, and is slightly positive (about 1.9 V, the potential. The position where the plasma potential reaches its maximum is slightly closer than about 1 cm from the cathode plasma potential) in the rest of the plasma. The position where the potential goes through zero defines the interface between for the cells with lengths and radii 2 cm (hence it is not always in the middle of the discharge cell, as one might expect), Cathode Dark Space (CDS) and negative glow (NG), and it is therefore indicated with a thicker line.The potential distri- and it was found to be at about 0.4–0.5 cm from the cathode for the cells with smaller dimensions. The cell dimensions butions calculated for the other cell dimensions are qualitatively the same. The length of the CDS is always more or less investigated here do not yet appear to be large enough to give rise to the formation of a Faraday dark space or positive similar, as can be seen from Table 2, except at the smallest lengths and radii investigated, since the current is lower there; column, although when l=2 cm and r=4 cm the calculated potential again becomes slightly negative (about -0.2 V) at a hence the electrons cannot give rise to so much ionization, and the CDS therefore has to be longer to sustain the discharge. radial position of about 3 cm from the cell axis, which indicates the beginning of a Faraday dark space being formed.The NG always fills up the rest of the discharge cell, thus being small for the small dimensions and large for the larger From the potential distributions, the electric field strengths throughout the discharge can also be calculated. The axial dimensions. The value of the plasma potential was calculated to be roughly constant for lengths and radii 2 cm, but electric field is extremely negative at the cathode (due to a large potential drop over a small distance), it increases rapidly increased for shorter lengths and radii (see Table 2).The reason for this is found in the phenomenon of ‘sheath formation’ in the CDS to small negative values at the interface with the NG, it crosses the zero-line always at about 0.5–1.2 cm from (Debye shielding):2 if the potential in the plasma is perturbed, the plasma reacts to oppose that change.Since the electrons the cathode (called ‘field-reversal;’ it occurs at the position where the potential reaches its maximum) and it increases to have a much higher mobility than the argon ions, they will diuse more rapidly to the walls, where they will be lost, and small positive values at the anode end-wall. The radial electric Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 753field is negative in the CDS at the edges of the cathode and it is slightly positive in the NG, increasing slightly towards the anode side-walls.These electric field distributions are qualitatively similar to those calculated for the VG9000 glow discharge cell,9 and are therefore not illustrated here again. Table 2 presents the maximum values of the axial electric field strengths at the cathode and at the anode end-wall, and the maximum values of the radial electric field strengths at the anode sidewalls for the dierent cell dimensions investigated.It appears that the axial electric field at the cathode increases for lengths and radii increasing from 0.5 to 2 cm (because the CDS becomes shorter and the potential has to fall over a shorter distance, giving rise to a higher electric field), and is approximately constant for the larger dimensions (because the CDS length was also found to be more or less constant). The axial electric field at the anode end-wall increases considerably for decreasing cell lengths, which is attributed to the shorter distance over which the potential has to drop to zero at the wall.The eect is most pronounced for the small values of l, since these are also characterized by a higher plasma potential that has to drop o. The eect of the increasing radius is only small, since the length (and hence the distance over which the potential has to fall in the axial direction) stays constant. The higher axial electric field at the anode end-wall in the case of the small radii is due to the higher plasma potential, but for radii ranging from 2 to 4 cm the axial electric field is constant, since both the plasma potential and the axial distance are the same.On the other hand, the values of the radial electric fields at the anode side-walls change considerably for the dierent radii investigated, as can be seen from Table 2, because (i) the distance over which the plasma potential has to fall to zero varies widely and (ii) at small values of r, a higher plasma potential has to drop o.Also, on decreasing the lengths, the radial electric field at the anode side-walls increases, but the eect is only visible at small values of l, owing to the higher plasma potential that has to drop o to zero, and it is absent at l2 cm, since both the value of the plasma potential and 2E+011 2E+011 2E+011 2E+011 1E+011 1E+012 1E+012 1E+011 1E+011 1E+011 Fig. 7 Calculated density profile of the argon metastable atoms for the cell with l=0.5 cm and r=2 cm, at 1000 V, 1 Torr and 0.65 mA in Fig. 6 Calculated density profile of the argon metastable atoms for an argon discharge with copper cathode. the standard cell under the same discharge conditions as in Fig. 2. in the model, or that further processes would have to be incorporated. Nevertheless, the overall agreement with experi- in these larger cells, owing to the higher slow electron densities ment was satisfactory, and therefore we believe that at least (see above).It was found that in the small cells, diusion and the trend observed in the present results will be correct. de-excitation at the walls are more or less the dominant loss process for the argon metastable atoms (i.e. about 54% at l= 0.5 and r=2 cm and about 41% at l=2 and r=0.5 cm, whereas Density of Sputtered Copper Atoms the values for this loss process at l=4 and r=2 cm and at l= 2 and r=4 cm are only about 5 and 20%, respectively), and Fig. 8 shows the sputtered copper atom density profile for the cell with l=2 and r=2 cm. The density is at its maximum in the larger cells electron collisional transfer to the nearby resonant levels is most important (i.e., about 88 and 70% for close to the cathode and decreases towards the cell walls. This sputtered atom density profile, calculated for tantalum, for a l=4 and r=2 cm and l=2 and r=4 cm, respectively, compared with about 35 and 50% for l=0.5 and r=2 cm and l=2 and comparable cell (six-way cross glow discharge cell) and discharge conditions, was in excellent agreement with results from r=0.5 cm, respectively).The relative contributions of the other loss processes were found to be of minor importance and were laser induced fluorescence experiments,13 which also supports the present results. The calculated sputtered copper atom comparable for all the cells investigated. The most important production process was in all cases electron impact excitation density is clearly higher than that calculated for tantalum, since copper has a much higher sputtering yield than tantalum.2 to the metastable levels although, especially in the small cells, argon ion and atom impact excitation were not negligible.For The copper atom density profiles, calculated for the other cell dimensions investigated, are similar to that in Fig. 8: a maxi- the cell of l=0.5 and r=2 cm, the importance of argon atom impact excitation was even found to be comparable to electron mum is always reached at about 0.05–0.1 cm from the cathode, whereafter the density decreases towards the cell walls.Table 2 impact excitation. It should be mentioned, however, that these calculated data concerning the argon metastable atoms have presents the maximum values for the dierent cell dimensions studied. The density appears to increase for lengths and radii to be considered with caution. The argon metastable atom density profile, calculated for a similar (so-called six-way cross) between 0.5 and 2 cm, but remains constant on further increasing the dimensions.This indicates again that the eect of the glow discharge cell, with comparable discharge conditions, was found not to be in complete agreement with results of laser cell walls is only important in the small cells, and that the cell with both length and radius equal to 2 cm is large enough to induced fluorescence measurements,14 which may indicate that the production and/or loss processes are not correctly described give high concentrations of sputtered atoms in the discharge. 756 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12Fig. 12 Calculated relative contributions of the fast argon atoms, Fig. 10 Calculated relative contributions of asymmetric charge trans- argon ions and copper ions to the sputtering process at the cathode fer, Penning ionization and electron impact ionization to the ionization as a function of the cell dimensions at 1000 V and 1 Torr (argon of the sputtered copper atoms as a function of the cell dimensions at discharge with copper cathode).The influence of the length (l ) is given 1000 V and 1 Torr (argon discharge with copper cathode). The by the full lines and that of the cell radius (r) by the dashed lines. influence of the length (l ) is given by the full lines and that of the cell radius (r) by the dashed lines. not dominant, but these processes are nevertheless not negli- role of asymmetric charge transfer becomes increasingly gible.5 In Fig. 11, the relative contributions of the ionization important for larger cells. It seems to be the dominant process mechanisms for argon are depicted for the dierent cell dimen- for all cell dimensions investigated, except for the two smallest sions under study. Electron impact ionization in the NG was cells (l=0.5 or r=0.5 cm), where Penning ionization is calcu- found to be clearly dominant for all cases, but, since the NG lated to be the most significant process.The argon metastable shrinks accordingly when the cell dimensions decrease (and, atom density was found to be relatively higher in the small moreover, the CDS becomes larger; see Table 2), the role of cells than in the larger cells (see above). Electron impact electron impact ionization in the CDS, and also of argon atom ionization appears to be of minor importance in all cases. It and ion impact ionization, becomes slightly more significant should be noted that the exact contributions of these three in the smaller cells.processes have to be considered with caution, since the rate coecients for Penning ionization, and especially for asymmet- Sputtering at the Cathode ric charge transfer, are not well known in the literature, and the values used (see ref. 11) are, therefore, subject to consider- Finally, since the ratio of copper ion flux to argon ion flux at able uncertainties.Nevertheless, the general trend is expected the cathode increases when the cell becomes larger for cells to be correctly predicted. with dimensions 2 cm, the relative contribution of copper ions to the cathode sputtering (i.e., self-sputtering) is also expected to rise. The relative contributions of copper ions, Ionization of Argon argon ions and fast argon atoms to sputtering are illustrated Whereas for the sputtered copper atoms asymmetric charge in Fig. 12 for the dierent cell dimensions studied. The role of transfer and also Penning ionization are more important than the fast argon atoms appears to be dominant in all cases; the electron impact ionization, the situation is completely dierent argon ions take second place and the copper ions contribute for the ionization of argon. Indeed, asymmetric charge transfer only a few percent. However, it can be noted that their role by argon ions and Penning ionization by argon metastables increases slightly when the cell dimensions change from 0.5 cm play, of course, no role in the ionization of argon, and electron (i.e., about 0.75%) to 2 cm (i.e., about 2.5–3%).On further impact ionization is the dominant process. Two other processes, increasing the cell dimensions, the relative contributions of the i.e., argon ion and atom impact ionization, come into play, copper ions, argon ions and fast atoms remain more or however. Since these ionization mechanisms are only important less constant.in the CDS, close to the cathode, their final contributions are CONCLUSION A number of three-dimensional models, developed for a direct current glow discharge in argon, have been applied to dierent cells with lengths and radii varying from 0.5 to 4 cm to investigate the influence of the cell dimensions on the typical quantities calculated by the models, e.g., the electric current as a function of voltage and pressure, the potential distributions and electric fields in the discharge and the densities of the plasma species.Special emphasis is placed on the use of the glow discharge as an ion source for mass spectrometry (GDMS) (i.e., calculation of the importance of dierent ionization processes and the role of the sputtered atoms and ions in the discharge), but the results of the present investigation can also be extended to other applications of glow discharges. Fig. 11 Calculated relative contributions of electron impact ioniz- It was found that the calculated results are qualitatively the ation in the NG and in the CDS and of fast argon atom and argon same for the dierent cell dimensions investigated, but the ion impact ionization to the ionization of the argon atoms as a absolute values are aected. The most important results are function of the cell dimensions at 1000 V and 1 Torr (argon discharge the following: on increasing the cell dimensions from 0.5 to with copper cathode).The influence of the length (l ) is given by the full lines and that of the cell radius (r) by the dashed lines. 2 cm, the electric current at the same voltage and pressure 758 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12increases slightly, and the same applies to the densities of the REFERENCES plasma species. Moreover, the ionization degree of copper 1 Marcus, R. K., Glow Discharge Spectroscopies, Plenum Press, increases and the copper ion peak (and hence the peaks of the New York, 1993.sputtered analyte ions in general) in the mass spectrum is 2 Chapman, B., Glow Discharge Processes, Wiley, New York, 1980. predicted to be higher, yielding a better analytical sensitivity. 3 Bogaerts, A., van Straaten, M., and Gijbels, R., Spectrochim. Acta, On further increasing the cell dimensions to 4 cm, the calcu- Part B, 1995, 50, 179. 4 Bogaerts, A., Gijbels, R., and Goedheer, W. J., J. Appl. Phys., lated results remain more or less the same.The main reason 1995, 78, 2233. is that, under the present discharge conditions, the range of 5 Bogaerts, A., and Gijbels, R., J. Appl. Phys., 1995, 78, 6427. the fast electrons for producing ionization collisions seems to 6 Bogaerts, A., and Gijbels, R., Phys. Rev. A, 1995, 52, 3743. be more or less limited to about 2 cm, and hence increasing 7 Bogaerts, A., and Gijbels, R., J. Appl. Phys., 1996, 79, 1279. the cell dimensions does not give rise to a higher degree of 8 Bogaerts, A., and Gijbels, R., Fresenius’ J.Anal. Chem., 1996, ionization, more electron multiplication and a higher plasma 355, 853. density. Consequently, also the ionization of copper, the copper 9 Bogaerts, A., Gijbels, R., and Goedheer, W. J., Anal. Chem., 1996, 68, 2296. ion density and the predicted analyte peaks in the mass 10 Bogaerts, A., van Straaten, M., and Gijbels, R., J. Appl. Phys., spectrum no longer increase, and a further gain in analytical 1995, 77, 1868. sensitivity is not expected. Therefore, under the present dis- 11 Bogaerts, A., and Gijbels, R., Anal. Chem., 1996, 68, 2676. charge conditions of 1000 V, 1 Torr and about 2 mA, it is 12 Bogaerts, A., PhD Dissertation, University of Antwerp, 1996. expected that a glow discharge cell with length and radius 13 Bogaerts, A., Wagner, E., Smith, B. W., Winefordner, J. D., both equal to 2 cm would yield the best analytical performance. Pollmann, D., Harrison, W. W., and Gijbels, R., Spectrochim. We have demonstrated that the modeling network is able Acta, Part B, 1997, 52, 205. to study the influence of cell dimensions on dierent plasma 14 Bogaerts, A., Guenard, R. D., Smith, B. W., Winefordner, J. D., Harrison, W. W., and Gijbels, R., Spectrochim. Acta, Part B, 1997, quantities. The models can, in principle, also be applied to 52, 219. specific cell geometries, and they can therefore be useful when 15 Bogaerts, A., and Gijbels, R., Spectrochim. Acta, Part B, in developing new cells, for predicting trends in the plasma the press. conditions and in the application results. 16 van Straaten, M., Gijbels, R., and Vertes, A., Anal. Chem., 1992, 64, 1855. 17 Vieth, W., Huneke, J. C., Spectrochim. Acta, Part B, 1991, 46, 137. A. Bogaerts is financially supported by the Flemish Foundation for Scientific Research (FWO). The authors also acknowledge Paper 6/08262I financial support from the Federal Services for Scientific, Received December 9, 1996 Technical and Cultural Aairs (DWTC/SSTC) of the Prime Minister’s Oce through IUAP-III (Conv. 49). Accepted February 28, 1997 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 759

ISSN:0267-9477    

DOI:10.1039/a608262i

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Transmission of X-rays From an Extended X-ray Source Through Parallelbore Glass Capillary Waveguides: Implications for the Design of a Laboratory X-ray Microprobe NORMAN R. CHARNLEYa AND PHILIP J. POTTS*† b aDepartment of Earth Sciences, University of Oxford, Parks Road, Oxford, UK OX1 3PR bDepartment of Earth Sciences, T he Open University, Walton Hall, Milton Keynes, UKMK7 6AA Results are presented from a ray tracing program, used to to-capillary distances are varied. An interpretation of these model the transmission eciency of parallel-bore glass transmission characteristics was given in terms of the finite capillaries configured as X-ray waveguides and coupled to number of reflections supported by the capillary.There are X-ray tube sources of finite size. A detailed explanation of the clearly some limitations in models based on idealised point transmission mechanisms is given in terms of regions of the sources and the purpose of the present paper is to extend these source that contribute to the transmission of rays.Results modelling studies to optimise parameters for the transmission (Fig. 7) predict that the highest flux is observed when the of rays through parallel-bore capillaries from X-ray sources of capillary is in contact with the source. As the capillary is finite size, representing, for example, the anode of a convenmoved away from the source, transmitted intensities first fall tional X-ray tube. Synchrotron beam lines, which have near o, and then rise again as a larger area of the source is able to parallel beams of low divergence, are not considered here.contribute rays for transmission. Intensities continue to rise Since our original paper was written, several other groups with increase in the source-to-capillary distance until the point have published work concerning the transmission of X-rays is reached where the entire area of the source can contribute to through glass capillaries, some using ray-tracing modelling the transmission characteristics.Transmitted intensities then procedures. Vincze et al.4 described the results from a computer begin to fall o rapidly. The results indicate that transmission ray-tracing code in which the shape of capillary reflection target tubes may have some favourable characteristics in this surfaces was defined numerically, allowing considerable flexiapplication and that the transmission eciency of bility in modelling a range of dierent types of capillary.The polycapillary (Kumakhov) lenses should not be seriously ray tracing code used random numbers to plot the trajectory compromised, provided that the source size, in relation to of rays and could be applied to both spiralling rays (that is, source-to-lens distance, is optimised. rays with trajectories outside the plane of a longitudinal section of the capillary) and to extended sources. Results in comparison Keywords: Ray tracing; X-ray microprobe; glass capillary; with experimental measurements on the transmission charac- waveguide; optimisation; microfluorescence teristics of parallel- and tapered-bore capillaries showed excellent fit for point sources but dierences became significant There is considerable topical interest in the use of glass when applied to extended sources.The code was used to model capillaries as ‘waveguides’ for X-ray fluorescence microfocusing the eect of surface roughness, to optimise the dimensions of devices.The flux of X-rays from a suitable source transmitted conical capillaries (for source-to-capillary distances of 5 cm through the capillary is enhanced by the total external reflec- and 19 m) and to compare ellipsoidal with conical capillaries. tion of rays from the internal walls of the glass capillary. The Cargill et al.5 described experiments with straight and tapered beam emerging from the exit end of the capillary may then be glass capillaries for 5–25 keV X-rays.Of specific relevance to used as an X-ray microprobe to undertake XRF microanalysis. the present work, they described the results from a ray-tracing Interest has arisen in the performance of parallel-bore capillar- program based on a Monte Carlo algorithm to investigate the ies, tapered-bore (conical) capillaries, capillaries with elliptical eects of source-to-capillary distance and source size on the internal bore, microchannel arrays and polycapillary ‘concen- transmitted X-ray flux from straight and narrow capillaries.trators’ (the Kumakhov lens). The capability of these devices Some of their simulations match work presented in the present in comparison with other grazing incidence focusing devices paper, but they caution that their results must be viewed has been summarised recently by Bilderback and Thiel1 and qualitatively and cautiously since their two-dimensional simu- Janssens et al.2 Sources to which these devices have been lations were rigorously corrected only for on-axis sources.coupled include synchrotron X-ray beam lines and more Dozier et al.6 used a set of ray tracing codes based on Monte conventional X-ray tubes and rotating anode instruments. Carlo simulations and discrete ray input patterns to model the In the original paper by the present authors,3 a ray tracing behaviour of glass capillaries coupled to point, line and disc modelling procedure was used to investigate parameters sources.Some of their conclusions do not agree with the results important in optimising the configuration of glass capillaries presented in this work, although this may be because their coupled to X-ray sources. This original paper described the work covered a range of X-ray energies (1–17.5 keV) whereas transmission characteristics of both parallel- and tapered-bore our work is restricted to modelling at 8 keV. Brewe et al.7 capillaries coupled to a theoretical point X-ray source, with described a technique for the fabrication of long glass capillar- particular interest in the transmission behaviour when source- ies, measured the transmission eciency of selected capillaries and compared results with data calculated using a simulation program.However, these data are mainly relevant to relatively † Currently on study leave at the Department of Geology, The Australian National University, Canberra, ACT 0200, Australia. long capillaries used on synchrotron beam lines, a source Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 (761–767) 761application not considered in the present work. Carpenter purposes of understanding the transmission behaviour of capillaries in the present work, reflection losses have been ignored et al.8 described a high-resolution X-ray microfluorescence imaging instrument and presented some ray tracing results on in initial calculations. Subsequent calculations showed that reflection losses reduced transmitted intensities (as expected), the improvement in transmission eciency when the capillary is moved closer to the focal spot of an X-ray tube.but did not aect the qualitative understanding of transmission characteristics presented here. This conclusion may have been Other recent work demonstrating the wide interest in this technology includes that of Rath et al.,9 who described an influenced by the fact that modelling was undertaken for relatively short, parallel-bore glass capillaries.If applied to automated test system for measuring X-ray transmission through glass polycapillaries, Yiming et al.,10 who described tapered-bore capillaries or very long parallel capillaries that support a large number of reflections, it would be appropriate further work with polycapillary lenses and Attaelmanan et al.,11 who described an ellipsoidal capillary optics instrument that to re-examine the applicability of this simplified model.A further assumption made here is that the flux of rays per unit oered better flexibility in how close the sample must be placed to the capillary exit to retain satisfactory resolution. area per unit solid angle from the source is constant. Since only rays with a small angle of divergence of ±Hc with respect The present paper, therefore, extends the earlier modelling studies of Charnley et al.3 to describe the intensity transmitted to the capillary axis are capable of being transmitted by the capillary, this assumption is considered to be reasonable. through parallel-bore capillaries from X-ray sources of finite size rather than just considering the behaviour of point sources.Finally, all modelling calculations have been undertaken in two dimensions. As in the work of Cargill et al.5 and several These calculations are more complex than those for a simple point source and must take into account the transmission other previous workers, the contribution of rays that spiral down the capillary has not been accounted for in this work.characteristics of o-axis as well as on-axis regions of the source. The overall aim here is to give a complete understand- In fact, this simplification probably represents an important limitation of the model, certainly as far as estimating the total ing of these transmission characteristics and to model the behaviour of transmitted rays suciently well to optimise transmission intensity is concerned.However, the interpretation of results in this work has been restricted to an evaluation design parameters for the construction of an X-ray microprobe based on an X-ray tube source. of relative intensities, and wherever possible, results of the ray-tracing calculations have been supported by qualitative interpretations based on the total-reflection behaviour of MODELLING STUDIES X-rays. In order to simplify calculations, modelling studies have been divided into separate parts.First, the behaviour of a point Change in Transmission Intensity From a Point Source as it is source has been modelled, as it is moved o the axis of the Moved O-axis capillary to a cut-o position where rays from the source can no longer be transmitted through the capillary. Second, a The aim of this first set of calculations is to identify the transmission characteristics of X-rays from a point source circular source of finite size (diameter) has been divided into annuli and the transmission contribution from each annulus starting from a position on the axis of a parallel-bore glass capillary and then moving o-axis in an orthogonal direction modelled as a function of source-to-capillary distance.Finally, data are derived for the complete transmission characteristics in 1 mm steps. These data are required subsequently to allow integration of the transmission intensity from the entire area of a source of finite size, again as a function of source-tocapillary distance.of an extended source. Calculations were undertaken as follows. Take a point source oset from the axis of a parallel-bore As in the earlier paper, it has been assumed that total reflection of an X-ray will occur from a glass–air boundary if capillary (of radius Ri) by distance d and calculate the angle representing: (i) the upper angle Hu, defined as the angle of a the angle of incidence of the ray is less than, or equal to, the critical angle (Hc).For this work, the critical angle has been ray projected from the point source subtended at the upper lip of the capillary, (ii) the lower angle Hl, measured as the taken to have a value of 0.005 rad, representative of X-ray photons of energy 7.6 keV. Rays interacting with the glass corresponding angle that is subtended at the lower lip of the capillary (see Fig. 1). If Hu or Hl are greater than the critical surface at a greater angle are assumed to be absorbed or scattered, but not transmitted by total external reflection. The angle (Hc), their values are reassigned the value of Hc since it is assumed that rays striking the wall of the capillary at an maximum intensity of rays that can, therefore, be transmitted by a parallel-bore capillary is represented by a cone of solid angle greater than Hc cannot be transmitted by total external reflection [Fig. 1(a)]. If the oset of the point source from the angle 2Hc aligned symmetrically on the axis of the capillary.In fact, this model of a sharp cut-o angle for total reflection axis of the capillary exceeds the radius of the capillary, transmission by reflection o the upper surface of the capillary is a simplification of the observation that the total reflection cut-o occurs progressively over a small range of angles. When bore is not possible, by straightforward consideration of the geometry [Fig. 1(b)]. In these circumstances Hu (which will considering the reflection of a monochromatic beam of X-rays near the critical angle, it is necessary, therefore, to consider a then have a negative value) represents a ray that just grazes the upper lip of the capillary and is reflected o the lower range of cut-o angles about the mean value with transmitted intensities for a specific angle being weighted by probability of surface and Hl equals the critical angle [Fig. 1(b)]. Clearly, as the oset of the source is increased further, a point is reached transmission based on experimental measurements.This more complicated approach was not thought to be justified in this where Hu=Hl=Hc. Beyond this cut-o point, the divergence of all rays entering the capillary orifice will be too great for work because the simple model was considered to be adequate for understanding the transmission mechanism using extended any reflection to occur and transmission through the capillary is extinguished. In all cases, the model calculated the sum of X-ray sources.However, the net eect will be to blur the edges of any ray tracing results that apparently give rise to a sharp Hu and Hl, that is, the angular range within which reflection can occur [note that when d>Ri, Hu has a negative value as cut-o boundary using the simplified model. Several estimates have been made of reflection losses during noted above, Fig. 1(b)]. The square of this angular range is then calculated to represent the solid angle of the cone of total external reflection; Stern et al.,12 for example, reported its magnitude as 6% per reflection.Reflection losses can, X-rays transmitted through the capillary, reflection losses being ignored. The results of calculations for a capillary of radius therefore, reduce significantly the intensity of rays transmitted through a capillary by multiple reflection. However, for the 50 mm, 50 mm long, for source-to-capillary distances varying 762 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12oset ratio representing 0 Ri source on-axis 0.51 Ri source at the centre of an annulus of radius 0.50 Ri to 0.52 Ri 0.99 Ri source at the centre of an annulus of radius 0.98 Ri to 1.00 Ri 1.00 Ri source at the centre of an annulus of radius 0.99 Ri to 1.01 Ri 1.01 Ri source at the centre of an annulus of radius 1.00 Ri to 1.02 Ri 1.51 Ri source at the centre of an annulus of radius 1.50 Ri to 1.52 Ri 2.01 Ri source at the centre of an annulus of radius 2.00 Ri to 2.02 Ri Interpretation of data in Fig. 2 is best undertaken with the Fig. 1 Schematic diagram of the transmission characteristics of a help of Fig. 3. Considering first a point source on-axis as the point source placed o-axis with respect to a parallel-bore capillary. source-to-capillary distance (s) is increased from zero up to the The largest cone shows the full solid angle (±Hc) that could be critical distance (as defined below) [Fig. 3(a)], a constant flux transmitted by total external reflection.(a) represents the source oset at which only part of the upper limb of the cone (having a maximum of X-rays will be transmitted by the capillary (a to b in Fig. 2), angle of divergence of Hu) falls within the capillary orifice. (b) represents because the entire ±Hc cone available for transmission falls a greater source oset at which the upper surface of the capillary within the capillary orifice. The critical distance represents the cannot contribute to transmission and the value of Hu is now negative.source-to-capillary distance at which rays from the source just In this and subsequent diagrams, the full ±Hc cone of rays that could subtend an angle Hc at the entrance lips of the capillary and be transmitted by the capillary is denoted by a single arc, whereas the corresponds to point (b) in Fig. 2. Beyond this distance, e.g., cone that is transmitted (taking into account geometric considerations) is denoted by a solid arc.All dimensions and angles are exaggerated position (c) in Fig. 3(a), a progressively smaller fraction of the for clarity. ±Hc cone strikes the capillary entrance leading to a continuing reduction in transmitted intensities, as represented by line b to c in Fig. 2. For a point source oset by a distance of less than the capillary radius [Fig. 3(b)], the trend is slightly dierent as shown by data for an oset of 0.51Ri in Fig. 2. For small source-to-capillary distances, the full ±Hc cone strikes the inner walls of the capillary and is available for transmission (a to d in Fig. 2). At slightly larger distances [between d and e in Figs. 2 and 3(b)], beyond the first critical distance for this configuration [represented by position d in Fig. 3(b)], only part of the upper Hc limb strikes the capillary and total transmission is reduced compared with the on-axis source (i.e., d to e in Fig. 2). At the second critical distance [e in Figs. 2 and 3(b)], the lower cone just clips the lower lip of the capillary. Fig. 2 Computed transmission characteristics of a point source oset from the axis of a parallel-bore capillary by specified amounts for source-to-capillary distances of 0–60 mm. Data are plotted for point source osets of 0Ri, 0.51Ri, 0.99Ri, 1.00Ri, 1.01Ri, 1.51Ri and 2.01Ri, corresponding, for a 50 mm radius capillary, to osets of 0, 25.5, 49.5, 50.0, 50.5, 75.5 and 100.5 mm. Letters denote configurations shown in Fig. 3. between 0 and 60 mm are shown in Fig. 2. To allow this diagram to be used generally for any capillary diameter and source oset distance, graphical data are presented for the oset distance from the capillary axis ratioed to the capillary radius (Ri). Thus, for the capillary modelled here, data for an oset ratio of 0.51 represent the behaviour of a source oset by a distance of 0.51×50=25.5 mm from the axis of the 50 mm radius parallel-bore capillary. The values of the osets plotted in Fig. 2 have been chosen as examples representing the centre of 1 mm width annuli on the surface of an extended source as Fig. 3 Schematic diagram showing the cone of rays transmitted from required in the next set of calculations. Data for an oset ratio a point source through a parallel-bore capillary, when the point source of 0.51 represent the centre of an annulus bounded by an inner is moved away from the capillary orifice. Schematic data are shown radius 0.50×50=25 mm and an outer radius of 0.52×50= for (a) an on-axis point source, (b) a point source oset by about 0.51Ri and (c) a point source oset by 1.51Ri. 26 mm. The other data plotted in Fig. 2 are as follows: Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 763At larger source-to-capillary distances, a cone of rays equival- To calculate the contribution of an individual annulus of an extended source to the intensity transmitted by a capillary, the ent in size to that from an on-axis source is available for transmission so that the transmission curve rejoins the original area of that annulus has been multiplied by the intensity transmitted from a point source oset from the capillary axis trend (e to c in Fig. 2). In this context, the first critical distance represents the source-to-capillary distance for a point source by a distance corresponding to the mean radius of the annulus in question. This calculation is shown schematically in Fig. 4. placed o-axis at which a ray emanating from the source subtends an angle of Hc at the upper lip of the capillary and The results of such calculations are shown in Fig. 5 for extended sources placed at distances of 0, 5, 10, 15, 20 and the second critical distance corresponds to the situation where the ray subtends an angle of Hc at the lower lip of the capillary 30 mm from a capillary of radius 50 mm and length 50 mm. Transmitted intensity data are plotted in Fig. 5 for successive (or vice versa).The case of a point source oset by a distance greater than annuli, 1 mm wide, the distance of the mean radius of the annulus from the capillary axis being plotted on the horizontal the radius of the capillary is illustrated by data for an oset of 1.51Ri in Fig. 2, the configuration of which is shown scale. This diagram is, in fact, generally applicable to any capillary of radius Ri by using the alternative scale for source diagrammatically in Fig. 3(c). No transmission at all is possible until the source is moved further from the capillary than the oset calibrated in units of Ri. For these calculations, it has been assumed that the source is ‘infinitely’ large in area so first critical distance [f in Figs. 2 and 3(c)]. At this point, the lower limb of the cone just grazes the upper lip of the capillary. that its size does not restrict transmitted intensities. The vertical scale is the relative transmitted intensity calculated as (planar As the point source is withdrawn further from the capillary, an increasing fraction of the lower cone becomes available for angle of transmittable rays)2×(area of annulus/p).Interpretation of these graphical data depends in part on transmission up to the second critical distance [g in Figs. 2 and 3(c)]. Beyond this distance, all rays striking the capillary the transmission characteristics of the capillary, considered above. The region in front of the capillary can be divided into orifice can be transmitted and the trend then follows that for an on-axis point source (g to c in Fig. 2). three regions, as shown in Fig. 6. Region 1 is represented by a cone projected from a point source placed on-axis at the When the source oset equals the radius of the capillary (data for 1.0Ri in Fig. 2), the fraction of rays transmitted by critical distance from the capillary (where the extremities of this cone subtend an angle equal to the critical angle at the the capillary remains constant as the source is moved further away from the capillary as only half the cone available for lip of the capillary).All rays projected towards the capillary orifice from any area of an extended source that fall within transmission strikes the capillary orifice (h to j in Fig. 2), until the source has been withdrawn to the point where the lower Region 1 can be transmitted by the capillary. The second lobe just grazes the lower limb of the capillary. Beyond this point, all rays striking the capillary orifice are capable of being transmitted and the transmitted intensity then follows the main trend for an on-axis source (j to c in Fig. 2). One way in which additional confidence can be given to the reliability of the computer calculations is to examine data for a source oset of 0.51Ri in Fig. 2 and calculate by simple geometry the source-to-capillary distance represented by points d and e in Fig. 2 [that is, the first (s1) and second (s2) critical distances].Simple geometry shows that s1=(Ri-d)/ tan(Hc) and s2=(Ri+d)/ tan(Hc), where d is the oset (=0.51Ri), Ri Fig. 4 Schematic diagram showing the surface of an extended source the radius of the capillary (=50 mm) and Hc the critical angle divided into annuli (of width 1 mm) as the basis of the model used to (=0.005 rad). Substituting these values, s1=4.9 mm (point d) compute the contribution made by individual annuli to the trans- and s2=15.1 mm (point e), distances that agree with the mission of rays through the capillary.For clarity, the face of the source computed data plotted in Fig. 2. has been turned through 90° towards the viewer and distances between successive annuli have been exaggerated. Profile of Transmitted Intensities From an Extended Source Having calculated the transmitted intensities derived from point sources, oset at various distances from the axis of a capillary, it is now possible to calculate the contribution made to the total transmitted intensity from dierent annuli of an extended source.The model used here is to divide the surface of an extended source into annuli of width 1 mm. It can be shown by simple geometry that the area of successive annuli follows a simple progression, such that if the annuli are numbered from the centre outwards, the corresponding areas are as listed in Table 1. Table 1 Area of successive annuli, each of which increases in radius by 1mm Annulus Area* Fig. 5 Computed transmission of rays from extended sources placed 1 1.pa2 2 3.pa2 at dierent distances from the entrance orifice of a parallel-bore capillary. The horizontal scale represents the distance of the mean 3 5.pa2 4 7.pa2 radius of an annulus from the axis of a 50 mm radius capillary.This diagram may be applied to a capillary of any size by substituting the 5 9.pa2 alternative horizontal axis scale labelled in units of Ri. Data are plotted for sources placed at 0, 5, 10, 15, 20 and 30 mm distances from *pa2 is the area of the first annulus represented by a circle of radius 1 mm.the capillary. 764 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12radius 2Ri (100 mm) lie within Region 2 (partial transmission). There can be no transmission from areas beyond 2Ri. Extended sources placed at greater distances from the capillary follow a similar trend except that transmission occurs from an increasingly large area of the source which falls into Region 2.The transmission intensity is then a compromise between the larger area of the source that can contribute rays for transmission through the capillary and the smaller solid angle that is then subtended at the capillary orifice. An interesting rule-of-thumb taken from Fig. 5 is that the annulus making the maximum contribution has a radius of about one third of the maximum that can contribute to the transmission of rays through the capillary.Fig. 6 Schematic diagram dierentiating the transmission characteristics of regions in front of a parallel-bore capillary. The full (±Hc) cone of rays emitted from any part of a source that falls within Region Total Transmission Intensities From Extended Sources at 1 (which also extends inside the capillary) can be transmitted through Various Source-to-capillary Distances the capillary, whereas only a reduced fraction of this cone can be transmitted from regions of the source that fall into Region 2.Rays Following the general procedure used to calculate data in from areas of the source that fall outside Regions 1 and 2 cannot be Fig. 5, it is now a relatively simple task to calculate the total transmitted through the capillary by total external reflection. transmission from an extended source of finite size as the source-to-capillary distance is changed. In principle, the total region (Region 2) corresponds to the truncated cone which transmitted intensity (for a specified distance) is the area under diverges out from the lips of the capillary at an angle of the appropriate curve plotted in Fig. 5. Total transmission divergence equal to the critical angle (but excluding Region 1). data for a glass capillary of radius (Ri) 50mm and length Only a fraction of rays emanating from any area of an extended 50 mm have been calculated for sources of 25, 50, 100, 150, source that falls within Region 2 can be transmitted by the 250, 500 and 1000 mm radius using source-to-capillary distances glass capillary.This fraction varies and can be calculated from of 0–200 mm. These data are plotted in Fig. 7. Again, this the solid angle of rays striking the capillary orifice, divided by diagram can be applied generally to any source/capillary the full 2Hc solid angle. The third region corresponds to all combination where source diameter is 0.5Ri, 1Ri, 2Ri, 3Ri, areas outside Region 2. None of the rays emanating from an 5Ri, 10Ri and 20Ri.extended source that lies in this third region can be transmitted The form of the curves in Fig. 7 can be explained in by the capillary because their angle of divergence with respect conjunction with diagrams shown in Fig. 8 as follows: to the capillary wall exceeds the critical angle. As can be For a source of radius 0.5Ri, with reference to Fig. 8(a), all seen from Fig. 6, at small source-to-capillary distances, only a rays projected towards the capillary with a divergence of ±Hc relatively small area of the source can be ‘seen’ by the capillary, can be transmitted at source-to-capillary distances of 0–5 mm and at zero source-to-capillary distance this area reaches a [corresponding to Fig. 8(a), points a and b], hence the constant minimum, equal in size to the capillary orifice. As the source- transmitted intensities in Fig. 7 out to 5 mm (also labelled a to-capillary distance is increased, two opposing factors aect and b).At distances of 5–10 mm, an increasing proportion of transmission intensities. First, a larger and larger area of the the source area falls within Region 2 (Fig. 6), and all of it at source can contribute to transmission. However, second, distances greater than 10 mm, so that transmitted intensities beyond the critical distance, only a decreasing fraction of the are then reduced in accord with the reduction in the solid possible maximum 2Hc cone that could be transmitted by the angle of the cone subtended at the capillary orifice, which capillary will actually fall within the capillary orifice.follows the inverse square law. With these transmission characteristics in mind, the form of For a source of radius 1Ri, calculations according to the data plotted in Fig. 5 (the transmitted intensity from successive model show that the maximum intensity is transmitted at a annuli each having a width of 1 mm) can now be explained. source-to-capillary distance of 0 mm [source touching the For an extended source at s=0 mm (i.e., touching the capillary capillary orifice, e in Figs. 7 and 8(b)]. This transmitted orifice), all rays projected towards the capillary at an angle of divergence of Hc can be transmitted, provided that they originate from the area of diameter 2Ri that coincides with the capillary orifice. Any rays emanating from regions of the source outside this area cannot enter the capillary orifice. The shape of the curve for s=0 mm (Fig. 5) increases linearly with the increase in area of the corresponding annulus (Table 1) up to an oset of Ri, the cut-o corresponding to the radius of the capillary. Data for a source placed at s=5 mm from the capillary show that the central annuli (which all lie in Region 1) have, as expected, the same transmission characteristics as the source in contact with the capillary. However, annuli of radius 0.5Ri (25 mm) to 1.5Ri (75 mm) lie in Region 2, where only partial transmission of rays propagated towards the capillary orifice can occur and there is a fall o in transmitted intensities compared with the source at s=0 mm.The cut-o now occurs Fig. 7 Transmission intensities of sources of finite size as a function at 75 mm, the oset distance beyond which all rays have too of source-to-capillary distance. Data are plotted for sources of radius high a divergence (>Hc) for transmission. 0.5Ri, 1Ri, 2Ri, 3Ri, 5Ri, 10Ri and 20Ri, which for the 50 mm radius An extended source placed at s=10 mm from the capillary source modelled here corresponds to source diameters of 50, 100, 200, lies at the critical distance (for a point source) for this configur- 300, 500, 1000 and 2000 mm.Letters correspond to source–capillary configurations marked on Fig. 8. ation. None of the source lies in Region 1, but annuli up to a Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 765that the distance at which the fall o commences varies with source size as follows: Source diameter Source fills transmission cone at: 100 mm (1Ri) 0mm 200 mm (2Ri) 10mm 300 mm (3Ri) 20mm 500 mm (5Ri) 40mm 1000 mm (10Ri) 90mm 2000 mm (20Ri) 190 mm The other interesting aspect of data presented in Fig. 7 is that even for the largest source investigated, computations show that the intensity transmitted never exceeds the intensity observed when the source is in contact with the capillary and this aspect is commented on below.IMPLICATIONS FOR THE DESIGN OF AN X-RAY MICROPROBE Interpretation of data in Fig. 7 gives a number of interesting results that are important when considering the design of an X-ray microprobe incorporating a parallel-bore capillary waveguide. (a) Although maximum transmission is observed when the capillary is in contact with the source, it is clearly impractical (and indeed self-defeating) to position a glass capillary in contact with the anode of a conventional X-ray tube.However, the shape of the curve in Fig. 7 indicates that optimum transmission intensities are likely to be achieved at greater, rather than shorter, distances. For example, for a 50 mm radius capillary coupled to a 250 mm (5Ri ) source, source-to-capillary distances of 40 mm will oer greater intensities with less sensitivity to position than smaller source-to-capillary distances. (b) If the above recommendation is followed, the diameter of the X-ray source should be significantly larger than the diameter of the glass capillary.However, there is little benefit in increasing the diameter much above about ten times that Fig. 8 Schematic diagram showing the source–capillary configur- of the capillary unless very large source-to-capillary distances ations for selected sources, data for which are plotted in Fig. 7. The source sizes plotted here are (a) 0.5Ri, (b) 1Ri and (c) 5Ri . The regions (>90 mm) are to be used. correspond to those delineated in Fig. 6. (c) In the design of excitation systems using a polycapillary (Kumakhov) lens, where practical considerations mean that it is impossible to position a bundle of capillaries very close to the X-ray source, data in Fig. 7 indicate that little reduction intensity is four times greater than that from the previous in transmitted intensities will be observed by the need to source (0.5Ri), a factor that is in proportion to the dierence position the lens at larger distances from the source, provided in areas. As this source is moved further away from the that the diameter of the source is suciently large.capillary, transmitted intensities again fall o for the same (d) Finally, one of the most interesting implications of data reasons as explained for the 0.5Ri source. in Fig. 7 concerns the nature of the excitation source. A Taking a source radius of 5Ri, as an example of a source conventional side window X-ray tube arrangement may not larger than 1Ri, the transmitted intensity at 0 mm [position e be the optimum for capillary waveguide excitation since the in Fig. 8(c)] is identical with that for the 1Ri source since rays maximum transmitted intensities are observed when the source propagated from regions outside a diameter of 1Ri at this touches the capillary. Rather, there may be some advantage in distance cannot contribute to the transmitted beam. As the the transmission target tube, where the capillary can be source is moved away from the capillary, the intensity falls o arranged to touch one side of a thin foil anode, the other side down to a minimum [position g in Figs. 7 and 8(c)], but this of which is excited by bombardment with electrons. fall o in intensity is not as great as that for the 1Ri source Further modelling is now being undertaken to investigate the because the larger area of the source in Region 2 (Fig. 6) can characteristics of tapered glass capillaries in this application.contribute to transmitted intensities. When the source is withdrawn further away than the critical distance, computations The initial discussions (including ‘back-of-the-envelope’ calcu- show that transmitted intensities increase, presumably because lations) with J.V.P. Long (University of Cambridge), that the larger area of the source ‘visible’ to the capillary more formed the basis of this work, are gratefully acknowledged. than compensates for the reduced solid angle from any particular region that can contribute to transmission. This increase in transmitted intensities continues up to point i [Figs. 7 and REFERENCES 8(c)], which corresponds to the distance from the capillary at 1 Bilderback, D. H., and Thiel, D. J., Rev. Sci. Instrum., 1995, which the source just fills the cone of rays that can contribute 66, 2059. to transmission. At greater distances [e.g., point j in Figs. 7 2 Janssens, K., Vincze, L., Rubio, J., Adams, F., and Bernasconi, G., and 8(c)], the source is not suciently large to fulfil its J. Anal. At. Spectrom., 1994, 9, 151. maximum transmission potential. Computations for all the 3 Charnley, N. R., Potts, P. J., and Long, J. V. P., J. Anal. At. Spectrom., 1994, 9, 1185. sources greater in size than 1Ri follow the same trend, except 766 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 124 Vincze, L., Janssens, K., Adams, F., and Rindby, A., X-ray 10 Yiming, Y., Xunliang, D., Dachun, W., Baozhen, C., Shengji, Z., Spectrom., 1995, 24, 27. and Andong, L., SPIE, 1994, 2321, 56. 5 Cargill, G. S., III, Hwang, K., Lam, J. W., Wang, P.-C., Liniger, E., 11 Attaelmanan, A., Voglis, P., Rindby, A., Larsson, S., and and Noyan, I. C., SPIE, 1995, 2516, 120. Engstro�m, P., Rev. Sci. Instrum., 1995, 66, 24. 6 Dozier, C. M., Newman, D. A., Gilfrich, J. V., Freitag, R. K., and 12 Stern, E. A., Kalman, Z., Lewis, A., and Lieberman, K., Appl. Kirkland, J. P., Adv. X-ray Anal., 1994, 37, 499. Opt., 1988, 27, 5135. 7 Brewe, D. A., Heald, S. M., Barg, B., Brown, F. C., Kim, K. H., and Stern, E. A., SPIE, 1995, 2516, 197. Paper 6/06885E 8 Carpenter, D. A., Taylor, M. A., and Lawson, R. L., J. T race Received October 8, 1996 Microprobe T ech., 1995, 13, 141. Accepted March 20, 1997 9 Rath, B. K., Youngman, R., and MacDonald, C. A., Rev. Sci. Instrum., 1994, 65, 3393. Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 7

ISSN:0267-9477    

DOI:10.1039/a606885e

出版商:RSC    

年代:1997

数据来源: RSC