摘要:

Method to Screen Urine Samples for Vanadium by Inductively Coupled Plasma Mass Spectrometry With Cryogenic Desolvation MICHAEL G. MINNICHa, R. S. HOUK*a , MARK A. WOODINb AND DAVID C. CHRISTIANIb aAmes L aboratory, US Department of Energy, Department of Chemistry, Iowa State University, Ames, IA 50011, USA bOccupational Health Program, Harvard School of Public Health, Boston, MA 02115, USA The determination of vanadium by inductively coupled plasma Most previous papers on vanadium measurements describe methodology for quantification of vanadium and employ mass spectrometry is complicated by ClO+ ions from chlorine matrices. Cryogenic desolvation reduces the amount of chloride chemical preconcentration and/or matrix removal procedures. 5,6 The present work describes a rapid method to reaching the plasma by condensing it as hydrogen chloride and reduces the amount of oxide formation by removing water screen urine samples for vanadium at the 12 ppb level without preconcentration.About half of the samples are from workers vapor. Urine samples are screened for vanadium at the 12 ppb level using scandium as an internal standard. The V+ and Sc+ exposed to vanadium. The remaining samples are from controls believed to be unexposed. For this application, a rapid method signals are aVected by the matrix in the same way, so the V+/Sc+ signal ratio corrects for signal suppression by the is needed to determine if vanadium levels are above the acceptable exposure levels; accurate quantification is not neces- matrix, as well as drift.Cryogenic desolvation also removes ArCl+, which should facilitate measurement of arsenic and sary here. The samples are simply diluted with aqueous 1% HNO3 and a scandium internal standard. The ratio of V+ selenium. signal to Sc+ signal obtained from a standard solution contain- Keywords: Inductively coupled plasma mass spectrometry; ing equal concentrations of the two elements is compared with cryogenic desolvation; vanadium; urine the V+/Sc+ signal ratio obtained from the urine samples.The vanadium concentration in each urine sample is determined to be either above or below the screening concentration Sources of vanadium exposure include industrial processes determined by the V+/Sc+ signal ratio from the standard which use vanadium as a catalyst and as an alloying agent in solution. As long as V+ and Sc+ signals are aVected in the steel, welding and metal plating processes, and the combustion same way by the matrix, this method accounts for diVerent of fuel oils.1,2 Vanadium is present in minute amounts in the amounts of signal suppression by various concentrations of typical diet, but the utility of vanadium in humans is not clear.matrix elements in diVerent samples. Since vanadium catalyzes a large number of reactions, the body could use it as a catalyst; however, the systemic toxicity of vanadium at relatively low doses argues against this hypoth- EXPERIMENTAL esis.Because vanadium–oxygen species are strong oxidizing Instrumentation agents, these species inhibit oxidative phosphorylation, in which a phosphate group is added to a compound and plays A Hewlett-Packard (HP) Model 4500 ICP mass spectrometer a key role in preventing cellular loss of metabolites by (Hewlett-Packard, Wilmington, DE, USA) with HP diVusion.3 VO3- is a potent inhibitor of the Na/K ATPase ChemStation software was used. The pneumatic nebulizer and pump which maintains the resting membrane potential of cooled spray chamber provided with the HP 4500 were neurons.3 Direct exposure of the lungs to vanadium leads to replaced by an ultrasonic nebulizer (Model USN 5000, Cetac respiratory symptoms from a mild cough to severe bronchiect- Technologies, Omaha, NE, USA) and a laboratory-designed asis.4 Since individuals vary in their response to vanadium cryogenic desolvation system.8 The nebulizer gas flow rate was exposure, urinary levels above 10 ppb are of interest.regulated by an extra mass flow controller (Matheson, Several recent papers describe the general capabilities of Secaucus, NJ, USA), not through the ChemStation software. various methods for measuring vanadium, such as liquid chromatography, neutron activation analysis, and either Samples and Reagents atomic absorption spectrometry or inductively coupled plasma mass spectrometry (ICP-MS) with electrothermal vaporiz- The urine samples were obtained from humans in an occupational population that participated in an epidemiological ation.5,6 The determination of vanadium by ICP-MS becomes diYcult when the sample matrix contains large amounts of study conducted by the Occupational Health Program at the Harvard School of Public Health.Each sample was identified chlorine.6,7 The diatomic ion 35Cl16O+ overlaps with the most abundant isotope of vanadium at m/z=51. A high resolution only by a number.No history was provided. Elemental standards used were from Plasmachem Associates (Farmingdale, mass spectrometer can separate 51V+ from 35Cl16O+; these instruments work best in conjunction with other methods to NJ, USA). The nitric acid used was Ultrex II Ultrapure reagent grade from J. T. Baker (Phillipsburg, NJ, USA). The 70% reduce the abundance of ClO+. Cryogenic desolvation reduces both the solvent load on the plasma and the abundance of concentrated reagent acid was diluted with de-ionized, distilled water to produce 1% m/v aqueous HNO3. The sodium oxide ions by removing nebulized water.8,9 Chloride is also removed as hydrogen chloride by the same method.7 For urine chloride used was from Fisher Scientific (Fair Lawn, NJ, USA).High-purity water (18 MV cm resistivity) was obtained from a and sea-water certified reference materials, the vanadium concentrations measured using cryogenic desolvation with five-stage Milli-Q Plus Water System (Millipore, Bedford, MA, USA).ICP-MS agree closely with the certified or suggested values.10 Journal of Analytical Atomic Spectrometry, December 1997, Vol. 12 (1345–1350) 1345Procedure Application of the dilution factor yields the vanadium concentration in the original urine sample. If the V+/Sc+ signal ratio The instrument was optimized by nebulizing a 10 ppb solution of the urine sample is greater than that for the standard of Li+, Y+ and Tl+ and adjusting the torch position, lens solution, then the concentration of vanadium in the sample voltages and aerosol gas flow rate to maximize the ion signals.exceeds the screening value. Blanks of the solvent were analyzed Resolution/mass axis and pulse counting/analog detection and did not contain significant amounts of vanadium compared factor were adjusted to HP specifications using the software. with the screening concentration. The instrument operating conditions are shown in Table 1.Data were acquired in the spectrum analysis mode for m/z= 45, 50–60, 72–78 and 203–209. The last two m/z windows were RESULTS AND DISCUSSION measured to look for As and Pb in the samples. Six points Mass Spectra were used to define a peak with an integration time of 0.5 s per point (3.0 s per mass). The total acquisition time was 82 s Vanadium sensitivity achievable with this instrument using per replicate. ultrasonic nebulization and cryogenic desolvation is 4.8×104 Approximately 2 g of each urine sample were diluted to a counts s-1 ppb-1 in a 1% HNO3 matrix and 1.0×104 final volume of 50 ml with 1% HNO3.A Sc+ internal standard counts s-1 ppb-1 in a 0.05% NaCl in 1% HNO3 matrix. Fig. 1 was added so that the concentration of scandium in the shows the mass spectrum for a matrix-matched blank. The solution analyzed was 0.5 ppb. Acidifying the urine sample sensitivity is more than suYcient to measure vanadium at helps keep the organic material in solution, lowers the concen- these levels.However, polyatomic ions and possible impurity tration of solids to reduce deposition on the sampler and ions from the nitric acid prevent a complete correction for skimmer, and provides theH+ necessary to remove the chloride spectral overlap interferences. One such interference is 51V+ as hydrogen chloride. A 25-fold dilution with 1% HNO3 and 35Cl16O+, which appear at the same nominal mass. Using suYces for these tasks. The samples were prepared the day the 351 isotopic ratio for the abundance of 35Cl+ to 37Cl+, before analysis in cleaned polyethylene vessels and refrigerated the signal at m/z=53 could possibly be used to correct for the overnight.No glassware was used in the sample preparation amount of ClO+ at m/z=51. Unfortunately, chromium, a steps. likely impurity in the nitric acid and a possible constituent of A solution of equal concentrations of Sc+ and V+ in 1% the urine sample, has a minor isotope (9.6% abundant) at HNO3 is used to determine the signal ratio of V+/Sc+. The m/z=53.Conceivably, another correction for 53Cr+ could be concentration of this screening solution is determined by determined by measuring 52Cr+, but this peak is too intense dividing the desired screening concentration by the dilution relative to the total signal at m/z=53. The additional signal at factor used in preparing the urine samples. Note that this m/z=52 is 40Ar12C+ from volatile organic compounds in the screening concentration is for the solution as analyzed.previous urine samples, which remain in the spray chamber. After analysis of several urine samples, much of the ArC+ remains even when 1% HNO3 alone is nebulized. This ion Table 1 ICP-MS operating conditions remains a problem with cryogenic desolvation of organic solvents.11 Note that the minor isotope of Cr+ at m/z=50 Plasma operating conditions— Sample uptake rate 2.0 ml min-1 (4.3% abundant) is also obscured by minor isotopes of V+ Forward power 1200 W (0.3% abundant) and Ti+ (5.2% abundant).Reflected power <5 W The source of ArC+ is presumably an organic compound RF matching 1.9 V that is not trapped in the cryogenic loops. This compound is Argon gas flow rates: probably urea, which slowly hydrolyzes in the presence of plasma 15.0 l min-1 acids to NH4+ and CO2.12 Any solid urea in the loops would auxiliary 1.00 l min-1 aerosol 1.35 l min-1 melt at 135 °C before decomposing to NH3 and CO2.Sampling position 3.6 mm from load coil Fortunately, the background at m/z=51 is attenuated by on center cryogenic desolvation substantially,8–10 so that corrections Torch-H -0.7 mm using the signal at m/z=53 are not necessary for this study. Torch-V 2.0 mm In fact, we believe that most of the signal at m/z=51 in Fig. 1 Ion lens settings— is actually vanadium impurity in the NaCl, HNO3 and Extract 1 -140 V de-ionized water. This illustrates the general caveat that it is Extract 2 -70 V better to remove interferences than to correct for them.Einzel 1,3 -100 V Fig. 2 shows a portion of the mass spectrum obtained from Einzel 2 8 V Omega bias -35 V Omega (+) 19V Omega (-) -19 V QP focus 3 V Ion def. 84 V Plate bias 0 V Pole bias 0 V Discriminator 11.8 mV EM voltage -1690 V Last dynode -300 V Mass and detector calibration— AMU gain 134* AMU oVset 179* Axis gain 1.0000* Axis oVset 0 P/A factor 111* P/A factor 111 P/A factor 76* * These are the numerical values listed by the control software for Fig. 1 Mass spectrum of 0.05% NaCl in 1% HNO3. the HP 4500. 1346 Journal of Analytical Atomic Spectrometry, December 1997, Vol. 12HNO3 as a function of time. Time zero was chosen arbitrarily to be the time of the first analysis of the 1% HNO3 blank after instrument warm-up (approximately 45 min) and optimization. The V+/Sc+ signal ratio was calculated from the count rate obtained at each peak maximum. This standard solution was analyzed at various times interspersed between measurement of several urine samples.Table 2 shows that the V+/Sc+ signal ratio is in the range 0.4–0.5, which is odd. Scandium is monoisotopic, vanadium is essentially so, and both elements have similar ionization energies and should be ionized with comparable eYciencies. The ions 45Sc+ and 51V+ also have similar m/z values, so the mass bias eVect should not cause the observed 2-fold diVerence in sensitivity.The basic reasons for this diVerence in sensitivity between V+ and Sc+ are not clear. Table 2 shows that the count rates at the peak maximum Fig. 2 Mass spectrum of the ‘low’ urine sample + 0.5 ppb Sc+ in for V+ and Sc+ both decrease with time. The V+ signal 1% HNO3. remaining at the end of the experiments is only about 5% of the original signal level. Deposition of solid material near the skimmer orifice, which is only 0.4 mm in diameter, most likely produces this result. A small, white solid sphere formed on the outside of the skimmer cone just downstream from the orifice after the analysis. The reader should note that, although the chloride is distilled oV as HCl, the matrix cations such as Na+, K+ and Ca+ remain as nitrate salts.The ultrasonic nebulizer introduces these matrix ions into the plasma at a rate 5–10 times higher than that from a typical pneumatic nebulizer, which also exacerbates deposition of solids. This substantial signal loss also changes the V+/Sc+ signal ratio.Because the V+ sensitivity is approximately half that of Sc+, the per cent decrease in the V+ signal is slightly greater than that for the Sc+ signal which may explain the decline in the V+/Sc+ signal ratio. Fig. 3 Mass spectrum of 0.05% NaCl in 1% HNO3. Spike Recovery A urine sample was collected from a subject whose exposure a urine sample believed to be low in vanadium content. The to vanadium was believed to be low. One aliquot of this ‘low’ magnitude of the signal at m/z=52 with respect to 40Ar14N+ urine sample was prepared by 25-fold dilution with 1% HNO3 and 40Ar16O+ is typical of that in other urine samples.and adding a 0.5 ppb Sc internal standard, as for the other Fig. 3 shows a spectrum in a diVerent mass range for the urine samples. A second and third aliquot were prepared in same matrix-matched blank. ArCl+ at m/z=75 and 77 has the same way as the first, but then spiked with 0.5 and 5 ppb been attenuated so that it is no longer the main ion observed V+, respectively, to measure vanadium recovery.here. Thus, this desolvation method should also be valuable Table 3 shows the results obtained from two separate analy- for the determination of As+ and Se+, which will be ses of each of the three aliquots. Between each group of three discussed below. samples, the 0.5 ppb Sc and V ratio solution was measured. The device was rinsed out with 1% HNO3 for approximately V+/Sc+ Signal Ratio and Drift 60 s between samples.The measured vanadium concentrations in the two sets of results in Table 3 are consistent (0.4 and Table 2 shows the V+ and Sc+ signals and V+/Sc+ signal 2.8–2.9 ppb), but are lower than the spike levels, for reasons ratio obtained for a solution of 0.5 ppb Sc+ and V+ in 1% that are unclear. Table 4 shows similar results obtained for the same ‘low’ Table 2 Signals and signal ratios for V+ and Sc+ from the 0.5 ppb urine samples analyzed at the end of the urine sample analyses standard ratio solution measured as a function of time (approximately 3.5 h).The result for the ‘low’ urine sample with only the 0.5 ppb Sc+ internal standard agrees well with V+ signal/ Sc+ signal/ the result obtained at the beginning of the day (Table 3). This Elapsed counts s-1 counts s-1 V+/Sc+ time/min* (at peak maximum) (at peak maximum) signal ratio is reassuring since the vanadium content of this sample is low. The determined concentration of V is now 0.43 ppb in the 0.5 14 24 800 49 000 0.506 ppb spike solution.Thus, the vanadium recovery is better in 17 24 400 49 000 0.497 40 13 700 28 400 0.483 this case than for the data in Table 4. 56 11 300 23 600 0.478 64 10 600 23 100 0.461 99 5080 12 100 0.420 Vanadium in Urine Samples 123 2940 7060 0.416 163 1410 3260 0.431 Fig. 4 shows a plot of the V+/Sc+ signal ratio in the standard 220 1290 3160 0.408 solution (denoted by the filled circles) and the V+/Sc+ signal ratio obtained for each urine sample (denoted by the open * Time zero was taken as the time of the first 1% HNO3 blank triangles) as a function of time.The urine samples were tested measurement after approximately a 45 min instrument warm-up and optimization. in random order. Recall that the level of interest in the Journal of Analytical Atomic Spectrometry, December 1997, Vol. 12 1347Table 3 Spike recovery at the start of the work for a urine sample with low vanadium content ‘Low’ urine sample in Vanadium concentration in 1% HNO3 V+/Sc+ signal ratio original urine sample (ppb) + 0.5 ppb Sc 0.06* 0.1 + 0.5 ppb Sc, 0.5 ppb V 0.38* 0.4 + 0.5 ppb Sc, 5 ppb V 2.91* 2.9 + 0.5 ppb Sc 0.08† 0.1 + 0.5 ppb Sc, 0.5 ppb V 0.37† 0.4 + 0.5 ppb Sc, 5 ppb V 2.72† 2.8 * The nearest (in time) V+/Sc+ signal ratio from the standard solution was 0.506.† The nearest (in time) V+/Sc+ signal ratio from the standard solution was 0.483. Table 5 V+/Sc+ signal ratio and estimated vanadium concentration Table 4 Spike recovery at the end of the work for the urine sample low in vanadium content in the original urine samples Urine sample V+/Sc+ V+ concentration in ‘Low’ urine sample V+/Sc+ Vanadium concentration in in 1% HNO3 signal ratio original urine sample (ppb) number signal ratio original urine sample (ppb) + 0.5 ppb Sc 0.06* 0.1 187 0.04* 1 195 0.27* 7 + 0.5 ppb Sc and V 0.43* 0.5 186 0.12* 3 184 0.14† 4 * The nearest (in time) V+/Sc+ signal ratio from the standard solution was 0.408. 230 0.33† 10 221 0.23† 7 200 0.12† 4 192 0.34† 10 185 0.40† 12 298 0.12† 4 219 0.06† 2 279 0.09† 3 188 0.19‡ 5 252 0.06‡ 2 296 0.21‡ 6 314 0.24‡ 7 212 0.04‡ 1 193 0.20§ 6 206 0.15§ 5 182 0.44§ 13 * V+/Sc+ signal ratio from standard solution is 0.46. † V+/Sc+ signal ratio from standard solution is 0.42. ‡ V+/Sc+ signal ratio from standard solution is 0.43. § V+/Sc+ signal ratio from standard solution is 0.41. Fig. 4 Comparison of the V+/Sc+ signal ratio between the standard solution (filled circles) and the urine samples (open triangles).spite of these diYculties, ClO+ levels are attenuated to suYcsolutions as analyzed is 0.5 ppb, which is 12 ppb in the original iently low levels that allow determination of vanadium at trace urine sample. Two urine samples have a vanadium concenlevels. From a risk assessment perspective, this method reptration close to the screening level. The vanadium concenresents the worst case scenario, since it assumes that all the trations in the remaining samples are well below the screening signal at m/z=51 is from vanadium in the original sample.concentration. The screening concentration could probably be reduced to about 5 ppb or less in the original samples, if desired. The vanadium concentration in the original urine sample ArCl+, As+ and Se+ can be estimated by multiplying the Sc+ count rate by the V+/Sc+ signal ratio for the nearest measurement of the As shown in Fig. 2, ArCl+ is also attenuated by cryogenic desolvation.7–9 This allows the current method to be extended standard ratio solution. The product gives the count rate expected for 0.5 ppb vanadium. By setting the expected V+ to the determination of As+ and possibly Se+, even though these elements were not the focus of this study. count rate per 0.5 ppb equal to the experimental V+ count rate per unknown concentration, an estimate of the unknown Fig. 5 shows a mass spectrum from a urine sample believed to be low in vanadium content. The determination of vanadium vanadium concentration in the solution as analyzed can be obtained.Multiplying this result by the dilution factor yields in this sample has been discussed previously. For the moment, if we assume that all the signal at m/z=75 (424 counts s-1) is the estimated concentration of vanadium in the original urine sample. Table 5 shows the results of this calculation for each due to 40Ar35Cl+, then the signal from 40Ar37Cl+ at m/z=77 should be one-third of that at m/z=75 according to the 351 urine sample in the order they were analyzed.This calculation yields a quantitative estimate of the vanadium concentration isotopic ratio of 35Cl+ to 37Cl+. Thus, the maximum contribution at m/z=77 due to 40Ar37Cl+ is 141 counts s-1. In this and accounts for mass bias because the urine samples are compared with the V+/Sc+ signal ratios from the standard case, the remaining signal at m/z=77 would be 3300 counts s-1.Interestingly, if all the signal at m/z=74 (400 solution. It should be noted that the background has not been counts s-1) is due to Se+ (0.9% abundant), the signal from Se+ at m/z=77 (7.6% abundant) is estimated to be 3400 subtracted from the V+ signals from the urine samples. Because of the spectral interferences described earlier, an accurate counts s-1, which is still below the total signal measured at m/z=77. However, if the maximum contribution to m/z=77 background correction cannot be obtained for the samples.In 1348 Journal of Analytical Atomic Spectrometry, December 1997, Vol. 12Fig. 5 Mass spectrum of the ‘low’ urine sample + 0.5 ppb Sc+ in 1% HNO3. (3440 counts s-1) is 77Se+ (3400 counts s-1), then only 40 counts s-1 remain from 40Ar37Cl+. Therefore, the signal at m/z=75 due to 40Ar35Cl+ is three times the signal at m/z=77 or 120 counts s-1. In this case, the remaining signal at m/z= 75 (300 counts s-1) is actually due to 75As+, not ArCl+.Fig. 6 shows the mass spectra for two urine samples which contain arsenic. Comparison of the signals at m/z=75 and 77 Fig. 7 Mass spectra of urine samples (a) 230 and (b) 195. with the 351 isotopic ratio for chlorine reveals that there is too much signal at m/z=75 to be due entirely to 40Ar35Cl+. The remaining signal is due to 75As+. Fig. 7 shows the mass spectra for two samples which possibly CONCLUSION contain selenium. In these two cases, as for the ‘low’ urine Cryogenic desolvation has been applied to the analysis of urine sample, the signal at m/z=77 is greater than the signal at samples with a wide range of vanadium concentrations.This m/z=75, so the peaks at these two m/z values are not due method attenuates ClO+ and ArCl+ to levels suYciently low solely to ArCl+. The remaining signal at m/z=77 is possibly that trace levels of vanadium and arsenic can be determined. due to Se+.The presence of Se+ is also indicated by its isotope Urine samples can be screened at the 12 ppb level using a Sc+ pattern at m/z=74 (0.9%) and m/z=76 (9.0%) where there is internal standard and a separate solution containing equal overlap with 40Ar36Ar+, m/z=77 (7.6%) and m/z=78 (23.5%). concentrations of Sc+ and V+ at the desired screening concentration. Semiquantitative measurements can also be extracted from the data. These results suggest that the screening concentration may be lowered, if necessary.An interesting question is whether chloride can be removed as eVectively using membrane desolvation. This will be addressed in a subsequent study.13 The loans of the ICP-MS instrument from Hewlett-Packard and the ultrasonic nebulizer from Cetac Technologies, Inc., are gratefully acknowledged. Ames Laboratory is operated by Iowa State University for the US Department of Energy under Contract No. W-7405-Eng-82. This research was supported by the OYce of Basic Energy Sciences, Division of Chemical Sciences. REFERENCES 1 Hauser, R., Elreedy, S., Hoppin, J., and Christiani, D. C., Am. J. Respir. Crit. Care Med., 1995, 152, 1478. 2 Sjoberg, S. G., AMA Arch. Ind. Health, 1955, 11, 505. 3 Raven, P. H., and Johnson, G. B., Biology, Times Mirror/Mosby College Publishing, St. Louis, MO, 2nd edn., 1989, pp. G-17 and G-21. 4 Lees, R. E. M., Br. J. Ind. Med., 1980, 37, 253. 5 Hastings, D. W., Emerson, S. R., and Nelson, B. K., Anal. Chem., 1996, 68, 371. 6 Iki, N., Irie, K., Hoshino, H., and Yotsuyanagi, T., Environ. Sci. T echnol., 1997, 31, 12. Fig. 6 Mass spectra of urine samples (a) 212 and (b) 314. 7 Tan, S. H., and Horlick, G., Appl. Spectrosc., 1986, 40, 445. Journal of Analytical Atomic Spectrometry, December 1997, Vol. 12 13498 Alves, L. C., Wiederin, D. R., and Houk, R. S., Anal. Chem., 1992, 12 Dictionary of Organic Compounds, ed. Buckingham, J., Chapman and Hall, New York, 5th edn., 5, 1982, pp. 5670–5671. 64, 1164. 9 Pin, C., Telouk, P., and Imbert, J.-L., J. Anal. At. Spectrom., 1995, 13 Minnich, M. G., and Houk, R. S., J. Anal. At. Spectrom., submitted. 10, 93. 10 Alves, L. C., Allen, L. A., and Houk, R. S., Anal. Chem., 1993, Paper 7/00172J 65, 2468. Received January 7, 1997 11 Alves, L. C., Minnich, M. G., Wiederin, D. R., and Houk, R. S., Accepted September 4, 1997 J. Anal. At. Spectrom., 1994, 9, 399. 1350 Journal of Analytical Atomic Spectrometry, December 1997, Vol. 12

ISSN:0267-9477    

DOI:10.1039/a700172j

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Improved Slurry Sampling Electrothermal Vaporization System Using a Tungsten Coil for Inductively Coupled Plasma Atomic Emission Spectrometry PETER BARTH SUSANNE HAUPTKORN AND VILIAM KRIVAN* Sektion Analytik und Ho� chstreinigung Universita�t Ulm D-89069 Ulm Germany vaporizers a much lower current is suYcient to reach very high heating rates and temperatures of up to 3000 °C. Therefore inexpensive and small power supplies are adequate. Sample volumes of up to 40 ml can be pipetted on to the coil and by using the slurry sampling technique the direct analysis of powdered solids is possible. The tungsten-coil vaporizer in combination with slurry sampling has already been applied to the determination of trace impurities in silicon carbide by ETV–ICP-AES18 and silicon dioxide by ETV–ICP-MS.19 Although the experimental set-up used for ETV–ICP-AES worked satisfactorily and provided accurate results,18 improvements concerning power supply data acquisition and processing spectrometer coupling automation and tungsten ablation were necessary.In this paper the improved ETV system is described. In addition the transport losses of several elements were determined by means of the radiotracer technique for diVerent matrices. The application of this system to the direct determination of trace impurities in silicon dioxide and silicon nitride by slurry sampling ETV–ICP-AES is described in a later paper.39 EXPERIMENTAL 2 An improved ETV system for the determination of trace elements in diverse samples by slurry sampling ETV–ICP-AES is presented.It consists of a tungsten coil vaporizer a simple computer controlled power supply with high reproducibility of the output voltage and a fast and eYcient data acquisition and processing system for the short transient signals. Coupling the ETV system with the spectrometer and control of additional functions were performed by means of an interface connected to the ETV computer allowing operation with a high degree of automation. The ablation of tungsten in the vaporization step could be significantly reduced by coating the coil with tungsten carbide and by a decrease in the concentration of traces of oxygen in the Ar–H carrier gas using a high-voltage discharge cell. The tungsten ablation was investigated for aqueous solutions and also for silicon carbide as an example of a refractory matrix.The transport losses of the analyte elements Au As Ca Cr Cu Co Fe La Mn Na Sb and Sc were determined for several matrices by means of the radiotracer technique. Transport losses were found to be in the range from 7% (La) to 54% (Cu). Keywords Inductively coupled plasma atomic emission spectrometry; electrothermal vaporization; tungsten coil; slurry sampling; silicon carbide; silicon dioxide; silicon nitride; transport losses; radiotracer technique Inductively coupled plasma atomic emission and mass spectrometry (ICP-AES and ICP-MS) are well established methods for the analysis of a wide variety of liquid and solid samples. However when these methods are applied to analysis of solid samples in their conventional form decomposition of the sample material is necessary.Particularly for refractory inorganic materials the decomposition is often diYcult and sometimes even impossible. In general this time consuming procedure represents a considerable limitation to the limits of detection and a source of systematic errors both caused mainly by blanks. For these reasons methods for the direct analysis of solids are of great interest. Among the solid sampling techniques that have been developed for ICP-AES and ICP-MS electrothermal vaporization (ETV) is increasingly gaining popularity for the direct determination of impurities in solids at the trace or ultratrace level.1–19 Graphite furnaces similar to those used in atomic absorption spectrometry (AAS) are the predominant type of ETV devices.1–11,20–25 Besides graphite refractory metals (tungsten or tantalum tubes and filaments) have been used as furnace materials.14,26–30 Double-layer tungsten coils normally manufactured in large numbers for low-voltage lamps also belong to this group and have been applied as vaporizers to AAS,31–37 ICP-AES18,38 and ICP-MS.19 The attraction of these coils is to be seen in their extremely low price and high reproducibility of geometric shape and physical properties.Mounted in a small quartz apparatus they form a simple inexpensive and eYcient ETV device. Compared with graphite or metal-tube Samples and Reagents Silicon carbide Type S933 (ESK Kempten Germany).The average particle diameter was at the sub-mm level and it did not exceed 5 mm. Silicon dioxide SiO2–1 99.9% pure -325 mesh lot no. X8653 (Cerac Milwaukee WI USA). SiO2–2 Aerosil 200 LOS 7638605 (Novartis Basle Switzerland) (high-purity sample). The particle size of both samples was estimated by electron microscopy to be less than 10 mm. Silicon nitride Type LC12 (H. C. Starck Goslar Germany). The average particle diameter was 0.48 mm with a Gaussian grain size distribution; 90% of the sample had a particle diameter of about 0.80 mm. element stock standard solutions (1 g l-1) (Merck Darmstadt Working standard solutions were prepared using single Germany). For dilution of the stock standard solutions and preparation of the sample slurries doubly distilled water was used.Hexane used for the coating of the coil and hydrofluoric acid (40%) and nitric acid (65%) used in the radiotracer experiments were of ‘reinst’ quality (Merck). An acid mixture containing HF and HNO3 (6 mol l-1 each) and dilute HNO3 (6 mol l-1) were prepared. In order to minimize the oxidation of the tungsten coil during heating an Ar–H mixture (6.5% v/v H2) (Linde Journal of Analytical Atomic Spectrometry December 1997 Vol. 12 (1351–1358) 2 1351 Munich Germany) normally used for welding purposes was used. Radiotracers Radiotracers were produced by irradiation of evaporated stock standard solutions of As Au Cu La Mn Na Sb and Sc in the FRM-1 reactor Garching Germany at a thermal neutron flux of 2.0×1013 cm-2 s-1.The irradiation and decay times for manganese and the other elements was 1 and 6 h and 1 and 2 d respectively. The irradiated compounds were dissolved in 1 ml of 6 mol l-1 HNO3. For copper a separate radiotracer solution was prepared. For the in situ experiments 100 mg portions of silicon carbide were irradiated. The radiotracers 47Ca 58Co 51Cr and 59Fe with specific activities of 5.4×103 4.6×108 1.03×107 and 1.02×105 Bq mg-1 respectively were supplied by Amersham-Buchler (Braunschweig Germany). The main nuclear data for the radionuclides are given in Table 1. Fig. 1 Schematic diagram of the set-up for the tungsten coil technique 1 carrier gas (Ar–H2; 6.5% v/v H2; 1 bar); 2 flow meter with built-in needle valve (0–1000 ml min-1); 3 flow meter with built-in needle valve (0–30 ml min-1); 4 high-voltage discharge cell (HVDC); 5 glass bottle with hexane (volume=10 ml); 6 coating valve (two-way valve electrically operated); 7 ETV device (quartz); 8 interface to the ICP (quartz tube with ball joint); 9 plasma torch; 10 computer (AT-286); 11 serial port; 12 parallel port; 13 interface; 14 power supply; 15 ultrasonic probe on/oV; 16 to spectrometer computer (triggering of the measurement); 17 ceramic stopper; 18 copper electrodes; 19 tungsten coil; 20 quartz stopper; 21 carrier gas inlet; A carrier gas stream (700 ml min-1); and B coating gas stream (9–16 ml min-1).The quartz apparatus for mounting of the tungsten coil and the quartz interface to the ICP were laboratory made.A 2 Type HD-70 ultrasonic probe (Bandelin Electronic Berlin Germany) was used for homogenization of the slurries. The laboratory made power supply was controlled by a portable Amount traced per vaporization/ ng Half-life Corresponding concentration in the matrix†/ mg g-1 Specific activity*/ Bq ng-1 c-Rays counted/ keV 559.1 411.8 1296.8 810.8 1.6 23 0.0007 52 750 5.4 455000 26.3 h 2.7 d 4.54 d 70.8 d 8 0.4 115 0.0035 0.14 0.8 1.5 0.8 10300 133 102 75 320.1 511.0 1099.2 1596.2 27.7 d 12.7 h 45.1 d 40.3 h 10 8 1 62 6.2 46 846.6 1368.6 564.0 2.58 h 15.0 h 2.72 d 0.7 4 7.5 4 50 40 5 4 0.8 57 889.3 83.6 d Instrumentation The instrumental set-up is shown in Fig.1. AES measurements were performed with a sequential JY-24 spectrometer extended with a JY-74 polychromator (Paschen–Runge mount 15 elements) both from Jobin-Yvon (Longjumeau France). The operating parameters are given in Table 2. Spectrometer control was eVected with the standard Jobin-Yvon software package running on a 386-IBM clone (33 MHz). For data acquisition a 16-channel 12-bit analogue-to-digital card (Type MC-PC20 BMC Puchheim Germany) plugged into an ISA slot of the 386 was used. The carrier and coating gas flow rates were adjusted by flow meters with built-in needle valves (Rota Wehr Germany; rotameter type). For the reduction of molecular oxygen in the carrier gas a high-voltage discharge cell (HVDC) was constructed.The Ar–H2 mixture flows through the space formed between two concentric quartz tubes. The electric discharge is generated between two copper electrodes located outside of this space (one inside the inner quartz tube and the other outside the outer quartz tube). The electrodes are connected to a highvoltage generator consisting of a pulse generator (rectangular wave shape frequency approximately 300 Hz) and a car ignition coil. The high voltage can be adjusted via the output voltage of the pulse generator. For coating the tungsten coil with tungsten carbide part of the carrier gas was led through a small glass bottle filled with hexane. The Ar–H –hexane mixture (‘coating gas’) could be switched into the carrier gas stream by means of an electrical valve (‘coating valve’).Table 1 Main data for the radiotracers used for the determination of transport eYciencies Radionuclide 76As 198Au 47Ca 58Co 51Cr 64Cu 59Fe 140La 56Mn 24Na 122Sb 46Sc * Specific activity corrected to the day when the experiments were performed. † Calculated for a slurry aliquot of 20 ml and a slurry concentration of 10 g l-1. 1352 Journal of Analytical Atomic Spectrometry December 1997 Vol. 12 Table 2 ICP-AES parameters used for slurry ETV–ICP-AES Plasma gas (Ar) flow rate Intermediate plasma gas (Ar) flow rate Aerosol carrier gas (Ar–H2; 6.5%v/v H2) flow rate Rf power Analyte emission line Element 396.152 208.959 313.042 393.366 226.502 228.616 267.716 324.754 259.940 279.553 257.610 231.604 220.353 334.941 213.858 193.001* 208.819* Al B Be Ca Cd Co Cr Cu Fe Mg Mn Ni Pb Ti Zn C W * Monochromator.286 computer (Type PP-1601 Charisma Taiwan). The coating valve the ultrasonic probe and the triggering of the data acquisition were controlled by a laboratory made interface connected to the serial port of the 286. Type 64655 HLX 24 V 250 W tungsten coils were supplied by Osram (Munich Germany). The diameter of the wire was 0.30 mm the area of the coil was 7.0×3.6 mm2 the distance between the double layers was 1.2 mm and the mass was 210.4±0.2 mg (n=20). The space formed between the double layers allows the introduction of up to 40 ml of sample solution or slurry.The tungsten coil was fixed in the quartz apparatus by means of a ceramic stopper with two copper electrodes for clamping and contacting the coil. Measurement of the tungsten coil temperature in the range 800–3000 °C was performed with an optical pyrometer (Type Cyclops 52 Land/Minolta Leverkusen Germany). For voltage measurements a true-r.m.s. digital multimeter was used (Type 87 Fluke Kassel Germany). Gamma-rays from the radiotracers were counted by a high resolution gamma-ray spec- Voltage/mV Step Drying Vaporization Clean-out Coating* 790 710 610 500 17100 19500 0 14500 11200 0 Cool down Additional functions— Action Baseline correction Measurement Coating valve Ultrasonic probe Table 3 ETV parameters used for slurry ETV–ICP-AES for a sample volume of 20 ml * Coating gas flow rate=9–16 ml min-1 depending on matrix and slurry concentration.From/s To/s Journal of Analytical Atomic Spectrometry December 1997 Vol. 12 Procedures For the determination of the power consumption of the tungsten coil vaporizer the actual electric current as a function of the output voltage was measured by the voltage drop at a high-precision resistor (0.001 V) switched in series with the tungsten coil. For measurement of the tungsten coil temperature the optics of the pyrometer were focused through the pipetting hole of the quartz apparatus on to the centre of the tungsten coil.A carrier gas flow rate of 700 ml min-1 was used throughout. For tungsten carbide coating executed after each sample vaporization the optimum coating gas flow rate was 9–16 ml min-1 depending on the type of matrix and slurry concentration. The coating was performed at 2400 °C and the coating gas was switched for 8 s into the carrier gas stream (see also Table 3). For coating a new tungsten coil this coating step was applied twice. For the optimization of the coating process the carbon and tungsten emission was monitored by setting the monochromator to 193.001 and 208.819 nm respectively. At a coating gas flow rate of 12 ml min-1 the maximum concenwas about 9.5 mg ml-1. tration of hexane in the ETV device during the coating step The slurries were prepared as described elsewhere.39 In the in situ radiotracer experiments the irradiated silicon carbide sample was used for slurry preparation.For all other radiotracer experiments inactive slurries or 10 ml of water were spiked with the radiotracer solution during ultrasonic agitation. As the activity of 64Cu can only be measured at the non-specific 511 keV annihilation line all experiments using 64Cu had to be performed separately from the other radionuclides. The amount traced in a 20 ml aliquot (sample volume used for vaporization) and the corresponding concentration in the solid for a 10 g l-1 slurry are given in Table 1. For the radiotracer experiments the ETV device was assembled in a hood. To retain the greatest part of the vaporized radionuclides a cellulose filter was placed at the end of the aerosol transport path.Aliqouts of the slurries (20 ml ) were pipetted on to the coil dried and vaporized using the voltage–time programme without a clean-out and coating step (see Table 3). After 20 Ramp/s 14 l min-1 0.3 l min-1 0.7 l min-1 900 W Wavelength/nm Interfering W emission line 228.629 267.728 259.964 257.617 Temperature/°C — — — — 2600 2700 Decreasing 2400 2200 Decreasing 99 110 94 100 125 175 117 160 trometer system consisting of an HPGe detector and a 16K multi-channel analyzer (EG&G Ortec Munich Germany). The gamma-rays from 64Cu were counted with a single channel analyzer equipped with an NaI(Tl ) detector and an automatic sample changer (Berthold Munich Germany).Elapsed time/s Hold/s 20 10 40 30 0 0 0 0 5 0 3 9 1 0 9 4 0 0 20 30 70 100 105 109 118 127 131 176 45 0 1353 3 3 (65%)+2 ml of HF 3 (65%)+0.5 ml of HF (40%) respect- observed so far indicating a remarkably high reproducibility sequential vaporization steps the ETV device was dismantled and the quartz parts were rinsed three times with 1 ml of the HF–HNO mixture. The tungsten coil and the filter were digested in a mixture of 1 ml of HNO (40%) and 2 ml of HNO ively. For the reference values the corresponding amounts of the radiotracers were pipetted directly into the vessels used for counting.Prior to the gamma-ray measurements all resulting solutions were made up to equal volumes. RESULTS AND DISCUSSION Fig. 2 Schematic diagram of set-up for the power supply used for the heating of the tungsten coil vaporizer. Power Supply and Data Acquisition Power supply When using a tungsten coil vaporizer as described above the requirements for a power supply are as follows. (a) For fast vaporization of the solvent or the suspension medium without any analyte losses during the drying step a precise and reproducible power setting in the low-power range must be possible. For example to remove a sample volume of 20 ml a power setting of 1.7W (790 mV 2.15 A) was used at the beginning of the drying step which to avoid analyte losses had to be reduced stepwise to a power setting of 0.91 W (500 mV 1.82 A).( b) For matrix vaporization and clean-out power settings in the range 150–200 W (16.2 V 9.3 A to 19.5 V 10.3 A) are necessary. (c) For comfortable operation and automation computer control of the power supply is desirable. We first used a 24 V ac power supply where the power setting was performed by means of a phase-shift control.18 However reproducible adjustment of low output power as needed during the drying step was not possible. To avoid analyte losses caused by too high temperatures of the tungsten coil the drying step had to be performed at lower output voltages resulting in a significantly prolonged drying step. The improved power supply presented in this work is based on a diVerent principle of power setting and combines simple construction low costs and very high reproducibility of the output voltage.The schematic set-up of the power supply is shown in Fig. 2. All functions are controlled by the parallel printer port of a computer. The output power can be digitally adjusted in 256 steps by pulse-width modulation (frequency approximately 122 Hz) of a stabilized dc voltage. To achieve a fine graduation of the output power in the low-power range the stabilized dc voltage can be switched from 22 to 5 V. As the pulse width is adjusted digitally the only part of the power supply which could influence the reproducibility of the power 1354 Journal of Analytical Atomic Spectrometry December 1997 Vol. 12 setting is the reference voltage needed for stabilizing the dc voltage.However only a few electronic components are necessary to achieve a stable and temperature compensated reference voltage. No drift of the dc voltage during operation has been of the power setting. As the temperature of the tungsten coil in the vaporization step was not controlled by a pyrosensor the temperature–time curve is essentially ballistic. Owing to the very high heating rates of the tungsten coil this heating characteristics do not have any disadvantageous eVect on the vaporization of the matrix. Simple and comfortable operation of the power supply is performed by a computer program written in PowerBasic (Kirschbaum Software Emmering Germany) which allows one to run voltage–time programmes similar to those in ETAAS (Table 3).The voltage values given in Table 3 (and at the beginning of this section) are the voltages of a dc source delivering the same electrical power to the tungsten coil as the pulsed dc output voltage of the power supply. The temperature and the power consumption of the tungsten coil as a function of the voltage of a dc source are shown in Fig. 3. The temperature values are the mean values of measurements performed with five diVerent new tungsten coils; the relative standard deviations were below 2.5%. The influence of the number of vaporizations coating and diVerent nature of the matrix on the temperature of the tungsten coil in the vaporization step was also investigated. The measured values were within the standard deviation of the results obtained for new tungsten coils.Data acquisition With computer programs normally used for ICP-AES instruments the registration of transient emission signals is often diYcult. Even with special software for transient signals as oVered by various manufacturers of ICP-AES systems the scanning rates could be too low for the very short signals produced by the tungsten coil vaporizer (about 1 s for most elements). In our previous work,18 only the transient signals of the polychromator could be integrated by the installed standard software. The integration had to be triggered manually before the vaporization step of the voltage–time programme. A long integration time was necessary to ensure complete integration of the emission signals of all elements which however caused an unfavourable signal-to-background ratio.Neither scanning of the emission signals nor coupling of the ETV system with the spectrometer was possible. For these reasons an improved data-acquisition and -processing procedure was developed. For the JY24/74 spectrometer the analog signals of the 16 photomultiplier–amplifiers 15 for the simultaneous channels and one for the monochromator are accessible with only minor modifications at the spectrometer. These signals are Fig. 3 Temperature (&) and power consumption ($) of the tungsten coil as a function of the output voltage of the power supply. recorded by means of a 16-channel analogue-to-digital card with a resolution of 12 bits.The data acquisition software written in PowerBasic has the following main features (a) coupling of the data acquisition system with the ETV system; (b) setting of individual integration times for each element; (c) automatic baseline correction; (d) scanning and storing of the intensity–time profiles of 16 elements with a scanning frequency of about 50 Hz each; (e) the data of the monochromator channel can be used for arithmetic correction of spectral interferences; (f ) the measured data can be exported to commercial computer progams (e.g. charting graphics and spreadsheet programs); and (g) automated measurement cycles are possible. The originally installed spectrometer software is only used for adjusting the mono- and polychromator before analysis.The data acquisition can be started by a trigger signal sent by the ETV computer (Fig. 1). With the above described improvements in instrumentation and software a high degree of flexibility and automation was achieved. Only the pipetting of the sample was done manually. However extension by an autosampler is possible. Reduction of Tungsten Ablation During the vaporization step of the voltage–time programme considerable tungsten ablation from the surface of the tungsten coil was observed.39 Owing to the extremely line-rich emission spectra of tungsten spectral interferences occurred for the analytes Mn Cr Co and Fe (see also Table 2) the extent of the interferences decreasing from Mn to Fe. As the polychromator used for simultaneous determinations (Paschen–Runge mount as utilized in many spectrometers) has a fixed wavelength for each element this problem could not be solved by simply choosing another analyte line.Furthermore the background has to be determined on the maximum of the emission lines making diYcult an appropriate background correction for the elements interfered with by tungsten. Hence the determination of these elements benefits from the reduction of tungsten ablation. As the vapour pressure of tungsten (mp 3410 °C bp 5660 °C)40 is very low (e.g. only 3 mPa at 2600 °C),40 the tungsten ablation during the vaporization step is mainly caused by volatile or easily decomposing compounds formed by the reaction of tungsten with the matrix or traces of oxygen and water in the carrier gas.Further investigations showed that the tungsten ablation caused by the carrier gas can mainly be attributed to traces of molecular oxygen. The concentration of molecular oxygen in the Ar–H2 carrier gas could be reduced by reaction with hydrogen induced by an electric discharge. For this purpose a simple HVDC was constructed and inserted into the carrier gas line after the flow meter (Fig. 1). A further possibility for reducing tungsten ablation is to coat the coil surface with a layer that is more resistant than tungsten to oxidation by oxygen water and many matrices. Tungsten carbide is a refractory compound (WC W2C mp approximately 2860 °C bp 6000 °C)40 of high hardness and chemical resistance. As a tungsten carbide layer on the surface of the coil can be easily formed by reaction with hydrocarbons at high temperatures the tungsten carbide coating (TCC) of the coil was investigated as a means of reduction of the tungsten ablation.Depending on the nature of the tungsten surface and on partial pressure of the hydrocarbon the reaction starts between 1000 and 1500 °C and the optimum reaction temperature is reached between 2000 and 2500 °C.41 Muzgin and co-workers42,43 used an argon–methane mixture for coating a tungsten coil atomizer for atomic absorption spectrometry with tungsten carbide. The aim was an improvement of the atomization and hence the sensitivity of several elements due to the reducing properties of carbon. However no data on tungsten ablation were given.In this work hexane was used as a coating reagent. For the Journal of Analytical Atomic Spectrometry December 1997 Vol. 12 formation of a tungsten carbide layer the coil was heated to 2400 °C in the presence of hexane. New tungsten coils were coated prior to analysis. With a coated coil tungsten ablation cannot be completely avoided obviously owing to partial removal of the carbide layer during the vaporization. To restore the carbide coating a coating step was inserted after each vaporization step (Table 3). In fact this successive carbide coating reduces the lifetime of the tungsten coil; the coil becomes increasingly brittle and the melting point decreases. For a maximum lifetime the carbide layer formed during the coating step should be as thin as possible but suYcient for a reliable decrease in tungsten ablation.Therefore the coating gas flow rate the duration of the coating and the timing of both the coating valve and voltage applied in the coating step (‘coating voltage’) had to be optimized for each matrix. If the coating valve and the coating voltage were switched oV simultaneously a significant decrease in the lifetime of the coil was observed. A possible explanation is the formation of a carbon layer on the surface of the coil during the cool down in presence of hexane the temperatures being suYcient for the pyrolysis of hexane but too low for the formation of tungsten carbide. In the subsequent vaporization step this carbon layer leads to a renewed formation of tungsten carbide which is responsible for the shortened lifetime.Therefore the coating valve has to be switched oV prior to the coating voltage (see also Table 3). In the following the eVect of the HVDC and TCC on tungsten ablation is discussed for the matrices water and silicon carbide. Water When processing water using an uncoated tungsten coil the tungsten emission signal during the vaporization step shows two diVerent sections. Section 1 consists of a strong and sharp signal at the beginning of the vaporization with a duration of about 0.4 s (see Fig. 4 trace A). This peak can be attributed to the vaporization of tungsten oxides previously formed during the drying step by reaction of tungsten with oxygen and water. The most likely reaction products are tungsten oxide WO (mp 1473 °C bp 1750 °C),40 and the tungsten 3 suboxides W2O5 and W4O11 (sublimation point 800–900 °C).40 In section 2 the tungsten signal increases continuously with increase in the tungsten coil temperature reaching its final value after about 1.1 s and then remaining constant until the end of the vaporization step (see Fig.4 trace A). This signal is caused by the vapour pressure of tungsten and by the corrosion of the hot tungsten coil by traces of oxygen and water in the carrier gas. As can be seen from Fig. 4 the HVDC mainly reduces the tungsten ablation in section 1 whereas with the TCC the tungsten ablation in section 2 can be almost Fig. 4 Signals measured using the monochromator at the tungsten emission line at 208.819 nm.Vaporization temperature 2600 °C (17 100 mV 165W). A Without HVDC and TCC; B with HVDC only; C with TCC only; and D with both HVDC and TCC. 1355 Table 4 Decrease in tungsten ablation using the HVDC and TCC for diVerent integration times measured at an interference free tungsten emission line (208.819 nm); n=4. Matrix water; vaporization temperature 2600 °C (17 100 mV 165W) Tungsten signal peak area/mV s 1.7 s 1.2 s TCC HVDC No No Yes Yes No Yes No Yes 2180±25 1305±35 850±25 175±2 1510±45 715±15 840±25 175±2 Fig. 5 Signals measured using the polychromator at the manganese emission line at 257.610 nm. Vaporization temperature 2600 °C (17 100 mV 165W). A–D Signals caused by tungsten interference; A without HVDC and TCC; B with HVDC only; C with TCC only; and D with both HVDC and TCC.Dotted line signal for 100 pg of manganese with HVDC and TCC corrected for tungsten interference by subtracting signal D. completely avoided. In Table 4 the eVect of the HVDC and TCC on the peak area is given for diVerent integration times at a vaporization temperature of 2600 °C. The influence of the HVDC and TCC on the spectral interference caused by tungsten is demonstrated in Fig. 5 for the manganese emission line at 257.610 nm as an example. With the HVDC and TCC the tungsten interference on a 100 pg manganese signal can be reduced from 218% to 24% (integration time 1.2 s see Table 5). Significant reductions were also achieved for the other emission lines interfered with by tungsten although not as pronounced as for the manganese line which suVered the strongest interference.Nevertheless the corrosion of the tungsten coil and thus the introduction of tungsten into the plasma cannot be completely avoided leading to a deterioration of the signalto-background ratio for the emission lines interfered with by tungsten. The optimum coating gas flow rate was found to be 11 ml min-1 resulting in a liftetime of the coil of about 80 vaporizations. Silicon carbide In the vaporization step silicon carbide reacts with tungsten at temperatures above 1400 °C44 to give tungsten carbides TCC HVDC Mn signal* (100 pg Mn) 124±2 No No No Yes Yes Yes No Yes * Mn signal corrected for tungsten interference.Table 5 Decrease in tungsten interference at the manganese emission line at 257.610 nm in processing water using the HVDC and TCC. Integration time=1.2 s; n=4. Vaporization temperature 2600 °C (17 100 mV 165W) Peak area/mV s Journal of Analytical Atomic Spectrometry December 1997 Vol. 12 Transport EYciency For the determination of transport losses i.e. the fraction of an analyte element vaporized in the ETV cell that does not reach the ICP for excitation the radiotracer technique is well suited. The vaporization of the radiotracers under conditions identical with those existing in sample analysis can only be achieved when using in situ labelling by irradiation of the sample with neutrons in a nuclear reactor.Owing to low analyte concentrations and/or low activation yields the activity of the radioisotopes in an irradiated sample was in general too low for the determination of transport losses by in situ labelling. Therefore the slurries were spiked with a radionuclide mixture leading to suYcient activity for the radiotracer experiments. If owing to a diVerent vaporization behaviour the analytes added by the spike are vaporized at a diVerent time to the analytes in the solid this selective volatilization of analytes without a concomitant matrix may lead to a change in transport eYciency.45–47 Therefore the influence of labelling in situ and by spiking the slurries with radiotracers was examined for silicon carbide as an example. The activity of the radioisotopes 76As 24Na and 122Sb deposited in the ETV device after 20 sequential vaporizations of 40 mg of irradiated silicon carbide was high enough to achieve good counting Signal caused by tungsten interference 270±7 124±4 158±2 128 30±1 24 1356 Decrease in tungsten ablation (-fold) 2.5 s 1.7 s 1.2 s 2.5 s 1.5 3.7 18.1 1.7 2.6 12.4 2.1 1.8 8.6 3180±40 — — — 2180±55 850±30 175±3 2 mp approximately 900 °C; W5Si3).41 At a (WC W2C mp approximately 2860 °C bp 6000 °C)40 and silicides (WSi vaporization temperature of 2600 °C which was normally used for analysis only the tungsten carbides are stable.When analysing a silicon carbide slurry (concentration 2 g l-1 sample volume 20 ml ) the carbide layer formed by reaction with the matrix has the same eVect on tungsten ablation as the carbide coating by hexane.With the HVDC the peak areas of the tungsten signal were found to be 165±14 166±15 and 167±14 mV s (mean±standard deviation of five consecutive measurements) for integration times of 1.2 1.7 and 2.5 s respectively. Thus the tungsten ablation caused by silicon carbide (only with the HVDC) is very similar to that given by water (with the HVDC and TCC). Using the HVDC and TCC the background correction for the elements interfered with by tungsten can be performed by measuring the intensity at the maximum of the emission line for the suspension medium water and subtracting this value from the signal for the silicon carbide slurry.Tungsten interference in relation to Mn signal (%) 218 100 Fig. 6 Transport losses for 12 elements determined by means of the radiotracer technique for the matrices water silicon carbide silicon nitride and silicon dioxide (20 sequential vaporization steps). Vaporization temperature 2600 °C; carrier gas flow rate 700 ml min-1. Slurry concentrations SiC 2 g l-1; and Si3N4 and SiO2–1 5 g l-1. Sample aliquot=20 ml. Values in parentheses are sample amounts per vaporization. statistics. The transport losses were found to be 41 44 and 40% for As Na and Sb respectively which are in good accordance with those obtained for the spiked silicon carbide (see Fig. 6). Obviously the very high heating rate of the tungsten coil vaporizer reduces the time diVerence between volatilization of diVerent species of the same element and consequently also the diVerences in transport eYciency between the analytes originating from the sample and from the spike.This is a presupposition for the applicability of a calibration by means of the standard addition method. In the presence of large amounts of a co-volatilizing matrix the absolute amount of analyte vaporized has only a minor influence on the transport eYciency.45–47 Nevertheless for most elements investigated the amount of each radiotracer present in a 20 ml slurry aliquot (sample volume used for vaporization) was kept at trace levels (see also Table 1). For this reason multiple sequential vaporization steps were necessary in order to obtain readily detectable activity of the radiotracers deposited in the ETV device for gamma-ray counting.When processing aqueous standard solutions or slurries of silicon carbide silicon nitride and silicon dioxide the RSDs of the peak areas of the analyte emisson signals were below 5% for 20 vaporizations indicating constant transport eYciencies during the enrichment procedure of the radiotracer experiments. Generally quantitative vaporization of the analytes was observed with the exception of Co Cr Fe and Sc for which residues on the tungsten coil of about 0.6% and 1.5% of the initial activity were found for SiC and Si3N4 respectively. As for a certain selected vaporization temperature the percentages of residues were the same for both coated and uncoated coils the TCC was not used in the radiotracer experiments.The influence of the slurry concentration on the transport losses was examined for silicon nitride as an example. Variations of the amount of sample applied in the range 40–160 mg caused only minor transport loss diVerences between 1.5% (Au) and 7.8% (Cu). Physical considerations of analyte transport losses suggest that the matrix acts as a physical carrier (transport modifier) and improves the transport eYciency with increasing amount of matrix.45–47 This could not be confirmed by the results of our radiotracer investigations under the experimental conditions used. Even with the lowest matrix concentration applied in our experiments suYcent transport modifier for the analytes was pro- Journal of Analytical Atomic Spectrometry December 1997 Vol.12 vided. Therefore a further increase in matrix concentration did not lead to an improvement in the transport eYciency. The transport losses of 12 elements obtained in processing aqueous solutions and slurries of silicon carbide silicon nitride and silicon dioxide are given in Fig. 6 (aqueous solution except for As Cu Mn Na; SiO2 except for Mn). Elements with very diVerent physical and chemical properties were selected permitting the estimation of the transport behaviour of other elements for which no suitable radiotracers were available. The volatilization characteristics of the matrix influence the release of the analytes and the particle size and concentration of the physical carrier formed by the evaporating matrix during the vaporization step.45 Therefore diVerent transport eYciencies would be expected for the matrices SiC Si3N4 and SiO2.However except for Ca La and Sb the transport losses were at about the same level for all matrices investigated; they were in the range 20–45% and the highest transport loss of 54% was obtained for Cu in SiO2. A possible explanation of the minor eVect of the matrix on transport eYciency could be seen again in the very high heating rate of the tungsten coil vaporizer which leads to similar vaporization rates even for very diVerent matrices. With aqueous solutions of Au Cr Fe La and Sc increased transport losses were observed obviously owing to the absence of a matrix (see Fig.6). However this was not found for Ca and Sb. For silicon carbide and silicon nitride the transport losses decreased in general with increasing boiling-point of the element this eVect being more pronounced for silicon nitride (see Figs. 7 and 8). For silicon dioxide the same trend was observed but with the boilingpoints of the respective oxides (see Fig. 9). The large diVerence between the boiling-points of calcium (1484 °C)40 and calcium oxide (2850 °C)40 could be a possible explanation for the significantly reduced transport loss of Ca obtained for silicon dioxide compared with the other matrices. With water as Fig. 7 Dependence of the transport losses for silicon carbide on the boiling-point of the elements. Slurry concentration 2 g l-1. Fig.8 Dependence of the transport losses for silicon nitride on the boiling-point of the elements. The given transport losses are mean values for three slurry concentrations (2 5 and 8 g l-1). 1357 Fig. 9 Dependence of the transport losses for silicon dioxide on the boiling-point of the oxides (As Sb2 2 O3). Slurry concentration 5 g l O-31 A . u2 O3 CaO Cr2O3 La2 O3 matrix the trends of the transport losses for the boiling-points of both the elements and the oxides were not as pronounced as with the other matrices. CONCLUSION Through improvements in instrumentation and software an ETV system for ICP-AES using a tungsten coil as vaporizer was optimized to give a high degree of reliability flexibility and automation. The ETV system was specially adapted for coupling with the ‘classical’ type of ICP spectrometers (separate mono- and polychromators photomultipliers polychromator in Paschen–Runge mount) which are still widely used.Compared with graphite furnace ETV systems the main advantages of the present tungsten coil ETV system include a simple small and inexpensive construction very high heating rates low dilution of the analyte vapours and the absence of memory eVects caused by the formation of stable carbides. On the other hand with graphite furnace ETV devices the introduction of larger sample amounts and direct solid sampling are possible and the furnace material carbon causes no spectral interferences in AES measurements. A substantial decrease in tungsten ablation during the vaporization step causing unspecific background and spectral interferences was achieved by means of a high-voltage discharge cell and tungsten carbide coating of the coil.The reduced entry of tungsten into the ICP has a positive eVect on both the unspecific background and spectral interferences. The radiotracer technique allowed an accurate determination of transport losses for 12 elements with very diVerent physical and chemical properties which were between about 10% and 50%. In general they decreased with increasing boiling-point of the element or the oxide. The described ETV system can be used for multi-element analyses of liquids but is especially advantageous for analysis of diYcult to digest powdered materials in the form of slurries.As only 10–40 ml sample volumes are required for an analysis cycle the method is well suited for the analysis of samples available in only small amounts. REFERENCES 1 Moens L. Verrept P. 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Zucheng J. and Zhenhuang L.Fresenius’ J. Anal. Chem. 1992 344 54. 27 Evans H. E. Caruso J. A. and Satzger D. R. Appl. Spectrosc. 1991 45 1478. 28 Shibata N. Fudagawa N. and Kubota M. Anal. Chem. 1991 63 636. 29 Tsukahara R. and Kubota M. Spectrochim. Acta Part B 1990 45 779. 30 Okamoto Y. Murata H. Yamamoto M. and Kumamuru T. Anal. Chim. Acta 1990 239 139. 31 Sanford C. L. Thomas S. E. and Jones B. T. Appl. Spectrosc. 1996 50 174. 32 Parsons P. J. Qiao H. Aldous K. M. Mills E. and Slavin W. Spectrochim. Acta Part B 1995 50 1475. 33 Bruhn C. G. Ambiado F. E. Cid H. J. Wo� rner R. Tapia J. and Garcia R. Anal. Chim. Acta 1995 306 183. 34 Krug F. J. Silva M. M. Oliveira P. V. and No� brega J. A. Spectrochim. Acta Part B 1995 50 1469. 35 Havesov I. Ivanova E. Berndt H. and Schaldach G. Fresenius J. Anal. Chem. 1990 336 484. 36 Berndt H. and Schaldach G. J. Anal. At. Spectrom. 1988 3 709. 37 Williams M. and Piepmeier E. H. Anal. Chem. 1972 44 1342. 38 Dittrich K. Berndt H. Schaldach G. and To� lg G. J. Anal. At. Spectrom. 1988 3 1105. 39 Barth P. Hauptkorn S. and Krivan V. J. Anal. Atom. Spectrom. 1997 12 1359. 40 Handbook of Chemistry and Physics ed. Weast R. C. CRC Press Boca Raton FL 62nd edn. 1982. 41 Gmelin Handbook of Inorganic Chemistry T ungsten System No. 54 suppl. vol. A5b Springer Berlin 8th edn. 1993. 42 Muzgin V. N. Atnashev V. B. Pupyshev A. A. and Atnashev Yu. B. Zh. Anal. Khim. 1986 41 1798. 43 Muzgin V. N. Atnashev Yu. B. Korepanov V. E. and Pupyshev A. A. T alanta 1987 34 197. 44 Gmelin Handbook of Inorganic Chemistry T ungsten System No. 54 suppl. vol. A7 Springer Berlin 8th edn. 1987. 45 Kantor T. Spectrochim. Acta Part B 1988 43 1299. 46 Ediger R. D. and Beres S. A. Spectrochim. Acta Part B 1992 47 907. 47 Kantor T. Fresenius’ J. Anal. Chem. 1996 355 606. Paper 7/05321E Received July 23 1997 Accepted

ISSN:0267-9477    

DOI:10.1039/a705321e

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Analysis of Silicon Dioxide and Silicon Nitride Powders by Electrothermal Vaporization Inductively Coupled Plasma Atomic Emission Spectrometry Using a Tungsten Coil and Slurry Sampling PETER BARTH SUSANNE HAUPTKORN AND VILIAM KRIVAN* Sektion Analytik und Ho� chstreinigung Universita�t Ulm D-89069 Ulm Germany Slurry sampling in combination with ETV-ICP-AES was employed for the direct determination of trace amounts of impurities in silicon dioxide and silicon nitride. The ETV device consisted of a double layer tungsten coil in a quartz apparatus. Spectral interferences and background emission caused by tungsten ablation of the coil were reduced by coating the coil with tungsten carbide. The background was measured either with a high-purity sample the suspension medium or close to the analyte emission line depending on matrix and analyte or it was calculated using relative emission intensities of tungsten.The concentrations of Al B Be Ca Cd Co Cr Cu Fe Mg Mn Ni Pb and Zn were measured simultaneously whereas K and Na were determined in the sequential mode. Calibration was performed using the standard additions method. The accuracy was checked by detection between 0.035 (Mg) and 130 mg g-1 (B) and between comparison with the results of independent methods. Limits of 0.01 (Be Mg) and 34 mg g-1 (B) were achieved in silicon dioxide and silicon nitride respectively. Keywords Inductively coupled plasma atomic emission spectrometry; electrothermal vaporization; tungsten coil; slurry sampling; silicon dioxide; silicon nitride Conventional solution techniques have serious limitations particularly for the analysis of refractory inorganic materials.The decomposition of these matrices is often time consuming involves highly toxic reagents such as hydrofluoric acid and is an important source of systematic errors caused by blanks or analyte losses. For these reasons methods for the direct analysis of solid samples are of great interest. Solid sampling techniques that have been developed for inductively coupled plasma atomic emission spectrometry (ICP-AES) include the direct insertion of solids,1–4 slurry nebulization,5–8 laser ablation,9–11 arc and spark erosion12–14 and electrothermal vaporization (ETV).15–25 The ETV technique oVers a number of advantages such as a high sample introduction eYciency low sample consumption the possibility of a simple in situ analyte–matrix separation and the use of aqueous standard solutions for calibration.Moreover the simple handling the comparatively inexpensive equipment and the ease of automation make it well suited for routine analysis. However its applicability can be limited by analyte vapour condensation on the cold walls of the transport tubing interferences caused by the material of the vaporizer or the sample matrix and the insuYcient data processing capabilities of most commercial spectrometers and their respective software concerning short transient signals. Graphite tubes are the most common vaporizers for ETV.26–38 As they are widely used in ETAAS their characteristics are well known and ETAAS atomizers can easily be modified to work as vaporizers for ETV.The furnace material Journal of Analytical Atomic Spectrometry December 1997 Vol. 12 (1359–1365) Instrumentation The instrumental set-up has been described in detail in a previous paper.48 All ICP-AES measurements were performed carbon causes no spectral interferences in AES measurements and its reducing properties can support the volatilization of some analytes. Furthermore the tubes allow the introduction of large sample amounts. Nevertheless they have several disadvantages including memory eVects caused by the formation of stable carbides and the necessity of strong and expensive power supplies to attain the temperatures needed for the vaporization step.The latter is also true for furnaces made of refractory metals mostly tungsten. Metal filaments on the other hand which consume less power only allow sample volumes of typically 3–5 ml. Moreover the laboratory-made vaporizers often suVer from low reproducibility. Double layer tungsten coils as manufactured for halogen lamps form simple and inexpensive but nonetheless eYcient ETV devices.39–45 They are produced in large numbers with highly reproducible physical properties allow high heating rates and temperatures of up to 3000 °C to be achieved with low cost power supplies and only a small quartz apparatus is necessary for mounting of the coil causing only low analyte vapour dilution. Vaporization from the tungsten coil has been applied in our laboratory to the analysis of silicon carbide by ETV-ICP-AES using the slurry technique.46 A very similar device was used for the trace characterization of silicon dioxide by slurry ETV-ICP-MS.47 In a previous paper we described an improved set-up for ETV-ICP-AES48 using a tungsten coil.In the present work this set-up was applied to the trace elemental analysis of silicon dioxide and silicon nitride. EXPERIMENTAL 2 2 Samples and Reagents The analysed samples were SiO -1 -325 mesh 99.9% pure lot No. X8653 (Cerac Milwaukee WI USA) SiO -2 Aerosil 200 LOS 7638605 (Novartis Basle Switzerland) and silicon nitride LC12 (H. C. Starck Goslar Germany). Particle sizes of the silicon dioxide samples were estimated by electron microscopy to be lower than 10 and 1 mm for samples SiO2-1 and SiO2-2 respectively.The silicon nitride sample had a median particle size of 0.48 mm with a Gaussian grain size distribution; 90% of the sample had a particle diameter of <0.80 mm. element stock standard solutions (1 g l-1 Merck Darmstadt Multi-element standard solutions were prepared using single Germany). The hexane used for the coating of the coil was of ‘reinst’ quality (Merck). Doubly distilled water additionally purified with a Milli-Q system (Millipore Neu-Isenburg Germany) was used throughout. 1359 on a JY-24 sequential spectrometer extended with a JY-74 polychromator (Jobin-Yvon Longjumeau France). The ETV device consisted of a double layer tungsten coil Type 64655 HLX supplied by Osram (Munich Germany) connected to a laboratory-made 0–24 V 250 W power supply.The quartz apparatus for mounting the tungsten coil and the quartz interface to the ICP were also laboratory-made. A Type HD 70 ultrasonic probe supplied by Bandelin Electronic (Berlin Germany) was used for homogenization of the slurries. The power supply the coating gas valve and the ultrasonic probe were all controlled by a portable 286-computer PP-1601 (Charisma Taiwan). Data acquisition and spectrometer control was performed on a 386-IBM clone. Procedure A description of the tungsten carbide coating (TCC) and the high voltage discharge cell (HVDC) is given in a previous paper.48 The signals of the 15 polychromator channels and the monochromator photomultiplier were recorded via a 16 channel analogue-to-digital card (resolution 12 bit).Data acquisition and processing was performed by a program written in PowerBasic (Kirschbaum Software Emmering Germany). The baseline was measured for 5 s before the vaporization step and corrected automatically. The data acquisition is also described in more detail in a previous paper.48 The entrance slit of the polychromator was calibrated using a Pb hollow cathode lamp. The monochromator was set to the peak maximum of the emission lines using continuous signals obtained by either heating the coil to 2000 °C (W) or by introducing the aerosol of a pneumatic nebulizer into the carrier gas stream (Na). Slurries were prepared by mixing up to 300 mg of silicon dioxide and 80 mg of silicon nitride with 10 ml of water in 15 ml polystyrene beakers.After conditioning and pre-coating a new tungsten coil slurry aliquots (20 ml ) were pipetted during ultrasonic agitation on the coil and the voltage-time programme was started. Depending on sample and analyte diVerent methods of background evaluation were applied which are discussed in detail under Results and Discussion.libration was performed using the standard additions method. The ETV and ICP-AES parameters are summarized in Tables 1 and 2. RESULTS AND DISCUSSION Spectral Interferences and Background Possible spectral interferences of the analyte elements caused by the matrix (silicon) or the vaporizer material (tungsten) were investigated by recording the emission spectra over a range of ±0.3 nm around the emission lines used in the determinations (see Table 2).For this purpose aqueous solutions of Si (1000 mg ml-1) W (100 mg ml-1) and a mixture of the elements to be determined (10 mg ml-1 for each element) were processed. Sample introduction was performed by pneumatic nebulization. The superimposed spectra showed no spectral interferences resulting from Si. However overlapping with tungsten emission lines was observed for the analyte lines of Mn Cr Co and Fe (see also Table 2) whereby the extent of the interference decreased from Mn to Fe. The significance of this interference depends on the amount of tungsten ablated from the coil during vaporization.As has been reported in our previous work,48 the tungsten emission signal obtained during the vaporization step when aqueous solutions are processed consists of two sections. Section 1 a sharp peak at the beginning of the vaporization stage is obviously caused by species volatilizing at comparatively low 1360 Journal of Analytical Atomic Spectrometry December 1997 Vol. 12 Table 1 ETV parameters used for slurry ETV-ICP-AES Voltage–time programmes Voltage/mV Temperature/°C Ramp/s Hold/s Pre-coating of the coil— 0 20 2400 Decreasing 2400 14 500 0 14 500 Decreasing 10 10 10 1 0 23 0 0 0 0 0 Coating gas 1st to 11th 21st to 31st and 41st to 51st second Measurement— (Drying) — — — — 2600 2700 (Vaporization) (Clean out) 90 710 610 500 17 100 19 500 (Re-coating) 0 14 500 11 200 Decreasing 2400 2200 Decreasing (Cool down) 20 10 40 30 5 3 9 9 4 45 0 0 0 0 0 1 0 0 0 0 0 Coating gas 117th to 125th second Coating gas flow 9–16 ml min-1 depending on matrix and slurry concentration Sample volume 20 ml Table 2 ICP-AES parameters used for slurry ETV-ICP-AES Plasma gas (Ar) 14 l min-1 0.3 (K Na 0.9) l min-1 Rf power Intermediate plasma gas (Ar) Aerosol carrier gas (Ar–H2 6.5% v/v H2) 0.7 l min-1 900 W Emission lines Wavelength/nm Interfering W emission line Analyte emission line Element Al B 228.629 267.728 259.964 Be Ca Cd Co Cr Cu Fe Mg 257.617 Mn Ni Pb Zn K Na W 396.152 208.959 313.042 393.366 226.502 228.616 267.716 324.754 259.940 279.553 257.610 231.604 220.353 213.858 766.490* 588.995* 208.819* *=Monochromator.temperatures while section 2 appearing only after about 1.1 s reflects the formation of volatile tungsten compounds at higher coil temperatures and/or the vaporization of tungsten metal. Both sections of the signal can be eVectively reduced by applying a HVDC reducing the trace amounts of oxygen in the carrier gas and TCC of the coil.48 In the following tungsten ablation and the eVect of a HVDC and TCC in the presence of silicon nitride and silicon dioxide are discussed in detail.Silicon nitride The vaporization of silicon nitride might lead to the intermediate formation of tungsten nitrides (WN W2N WN2) and/or silicides. Tungsten nitrides are unstable compounds easily decomposing into the elements.49 Without using a HVDC and TCC section 1 and section 2 of the tungsten signal are significantly reduced compared with water when silicon nitride slurries are processed (see Table 3). With increasing slurry concentrations section 1 of the signal increases whereas section 2 decreases [see Fig. 1(i )]. As can be seen from Fig. 1(i ) and (ii ) the HVDC and TCC have virtually no eVect on section 1 of the tungsten signal. For high sample amounts e.g. 160 mg as used for analysis this part of the signal was even slightly increased.Section 2 of the tungsten signal was totally suppressed by the HVDC and TCC under all conditions [see Fig. 1(ii )]. As a consequence of the diVerent behaviour of section 1 and section 2 of the tungsten signal a significant reduction of tungsten ablation by the HVDC and TCC was only achieved for low sample amounts and long integration times (see Table 3). Under the conditions used for analysis (sample amount per vaporization 160 mg) however the application of the HVDC and TCC was generally found to be dispensable. The optimum coating gas flows for sample amounts of 40 and 160 mg were 11 and 9 ml min-1 respectively. Silicon dioxide Compared with silicon nitride silicon dioxide causes a much higher tungsten ablation. Applying a sample amount of 40 mg per vaporization tungsten ablation was approximately a factor of 4 higher for silicon dioxide than for silicon nitride.The pronounced release of tungsten during the vaporization step is obviously caused by the formation of volatile tungsten oxide WO3 (mp 1472 °C bp 1750 °C),50 and tungsten Fig. 1 Tungsten ablation during the vaporization of diVerent amounts of silicon nitride. Signal measured via the monochromator at the tungsten emission line at 208.819 nm. Vaporization temperature 2600 °C. (i) without high-voltage discharge cell (HVDC) and tungsten carbide coating (TCC); (ii) with HVDC and TCC. (A) 40; (B) 100; and (C) 160 mg silicon nitride per vaporization. Table 3 Tungsten ablation during the vaporization of diVerent amounts of silicon nitride measured via the monochromator at an interferencefree tungsten emission line (208.819 nm); n=10.Vaporization temperature 2600 °C Integration time=1.2 s Amount of Si3N4 per vaporization/mg II† I* 0 40 100 175±2 148±19 245±29 1510±45 159±25 263±32 160 860±145 730±90 * I Without high-voltage discharge cell (HVDC) and tungsten carbide coating (TCC). † II With HVDC and TCC. Journal of Analytical Atomic Spectrometry December 1997 Vol. 12 2O5 W4O11 (subl. p. 800–900 °C).50 Tungsten suboxides W ablation increased with increasing sample amount. Aside from the tungsten interference the vaporization of silicon dioxide in amounts of 40 mg or more also caused an increased unspecific background emission influencing analyte lines not directly interfered with by tungsten.This observation was not made when processing silicon nitride. Possible reasons for the observed unspecific background emission are a high plasma loading leading to an increased continuum emission a shift of the background emission intensity maximum to a diVerent height in the plasma and especially at higher wavelengths an enhancement of stray light. These eVects are more pronounced for silicon dioxide than for silicon nitride because tungsten ablation from the coil and therefore tungsten introduction into the plasma is much higher with the former matrix. Another explanation for the observed diVerences between the two matrices could be their diVerent thermal properties leading to diVerent behaviour in the plasma.Molecular bands of SiO (between 210.0 and 292.5 nm)51 and WO (between 350 and 590 nm)52 may also contribute to the background. Molecules containing Si alone however can be excluded as a source of unspecific emission because otherwise silicon nitride would show a similar unspecific background increase to silicon dioxide. Both the unspecific emission and the interference by tungsten can be significantly reduced by TCC. The eVect of TCC on the background of 12 elements measured using the high-purity sample SiO2-2 is shown in Fig. 2. The TCC improves the reproducibility of the measured background values and reduces the background by a factor of 2.3 (Fe) to 3.1 (Pb) with a mean value for all elements of 2.6±0.3 (as the SiO2-2 sample still 2 Fig.2 EVect of tungsten carbide coating (TCC) on the background of 12 elements measured using the high-purity sample SiO -2. Concentrations of all elements except for Ca and Mg below the limits of detection; sample amount 200 mg per vaporization. High-voltage discharge cell used throughout. Vaporization temperature 2600 °C; five replicate measurements; integration time=2.5 s. Tungsten signal; peak area/mV s Integration time=2.5 s Integration time=1.7 s II† I* II† I* 175±3 162±20 252±30 3180±40 795±40 336±24 175±2 157±20 249±29 2180±75 252±26 266±32 870±150 750±95 865±150 735±90 1361 contains detectable amounts of Ca and Mg these elements were not considered for factor calculation).Obviously the refractory tungsten carbide can withstand the attack of the matrix much better than tungsten. Nevertheless when analysing a slurry the corrosion of the carbide layer and thus the introduction of tungsten into the plasma cannot be completely avoided. The corrosion increases with increasing slurry concentrations leading not only to a higher background emission but also requiring higher coating gas flows to restore the tungsten carbide layer. For the maximum applicable sample 16 ml min-1. amount of 600 mg the optimum coating gas flow was Background Correction In ETV-ICP-AES background correction is aggravated by the transient nature of the emission signals because a simultaneous determination of the background beside the emission line and the emission value at the line maximum is not possible with a Paschen–Runge mount still utilized in many spectrometers including that used in the present work.Moreover for the analyte lines directly interfered with by tungsten the background has to be determined on the peak maximum. Therefore diVerent background correction strategies were developed depending on the matrix type the concentrations of the respective analyte elements and the nature of the background. As mentioned above the presence of the silicon dioxide matrix causes a considerable increase in background emission. Therefore it is in principle not possible to determine the background by simply using the suspension medium water. There are two possible ways of estimating the background when processing the sample slurry.The measurement could be performed close to the analyte emission line with any sample or on the line maximum using a sample with analyte concentrations below the limit of detection. However only the second method allows a correction of the direct line interferences by tungsten. Fortunately the high-purity sample SiO2-2 was appropriate for this kind of background evaluation. This was further verified by measuring the emission intensities at the respective line maxima and near to the lines when processing slurries of both silicon dioxide samples with equal matrix concentrations. Except for those elements interfered with by tungsten the background values obtained for the two diVerent samples were within the measurement uncertainties the same.It can be assumed that this is also true for the analyte lines that overlap the tungsten lines. In spite of its high purity the sample SiO2-2 still contains detectable amounts of Ca Mg and Na and can therefore not be used for the evaluation of the background at the emission lines of these elements. For high analyte concentrations i.e. Ca and Mg in both samples and Na in the SiO2-1 sample water could be employed in the correction of the background because very low matrix concentrations (0.1–1 g l-1) were suYcient for the analysis and therefore the unspecific background caused by the matrix was low compared with the respective analyte and blank signals. For the determination of Na in the SiO2-2 sample and in silicon nitride the background had to be measured beside the analyte emission line.On account of an argon emission line close to the Na line used the background measured at wavelengths below the line is significantly higher than the background measured above it. Thus background correction was performed by measuring the background at both sides of the analyte line using a sample slurry and interpolating the background at the line maximum. As the silicon nitride matrix caused no significantly increased background emission it was possible to evaluate the background for those elements not interfered with by tungsten at the maximum of the emission lines using water. Nevertheless 1362 Journal of Analytical Atomic Spectrometry December 1997 Vol.12 Analysis of Samples Fig. 3 shows the ETV signals obtained for an aqueous standard solution a slurry and a slurry spiked with a standard solution for Fe in silicon dioxide and Mn in silicon nitride as typical examples. On account of the high heating rate achievable by the tungsten coil very short signals with durations usually below 1 s were obtained for both slurries and standard solutions. Nevertheless the signals obtained for slurries were Fig. 3 ETV-ICP-AES signals obtained for (i) Fe in silicon dioxide (A) background signal (50 mg SiO2-2) (B) silicon dioxide slurry (50 mg SiO2-1) (C) silicon dioxide slurry spiked with 20 ng Fe and (D) aqueous standard solution containing 10 ng Fe; and for (ii) Mn in silicon nitride slurry (A) background signal (B) silicon nitride slurry (160 mg Si3 N4) (C) silicon nitride slurry spiked with 1 ng Mn and (D) aqueous standard solution containing 1 ng Mn.the true background can only be determined in the presence of the matrix. Therefore for the analyte elements (B Cd Ni Pb and Zn) with concentrations below their respective limit of detection in the analysed sample a sample slurry was employed for background determination. For the elements Co Cr Fe and Mn which were directly interfered with by tungsten neither of the above-mentioned correction methods is applicable to the silicon nitride matrix as on the one hand the tungsten emission was not identical for water and silicon nitride and on the other hand no adequately pure sample was available for a direct background evaluation.For this reason the tungsten emission constituting the greatest part of the background on the interfered analyte lines was calculated via the emission on the interference-free tungsten line at 208.819 nm. The relative emission intensities of tungsten on the interfered lines (=background at the interfered analyte lines measured with the polychromator divided by the tungsten emission signal at 208.819 nm measured with the monochromator) could be determined in extra runs prior to the analysis using water provided that they do not diVer for water and slurry. In fact a diVerence of about 15% for the relative emission intensities of water and silicon nitride was observed. This could be attributed to a shift of the emission intensity maximum in the plasma in the presence of the matrix and the diVerent observation heights of the monoand polychromator.Consequently all determined relative emission intensities were corrected accordingly. The calculation of the relative emission intensities and the correction of the analyte emission intensities were performed automatically by the data processing program. usually broader than those obtained for aqueous standard solutions whereby the width of the signals increased with increasing slurry concentrations. However this did not impede the analysis because integrated signals were used for quantification. Apparently the excitation conditions in the plasma for the slurry and standard solution were not identical as for both matrices the sensitivity obtained for the slurry was lower than that obtained for the aqueous standard.Therefore the calibration had to be performed using the standard additions method. Maximum applicable matrix amounts per vaporization for silicon dioxide were found to be 50 mg for Mn and Cr 200 mg for Na and 600 mg for all other elements. For Mn Cr and Na the matrix mass applied was limited by the background emission and for the remaining elements by the signal suppressing eVect of the matrix. The matrix amount of silicon nitride slurries was limited to a maximum value of 160 mg by the lifetime of the tungsten coil. Under these conditions approximately 25 vaporizations could be performed with a single tungsten coil before it broke or melted which is suYcient for one entire analysis including blank sample slurry and standard additions with two diVerent concentrations.The lifetime of the coil was considerably longer when analysing silicon dioxide reaching up to about 100 vaporizations. It has to be pointed out that the tungsten coils employed are extremely inexpensive and could easily be exchanged in less than 1 min without switching oV the plasma. The contents of the analytes determined in silicon dioxide and silicon nitride by the developed slurry ETV-ICP-AES method are listed in Tables 4 and 5 together with results obtained by several independent methods. With the exception of Na in SiO2-2 and in Si3N4 and Cr in Si3N4 the agreement of the results can be considered as excellent.This confirms the applicability of the proposed background correction procedures and the calibration. The concentration of Na in the SiO2-2 sample determined by slurry ETV-ICP-AES is higher by a factor of about 5 than Al B Ca Co Cr Cu Fe Mg Na Ni Pb Zn Table 4 Results of the determination of 13 elements in silicon dioxide by slurry ETV-ICP-AES and comparison with results by independent methods (all values in mg g-1) 8±1 0.4±0.1 the values obtained by the other methods excluding ICP-MS. The results for Cr and Na in silicon nitride are higher by factors of about 2 and 1.5 respectively than the ETAAS results. One possible reason for the observed deviations is a faulty background correction especially for the determination of Na in SiO2-2 for which the background amounts to about 50% of the overall signal.Moreover the result obtained for the SiO2-1 sample which has a much higher Na content is in good agreement with the results of the independent methods indicating that the calibration is accurate. The calibration can become a problem despite the use of the standard additions method if either the vaporization or the excitation conditions diVer for analyte contained in the solid material and analyte originating from the spike. Whereas the former is less likely to cause errors on account of the quantification via integrated signals the latter cannot be excluded and is probably responsible for the high Cr result in Si3N4. The eVect was still more pronounced for the determination of K for which slurry ETV-ICP-AES gave a result that was approximately a factor of 150 higher than the results obtained by ETAAS similarly also for K in silicon dioxide.ICP-MS55 2500±3 — 1.6±0.1 3.30±0.01 <0.15 109±1 — — 0.4 — 3 0.02 0.8 <0.03 400 0.3 289 19 0.029 198 2.6 — — 5.5 <0.03 1 <0.25 This work Sample Slurry ETAAS53 Slurry ETV-ICP-MS47 Slurry ETV-ICP-AES 3100±250 3300±400 1.1±0.2 — — 3200±600 <70 — — SiO2-1 SiO2-2 SiO2-1 SiO2-2 SiO2-1 SiO2-2 — — — <2 <130 <130 131±19 0.4±0.1 <70 <70 — — 0.8±0.15 <0.014 2.1±0.5 — 3.7±0.6 <0.02 1.62±0.06 SiO2-1 SiO2-2 SiO2-1 SiO2-2 SiO2-1 SiO2-2 <9 <9 <3 <3 294±30 <0.02 1.7±0.2 <0.05 369±18 <3 <0.07 360±30 0.4±0.1 SiO2-1 SiO2-2 SiO2-1 SiO2-2 — <2 62±9.5 1.2±0.1 0.50±0.01 <6 — — — — <0.1 3.1±0.6 6.1±0.5 SiO2-2 SiO2-1 SiO2-2 SiO2-1 SiO2-2 SiO2-1 <2 125±13 128±15 130±30 150±10 0.4±0.1 <0.7 0.21±0.03 0.19±0.01 18±3 14±3 17±4 17.9±0.6 <0.04 0.012±0.002 <0.03 73±4 6±9 8±10 9 0 1.0±0.2 1.5±0.2 1.1±0.2 — <0.006 1.0±0.1 <0.4 5±1 <6 <20 <20 <2 <2 — — — — SiO2-2 SiO2-1 SiO2-2 Journal of Analytical Atomic Spectrometry December 1997 Vol.12 <5 0.21±0.09 Table 5 Results of the determination of ten elements in silicon nitride by slurry ETV-ICP-AES and comparison with results obtained by ETAAS (all values in mg g-1) Independent methods This work Slurry ETV-ICP-AES Slurry ETAAS56 Solution ETAAS56 470±40 — 380±20 — 371±15 0.055±0.006 43±2 — — 3.8±0.1 3.5±0.3 7.0±0.8 1.1±0.1 1.56±0.07 1.2±0.1 62±2 59±1 50±5 2.3±0.1 42±3 55±4 46±8 1.8±0.1 12.0±0.6 <1.5 Al Be Ca Cr Cu Fe Mg Mn Na Zn 2.0±0.1 6.5±0.4 0.34±0.02 Independent methods ICP-AES54 INAA54 Solution ETAAS53 — — — — — — — 2990±150 0.8±0.1 — — — — — <2.5 — — — 0.55±0.1 0.0017±0.0001 3.0±0.4 — 2.1±0.1 <0.02 1.60±0.01 — 3.5±0.4 0.007±0.001 1.8±0.1 0.015±0.004 — — 348.0±0.2 0.5±0.2 <0.007 390±24 0.7±0.1 <0.02 233±1 0.80±0.01 147±1 — — 14.0±0.2 <0.02 79±1 10.9±0.1 <0.05 61.6±0.5 0.8±0.1 — — — — — — — — <0.01 1.1±0.1 0.10±0.01 1363 On closer investigation it was found that the emission intensity maximum for K originating from the standard solution appeared very early in the plasma inside the load coil.The emission maximum for K originating from the matrix however appeared much later near the observation height of the monochromator. This shift is obviously caused by the additional energy needed for the release of the analyte from the matrix aerosol.As a result the sensitivity obtained at the chosen observation height for K originating from the spike is lower than the sensitivity for K originating from the matrix. For a wet aerosol i.e. when using pneumatic nebulization the emission intensity maximum for K is also shifted to a higher position in the plasma near the observation height owing to the energy needed for the evaporation of the water. As this is the case for both the analyte from the sample and from the standard the problem described above is specific for sample introduction via ETV. However none of the above factors should influence the determination of Na in Si result obtained for a second sample was 9.4±0.5 mg g-1 which 3N4 by slurry ETV-ICP-AES as the is in good agreement with the results obtained by slurry ETAAS (8.2±0.5 mg g-1) and solution ETAAS (12±2 mg g-1).The limits of detection (LODs) achievable for the analysis of silicon dioxide and silicon nitride by slurry ETV-ICP-AES are listed in Table 6 and were calculated as three times the standard deviations of background measurements. For silicon nitride the LODs were mainly limited by the small applicable sample amount of only up to 160 mg per vaporization. However the LODs achievable in silicon dioxide are between a factor of 1.3 and 67 higher than those in silicon nitride although 3.75 times higher sample amounts can be applied. The reason for this is the considerably higher matrix-produced background showing also higher fluctuations.LOD/mg g-1 SiO2 Si3N4 2 130 — 0.4 — 70 9 3 3 0.035 2 1 6 20 2 1 34 0.01 0.03 0.8 3 0.4 0.3 0.4 0.01 0.03 0.04 3 14 1.5 CONCLUSIONS For most analytes the combination of a tungsten coil as a simple and inexpensive ETV device with ICP-AES and the slurry sampling technique provides a reliable method for the analysis of silicon-based materials. Compared with the conventional nebulization of solutions the developed method considerably reduces the risk of contamination is less time consuming easier to apply and avoids the use of the highly toxic hydrofluoric acid as digestion medium. However owing to rather small applicable sample portions and tungsten interferences for the analytes Mn Cr Co and Fe only moderate LODs can be achieved.The method is well suited for the fast routine determination of impurities at the mg g-1 level. Table 6 Limits of detection (LODs) achieved by slurry ETV-ICPAES for the analysis of silicon dioxide and silicon nitride Element Al B Be Ca Cd Co Cr Cu Fe Mg Mn Na Ni Pb Zn 1364 Journal of Analytical Atomic Spectrometry December 1997 Vol. 12 However coupling of the tungsten coil vaporizer to an ICP-MS instrument allows considerably lower LODs to be achieved.47 REFERENCES 1 Umemoto M. Hayashi K. and Haraguchi H. Anal. Chem. 1992 64 257. 2 Blain L. and Salin E. D. Spectrochim. Acta Part B 1992 47 205. 3 Fujimoto K. Okano T. and Matsumura Y.Anal. Sci. 1991 7 549. 4 Umemoto M. and Kubota M. Spectrochim. Acta Part B 1991 46 1275. 5 Farinas J. C. Moreno R. and Mermet J. M. J. Anal. At. Spectrom. 1994 9 841. 6 Halicz L. Brenner J. B. and YoVe O. J. Anal. At. Spectrom. 1993 8 475. 7 Ebdon L. and Goodall P. J. Anal. At. Spectrom. 1992 7 1111. 8 Lobinski R. Van Borm W. Broekaert J. A. C. Tscho� pel P. and To� lg G. Fresenius’ J. Anal. Chem. 1992 342 563. 9 Baldwin D. P. Zamzov D. 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Accepted September 12 1997 1365 Journal of Analytical Atomic Spectrometry D

ISSN:0267-9477    

DOI:10.1039/a705322c

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Feasibility of Identification and Monitoring of Arsenic Species in Soil and Sediment Samples by Coupled Highperformance Liquid Chromatography — Inductively Coupled Plasma Mass Spectrometry† P. THOMAS*a , J. K. FINNIE‡b AND J. G. WILLIAMSc aInstitut Pasteur de L ille, Service Eaux Environnement, 1 rue Calmette, BP 245, F-59019 L ille Cedex, France bDepartment of Geology, Royal Holloway College, University of L ondon, Egham, Surrey, UK TW200EX cNERC ICP-MS Facility, Centre for Environmental T echnology, Imperial College, Silwood Park, Ascot, Berkshire, UK SL 5 7T E The determination of four arsenic species (AsIII, non-toxic. The diVerence in the toxicity of species is dramatic, dimethylarsinic acid, monomethylarsonic acid and AsV ) in soil with the inorganic forms having a toxicity comparable to that and sediments following a single microwave extraction of strychnine whereas the toxicity of the organic species is procedure was investigated using an on-line system involving similar to that of aspirin.2,3 high-performance liquid chromatography (HPLC) coupled Of interest to this study are the anthropogenic sources of with inductively coupled plasma mass spectrometry (ICP-MS).arsenic found in soils and sediments, such as arsenical pestic- Phosphoric acid was used in conjunction with an open focused ides, fertilisers, irrigation, dust from the burning of fossil fuels microwave system to extract arsenic compounds. This system and disposal of industrial wastes.Metal-containing waste was optimised with respect to acid concentration, microwave materials which may aVect groundwater pollution include solid power and time in order to obtain the maximum rate of wastes, dredged material and industrial by-products. More recovery whilst retaining the integrity of arsenic species. Using recent surveys4 undertaken in five EC countries indicate that an anion-exchange column and buVered phosphate solution about 89 000 contaminated industrial sites exist, which require with methanol added as the mobile phase, good separation and immediate treatment because they either present an environsensitivity were achieved.Under these conditions, recoveries mental health problem or cannot be re-used without being between 60 and 80% of the total As content were obtained, de-contaminated. With regard to loss of groundwater resources and the detection limits were in the range 1–2 mg kg-1 for all ascribable to contamination, it is important to address arsenic species.The HPLC–ICP-MS system was used for the species in order to assess the behaviour of these compounds determination of arsenic species in acid extracts of in-house in case of remediation or land usage. reference materials (soil and sediment samples from polluted A number of coupled techniques have been developed which areas). Only arsenate was found in soil but arsenite was the allow the speciation of arsenic in a variety of media.Highmain species found in a polluted river sediment. Three certified performance liquid chromatography (HPLC) coupled with reference materials were analysed to determine the inductively coupled plasma mass spectrometry (ICP-MS) proconcentrations of the four arsenic species, and the sum of the vides an ideal combination for the eYcient separation and arsenic concentration was compared with the certified total detection of arsenic species. Previously the separation has been arsenic value for each of the reference materials. This method achieved for water and marine biota samples5–8 using reversedallows the speciation of arsenic species in soil and sediment phase anion and cation pairing modes or ion-exchange colsamples to be determined at natural levels, and enables their umns.For soil extracts better results have been achieved with behaviour in the environment to be monitored. the anion-exchange mode.9,10 The main advantages of ICP-MS over other detection techniques is that it allows both on-line Keywords: Arsenic speciation; inductively coupled plasma mass real time analysis of the HPLC eluate with a high level of spectrometry; ion exchange; high-performance liquid sensitivity.ICP-MS is also an element specific detection chromatography; microwave extraction; soil; sediment method which allows the majority of interferences to be resolved from the analytical peaks. The main drawback of this Arsenic has a wide range of industrial uses and, as a conse- instrumentation is that arsenic has only one isotope (m/z 75), quence, anthropogenic emissions greatly exceed natural levels.which can suVer interference from the ArCl polyatomic ion The toxicity and mobility of arsenic in the environment are produced during extraction of the plasma through the interface. dependent on the chemical form or species in which it exists. However, this can be successfully attenuated by the addition It is well known that inorganic arsenic such as arsenite (AsIII) of organic modifiers.11–16 and arsenate (AsV) are the most toxic arsenic species.1 This technique requires that soil and sediment samples must Methylated arsenic such as monomethylarsonic acid (MMA), be in solution for analysis, and therefore the arsenic species dimethylarsinic acid (DMA) and trimethylarsine oxide must be extracted both quantitatively and in such a way that (TMAO) are less toxic and arsenic compounds such as arseno- the integrity is maintained.Approaches such as nitric acid or choline (AsC) and arsenobetaine (AsB) are considered to be perchloric acid digestion in conjunction with hydrofluoric acid are too aggressive and lead to the destruction of arsenic species. Acid leaches are an alternative, but the most commonly used † Presented at the 1997 European Winter Conference on Plasma of these, an aqua regia leach,17 is unsuitable for this technique Spectrochemistry, Gent, Belgium, January 12–17, 1997.owing to the presence of chlorine. A recent study to produce ‡ Present address: Institute of Terrestrial Ecology, Monks Wood, Abbots Ripton, Huntingdon, Cambridgeshire, UK PE17 6LS. CRMs for speciation reported favourable results with the use Journal of Analytical Atomic Spectrometry, December 1997, Vol. 12 (1367–1372) 1367Table 1 ICP-MS operating conditions of phophoric acid combined with microwave heating for the extraction of arsenic from soils.18 That study used a low Forward power 1350 W concentration of acid (0.3 M) and the results were promising.Reflected power 1 W Nevertheless, the idea of certifying soils and sediment was Plasma gas 14 l min-1 abandoned because of several diYculties that arose during the Auxiliary gas 1.1 l min-1 Nebuliser gas 0.80 l min-1 first trial. In our study, we decided to investigate further the Nebuliser Glass concentric nebuliser, TR30A use of phosphoric acid as an extraction agent.Microwave (Glass Expansion, Camberwell, heating was chosen as it allows better temperature control Victoria, Australia) than heating blocks, which in turn leads to greater reproduc- Spray chamber Borosilicate glass Scott double pass, ibility of results. In addition, there is a reduced chance of water cooled (5 °C) arsenite oxidation with shorter heating times. Ion sampling— Sampling cone Nickel, 1.0 mm orifice This feasibility study involved work undertaken to produce Skimmer cone Nickel, 0.75 mm orifice an extraction procedure for arsenic in soils and sediments Sampling depth 13 mm from load coil using phosphoric acid which allows the integrity of the species T ime-resolved analysis peak-jumping parameters— to be maintained. The results are reported, together with those Time per slice 1.00 s from an anion-exchange HPLC–ICP-MS method.This method Points per peak 3 was found to be suitable for the qualitative and quantitative Detector mode Pulse counting Selected isotopes As at m/z 75, ClO at m/z 51 analysis of the acidic extracts produced. Composition of mobile phase A and B— A: (NH4)2HPO4–(NH4)H2PO4510 mmol l-1, pH 7.0, 3% EXPERIMENTAL MeOH B: (NH4)2HPO45100 mmol l-1, pH 8.5, 3% MeOH Standards and Reagents Arsenic standard solutions (1000 mg l-1) were prepared as follows: arsenite, 1.31 g of As2O3 (Aldrich, Milwaukee, WI, eVected by simply connecting a length of PEEK tubing USA) dissolved in 4 g l-1 of NaOH (Merck, Darmstadt, (0.17 mm id) from the exit of the column directly to the Germany); arsenate, 4.15 g of Na2HAsO4 7H2O (Aldrich) disnebulizer.The capillary tube was kept as short as possible in solved in distilled water; monomethylarsonic acid (MMA), order to minimise the dead volume. Prior to coupling the 3.91 g of CH3AsO(ONA)2 6H2O (Carlo Erba, Milan, Italy) HPLC system to the ICP-MS instrument, a peristaltic pump dissolved in distilled water; and dimethylarsinic acid (DMA), was used to aspirate an aqueous solution of arsenic as arsenite 2.90 g of (CH3)2AsO(ONa) 3.5H2O (Sigma, St.Louis, MO, (20 mg l-1) in the selected mobile phase; this was used to USA) dissolved in distilled water. optimise the ion lenses and nebuliser flow. For the hydride generation measurements, working standard For chromatographic separations, a gradient CM 41000 solutions were prepared by the appropriate dilution of a stock pump (LDC Analytical, Riviera Beach, FL, USA) was used standard solution of 1000 mg l-1 arsenic(III) chloride (Johnson with a 10 mm particle size (250×4.6 mm id) Hamilton PRP- Matthey, Karlsruhe, Germany) in 1% hydrochloric acid X100 anion-exchange column with a guard column fitted.A prepared from 32% general reagent acid (Merck). Rheodyne (Rheodyne, Cotati, CA, USA) Model 7125 injection For HPLC, mixtures of arsenic species were prepared daily valve with a 20 ml injection loop was used for sample introduc- in distilled water after appropriate dilution.Gradient elution tion. Gradient elution was carried out at a flow rate of was employed and the constituents of the two mobile phases 1.0 ml min-1. are given in Table 1. The chemicals used for the mobile phase An M301 microdigester (Prolabo, Fontenay-sous-Bois, were all of Fluka purum p.a quality (Sigma–Aldrich, Buchs, France) was used for the extraction procedure. Temperature Switzerland) and the water of HPLC grade was provided by measurements were made using a digital air thermometer in a Scharlau (FEROSA, La Jota, Barcelona, Spain).Methanol of borosilicate glass sheath (Prolabo Megal 500). It has a tem- HPLC grade (Carlo Erba) was also added to the mobile phase perature range of 0–500 °C with a precision of<3%. In-house as it has been shown to increase the signal sensitivity.7 The reference materials (soil and river sediment), after collection, resulting mobile phase was filtered through a 0.45 mm filter were freezed-dried, crushed and passed through a seive of mesh and degassed before use.Orthophosphoric acid (85%) (Merck) size 100 mm and bottled. A 0.3 g aliquot of sample was accu- was diluted to the appropriate concentration and used for the rately weighed into the digester flask and 50 ml of 1 M phos- microwave extraction procedure. phoric acid were added. An air condenser was fitted to the top of the flask and the latter was placed in the cavity of the Instrumentation and Sample Preparation microwave digestor, processed and allowed to cool.The contents were then filtered into a 100 ml calibrated flask and A PlasmaQuad PQ II+ ICP-MS instrument (VG Elemental, Winsford, Cheshire, UK) was used in the standard configur- diluted to volume with distilled water. From this 100 ml of solution, 5 ml were taken and diluted to 10 ml with distilled ation. The operating conditions are given in Table 1. The measurements were carried out using time resolved analysis water, ready for chromatographic analysis.Total arsenic determination was carried out using continu- (TRAVision). This is an option in the instrument software suite, designed specifically for the acquisition of multi-element ous flow hydride generation atomic fluorescence spectrometry (HG-AFS) (Excalibur Plus System, PSA, Orpington, Kent, time resolved signals. Data for arsenic (m/z 75) were collected using the peak jumping acquisition mode and stored in defin- UK).Samples were prepared using the dry ashing digestion procedure as follows: 1 g of sample with 2 ml of ashing aid able time slices and displayed as mass–intensity–time plots. Mass 51 (35Cl16O+) was also monitored as an indicator of (10% m/v ammonium nitrate, Fluka puriss. p.a. grade) at 450 °C for 2 h in a programmable muZe furnace. After ashing, potential interference at m/z 75 from 40Ar35Cl. Multi-element time resolved data were displayed in real time during the the residue was dissolved in 30 ml of 6 M HCl on a sand-bath and, when cool, the digested solution was filtered into a 50 ml chromatographic run.The acquired data were exported to additional software for more specialised processing,19 and a calibrated flask and diluted to volume with distilled water. An aqua regia leach was used to determine the proportion linear calibration curve was constructed for each As species with a top standard of 50 mg l-1. ofaqua regia-soluble arsenic in the sample; 0.5 g of freezedried sample was digested in 20 ml of aqua regia for 30 min at Coupling of the HPLC system to the ICP-MS system was 1368 Journal of Analytical Atomic Spectrometry, December 1997, Vol. 12a microwave power of 40 W. When cold, the digest solution in view of the possibility of other metals found in soil or sediment extracts precipitating in the column at high pH was transferred into a 100 ml volumetric flask and diluted to volume with distilled water. To determine the arsenic leached values.A pH of 8.5 is required to elute the arsenate in as short a retention time as possible; this was also found by Branch from the samples, HG-AFS was used. The conditions of this analysis were described in a previous paper.7 The same method et al.13 On increasing the pH of mobile phase A to 7, the resolution of DMA and MMA was improved and the retention of quantification was used to determine the ‘total’ arsenic extracted using phosphoric acid.The accuracy of this method time of arsenate was further reduced. It is thought that this is because there is less of a diVerence and so the column was assessed by using three reference materials (RMs), BCR 320, BCR 141 and IAEA soil-7. ‘conditions’ itself to mobile phase B more quickly. The starting mixture of the mobile phase was considered and the optimum conditions were found to be a 1+1 A–B mixture. With a higher proportion of mobile phase A, the RESULTS AND DISCUSSION retention time of arsenate was increased, and with a higher Optimisation of Anion-exchange Separation of the Analytes proportion of mobile phase B arsenite started to be retained and the peak to peak resolution between arsenite and DMA The methodologies employed by Demesmay et al.10 were used as a starting point for the study.The anion-exchange column was reduced. The chromatogram in Fig. 1 shows the separation of the four arsenic species using the selected gradient was used in conjunction with a phosphate based mobile phase for the separation of the arsenic species.Anion exchange allows programme. It was found that the concentration of acid in the extracts the use of a mobile phase with a higher buVer salt concentration than those used with an ion-pair reversed-phase column and had a significant eVect on the arsenate peak. Standards were produced for arsenite, MMA and arsenate with diVerent is thought to be less susceptible to matrix interferences such as from acids and other concomitant elements which may concentrations of phosphoric acid.Fig. 2 shows the eVect of increasing concentration of acid on the arsenate peak. These degrade the resolution and eYciency of the column. Phosphate has been used eVectively in a number of arsenic speciation results indicated that although there is an element of band broadening at 0.25 M H3PO4, it is not significant and these studies with either NH3 or Na being used as a counter ion.Sodium is unsuitable for use in conjunction with ICP-MS. The results were used when designing the extraction methods. As stated previously, arsenic has a single isotope at m/z 75 presence of alkali metals can greatly aVect the signal and lead to a build-up of salt deposits in the sampling cone. which can suVer from interference from the polyatomic ion 40Ar35Cl+ due to argon in the plasma and chlorine from the The exact mechanisms of ion-exchange chromatography are complex and not fully understood. However the resolution sample.A number of techniques have been employed to deal with this interference. Hydride generation has been used as a and the retention times of the analyte ions can be optimised through adjustments to the pH and ionic strength of the means of separating the Cl from the analyte in both total arsenic determinations14 and in arsenic speciation studies.15 mobile phase. With the literature pKa values,20 it is possible to predict how the species will behave at diVerent pH values; however, the total ionic strength of the mobile phase must also be considered and this is also related to the concentration. The behaviour of the ions was investigated through isocratic elution of mobile phases A and B at diVerent pH values.The pH ranges investigated were 6–7 for A and 7.95–9 for B. With the exception of arsenate, the species behave as predicted from their pKa values. Arsenite has an acidic proton and is not ionised until the pH is greater than 9, and it is therefore not retained by the column.DMA is fully protonated at pH 6 and is therefore also unaVected by changes in the pH. The first proton of MMA is removed at pH 6 and the second at pH 8; the retention time of MMA increased with increase in pH and indeed was very sensitive to changes in pH. Arsenate exhibited a reversal of the expected behaviour. For isocratic elution with mobile phase A at pH 6 and 6.5 arsenate was not eluted in an Fig. 1 Anion-exchange chromatogram of arsenic species in aqueous analysis time of 15 min. As the pH is increased it would be solution; 400 pg of each species injected. Peaks: 1, AsIII; 2, DMA; 3, MMA; 4, AsV. Response is normalised to the highest peak. expected that the retention time would increase; however, the opposite is observed and at pH 7.95 with mobile phase A, arsenate was eluted within 15 min; as the pH was further increased the retention time decreased. It would appear that there are interactions taking place other than those solely due to the ionic strength of the buVer.This behaviour can be used to reduce the total time of analysis and in doing so reduce band broadening eVects on the arsenate peak caused by long retention times. It was not found possible with isocratic elution to reduce the retention time of arsenate to around 10 min and optimise the conditions such that DMA and MMA were suYciently resolved for quantitative purposes.A gradient programme was developed using the results of the pH study on the individual species, and additional work was undertaken to examine the eVect of diVerent mixtures of the two mobile phases as starting points for the gradient. It was felt that it was desirable to keep Fig. 2 Magification of arsenate peak: eVect of acid concentration. 1, the programme as simple as possible by utilising a single ramp. 1.0 M H3PO4; 2, 0.50 M H3PO4; 3, 0.25 M H3PO4; 4, 1% nitric acid.Response is normalised to the highest peak. It was decided that a maximum pH of 8.5 should be imposed Journal of Analytical Atomic Spectrometry, December 1997, Vol. 12 1369The alternative is to add molecular gases to the argon flow16 dilution was required. This would have brought the typical concentrations of arsenic species below the detection limit or organic solvents to the solution phase to act as suppressants. 11,12 The concentration of the organic solvents can achieved, so it was decided to lower the phosphoric acid concentration to 1 M, which meant that only a fourfold dilution be optimised to act as a Cl attenuator.Initially, 2% v/v methanol was added to the mobile phase to act as a dual of the neat extract was necessary. The eVect on the recovery of the heating time and power of purpose organic modifier to attenuate interferences and to increase the analyte signal. However, an interference was still the microwave oven was investigated by extracting the in-house reference at diVerent settings.The variation in the arsenic observed with the arsenate peak. The scan for ClO, m/z 51, showed a large peak at the corresponding retention time, extracted with respect to the total arsenic with the diVerent microwave programmes was negligible and so it was decided indicating that the problem was caused by chlorine. The methanol concentration was subsequently increased to 3%, to reduce the possibility of oxidation by keeping the power setting and time to the minimum.which attenuated the interference. The scan for m/z 51 did not return to the original background signal, but showed no peaks The stability of the arsenic species under the extraction conditions is an important factor, but the absence of RMs as had been previously observed. This had the eVect of increasing the retention time of arsenate, but compensation makes this very diYcult to assess. Experiments on simulated extracts were carried out.Solutions of 100 mg ml-1 of arsenite, was achieved by adjusting the starting mixture of the gradient to A–B 45+55. The final programme used was from 55% to DMA and arsenate were prepared; one 50 ml portion was put aside as a control and then 50 ml aliquots were taken and 100% B in 3 min, held at 100% B for 11 min and then ramped back down to 55% B in 2 min and allowed to equilibrate for measured at diVerent power and time settings. The results are summarised in Table 3.As can be seen, the two samples 10 min before injecting the next sample. subjected to 20% power for 20 min have a reasonably constant arsenite concentration that is similar to that of the control, Assessment of Results whereas samples treated at higher power or for a longer time showed a decrease in the amount of arsenite and an increase Currently no RMs are available for arsenic speciation studies. in the amount of arsenate greater than the margins of error. The same procedure was carried out, as detailed by Thomas Although inconclusive, these results suggest that with 20% and Sniatecki,7 of determining the total arsenic content in a power (40 W) for 20 min there is no significant oxidation of number of soil and sediment RMs by hydride generation.AsIII to AsV. This was confirmed by a limited spiking experi- These results were compared with the sum of total arsenic ment. Three portions of in house soil reference material were species found by phosphoric acid extraction and HPLC– taken, two of which were spiked with AsIII. All three were ICP-MS.This can only provide an indication of the level of extracted under the same conditions and analysed by HPLC– accuracy of the techniques; a true determination is only possible ICP-MS. The amount of AsIII recovered was less than the with appropriate RMs or by comparison with the results amount spiked; however, the amount of AsV remained the obtained using alternative methods.same within the margins of experimental error. Again, these The limit of detection for the system was determined with results are inconclusive and not extensive enough to allow 3% v/v methanol in the mobile phase and a 20 ml injection statistical analysis, but they do concur with the results of the loop. A blank solution of 0.25 M phosphoric acid was injected, stability experiments that there is no significant oxidation of the peak to peak amplitude was measured and this process AsIII to AsV under these operating conditions.was repeated five times. The mean was taken and the 3s The recoveries achieved with the phosphoric acid leach were response determined. The corresponding concentration for compared with those with the most commonly used leach, each species was calculated from the 50 mg l-1 standard run aqua regia. Five diVerent soil and sediment reference materials after the blanks. The precision of the instrumental technique were extracted with aqua regia and phosphoric acid.The total was determined by measuring a 20 mg l-1 standard solution amount of arsenic extracted was determined by HGFAS. The three times under the same conditions as for the limit of results are given in Table 4 along with the certified total arsenic detection. The RSD for each species was calculated. A summary content; the value for CRM 141 is an indicative value only. of these results is given in Table 2. As can be seen, the recoveries with the phosphoric acid leach are the same as or better than those with the aqua regia leach, Optimisation of the Extraction Procedure indicating that this extraction is suitable for a wide range of soil and sediment samples.To determine the optimum concentration of acid, the extraction A portion of each of the five phosphoric acid extracts was carried out with distilled water and then with increasing referred to above was analysed by HPLC–ICP-MS. Table 4 concentrations of phosphoric acid up to 3 M using the in-house gives the concentration of each species found and the sum of soil reference material.The total arsenic was determined by all the species analysed to allow comparison with the results HG-AFS. Initially 2 M phosphoric acid was chosen because with this concentration a higher percentage of total arsenic was extracted; however, this was shown to be unsuitable owing Table 3 Results from simulated extraction solutions (standard solutions of 100 mg l-1 of AsIII, AsV, MMA) to check the integrity of the to interferences as discussed earlier.To minimise the interspecies under various microwaves conditions ferences and bring the final acid content to 0.25 M an eightfold Microwave conditions AsIII (±6%) MMA (±3%) AsV(±4%) Table 2 Detection limits and precision with 3% v/v methanol in the Control 1 103–117 100–106 80–86 mobile phase using the HPLC–ICP-MS method with 20 ml injections 20%, 20 min 102–114 99–105 80–86 20%, 20 min 100–112 98–104 80–86 Species LOD* mg kg-1 RSD (%) 25%, 20 min 103–115 95–101 78–84 AsIII 1.3 8 Control 2 98–110 96–102 73–79 DMA 1.3 10 25%, 20 min 92–104 101–107 82–88 MMA 1.3 4 25%, 10 min 103–115 102–108 78–84 AsV 1.7 10 25%, 10 min 94–106 108–104 90–98 30%, 10 min 101–113 96–102 79–85 * Detection limits are based on three times the amplitude of the 30%, 10 min 104–118 109–115 85–93 baseline noise and are given as elemental As. 1370 Journal of Analytical Atomic Spectrometry, December 1997, Vol. 12Table 4 Results for each species found in some soil and sediment reference materials. Comparison between results obtained with phosphoric acid extract analysed by HPLC–ICP-MS, soluble aqua regia content and total As certified value. Results are expressed in mg kg-1 HPLC–ICP-MS HG-AFS H3PO4 Aqua regia Total As AsIII DMA MMA AsV Sum of extract extract certified Sample (±8%) (±10%) (±3%) (±10%) species (±5%) (±5%) values In-house soil nd* nd nd 10.5 10.5 11.1 11.6 13.7±0.3 In-house river 21.3 nd nd 4.0 25.3 41.1 37 41.2±1.4 sediment CRM 320 2.5 nd nd 39 41.5 62.8 59.8 76.7±3.4 CRM 141 nd nd nd 4.3 4.3 5.3 4.5 10.5 IAEA Soil-7 nd nd nd 8.0 8.0 11.6 9.0 13.4±0.9 * Below detection limit shown in Table 2.Fig. 5 Example of chromatogram of CRM 320 sediment. Two peaks Fig. 3 Example of chromatogram of in-house soil. Two peaks identidentified : 1, AsIII; 2, AsV. Total As content, 76.0 mg kg-1. Response ified: 1, AsIII; 2, As V .Total As content, 13.7 mg kg-1. Response is is normalised to the highest peak. normalised to the highest peak. industrial activity (i.e., a chlorine–alkali plant) and comes airdried, crushed, sieved and bottled.21 The results show that the main species is arsenate with a small amount of arsenite, indicating that this sediment comes from an aerobic situation. These results show that arsenic species found in river sediments seem to be very dependent on the sampling environment.CONCLUSIONS This method has the potential to form the basis of a routine procedure for the speciation of arsenic in soils and sediments, but issues of accuracy in the extraction process still need to be addressed. Further work is required on the behaviour of arsenic species during the sample pre-treatment to improve upon what Fig. 4 Example of chromatogram of in-house sediment. Two peaks is known about the changes to the element that take place identified : 1, AsIII; 2, AsV.Total As content, 41.2 mg kg-1. Response after sampling. is normalised to the highest peak. REFERENCES of the HG-AFS analyses of the same extracts. The concentration of arsenic species determined by HPLC–ICP-MS is 1 Leonard, A., in Metals and T heir Compounds in the Environment, lower than that determined by HG-AFS; this could be because ed. Merian, E., VCH, Weinheim, 1991, p. 751. the individual species are present at levels lower than the limits 2 Pershagen, G., in Environmental Carcinogens, Selected Methods of Analysis, ed.O’Neill, I. K., Schuller, P., and Fishbein, L., Oxford of detection. Three chromatograms (Figs. 3–5) are presented University Press, Oxford, 1985, vol. 8. to illustrate the main species in each diVerent sample. Fig. 3 is 3 Nriagru, J. O., Arsenic in the Environment. Part 1: Cycling and for in-house soil and shows a small arsenite peak; however, Characterisation, Wiley, New York, 1994. this was not quantifiable as it was below the limit of detection. 4 Fo� rstner, U., in Metal Speciation and Contamination of Soil, ed. Fig. 4 is a chromatogram of the in-house sediment RM, the Allen, H. E., Huang, C. P., Bailey, G. W., and Bowers, A. R., major species in this sample is arsenite, confirming that there CRC Press, Boca Raton, FL, USA, 1995. 5 Thomas, P., and Sniatecki, K., Fresenius’ J. Anal. Chem., 1995, is minimal oxidation, and there is also a small arsenate peak. 351, 410. This sediment was taken from a polluted river where the 6 Corr, J.J., and Larsen, E. H., J. Anal. At. Spectrom., 1996, 11, 1215. sediment is in a reducing environment. Although the sample 7 Thomas, P., and Sniatecki, K., J. Anal. At. Spectrom., 1995, 10, 615. had been freeze-dried, crushed and sieved, it was observed that 8 Larsen, E. H., PhD T hesis, National Food Agency of Denmark, AsIII seems unaVected by this kind of treatment. Fig. 5 is a 1993. chromatogram from CRM 320, a BCR river sediment. This 9 Beauchemin, D., Siu, K. W. M., McLaren, J. W., and Berman, S. S., J. Anal. At. Spectrom., 1989, 4, 285. CRM is typical of an aerobic situation in rivers with prolonged Journal of Analytical Atomic Spectrometry, December 1997, Vol. 12 137110 Demesmay, C., Olle, M., and Porthault, Fresenius’ J. Anal. Chem., 18 Amran, B., Lagarde, F., Leroy, M. J. F., Lamotte, A., 1994, 348, 205. Demesmay, C., Olle�, M., Albert, M., Rauret, G., and Lopez- 11 Larsen, E. H., and Studrup, S., J. Anal. At. Spectrom., 1994, 9, 1099. Sanchez, J. F., in Quality Assurance for Environmental Analysis, 12 Evans, E. H., and Ebdon, L., J.Anal. At. Spectrom., 1990, 5, 425. ed. Quevauviller, Ph., Maier, E. A., and Griepink, B., Elsevier, 13 Branch, S., Ebdon, L., Hill, S., and O’Neill, P., Anal. Proc., 1989, Amsterdam, 1995, vol. 17, p. 285. 26, 401. 19 Thomas, P., Koller, D., and Perriera, K., Analusis, 1997, 25, 19. 14 Sheppard, B. S., Caruso, J. A., Heitkemper, D. T., and Wolnik, 20 Kortum, G., Vogel, W., and Andrussov, K., Dissociation Constants K. A., Analyst, 1992, 117, 971. in Aqueous Solutions, Butterworths, London, 1961, p. 492. 15 Sheppard, B. S., Caruso, J. A., Heitkemper, D. T., and Perkins, L., 21 Griepink, B., and Muntau, H., EUR Report 11850, CEC, Brussels, J. Chromatogr. Sci., 1992, 30, 427. 1988, p. 4. 16 Hansen, S. H., Larsen, E. H., Pritzl, G., and Cornett, C., J. Anal. At. Spectrom., 1992, 7, 629. Paper 7/04149G 17 Lobinski, R., and Marczenko, Z., in Comprehensive Analytical Received June 13, 1997 Chemistry, ed. Weber, S. G., Elsevier, Amsterdam, 1996, vol. 30, p. 7. Accepted September 12, 1997 1372 Journal of Analytical Atomic Spectrometry, December 1997, Vol

ISSN:0267-9477    

DOI:10.1039/a704149g

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Microconcentric Nebuliser for the Analysis of Small Sample Volumes by Inductively Coupled Plasma Mass Spectrometry SIMON D. LOFTHOUSEa , GILLIAN M. GREENWAY*a AND SHARON C. STEPHENb aUniversity of Hull, Cottingham Road, Hull, North Humberside, UK HU67RX bZeneca Specialities, Blackley,Manchester, UKM9 8ZS A microconcentric nebuliser was evaluated for the analysis of Although DIN is a viable alternative it is not straightforward to use on a routine basis. small volumes of sample. An optimum set of conditions was obtained for a range of analytes by optimisation of the A microconcentric nebuliser has been reported11,12 which provides stable sample introduction into an ICP at flow rates nebuliser gas flow rate.The technique was evaluated by accurate analysis of reference material NIES No. 5 using just 30 ml min-1. The basic micronebuliser is inexpensive, easy to assemble and has a very high nebulisation eYciency. This 100 ml of sample. Recoveries between 95 and 106% were obtained for all the elements determined.The technique was should provide great advantages for low volume samples. This is because smaller sample volumes can be used to obtain limits then applied to the analysis of biological samples and to the analysis of a peptide, with multi-element analysis on less than of detection comparable to those with conventional methods as more of the sample will enter the plasma. The performance 100 ml of sample. Acceptable reproducibility and precision were obtained for both sample types.Recoveries between 95 and of a commercially available microconcentric nebuliser (MCN-100 M2, Cetac Technologies, Omaha, NE, USA) suit- 118% were obtained for the biological samples and between 72 and 104% for the peptide samples. The microconcentric able for the introduction of low sample flow rates was evaluated and applied to the analysis of some batches of polymerase nebuliser permits a significant decrease in sample volume for multi-element analysis without loss of accuracy.chain reaction (PCR) based DNA test kits. Variations in performance of these test kits had been noted, but the reason Keywords: Microconcentric nebuliser; inductively coupled is not understood. Magnesium is present as an essential plasma mass spectrometry; small volume samples; biological ingredient in biological test kits, but it is suspected that trace samples transition metals, present as contaminants, may aVect the eYciency of the test kit reaction.The 50 ml test kit volume is diluted to 5 ml, which is suYcient for a single analysis without ICP-MS has evolved into a powerful and widely applied dilution using conventional nebulisation. In addition to accu- technique for the determination of trace and ultra-trace rate magnesium measurement, multi-element analysis was elements in a variety of matrices. performed on the samples using the microconcentric nebuliser. Aqueous solutions make up the majority of samples analysed The MCN-100 was also used for the analysis of a peptide.by ICP-MS, although with conventional nebulisation tech- Quantitative multi-element analyses of specific metals are niques there is a need for relatively large volumes of solution. required for US Food and Drugs Administration (FDA) Some samples, particularly biological types, are provided in compliance for pharmaceutical products. These peptides are very small volumes and dilution may take elements of interest manufactured in 5 g amounts for early clinical trials and at below the limits of detection.Over the years, alternative sample least 1 g is required for analysis and retention samples. introduction systems have been developed, including direct Therefore, a high percentage of the sample and costs is injection nebulisation (DIN)1,2 and electrothermal vaporisattributed to analysis and any reduction in sample size for ation (ETV),3–5 for the introduction of low volume liquid analysis is desirable. Previously, 50 mg of peptide, in duplicate, samples into ICP sources.ETV seems a promising technique was digested in nitric acid and made up to 50 ml for conven- for routine analyses as it needs little sample preparation and tional nebulisation into the plasma. The microconcentric nebu- calibration with aqueous standards is possible. Matrix constituliser oVers the possibility of a significant reduction in the ents can, however, reach the plasma simultaneously with the sample volume and hence sample mass needed for analysis analytes, possibly leading to matrix and memory eVects.ETV and the important benefit of reduced costs. furnace material can also lead to possible interferences and analyte losses can occur during aerosol transport. Both of these eVects can result in diminishing sensitivity and reproducibility. 6–8 Achieving ultra-trace detection limits with ETV EXPERIMENTAL proves diYcult on a multi-element basis, it is very time Instrumentation consuming and the added cost of the ETV instrumentation means it may not be cost eVective.The instrument used was a VG Elemental PlasmaQuad II+ (VG Elemental, Winsford, Cheshire, UK). Operating The DIN is positioned inside the injector tube in the ICP torch and eliminates the need for a spray chamber. Its low conditions are given in Table 1. The atomic emission spectrometer was a Thermo Electron dead volume oVers faster sample introduction and wash-out times, resulting in multi-element analysis on low sample vol- Atomscan 16 (Thermo Electron, Cambridge, UK).The microconcentric nebuliser was an MCN-100 M2 (Cetac umes with minimal waste.9,10 The DIN can be very diYcult to install and optimise on ICP-MS instruments and its high cost Technologies), which was supplied with an end-cap that fits directly on to the Scott double pass glass spray chamber. The is also a consideration. There is also a need for an extra argon gas cylinder to operate the gas displacement pump of the DIN.sample was fed using special low flow capillary tubing supplied Journal of Analytical Atomic Spectrometry, December 1997, Vol. 12 (1373–1376) 1373Table 1 Instrumental conditions and measurement parameters for the VG PlasmaQuad II+ICP mass spectrometer Rf forward power 1350 W Reflected power 0 W Coolant gas flow rate 14 l min-1 Auxiliary gas flow rate 1.2 l min-1 Nebuliser gas flow rate Optimised Spray chamber Scott double pass, water cooled at 4 °C Data acquisition mode Peak jumping Points per peak 3 Dwell time 10.24 ms Quadrupole settle time 10 000 ms Detector mode Pulse counting Isotopes measured 24Mg, 26Mg, 52Cr, 53Cr, 55Mn, 56Fe, 57Fe, 3.50E+05 3.00E+05 2.50E+05 2.00E+05 1.50E+05 1.00E+05 5.00E+04 0.00E+00 0.70 0.75 0.80 0.85 0.90 0.95 1.00 Nebuliser gas flow/l min–1 Intensity/counts s–1 Be In U 60Ni, 62Ni, 59Co, 63Cu, 65Cu, 66Zn, 68Zn, Fig. 1 Plot of nebuliser gas flow and ion signal intensity for the 75As, 107Ag, 109Ag, 112Cd, 114Cd, 118Sn, MCN-100. 120Sn, 200Hg, 202Hg, 206Pb, 208Pb, 209Bi with the nebuliser. The polyamide nebuliser capillary is 150 mm in diameter with a dead volume of 2 ml. The pressure controlled microwave system used was a CEM MDS 81D (CEM, Microwave Technology, Buckingham, UK). The electronic balance used was a Mettler MT5 (Mettler- Toledo, Greifensee, Switzerland). Reagents High purity de-ionised water (18MV cm resistivity) (Elgstat UHQ PS, Elga, High Wycombe, UK) and super purity nitric Fig. 2 Plot of nebuliser gas flow and stability (RSD) for the acid (Romil, Cambridge, UK) were used throughout. Elemental MCN-100. stock standard solutions [1000 mg ml-1, SpectrosoL (Merck, Poole, Dorset, UK)] were used in the preparation of calibration element tune solution including Be, In and U to cover the and spiked solutions. whole mass range. Ideally, optimisation would have been performed using the actual samples but owing to the small amounts available this was not possible.The shape of the Sample Preparation curve of signal intensity versus nebuliser gas flow rate was NIES No. 5 reference hair was digested (0.05 g+1 ml of found to be similar for all elements contained in the solution. HNO3) using a pressure controlled microwave oven for 5 min Fig. 1 shows the variation in signal intensities for Be, In and at 30% power followed by 5 min at 50% power controlled to U as a function of nebuliser gas flow rate and hence is assumed 100 lb in-2.The digests were transferred into 5 ml calibrated to be representitive of the whole mass range. There is a slight flasks with the addition of 20 ng ml-1 of In as an internal variation in the position of the maximum signal intensity standard. Spiked samples were also prepared by digestion of across the mass range, which is probably due to mass transport the hair in a spiked nitric acid solution. eVects. The shift is only over a small range of nebuliser gas The biological test kit samples were stored at 4 °C until they flow values and therefore a compromise of conditions can be were used.Aliquots of 100 ml were taken from each sample used with little degradation in response. and diluted with 100 ml of 2% HNO3 containing an internal The stability of the signals over a 10 min period at the standard, In at 20 ng ml-1. diVerent nebuliser gas flow rates was measured. Fig. 2 shows Samples of 5 and 2 mg of the peptide were accurately that low gas flow rates (<0.7 l min-1) result in less stable weighed into PTFE digestion vessels, digested in 0.5 and 0.1 ml aerosol formation and hence result in high relative standard of HNO3, respectively, and made up to 5 and 2 ml, respectively, deviations.If the sample was pumped to the nebuliser at these with the addition of 20 ng ml-1 of In as an internal standard. nebuliser gas flow rates then the stability would be improved, As all the samples were real, small volume, samples, only a but the noise from the pumped sample system would have to limited number of spiked samples could be prepared and be considered. No long term stability data were obtained as analysed.only small numbers of samples were used in this application. A nebuliser gas flow rate of 0.86 l min-1 was chosen to obtain the best response over the mass range with optimum RESULTS AND DISCUSSION stability. The uptake of the sample at this flow rate was Optimisation 30 ml min-1.Multi-element analysis could therefore be performed using only 100 ml of sample. This incorporated a 90 s The microconcentric nebuliser fits easily to the conventional uptake time and three replicate measurements each at 30 s spray chamber of the VG PlasmaQuad II instrument. It acquisition in the peak jumping mode. A 2% nitric acid wash operates at low sample flow rates by freely aspirating or solution was aspirated for 100 s between samples.pumping the sample to the nebuliser. In order to maximise the response and to produce stable signals, the nebuliser was optimised in terms of nebuliser gas Method Validation flow rate. The sample was freely aspirated into the plasma to lower the flow rate and reduce sample consumption. Free Validation of the technique included analysis of the reference hair material NIES No. 5. This reference material was chosen aspiration of the sample into the plasma also reduces noise arising from a pumped sample system.as it was the closest available to a biological sample and is certified for the analytes of interest to be determined in the The MCN-100 was optimised using a 1 ng ml-1 multi- 1374 Journal of Analytical Atomic Spectrometry, December 1997, Vol. 12Table 2 Analyte concentrations in mg g-1 in dry solid hair samples (NIES No. 5). Uncertainties expressed as 2 standard deviations of the instrumental response to each analyte (95% confidence limit, n=3) Mg Cr Mn Fe Ni Co Cu Zn Measured 196±12 1.3±0.1 4.5±0.5 223±12 2±0.2 0.12±0.02 17.3±1.0 159±10 Reference 208±10 1.4±0.2 5.2±0.3 225±10 1.8±0.1 0.10 16.3±1.2 169±10 Table 5 Spike recoveries (%) for spiked biological test kit samples Table 3 Spike recoveries (%) for the spiked hair samples Spike Spike Cr Mn Fe Ni Co Cu Zn no.no. Mg Cr Mn Fe Ni Co Cu Zn 1 100 103 95 100 102 95 118 1 96 100 98 101 106 101 95 102 2 98 97 95 105 102 98 96 96 2 111 114 107 115 112 104 111 biological samples.The amount of reference hair digested for Table 6 Analyte concentrations in solid peptide samples (mg g-1) analysis was less than recommended, which may contribute to Element 50 mg* 5 mg† 2 mg† imprecision but demonstrates analysis on small sample volumes. Six acidified multi-element standards in the range Cu 45 48±1.0 47±1.0 As <1 0.1±0.01 0.1±0.01 0–200 ng ml-1 (Mg, Cr, Mn, Fe, Ni, Co, Cu and Zn) were Ag <1 0.14±0.03 0.19±0.03 used for external calibration.The calibration graphs showed Cd <1 0.10±0.005 0.11±0.005 good linearity with least squares regression coeYcients of Sn 2.1 2.5±0.06 2.5±0.06 0.998–0.999 over the elements analysed. Limits of detection in Sb <1 0.1±0.004 0.1±0.004 the solution, calculated as three times the standard deviation Hg <1 <1 <1 of the blank plus the blank, were 0.8, 0.5, 0.1, 5.0, 3.5, 0.2, 1.0 Pb 3.7 4.9±0.08 4.3±0.08 Bi <1 0.16±0.001 0.14±0.001 and 3.5 ng ml-1 for Mg, Cr, Mn, Fe, Ni, Co, Cu and Zn, respectively.The sensitivity of the method was equivalent to * Concentrations provided with the peptide sample. that obtained by the conventional method. The certified refer- † Uncertainties expressed as 2 standard deviations of the instrumenence material was analysed in triplicate and the results and tal response to each analyte (95% confidence limit, n=3). spike recoveries are given in Tables 2 and 3. Good agreement between the measured and reference values was obtained for Table 7 Spike recoveries (%) for the spiked peptide samples all the elements and the spiked samples showed acceptable recoveries. Spike Amount/mg As Ag Cd Sn Sb Hg Pb Bi no. 1 5 100 85 98 86 98 72 88 102 Biological Test Kit Analysis 1 2 86 96 91 81 88 104 89 100 External calibrations were produced with six acidified stan- 2 2 89 95 94 88 85 98 92 102 dards across the concentration ranges 0–100 ng ml-1 (Cr, Mn, Fe, Ni, Co, Cu and Zn) and 0–500 ng ml-1 (Mg).These showed good linearity with least squares regression coeYcients calibrations showed acceptable linearity with least squares regression coeYcients of 0.998–0.999. Both the 5 and 2 mg of 0.997–0.999. The samples and spiked samples were analysed in triplicate with a check standard at regular intervals to samples were analysed in triplicate along with blanks and spiked samples. Calculated limits of detection in the peptide monitor instrumental drift. The calculated limits of detection for the sample solutions were 0.5, 0.2, 0.04, 7.0, 4.0, 0.1, 1.2 solutions were 1.1, 0.03, 0.1, 0.1, 0.02, 0.01, 1.0, 0.1 and 0.03 ng ml-1 for Cu, As, Ag, Cd, Sn, Sb, Hg, Pb and Bi, and 4.0 ng ml-1 for Mg, Cr, Mn, Fe, Ni, Co, Cu and Zn, respectively.The analyte concentrations and spike recoveries respectively. There is good agreement between the three sample masses and acceptable recoveries for the spiked samples. The are given in Tables 4 and 5. The spike recovery values are all close to 100%.results are given in Tables 6 and 7. CONCLUSION Peptide Analysis Six acidified standards were used to produce external cali- The MCN-100 is a simple but eVective solution to the analysis of small volumes of sample by ICP-MS. In addition to the brations with concentration ranges of 0–50 ng ml-1 (Cu) and 0–10 ng ml-1 (As, Ag, Cd, Sn, Sb, Hg, Pb and Bi). The low cost of the nebuliser and ease of operation at very low Table 4 Analyte concentrations in biological test kit samples (ng ml-1).Uncertainties expressed as 2 standard deviations of the instrumental response to each analyte (95% confidence limit, n=3) Sample no. Mg Cr Mn Fe Ni Co Cu Zn S1 197±11 2.57±0.3 8.0±0.2 <7 <4 0.36±0.01 42±2 172±11 S2 169±9 0.44±0.01 0.22±0.02 <7 8±0.3 0.36±0.01 15±1 16±2 S3 369±15 0.24±0.01 0.17±0.02 <7 <4 0.20±0.01 13±1 <4 S4 399±16 <0.2 0.15±0.02 <7 <4 0.19±0.01 11±1 8±1 S5 381±15 <0.2 0.17±0.02 <7 <4 0.17±0.01 12±1 <4 S6 361±15 <0.2 <0.04 <7 7±0.3 0.20±0.01 18±1 15±1 S7 250 000* 37±2 13±0.2 22±1 21±1 1.14±0.02 31±1 6±1 S8 300 000* 46±2 66±2 38±1 20±1 1.66±0.02 134±6 72±2 S9 182±10 0.36±0.01 0.39±0.02 <7 18±1 0.18±0.01 11±1 <4 S10 185±10 <0.2 0.41±0.02 <7 16±0.7 0.19±0.01 15±1 12±1 * Determined by MCN ICP-AES.Journal of Analytical Atomic Spectrometry, December 1997, Vol. 12 13752 Wiederin, D. R., Smith, F. G., and Houk, R. S., Anal.Chem., flow rates, stable introduction of the sample in to the ICP can 1991, 63, 219. be achieved. This not only provides multi-element analysis 3 Ulrich, A., Dannecker, W., Meiners, S., and Vollkopf, U., Anal. on a 100 ml aliquot of sample but also reduces the waste Proc., 1992, 29, 284. produced by the ICP-MS by using less sample and shorter 4 Yu, L., Koirtyohann, S. R., Rueppel, M. L., Skipor, A. K., and wash-out times. Jacobs, J. J., J. Anal. At. Spectrom., 1997, 12, 69.The MCN-100 has been invaluable for the analysis of two 5 Chang, C., and Jiang, S., J. Anal. At. Spectrom., 1997, 12, 75. 6 Kantor, T., Spectrochim. Acta Part B, 1988, 43, 1299. industrial biological products where the sample size is very 7 Sparks, C. N., Holcombe, J. A., and Pinkston, T. L., Spectrochim. limited. Dilution of the biological test kit solutions for standard Acta Part B, 1993, 48, 1607. nebulisation would have precluded accurate measurement of 8 Ediger, R. D., and Beres, S. A., Spectrochim. Acta, Part B, 1992, the range of metals determined and therefore would have been 47, 907. less eVective in the provision of critical elemental information 9 Christodoulou, J., Kashani, M., Keohane, B. M., and Sadler, P. J., to the customer. J. Anal. At. Spectrom., 1996, 11, 1031. Significant cost reductions may result as a consequence of 10 Crain, J. S., and Kiely, J. T., J. Anal. At. Spectrom., 1996, 11, 525. 11 Vanhaecke, F., Van Holderbeke, M., Moens, L., and Dams, R., the implementation of the MCN-100 nebuliser, which has J. Anal. At. Spectrom., 1996, 11, 543. permitted a reduction in sample size from 100 to<5 mg for a 12 Augagneur, S., Medina, B., Szpunar, J., and Lobinski, R., J. Anal. duplicate analysis. No decrease in accuracy has been noted. At. Spectrom., 1996, 11, 713. REFERENCES Paper 7/05047J Received July 14, 1997 1 Wiederin, D. R., Smyczek, R. E., and Houk, R. S., Anal. Chem., 1991, 63, 1626. Accepted September 11, 1997 1376 Journal of Analytical Atomic Spectrometry, December 1997, Vol. 12

ISSN:0267-9477    

DOI:10.1039/a705047j

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Direct Determination of Arsenic in Sea-water by Continuous-flow Hydride Generation Atomic Fluorescence Spectrometry JORGE MOREDA-PIN� EIROa, M. LUISA CERVERAb AND MIGUEL DE LA GUARDIA*b aDepartment of Analytical Chemistry, Nutrition and Bromatology, Faculty of Chemistry, University of Santiago de Compostela, Av. de L as Ciencias, s/n 15706-Santiago de Compostela, Spain bDepartment of Analytical Chemistry, Faculty of Chemistry, University of Valencia, Dr. Moliner 50, 96100-Burjassot, Valencia, Spain A highly sensitive and simple procedure was developed for the The use of in situ trapping of the generated hydride vapour direct determination of total As in sea-water samples by hydride in a hot coated graphite furnace (HG-ETAAS) presents higher generation atomic fluorescence spectrometry.The method sensitivity than HGAAS owing to the avoidance of hydride involves the generation of arsenic hydride from sea-water dilution in the Ar flow and the use of atomization temperatures samples, diluted with HCl to a final HCl concentration of higher than 1000 °C.Thus, this technique has been recognized 2 mol l-1, which were merged with a reducing solution, viz., as the most sensitive detection system for trace metal determi- 3% m/v NaBH4. The sample and NaBH4 were pumped at flow nation.6 However, the main disadvantages of this technique rates of 6.0 and 1.3 ml min-1, respectively, and allowed to react are the necessity of using porous graphite structures to obtain in a 200 cm×0.8 mm id reaction coil.The evolved arsenic adequate hydride trapping and the reduced possibility of hydride was removed using an argon flow rate of 400 ml min-1 routine monitoring. and passed to a hydrogen diVusion flame where the atomic The use of atomic fluorescence presents a sensitivity compar- fluorescence of As was measured at 193.7 nm. With the proposed able to the highest sensitivity oVered by HG-ETAAS with a procedure a detection limit of 5.0 ng l-1 was achieved.The reduced cost because graphite tubes are unnecessary, thus oVerrepeatability of the determination varied between 1.5 and 4.0%. ing an attractive detection system for the direct determination of The accuracy was confirmed by the analysis of two sea-water As in liquid samples at trace levels. The literature concerning As reference materials (NASS-4 and CRM-403) and by recovery determination in natural waters and geological and biological studies on natural samples spiked with known concentrations of samples by HGAFS is extensive;12–25 however, only a single AsIII and AsV.The proposed method was successfully applied to paper concerning As determination in sea-water samples by the determination of As in several sea-water samples. The HGAFS was found in the literature,26 which used non-dispersive number of samples that can be analysed is 40 per hour. AFS and radiofrequency-excited electrodeless discharge lamps. The purpose of this paper was the development of a highly Keywords: Arsenic; sea-water; hydride generation; atomic sensitive and accurate method for the direct determination of fluorescence trace amounts of As in sea-water samples using HGAFS.The method should be applicable to routine analysis and Electrothermal atomic absorption spectrometry (ETAAS) has monitoring studies. been extensively used for As determination in several samples owing to its sensitivity and accuracy. However, for complex EXPERIMENTAL samples with a high saline content, such as sea-water, several problems occur owing to the important interference eVects Apparatus from NaCl and K2SO4.1,2 In addition, the As concentration A Unicam VP-90 continuous-flow vapour system equipped in sea-water3 samples is below the detection limit of this with a B-type gas–liquid separator (Cambridge, UK) and a technique, around 1.1–1.9 mg l-1.These levels are reduced Perma pure drier tube (PS Analytical, Sevenoaks, Kent, UK) considerably for unpolluted areas.Thus, the use of diVerent was used. An Excalibur atomic fluorescence detector (PSA preconcentration procedures is necessary, methods being 10033; PS Analytical), equipped with a boosted discharge unavailable for routine analysis. hollow cathode lamp (BDHCL) (Superlamp; Photon, Victoria, The main advantage of analytical methodology based on Australia) for As as the excitation source, a hydrogen diVusion covalent hydride generation (HG) is that it allows the separa- flame as the atom cell, a series of lenses to collect and focus tion of the matrix and aVords increased sensitivity as compared useful radiation, and a specific filter to achieve isolation and with ETAAS, while also oVering automation facilities and the possibility of speciation.4 reduction of flame emission in conjunction with a solar blind Hence, HG combined with atomic absorption spectrometry photomultiplier, was used.Measurements were carried out at (AAS and ETAAS) as the detection system has been used for the resonance wavelength of As (193.7 nm).the direct determination of As in several samples. The application of this technique is well documented and has recently Reagents been reviewed by Fang et al.5 and Dedina.6 However, the All solutions were prepared from analytical-reagent grade sensitivity achieved using HGAAS is often inadequate for nonchemicals using ultrapure water, with a resistivity of 18MV cm, contaminated sea-water samples, making previous preconcenwhich was obtained from a Milli-Q water-purification system tration steps necessary.Thus, diVerent preconcentration (Millipore, Bedford, MA, USA). An AsIII stock standard solu- procedures such as liquid–liquid extraction,7 flotation,8,9 tion (1.000 g l-1) was prepared by dissolving 1.320 g of As2O3 preconcentration on solid resins10 and cryogenic trapping11 (Riedel-de Hae�n, Hannover, Germany) in 25 ml of 20% m/v have been used, increasing the sensitivity but also the analysis time.KOH solution, neutralizing with 20% v/v H2SO4 and diluting Journal of Analytical Atomic Spectrometry, December 1997, Vol. 12 (1377–1380) 1377Table 1 Operating conditions for HGAFS to 1 l with 1% v/v H2SO4. The AsV stock standard solution (Titrisol; 1.000 g l-1) was obtained from Merck (Darmstadt, Parameter Germany). Sodium tetrahydroborate (Fluka, Buchs, Spectrometer operating conditions — Switzerland) dissolved in 0.5% m/v NaOH (Carlo-Erba, Milan, Resonance wavelength/nm 197.3 Italy) was used as the reducing solution.This solution was Bandpass/nm 0.5 prepared daily and filtered before use. Hydrochloric acid Primary current/mA 27.0 solution was prepared from 37% HCl (Merck). NASS-4 Open Boost current/mA 35.0 Ocean Seawater Reference Material for Trace Metals (National Gain ×10 Hydride generation conditions — Research Council of Canada) and CRM-403 Sea Water, sup- [HCl]/mol l-1 2.0 plied by the Commission of the European Communities, were [NaBH4] (%m/v) 3.0 used to evaluate the accuracy of the developed procedure.Reaction coil length/cm 200 Argon C-45 (purity 99.995%) was used as the carrier gas Sample/HCl flow rate/ml min-1 6.0 for the atomizer and as the internal purge gas and was obtained NaBH4 flow rate/ml min-1 1.3 from Carburos Metalicos (Barcelona, Spain). Synthetic air Ar flow rate/ml min-1 400 Atomic fluorescence measurement — (Carburos Metalicos) was used to dry the generated vapour Delay time/s 15 phase in the Perma pure drier tube. Rinse time/s 30 Measurement time/s 40 Memory time/s 30 Procedure for Sample Collection Measurement mode Peak height Sea-water samples were collected from coastal surface water of the Mediterranean Sea near to Valencia in 100 ml polyethyl- Hydrochloric acid concentration ene bottles.The samples were immediately acidified with 100 ml of concentrated HNO3, which provided a pH lower than 2, to The eVect of varying the HCl concentration on the atomic avoid the adsorption of As onto the polyethylene bottle walls.fluorescence from AsIII and AsV is shown in Fig. 2. As can be seen, the signals related to AsIII and AsV increase up to a concentration of 2 mol l-1 HCl. In addition, the response Procedure for Measurement obtained from AsV is around 30% lower than that obtained The method involves the continuous gation of arsenic from AsIII.This is due to the poor eYciency of arsine generation hydride from sea-water samples diluted with HCl to a final obtained using a low NaBH4 concentration (1.5%) and the HCl concentration of 2 mol l-1, which were merged with short length of the reaction coil employed (11 cm), as will be a reducing solution, viz., 3.0% m/v NaBH4. The evolved seen in the following sections. arsenic hydride was transferred, using an argon flow rate of 400 ml min-1, to the atomic fluorescence detector. Reaction coil Fluorescence measurements for samples were interpolated using the calibration line obtained with AsIII standards.A The variation of the eYciency of arsenic hydride generation schematic diagram of the continuous flow injection system is from AsIII and AsV was evaluated using reaction coils of shown in Fig. 1. The operating conditions for HGAFS are diVerent lengths (11, 100, 200 and 300 cm). Results (Fig. 3) shown in Table 1. show that a reaction coil longer than 200 cm is necessary to obtain complete volatilization of AsV.For AsIII no variation of the hydride generation eYciency with the length of the RESULTS AND DISCUSSION reaction coil was observed. Thus, a reaction loop of 200 cm Evaluation of Arsenic Hydride Generation Conditions was selected for further experiments. All experiments were performed on natural sea-water samples and aqueous standard solutions of 1 mg l-1 As expressed as Sodium tetrahydroborate concentration AsIII and AsV.The NaBH4 concentration is an important parameter for arsine generation because arsenic hydride is formed in the presence Fig. 2 EVect of HCl concentration on arsine generation from AsIII and AsV solutions. The concentration of NaBH4 and reaction coil length were 1.5% m/v and 11 cm, respectively. The AsIII and AsV concentrations were 1.0 mg l-1. Error bars indicate the variability of fluorescence measurement as±the standard deviation of three indepen- Fig. 1 Schematic diagram of the continuous-flow system used.dent measurements. 1378 Journal of Analytical Atomic Spectrometry, December 1997, Vol. 12Fig. 3 EVect of the reaction coil length on the integrated signal of AsIII and AsV solutions. The concentrations of HCl and NaBH4 were 2.0 mol l-1 and 1.5% m/v, respectively. The AsIII and AsV concentrations were 1.0 mg l-1. of hydrogen generated by NaBH4 in an acidic medium and because the flame in which the generated hydride is atomized is maintained by the excess of hydrogen produced in this reaction.As can be seen in Fig. 4, an NaBH4 concentration higher than 3.0% m/v is required to obtain complete arsenic hydride generation from both AsIII and AsV. For NaBH4 concentrations lower than 1.5% m/v, the flame is extinguished, while for NaBH4 concentrations higher than 3.5% the instability in the flame caused by the excess of hydrogen gives a poor Fig. 5 EVect of the standard/sample (a) and NaBH4 (b) flow rates on reproducibility and sensitivity of the fluorescence measure- the fluorescence of AsIII and AsV solutions.The HCl and NaBH4 concentrations were 2 mol l-1 and 1.5% m/v, respectively; the reaction ments. Thus, an NaBH4 concentration of 3.0% m/v was coil length was 200 cm. The AsIII and AsV concentrations were selected in order to obtain the best analytical performance. 1.0 mg l-1. As can be seen, the hydride generation from AsV exhibits slow kinetics compared with AsIII, making it necessary to increase the NaBH4 and HCl concentrations to obtain a good mentioned earlier.For an NaBH4 flow rate lower than comparability between the results found for both species.27,28 1.0 ml min-1 the flame is extinguished owing to the low hydrogen concentration obtained. Therefore, a sample flow Flow rate parameters for standard/sample and NaBH4 rate of 6 ml min-1 and an NaBH4 flow rate of 1.3 ml min-1 were selected for further experiments. The eYciency of the arsenic hydride generation was studied for diVerent flow rates of standard/sample in 2.0 mol l-1 HCl and NaBH4.Results shown in Fig. 5(a) indicate an increase in Argon flow rate the As fluorescence signals with an increase in standard/sample An increase in the Ar flow rate produces an increase in the flow rate up to 6.0 ml min-1. However, a decrease in the atomic fluorescence up to an Ar flow rate of 400 ml min-1. hydride generation eYciency was obtained for an NaBH4 flow However, for Ar flows higher than this value, samples were rate higher than 2.0 ml min-1 [see Fig. 5(b)], for the reasons diluted and fluorescence signals reduced. Hence, a 400 ml min-1 Ar flow was selected as the most convenient. Analytical Figures of Merit The calibration and standard additions equations obtained for aqueous standard solutions and natural sea-water samples spiked with AsIII and AsV are shown in Table 2. As can be seen, no matrix eVect was observed; in addition, the slopes obtained for the AsIII and AsV calibration and standard additions equations are similar.The calibration and standard additions graphs Table 2 Calibration and standard additions graphs* AsIII AsV Calibration I=1.5+86.9[As] I=1.7+79.7[As] r=0.9999 r=0.9999 Fig. 4 EVect of NaBH4 concentration on arsine generation from AsIII Standard additions I=85.7+88.9[As] I=84.3+82.2[As] (natural sea-water) r=0.9987 r=0.9999 and AsV solutions. The concentration of HCl and reaction coil length were 2.0 mol l-1 and 200 cm, respectively.The AsIII and AsV concentrations were 1.0 mg l-1. * [As] expressed as mg l-1. Journal of Analytical Atomic Spectrometry, December 1997, Vol. 12 1379obtained were linear up to a concentration of 1.25 mg l-1 for indicative value of 1.461 mg kg-1. Results obtained were 1.28±0.03 mg l-1 and 1.51±0.02 mg kg-1, respectively. These which a signal intensity lower than 200 was obtained. For samples with a high As concentration, the use of a low gain was results are in good agreement with the certified values.selected to obtain an atomic fluorescence intensity below 200. The detection and quantification limits, defined as 3sb/m Application and 10sb/m, where sb is the standard deviation of 11 measurements of a blank and m is the slope of the calibration graphs, The proposed method was applied to the determination of As were 5.0 and 17 ng l-1, respectively. As can be observed, the in diVerent samples from the Mediterranean Sea.Results sensitivity achieved is adequate for As determination in non- ranged from 1.1 to 1.6 mg l-1, with RSDs from 0.5 to 1.7%. polluted sea-water samples and in open ocean sea-water Hence, the developed procedure is suitable for the direct samples. The sensitivity achieved by using this direct procedure determination of As in natural sea-water samples with good for As determination is considerably improved with respect to precision. published literature values concerning the use of HG-ETAAS.Hence, Tsalev et al.29 reported characteristic masses of 31–35 pg for inorganic and organic As species using Ir–Zr- CONCLUSION coated graphite tubes. Ding and Sturgeon30 reported a detec- The use of HGAFS provides adequate sensitivity and accuracy tion limit of 84 ng l-1 using Ir- and Pd-coated graphite tubes for the direct determination of As in coastal and open ocean and electrochemical hydride generation. Willie31 reported a sea-water samples, avoiding the tedious preconcentration pro- detection limit of 140 ng l-1 using an Ir-coated graphite tube, cedures required by using other AAS techniques.The number corresponding to a characteristic mass of 41 pg. of samples that can be analysed is 40 per hour, making the As compared with the previously reported procedures for method suitable for routine analysis and monitoring studies. the simultaneous determination of As, Se, Sn and Hg by nondispersive atomic fluorescence,21 the procedure developed here is simpler, because it does not require the use of expensive REFERENCES radiofrequency-excited discharge lamps.Additionally, the detection limit found by us is four times lower than that 1 Chakraborti, D., de Songhe, W., and Adams, F., Anal. Chim. reported for a 5 ml sample volume. Acta, 1980, 119, 331. 2 Saeed, K., and Thomassen, Y., Anal. Chim. Acta, 1981, 130, 281. The repeatability (relative standard deviation for seven repli- 3 Metals and T heir Compounds in the Environment, ed.Merian, E., cate measurements of the same sample spiked at diVerent VCH, New York, 1991, vol. II.22, p. 1101. concentration levels) [RSD (%)] obtained was lower than 4 Guo, X., and Guo, X., Anal. Chim. Acta, 1995, 310, 377. 4.0% for all concentrations tested, as can be seen in Table 3. 5 Fang, Z., Xu, S., and Tao, G., J. Anal. At. Spectrom., 1996, 11, 1. Analytical recovery values close to 100% were obtained for 6 Dedina, J., and Tsalev, D.L., Hydride Generation Atomic experiments carried out using sea-water samples spiked with Absorption Spectrometry, Wiley, Chichester, 1995. 7 Amankakwash, S. A., and Fasching, J. L., Talanta, 1985, 32, 111. AsIII and AsV. The values obtained, shown in Table 4, were 8 Nakashima, S., Fresenius’ J. Anal Chem., 1991, 341, 570. calculated using the slope of the calibration graph for AsIII. As 9 Nakashima, S., and Yagi, M., Bunseki Kagaku, 1983, 32, 535. can be seen, AsV is reduced eYciently using the selected 10 van Elteren, J.T., Gruter, G. J., Das, H. A., and Brinkman, U. A. experimental conditions. Th., Int. J. Environ. Anal. Chem., 1991, 43, 45. The accuracy of the method was evaluated by analysing 11 van Cleuvenbergen, R. J. A., van Mol, W. E., and Adams, F. C., diVerent reference materials: NASS-4, with a certified inorganic J. Anal. At. Spectrom., 1988, 3, 169. 12 Hueber, D., Smith, B. W., Madden, S., and Winefordner, J. D., As concentration of 1.26±0.04 mg l-1, and CRM-403, with an Appl. Spectrosc., 1994, 48, 1213. 13 Stockwell, P. B., and Corns, W. T., Analyst, 1994, 119, 1641. 14 Heitmann, U., Sy, T., Hese, A., and Schoknecht, G., J. Anal. At. Spectrom., 1994, 9, 437. Table 3 Analytical recovery studies* 15 Stockwell, P. B., and Corns, W. T., J. Autom. Chem., 1993, 15, 79. 16 Waldock, M. J., Mikrochim. Acta, 1992, 109, 23. [As] added/mg l-1 Analytical recovery (%) 17 Corns, W. T., Stockwell, P. B., Ebdon, L., and Hill, S.J., J. Anal. At. Spectrom., 1993, 8, 71. AsIII AsV 18 Guo, T., Liu, M., and Schrader, W., J. Anal. At. Spectrom., 1992, 7, 667. 0.25 101±1 103±3 0.5 107±3 102±2 19 Ebdon, L., and Wilkinson, J. R., Anal. Chim. Acta, 1987, 194, 177. 20 Rigin, V. I., Zh. Anal. Khim., 1986, 41, 46. 0.75 100±4 108±2 1.0 101±1 100±1 21 D’Ulivo, A., PapoV, P., and Festa, C., Talanta, 1983, 30, 907. 22 Rigin, V. I., Zh. Anal. Khim., 1983, 38, 1060. 1.25 99±2 104±2 23 Ebdon, L., Wilkinson, J. R., and Jackson, K. W., Anal. Chim. Acta, 1982, 136, 191. * Natural sea-water samples were spiked with diVerent concentrations of AsIII or AsV and analysed by the proposed procedure using 24 Kuga, K., and Tsujii, K., Anal. L ett., 1982, 15, 47. 25 Azad, J., Kirkbright, G. F., and Snook, R. D., Analyst, 1980, AsIII standards for calibration. 105, 79. 26 D’Ulivo, A., Fuoco, R., and PapoV, P., T alanta, 1985, 32, 103. 27 Lo� pez, A., Torrealba, R., Palacios, M. A., and Ca�mara, C., T alanta, 1992, 39, 1342. Table 4 Repeatability of As determination by HGAFS* 28 Torrealba, R., Bonilla, M., Palacios, M. A., and Ca�mara, C., Analusis, 1994, 22, 478. AsIII concentration/mg l-1 RSD (%) 29 Tsalev, D. L., D’Ulivio, A., Lampugnani, L., Di Marco, M., and 0 3.1 Zamboni, R., J. Anal. At. Spectrom., 1996, 11, 989. 0.25 2.1 30 Ding, W. W., and Sturgeon, R. E., Spectrochim. Acta, Part B, 0.5 3.2 1996, 51, 1325. 0.75 4.0 31 Willie, S. N., Spectrochim. Acta, Part B, 1996, 51, 1781. 1.0 1.5 Paper 7/05264B 1.25 1.7 Received July 22, 1997 * Values reported were found for seven independent measurements Accepted September 22, 1997 of each solution. 1380 Journal of Analytical Atomic Spectrometry, December 1997, V

ISSN:0267-9477    

DOI:10.1039/a705264b

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Fast Species-selective Screening for Organolead Compounds in Gasoline by Multicapillary Gas Chromatography With Microwave-induced Plasma Atomic Emission Detection ISAAC RODRIGUEZ PEREIRO AND RYSZARD £OBIN� SKI* CNRS URA 348, Universite� Bordeaux I, 351, Crs de la L iberation, F-33 405 T alence, France Multicapillary GC is proposed and optimized as a tool for the separation method of choice. AAS in both the flame and electrothermal modes,18–22 hydrogen flame ionization (FID),23 introduction of gasoline samples into an MIP atomic emission spectrometer for the fingerprinting and species-selective surface emission flame photometry (FPD),24 alternating current plasma (ACP),25 MIP equipped with the Beenaker TM010 analysis for organolead additives.The coupling of multicapillary GC with MIP-AES not only allows speciation cavity,26 and electron ionization mass spectrometry (MS)27 have been used for detection. Temperature programmed runs of alkyllead but is also faster than analysis for total lead in gasoline by flame AAS or ICP-AES.All five tetraalkyllead of 20 min are common to achieve the separation and the need to cool down the oven to the starting temperature (usually species (MenEt4-nPb, n=1–4) are baseline separated and quantified within 10 s in comparison with about 10–20 min about 60 °C) limits the throughput to 1–2 samples h-1. Recently, the use of a multicapillary column (a bundle of required by conventional GC procedures.Separation is carried out in the isothermal mode but neither resolution nor sample about 1000 1 m long, 40 mm id wall-coated open-tubular capillaries) was shown to shorten dramatically the GC analysis capacity is sacrificed in comparison with conventional capillary GC with oven temperature gradient programming. The for organohalogens and tin.28,29 The objective of this paper was to evaluate multicapillary GC as a tool for sample throughput reaches about 100 samples h-1. The detection limit is below 1 mg l-1 and the linearity range spans four introduction into an MIP atomic emission spectrometer for the fingerprinting and species-selective analysis of petrol for concentration decades.The method was validated by the analysis of the NIST SRM 2715 (Lead in Fuel ) reference organolead additives. material. EXPERIMENTAL Keywords: T etraalkyllead; multicapillary gas chromatography; microwave-induced plasma atomic emission detection; Instrumentation speciation; gasoline Chromatographic separations were carried out using an HP Model 6890 gas chromatograph (Hewlett-Packard, Tetraalkyllead (TAL) compounds are highly toxic but, despite Wilmington, DE, USA) equipped with a capillary split/splitless severe restrictions in many countries, they are still used as injection port with electronic pressure control.Detection was antiknock agents in petrol.1,2 Tetramethyllead (Me4Pb), tetra- achieved with an HP Model G2350A atomic emission detector.ethyllead (Et4Pb) and the products of their catalytic redistri- Injections were made by means of an HP 6890 series automatic bution: Me3EtPb, Me2Et2Pb and MeEt3Pb, are commercially sampler. Data were handled using an HP Model D3398A used. Leaded petrol contains 0.3–1.5 mg l-1 of lead; this con- ChemStation. Analyte species were separated on a multicapiltent drops below a few mg l-1 for unleaded petrols. The need lary column which consisted of about 900 1 m longs 40 mm id for a rapid screening and discrimination of regular and capillaries coated with 0.2 mm of SE-30 (Alltech Associates, unleaded petrol from various sources, studies of automotive San Jose, CA, USA).It was connected at both ends to lead air pollution,3–5 early warning of accidental petrol spill- deactivated alumina tubes (about 0.3 m×0.53 mm id). Because ages6,7 and fingerprinting of accelerants from arson debris8 the alumina tube connector delivered with the multicapillary make species-selective analysis of organolead in petrol of column could not be introduced into the microwave discharge paramount importance from industrial, environmental and tube, a piece of deactived silica tube (about 0.6 m×0.32 mm) forensic points of view.was used as the transfer line. An HP-5 (30×0.32 mm×0.25 mm) In order to circumvent the diVerence between the analytical capillary column was used for comparison studies. responses from various organolead compounds in flame AAS (FAAS) or ICP-AES, lead in petrol is usually determined upon Reagents, Standards and Samples reduction of TAL compounds by ICl (iodine monochloride) followed by the extraction of ionic organolead compounds Analytical-reagent grade pentane and hexane obtained from Aldrich (Milwaukee, WI, USA) were used throughout.into water prior to determination.9 This method is time consuming and does not allow for speciation of lead. Some Commercial solutions of tetramethyllead (39.3% Me4Pb, 19% 1,2-dichloroethane, 18% 1,2-dibromoethane and 13% toluene) success in speciation by vapour phase sample introduction into a flame was reported,10,11 but it is the coupling between and tetraethyllead (>99% Et4Pb) were obtained from Octel (Paris, France).Individual standard solutions and mixtures of chromatography (to separate the species from the bulk sample and from each other) and atomic spectrometry (to quantify both species were prepared in pentane. Commercial solutions and concentrated standards were manipulated in a separate them) that is now considered a reference method for speciesselective analysis for lead in gasoline.12–14 room from where the GC–MIP-AES system was installed and stored in double glass containers to avoid contamination Despite some eVorts in speciation of TAL compounds by HPLC with ICP source detection,15–17 GC is usually the problems.Petrol (regular and unleaded) samples were obtained Journal of Analytical Atomic Spectrometry, December 1997, Vol. 12 (1381–1385) 1381at local filling stations. A reference material with a certified selectivity of the detector minimizes the background even in the analysis of complex samples. content of Et4Pb, SRM 2715 Lead in Fuel, was obtained from the National Insitute of Standards and Technology (NIST, Gaithersburg, MD, USA). This material is a mixture of iso- Optimization of the Separation Conditions octane (91% v/v) and heptane (9% v/v) spiked with a tetraethyllead motor mixture.The certified value for lead is based The goal of optimization was to obtain a baseline separation on results obtained by thermal ionization dilution mass of the five organolead compounds within a time equal to the spectrometry (TIMS). half-width of a peak in conventional flow-injection hydride generation atomic spectrometric analysis (about 10 s). The parameters to be optimized include the separation temperature Procedure and the column flow rate.The limitations to be taken into consideration during the optimization included: (1) separation Optimization was carried out using an authentic lead petrol sample diluted about 3000 times in pentane. Leaded petrol of the first eluting peak, Me4Pb, from the solvent peak; (2) baseline separation of the analyte compounds; (3) acquisition samples were diluted about 2000–4000-fold with pentane prior to injection into the GC–MIP-AES system. Diesel and of a representative number of data points for a short transient signal; and (4) matching the optimum detector make-up gas unleaded petrol samples were diluted about 10 times.Optimum gas chromatographic and detection conditions are given in flow. These many limitations and only two parameters to be optimized allowed the use of a manual univariate optimization Table 1. rather than a factorial or a simplex experiment. RESULTS AND DISCUSSION Separation eYciency using a multicapillary column The eYciency (the height equivalent to a theoretical plate or the number of theoretical plates per unit length) of gas chroma- Fig. 1 shows the Van Deemter (Golay–Giddings) curve for a standard of Et4Pb in pentane, obtained using the multicapillary tographic separations is known to increase with the decreasing inner diameter of the capillary.30,31 In consequence, shorter column. Whereas a conional 0.32 mm capillary with a similar coating (100% polydimethylsiloxane) shows a narrow columns can be used which result in faster separations.The decrease in the column diameter reduces the column capacity maximum of eYciency, defined as the minimum value of the height equivalent to a theoretical plate (HETP), with a gas (allowable sample load) and increases the working column head pressure. Assembling a large number (about 1000) of velocity range of 20 cm s-1 of He, corresponding to a column flow of 1 ml min-1, the multicapillary column presents a small diameter capillaries into a bundle increases the crosssection of the carrier flow rate and allows the capacity to be very broad maximum (100–180 cm s-1 or 65–110 ml min-1).This proves that high flow rates can be used to shorten increased without compromising eYciency. The carrier gas flow rates in multicapillary chromatography chromatographic separations without sacrificing peak resolution. reach the range of 50–300 ml min-1 and thus match those required by an MIP torch. This makes multicapillary chromatography an attractive sample introduction technique for MIPSeparation ofMe4Pb from the solvent peak AES.Other advantages of MIP are the low dead volume of the detector, which does not distort the sharp (half-width Gasoline is a mixture of various light hydrocarbons with down to 0.1 s) chromatographic peaks, and a fast response and 2,2,4-trimethylpentane and toluene being major compounds. a detection limit for lead below 0.1 pg. The high elemental These compounds elute close to Me4Pb and interferes with the detection of the latter leading even to extinguishing of the plasma.Because of the proximity of the boiling-points it proved to be impossible to find optimum separation conditions Table 1 Optimum conditions for speciation of organolead in gasoline by capillary and multicapillary chromatography with MIP-AES which would allow one to vent the bulk of the matrix before detection the Me4Pb band reaches the detector. The only solution to this problem appeared to be a reduction of the amount of GC Parameters — gasoline sample to a level tolerated by the detector.Injection port Split/splitless Splitting the sample at the injector up to a ratio of 651 was Injection mode Split found insuYcient to achieve undisturbed detection of Me4Pb; Injection port temperature 250 °C Injected volume 1 ml Multicapillary column Column flow 95 ml min-1 Split ratio 651 Oven programme Isothermal 130 °C HP-5 column Column flow 1.5 ml min-1 Split ratio 2051 Oven programme Temperature programmed Initial temperature 60 °C (1 min) Rate 20 °C min-1 Final temperature 200 °C (2 min) AES Parameters — Transfer line temperature 250 °C Cavity block temperature 250 °C Wavelength 405.78 nm 261.42 nm Helium make-up flow 285 ml min-1 Spectrometer purge flow 2 l min-1 Solvent vent 0.07 min Fig. 1 Van Deemter curve obtained for an Et4Pb solution using the H2 pressure 25 psi multicapillary column at 120 °C. HETP, height equivalent to a theoreti- O2 pressure 30 psi cal plate. 1382 Journal of Analytical Atomic Spectrometry, December 1997, Vol. 12higher split ratios were not examined because of their being possible chromatogram with baseline resolution of analytes) was chosen. impractical for technical and economic (helium consumption) reasons. Dilution of a sample was thus necessary. Pentane, An increase in temperature [Fig. 2(b)] leads to a continuous decrease in retention time but over 130 °C the bulk of the being the most volatile hydrocarbon in liquid state at room temperature, was chosen as a diluent.The separation of the solvent (pentane) is no longer separated from the most volatile organolead compound (Me4Pb). The optimum conditions bulk of the pentane from Me4Pb depends on the column flow rate and temperature and is possible at a large number of chosen were thus 95 ml min-1 and 130 °C, which ensured the elution of Et4Pb in less then 15 s whereas the elution envelope combinations of these parameters. The column flow rate should be set within the broad minimum of the Van Deemter curve, of all the alkylleads (the diVerence between the earliest and the latest peak) was about 10 s.i.e., between 65 and 105 ml min-1. The working temperature range would then be between 145 and 125 °C. In order to reduce further the duration of a GC run, an attempt was made to increase rapidly (100 °C min-1) the Under these and the intermediate conditions the majority of pentane is vented whereas the background signal from the temperature after the Me4Pb peak had passed.The resulting gain in retention time of 2–3 s obtained did not, however, residual matrix is dealt with by the high elemental selectivity of the detector. A minimum dilution factor of 10 was applied. justify the extra time necessary to cool down the oven to the initial temperature. Baseline separation between alkyllead compounds Acquisition of a representative number of data points for a Many combinations of lower or higher column flows (within transient signal the minimum of the Van Deemter curve) with higher or lower oven temperatures allow organolead separation within 15–30 s.The high linear flow rates (up to 3 m s-1) through a multicapil- In the optimum column eYciency range the eVect of flow rate lary column set strong requirements in terms of the on the duration of the GC run (retention time of Et4Pb) is time constant of the detector to ensure the acquisition of a negligible [Fig. 2(a)] compared with the eVect of the column representative number of data points for a short transient temperature [Fig. 2(b)]. Fig. 2(a) shows that for column flow signal. rates over 100 ml min-1 (corresponding to a linear velocity of Under the optimized conditions the typical peak half-width 150 cm s-1) hardly any gain in the retention time of Et4Pb is 0.06, which is very close to the maximum data acquisition (duration of the GC run) is observed.Because of the necessity rate of the AES system (Fig. 3). Speeding up the separation for low helium consumption (the column flow is multiplied by by increasing the flow rate or temperature will inevitably lead the split ratio!) this flow rate (the lowest allowing the fastest to a loss of precision and even missing peaks. Optimization of Detector Parameters When multicapillary GC is coupled to AES detection the contribution of the column flow to the cavity flow is much higher than in conventional capillary GC.This makes optimization of the detection conditions necessary. The optimum make-up gas flow was 285–290 ml min-1. The optimum oxygen pressure was 25–30 psi; the optimum hydrogen pressure was also 25–30 psi. Only for hydrogen was the optimum pressure and the shape of the curve (a sharp maximum) diVerent from that reported previously (90 psi) for the HP 5921A detector.32 The system software allows the lead response to be recorded at two diVerent emission lines: 261.42 and 405.78 nm.The latter was found to be twice as sensitive as the former and was therefore chosen. High concentrations of alkylleads in petrol samples allow their determination at either of the above lines. Fig. 2 Influence of: (a) carrier gas flow rate (oven temperature 120 °C); (b) oven temperature (column flow rate 95 ml min-1), on peak width, retention time and peak height for Et4Pb. A 100% response corre- Fig. 3 Sampling of data points in a transient signal after sample sponds to a retention time of 0.88 min, a peak width of 1.4 s and a peak height of 30 arbitrary units.Above 130 °C the Me4Pb signal is introduction by multicapillary chromatography. 1, Me4Pb; 2, Me3EtPb. unresolved from the solvent peak. Journal of Analytical Atomic Spectrometry, December 1997, Vol. 12 1383Analytical Figures of Merit 20 ng ml-1) in pentane was about 3% for Me4Pb and 1–2% for Et4Pb. Fig. 4 compares typical chromatograms for a sample of gaso- Taking into account the duration of a GC run a sample can line obtained using a multicapillary [Fig. 4(a)] and a convenbe injected every 20 s, which results in a maximum theoretical tional [Fig. 4(b)] column under optimum conditions. It should throughput of 180 samples h-1. This rate can be achieved for be noted that the chromatogram on a conventional column is diluted gasolines (leaded gasolines) because no dirt is accumualso, to our knowledge, the shortest reported.A duration of lated on the column. Lower dilutions require more time for 12–20 min is usual in the literature.22,25–27,32 cleaning of the column. In practice, however, the sample In the isocratic elution mode the sensitivity measured in throughput is limited by the autosampler and the software, peak height mode decreases linearly with the retention time which requires the acquisition to be accomplished before because of peak broadening. Indeed, the values obtained for preparing for a new injection.Samples can be injected every Me4Pb are superior to those for Et4Pb by a factor of 3–4 2 min which results in a throughput of 30 samples h-1. (Table 2). The absolute detection limits reach 0.1 pg. These values, taking into account the split ratio, are similar to those published elsewhere.32 When using a multicapillary column an Analysis of Petrol Samples autosampler should be used and the split should always be Table 3 summarizes results obtained for the analyses of leaded open to ensure a fast injection.and unleaded gasoline samples and that of the reference Calibration graphs for Me4Pb and Et4Pb standards in material. Except for Me4Pb and Et4Pb, Me3EtPb, Me2Et2Pb pentane were obtained by working under the conditions given and MeEt3Pb are present in leaded gasoline at similar concenin Table 1. Standards of both compounds at five diVerent levels tration levels. Owing to isothermal elution, these species could of concentration between 0 and 100 ng ml-1 as lead were be quantified using response factors obtained by interpolation injected in duplicate.A good linearity was observed at both of the response factors of Me4Pb and Et4Pb. Similar results emission lines with correlation coeYcients (r2) from 0.9990 to (not shown) were obtained in the peak area quantification 0.9996 (Table 2). mode which oVers the same response factor for each of the Typical precision in peak height and area for ten consecutive TAL compounds, irrespective of the retention time.injections of a standard of Me4Pb and Et4Pb (about Several diesel and unleaded petrol samples were analyzed and in all the samples the presence of Et4Pb at levels around 0.1–0.2 mg ml-1 (as Pb) was detected, which is about 1000 times less than in leaded petrol. The origin of these concentrations remains unclear but it is likely to be a result of contamination of tanks by leaded petrol during transport and/or storage.The procedure developed was validated with the reference fuel SRM 2715. A good agreement (Table 3) was obtained between the certified and found values for Et4Pb (the only compound present in the mixture). CONCLUSIONS Multicapillary GC is an attractive sample introduction technique for MIP-AES analysis of gasoline, oVering a 10-fold increase in speed over conventional capillary chromatography Table 3 Determination of organolead species in fuel samples Concentration as Pb/mg g-1±s* Sample Me4Pb Me3EtPb Me2Et2Pb MeEt3Pb Et4Pb Leaded 11.5±0.9 6.8±0.3 6.7±0.2 5.5±0.2 130±4 petrol A Leaded 15.4±0.4 6.6±0.5 6.4±0.3 4.8±0.1 120±1 petrol B Unleaded <0.002 <0.002 <0.002 <0.002 0.2±0.0 petrol A Unleaded <0.002 <0.002 <0.002 <0.002 0.1±0.0 petrol B Diesel <0.002 <0.002 <0.002 <0.002 0.2±0.0 Fig. 4 Chromatograms for a mixture of alkyllead compounds in SRM 2175† <0.002 <0.002 <0.002 <0.002 792±20 leaded gasoline under the optimum conditions (cf. Table 1).(a), Multicapillary column; (b) HP-5 column. 1, Me4Pb; 2, Me3EtPb; 3, * Standard deviation for seven measurements. † Certified value, 784±4 mg g-1. Me2Et2Pb; 4, MeEt3Pb; and 5, Et4Pb. Table 2 Calibration graphs and detection limits (DL) for Me4Pb and Et4Pb by multicapillary GC–MIP-AES 405.78 nm 261.42 nm DL/ng ml-1 as Pb Standard Correlation DL/ng ml-1 as Pb Standard Correlation Compound (S/N=3) Slope error coeYcient (r2) (S/N=3) Slope error coeYcient (r2) Me4Pb 0.07 3.38 0.05 0.9992 0.10 2.39 0.02 0.9996 Et4Pb 0.18 1.36 0.01 0.9994 0.42 0.56 0.01 0.9995 1384 Journal of Analytical Atomic Spectrometry, December 1997, Vol. 1211 Bagdi, G., Lakatos, J., and Lakatos, I., J. Anal. At. Spectrom., without loss of eYciency. The compatibility of flow rates with 1992, 7, 769. the high make-up gas flow required for detection of lead by 12 �©obin¢¥ ski, R., Dirkx, W. M. R., Szpunar-�©obin¢¥ ska, J., and Adams, MIP-AES make it an attractive tool for speciation analysis of F.C., Anal. Chim. Acta, 1994, 286, 381. this element. Carrying out the separations in the isothermal 13 Baxter, D. C., and Frech, W., Pure Appl. Chem., 1995, 67, 615. mode instead of in the temperature programming mode permits 15 Al-Rashdan, A., Vela, N. P., Caruso, J. A., and Heitkemper, D. T., a cumbersome GC oven to be avoided and the dimensions of J. Anal. At. Spectrom., 1992, 7, 551. 16 Ibrahim, M., Gilbert, T.W., and Caruso, J. A., J. Chromatogr. the separation unit to be reduced. The rapidity of analysis Sci., 1984, 22, 111. makes multicapillary chromatography worthy of consideration 17 Cammann, K, Robecke, M., and Bettmer, J., Fresenius¡� J. Anal. as a sample introduction tool for volatile compounds in organic Chem., 1994, 350, 30. solvents because it is faster than the washout times of the 18 De Jonghe, W., Chakraborti, D., and Adams, F. C., Anal. Chim. spray chamber in ICP techniques.Acta, 1980, 115, 89. 19 Radojevic, M., Allen, A., Rapsomanikis, S., and Harrison, M., This study was financially supported by the EC (Contract: Anal. Chem., 1986, 58, 658. 20 Blais, J. S., and Marshall, W. D., J. Environ. Qual., 1986, 15, 255. SMT4-CT96.2044 Automated Speciation Analyser). I. R. P. 21 Forsyth, D. S., Anal. Chem., 1987, 59, 1742. acknowledges a post-doctoral fellowship from the Spanish 22 Brunetto, M. R., Burguera, J. L., Burguera, M., and Chakraborti, government.D., At. Spectrosc., 1992, 13, 123. 23 Gallagher, M. M., and Hill, H. H., J. High Resolut. Chromatogr., 1990, 13, 694. REFERENCES 24 Xu, F. Z., Jiang, G. B., and Zhao, J. M., Fenxi Huaxue, 1995, 1 Van Cleuvenbergen, R., and Adams, F. C., in Handbook of 23, 1165. Environmental Chemistry, ed. Hutzinger, O., Springer, Berlin, 25 Constanzo, R. B., and Barry, E. F., J. High Resolut.Chromatogr., 1990, vol. 3E, p. 99. 1989, 12, 522. 2 Nriagu, J. O., Sci. T otal Environ., 1990, 92, 13. 26 Estes, S. A., Uden, P. C., and Barnes, R. M., J. Chromatogr., 1982, 3 De Jonghe, W. R. A., Chakraborti, D., and Adams, F. C., Environ. 239, 181. Sci. T echnol., 1981, 15, 1217. 27 Nerin, C., Pons, B., Martinez, M., and Cacho, J., Mikrochim. 4 US Environmental Protection Agency, Environmental Criteria Acta, 1994, 112, 179. and Assessment OYce, Air Quality Criteria for L ead, Research 28 Cooke, W. S., Wals, J. W., and Wiedemer, R. T., Book of Abstracts Triangle Park, NC, 1986. of Pittcon ¡�97, Atlanta, GA, p. P19. 5 �©obin¢¥ ski, R., Analyst, 1995, 120, 615. 29 Schmitt, V. O., Rodriguez Pereiro, I., and �©obin¢¥ ski, R., Anal. 6 Wong, P. T. S., Chau, Y. K., Yaromich, J., Hodson, P., and Commun., 1997, 141. Whittle, M. Can. T ech. Rep. Fish. Aquat. Sci. 1602, Ministry of 30 Grant, D. W., Capillary Gas Chromatography, Wiley, Chichester, Supply and Services Canada, Ottawa, 1988. 1995. 7 Harrison, G. F., in L ead in the Marine Environment, ed. Branica, 31 Grob, R. L., Modern Practice of Gas Chromatography, Wiley, M., and Konrad, Z., Pergamon, Oxford, 1980, p. 305. Chichester, 3rd edn., 1995. 8 Frontela, L., Pozas, J. A., and Picabea, L., Forensic Sci. Int., 1995, 32 �©obin¢¥ ski, R., and Adams, F. C., Anal. Chim. Acta, 1992, 262, 285. 75, 11. 9 Ouyang, Y., Mansell, R. S., and Ou, L. T., Bull. Environ. Contam. Paper 7/04050D T oxicol., 1994, 52, 760. Received June 10, 1997 10 Maur©¥¢¥, A. R., de la Guardia, M., and Mongay, C., J. Anal. 1989, 4, 539. Accepted September 29, 1997 Journal of Analytical Atomic Spectrometry, December 1997, Vol. 12 1385

ISSN:0267-9477    

DOI:10.1039/a704050d

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Application of a Rapid Sequential Inductively Coupled Plasma Optical Emission Spectrometric Method for the Analysis of Materials With Linerich Emission Spectra by Different Means of Sample Introduction D. MERTENa , J. A. C. BROEKAERT* a AND A. LE MARCHAND aUniversity of Dortmund, Department of Chemistry, D-44221 Dortmund, Germany aISA Jobin Yvon, 16–18 Rue du canal, F-91165 L ongjumeau Cedex, France Rapid sequential atomic emission spectrometry with the respect a proper line selection is required and rapid sequential spectrometry to a certain extent can oVer possibilities here.IMAGE system (ISA Jobin Yvon, Longjumeau, France) in combination with an inductively coupled plasma is described. This is the case when rapid sequential emission spectrometry in an ultra-rapid slew-scan mode is performed, as it is possible As analytical figures of merit the precision, limits of detection, spectral resolution and linear dynamic range are discussed in with the IMAGE system (ISA Jobin Yvon, Longjumeau, France). comparison with those of conventional sequential spectrometry.The IMAGE system is shown to allow a registration of the In the work presented, the application of a JY 24 spectrometer equipped with the IMAGE system for sequential emission spectrum in the wavelength range 165–750 nm in less than 2 min (compared with several hours when applying ICP atomic emission spectrometry to the analysis of samples with a linerich Zr matrix is described for diVerent means of conventional slew times and scanning at similarly high resolution) without any loss of spectral resolution.Thus, sample introduction. Results for the determination of Fe, Co, Mn and Ni in the presence of Zr as sample matrix for both spectral interferences can easily be detected, suitable lines for quantitative determinations can be selected and standard aqueous solutions and in work with slurries of ZrO2 ceramic powders are discussed. Figures of merit are compared with solutions used for calibration could even be adapted ‘on-line’ to the matrix of the sample to be analyzed.However, a those obtained by sequential ICP-OES with conventional spectrometry. Further, the possibilities for a direct survey deterioration in the detection limits by a factor of 3–10 has to be taken into account. Results are given for several means of analysis for ZrO2 powders by ICP-OES combined with laser ablation are discussed and compared with qualitative analysis sample introduction.It is shown for a solution of 2 mg l-1 Fe in the presence of a 5000-fold excess of Zr that the method making use of multi-channel detectors such as an SCD.1 can be used for accurate quantitative determinations at the minor and trace element concentration level in work with EXPERIMENTAL solutions and slurries. For laser ablation coupled to ICP-OES of briquetted ZrO2 ceramic powders, the potential for The equipment consists of a sequential ICP-OES instrument qualitative multi-element survey analysis from a large number (JY 24, ISA Jobin Yvon) utilizing a 0.64 m monochromator of laser impacts is shown.with a grating having 2400 grooves mm-1 and the IMAGE data acquisition system. The widths of the entrance and exit Keywords: Inductively coupled plasma; atomic emission slits are 30 and 20 mm, respectively. The IMAGE system spectrometry; laser ablation; slurry nebulization; fast scanning consists of high-speed electronics as well as a HDD (High acquisition; zirconium matrix Dynamic Detector, ISA Jobin Yvon) measurement system including powerful software for data acquisition and manipulation.In ICP-OES spectral interferences limit both the accuracy The instrumental parameters used for ICP-OES are given and the precision of trace analysis, especially for samples in Table 1. The emission spectra in the range from 165 to with matrices emitting linerich atomic emission spectra. Simultaneous spectrometers allow rapid analysis and high precision measurements when internal standardization is used.Table 1 Instrumental parameters However, the flexibility in line selection is very low and spectral interferences cannot easily be detected. Multi-channel detectors Sequential ICP-OES instrument, ICP-OES such as charge coupled devices (CCDs), charge injection devices JY 24 (ISA Jobin Yvon, Longjumeau, France) (CIDs) and segmental charge coupled devices (SCDs) make it Generator 800 W at 40.68 MHz possible to acquire parts of the emission spectrum of a sample Monochromator 0.64 m Czerny–Turner mounting simultaneously, but the covered wavelength range, the grating: 2400 grooves mm-1 obtainable spectral resolution and the dynamic range are Nebulizer for aqueous solutions Cross-flow (ISA Jobin Yvon) compromised. In contrast, conventional sequential spec- Nebulizer for slurries G.M.K.(Labtest, Ratingen, trometers oVer high flexibility and excellent detection limits. Germany) Peristaltic pump Perimax 12 (Spetec, Munich, Nevertheless, they are slow and, in particular, when with Germany) conventional sequential spectrometers the intensities of the Laser ablation system LSX-100 (FMS CETAC, spectral background are measured at arbitrary wavelengths Freudeuberg, Germany), and when the resolution of the spectrometer is low, spectral Nd–YAG (266 nm) interferences lead to systematic errors.For progress in this Journal of Analytical Atomic Spectrometry, December 1997, Vol. 12 (1387–1390) 1387750 nm are recorded in about 2 min. In a second step a which are recorded with the JY 24 spectrometer in the second order. wavelength calibration is carried out using several reference lines and the spectrum is converted digitally to make the In Fig. 1(b) the profile recorded for the Ni I 361.939 nm line in the first order is shown. The values for the full-width at individual intensity signals accessible for the software.The error in wavelength calibration is typically in the range half-maximum measured for these profiles, the physical linewidths and the resulting values for the spectral resolution are of only a few pm. Further, it is possible for narrow wavelength ranges to shift manually the measured peaks to the theoretical listed in Table 2. As can be seen from these results the spectral resolution in the second order is a factor of 2 better than in positions that are indicated by a database of atomic emission lines included in the software.the first order. These values also show that the spectral resolution of ICP-OES with the IMAGE approach is as good The linear dynamic range comprises six orders of magnitude. This is achieved by automatically adapting the photomultiplier as the resolution obtained with conventional sequential ICPOES instruments with medium resolution (7–11 pm).3 Thus, it voltage depending on the intensity of the signal. The scanning step for the grating used is 1.9 pm in the first order (305– can be concluded that no distortion in the peak shape as a result of detector hysteresis, which could influence the spectral 750 nm) and 0.95 pm in the second order (165–305 nm).The integration time per point is 0.5 ms. Grams/386 software resolution, is obtained. (Galactic Industries, Salem, USA) was used to process the data. Precision and limits of detection RESULTS AND DISCUSSION In Table 3 the detection limits obtained when using the IMAGE approach are compared with those obtained when Analytical Figures of Merit the same spectrometer is used in the conventional way of The achievable spectral resolution, the precision and the sequential analysis.It was found that the detection limits detection limits are important analytical figures of merit, which obtained with IMAGE (based on the standard deviations in well describe the spectrometric features of the equipment used the intensity of four replicates) are higher than those obtained in combination with the IMAGE system. This allows a comin the conventional mode of operation.This is understandable parison of the capabilities of the IMAGE approach with those from the fact that the relative standard deviations of the of conventional sequential spectrometers using integration intensity signals obtainable when using the same spectrometer times of a few hundred ms. in the conventional way are about 1% whereas those obtainable with the IMAGE system are much higher.However, the IMAGE system allows a registration of the emission spectrum Spectral resolution in the wavelength range 165–750 nm in less than 2 min, com- From the values of the full-width at half-maximum Dlexp of pared with a few hours with the conventional way of scanning recorded line profiles and tabulated physical widths Dlphys for using integration times per data step of 0.3 s with slew times atomic emission lines in ICP-OES, the instrumental resolution of 200 steps min-1, and without any loss of resolution.Dlinstr can be calculated.2 Dl2instr=Dl2exp-Dl2phys (1) Survey Analysis In Fig. 1(a) the profile recorded for the Fe II 239.562 nm line One of the major advantages of the IMAGE system is that is shown. The full-width at half-maximum (7.0 pm) is typical within 2 min the emission spectrum in the complete wavelength for atomic emission lines in the wavelength range 165–305 nm range 165–750 nm can be recorded with high resolution (about 7 pm in the second order and about 14 pm in the first order, Table 2) and with a high dynamic range (6 decades).Even for materials with linerich spectra, such as metals and advanced ceramic materials such as ZrO2-based ceramics, spectral interferences can easily be evaluated. This is of prime importance for the choice of suitable analytical lines for the analysis of these materials by atomic emission spectrometry. In Figs. 2 and 3 the spectra obtained with ICP-OES using diVerent methods for sample introduction of ZrO2 ceramics Table 2 Calculation of the spectral bandwidth Dlinstr2 Line/nm Dlphys/pm2 Dlexp/pm Dlinstr/pm Fe II 239.562 2.1 7.0 6.7 Ni I 361.939 2.7 14.6 14.4 Table 3 Detection limits (cL) and relative standard deviations (RSD) with the IMAGE approach as compared with conventional sequential spectrometry for the analysis of aqueous solutions by ICP-OES (four replicates) Conventional IMAGE RSD Line/nm cL/mg l-1 cL/mg l-1 (%) Fe II 238.204 6.3 38 1.3 Fe II 259.940 14 41 3.4 Co II 228.616 3.3 60 3.5 Mn II 257.610 4.3 16 13 Fig. 1 Profiles recorded for the lines Fe II 239.562 nm (a) and Ni I Mn II 259.373 15 22 16 361.939 nm (b) in ICP-OES using the JY 24 spectrometer in combi- Ni II 231.604 33 94 0.2 nation with the IMAGE system. 1388 Journal of Analytical Atomic Spectrometry, December 1997, Vol. 12Fig. 4 Calibration graph and its 90% confidence intervals for the Fe II 239.562 nm line in ICP-OES of aqueous solutions containing Fe Fig. 2 ICP atomic emission spectrum in the wavelength range 165– only by using the IMAGE approach. 750 nm, obtained for a ZrO2 powder by laser ablation (Powder 2, Cerasiv, Plochingen, Germany). Fig. 3 ICP atomic emission spectra for two ZrO2 powders (Cerasiv) obtained with slurry nebulization (Powder 2: with Y and Powder 3: without Y). are shown. In Fig. 2 the ICP emission spectrum for a ZrO2 powder, that was introduced into the ICP by means of laser ablation, is shown.The powder was pressed in a sample holder, the ablated amount of material was introduced into the ICP by means of a 0.8 l min-1 carrier gas flow and the emission spectrum was recorded. From this spectrum it can be seen that Fig. 5 ICP atomic emission spectra for a 10 g l-1 Zr solution with Zr has an extremely linerich emission spectrum with atomic and without 2 mg l-1 Fe in the vicinity of the Fe II 234.349 nm (a) spectral lines of largely varying intensities and that a qualitative and Fe II 239.562 nm (b) lines obtained with the IMAGE approach. analysis is possible.For the analysis of powders used for the production of advanced ceramics, slurry nebulization is commonly used. In excess of Zr. In particular, the influence of spectral interferences this technique the ceramic powder is suspended in water and on the accuracy of the trace determination can be evaluated. the slurry is directly aspirated and nebulized by a Babington On the basis of calibration graphs such as that for Fe II nebulizer.In Fig. 3 part of the emission spectrum in the vicinity 239.562 nm (Fig. 4), quantitative determinations can be carried of one of the most sensitive atomic emission lines of Y is out. The calibration graphs for spectral data acquisition with shown. It is obvious that by the use of the IMAGE system the the IMAGE approach were found to be linear up to 0.5 g l-1. information required for qualitative analyses as well as for Both the calibration and the analysis are based on four quantitative determinations can be obtained within a very replicates.The location of the background correction was set short time. For example, it can be concluded that from the manually to detect and overcome spectral interferences in the samples investigated only Powder 2 contains a significant linerich spectra. amount of Y, a point that is technologically relevant, as the In Fig. 5 the superimposed spectra for a solution of 10 g l-1 addition of oxides of Ca, Y or Mg is necessary for realizing a Zr and for a 10 g l-1 Zr solution containing additionally high temperature stability of ZrO2-based ceramics.The results 2 mg l-1 Fe (200 mg Fe per gram of Zr) in a spectral window in Fig. 3 thus demonstrate that a rapid identification of a given around the Fe II 234.349 nm line [Fig. 5(a)] and around the ZrO2 powder with respect to its Y content is possible with the Fe II 239.562 nm line [Fig. 5(b)] are shown. aid of the IMAGE approach. From the spectra it can be seen that the Fe II 234.349 nm line is strongly interfered with a Zr line, whereas for Fe II 239.562 nm no interferences are present. The influence of Quantitative Determinations spectral interferences on the accuracy of quantitative determinations is clearly shown by the recovery found for Fe in the With the aid of the approach described, quantitative determinations are simplified, since spectral interferences can presence of a Zr matrix as determined by a calibration with aqueous solutions containing Fe only.The results obtained for easily be detected and corrected for. This is shown by the determination of 2 mg l-1 Fe in the presence of a 5000-fold diVerent Fe lines are given in Table 4 where the interfered Journal of Analytical Atomic Spectrometry, December 1997, Vol. 12 1389Table 4 Quantitative determination of 2 mg l-1 Fe in the presence of within a short time and standard solutions for calibration can 10 g l-1 Zr be adapted eventually by automated dilutors (‘on-line’) to the matrix of the sample to be analyzed.With respect to the Line/nm Concentration/mg l-1 Interference recovery of Fe in the presence of an excess of Zr, it was shown Fe II 234.349 6.1±0.7 + that with the IMAGE system accurate quantitative determi- Fe II 239.562 2.0±0.4 - nations are possible as a result of the ease of selection of Fe II 238.204 2.4±0.5 - interference-free atomic emission lines.Further, it was shown Fe II 240.488 2.6±0.4 - that for diVerent types of sample introduction systems yielding steady-state signals, such as slurry nebulization and multiimpact laser ablation, data for a rapid survey, as required in Fe II 234.349 nm line is marked. Indeed, for the interfered Fe II 234.349 nm line the concentration found is much too high, qualitative analysis, can be collected. Thus, rapid sequential ICP atomic emission spectrometry is an interesting alternative whereas for the other lines used the concentrations found are in good agreement with the concentration of Fe provided in to multi-channel detectors.Nevertheless, the short integration time leads to a the solutions. deterioration of the precision as compared with conventional systems and, therefore, also to higher detection limits. CONCLUSIONS It has been shown that with rapid sequential spectrometry using the IMAGE approach the identification of spectral REFERENCES interferences for linerich ICP atomic emission spectra is easily 1 Rivier, C., and Mermet, J. M., Appl. Spectrosc., 1996, 50, 959. possible, which is of paramount importance for avoiding 2 Boumans, P. W. J. M., and Vrakking, J. J. A. M., Spectrochim. systematic errors. All line intensities within the wavelength Acta, Part B, 1986, 41, 1235. range 165–750 nm can be recorded within 2 min without any 3 Mermet, J. M., and Poussel, E., Appl. Spectrosc., 1995, 49, 12A. loss of resolution compared with conventional sequential ICPOES. Thus, the qualitative detection of metals and non-metals Paper 7/03388E is possible within a single spectrum and the choice of lines is ReceivedMay 16, 1997 not limited, due to the completely covered wavelength range. Accepted September 18, 1997 Accordingly, for qualitative analysis the data can be acquired 1390 Journal of Analytical Atomic Spectrometry, December 1997, Vol. 12

ISSN:0267-9477    

DOI:10.1039/a703388e

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Thermal Stabilisation of Phosphorus During Electrothermal Atomic Absorption Spectrometry Using Sodium Fluoride as Chemical Modifier EDWIN HERNA� NDEZa, JOSE� ALVARADO*a , FREDDY ARENASb AND MARIANO VE� LEZc aUniversidad Simo�n Bolý�var, Departamento de Quý�mica, Apartado Postal 89.000, Caracas 1080-A, Venezuela bInstituto Universitario de T ecnologý�a ‘Dr. Federico Rivero Palacio’, Departamento deMateriales, Apartado Postal 40347, Caracas 1040-A, Venezuela cUniversidad Simo�n Bolý�var, Departamento deMateriales, Apartado 89.000, Caracas 1080-A, Venezuela The stabilisation mechanism of phosphorus during its was found to provide the maximum stabilisation eVect.8 However, a drawback to its use is the markedly low sensitivity determination by electrothermal atomic absorption spectrometry in the presence of sodium fluoride as a chemical of the measurements, approximately 38% less than that achieved using Pd as the modifier.modifier has been studied. The study includes atomisation from dual-cavity platforms and analysis by scanning electron The aim of this work was to gain information on the mechanism by which NaF stabilises phosphorus, and also on microscopy, energy dispersive X-ray spectrometry and X-ray photoelectron spectrometry of the material accumulated onto the cause of the reported low sensitivity.To this purpose, atomisation from dual-cavity platforms, scanning electron totally pyrolytic graphite platforms. The results obtained from this study point toward a stabilisation mechanism which is of a microscopy (SEM), energy dispersive X-ray spectrometry (EDS) and X-ray photoelectron spectrometry (XPS) were used physical nature, whereby the analyte is retained in the matrix of the modifier until temperatures around 1350 °C are reached.to study the morphology as well as the chemical composition of the material formed at diVerent stages of an ETAAS heating The eVect of the modifier on the sensitivity of measurements of diVerent phosphorus compounds was also studied.A possible programme for phosphorus in the presence of NaF. In the field of ETAAS, SEM has been mainly used to study the explanation of the comparatively low sensitivity of phosphorus measurements with this particular modifier, approximately morphology of the atomisers which in turn can be related to the behaviour of the analyte through the shape of the analytical 38% less as compared with palladium, is given.signal.9–11 Only a few authors have reported SEM analysis as Keywords: Electrothermal atomic absorption spectrometry; a tool for the elucidation of stabilisation mechanisms of phosphorus determination; sodium fluoride; modification; analytes by chemical modifiers.7,12–15 The lack of information stabilisation mechanism dealing with the use of XPS16–19 for stabilisation mechanism interpretation is also notable. However, these techniques are used in this work to put forward arguments supporting a Electrothermal atomic absorption spectrometry (ETAAS) is possible mechanism for stabilisation of phosphorus using NaF not the most sensitive technique for phosphorus determination, as a modifier in ETAAS. because of both the design of modern spectrometers and the nature of this particular element.In fact, since the resonance lines of phosphorus lie in the UV–vacuum (l<190 nm), where absorption from the light source by oxygen in the air occurs, EXPERIMENTAL it is necessary to resort to a non-resonant doublet at Instrumentation 213.5–213.6 nm, with the corresponding reduction in sensitivity, for the ETAAS determination of this element.1 A Perkin-Elmer (U� berlingen, Germany) Model 2100 atomic absorption spectrometer was used together with a HGA 700 Additionally, phosphorus losses in the form of molecular species have been shown to occur at low pretreatment tempera- graphite atomiser system and an AS-70 auto sampler.Measurements were made at the non-resonant doublet at tures.2 Even though a few authors have reported successful phosphorus determinations in the absence of chemical modifi- 213.5–213.6 nm with a Perkin-Elmer phosphorus hollow cathode lamp.Dual-cavity platforms made of pyrolytic graphite cation,3,4 the use of a modifier is practically mandatory for the ETAAS determination of this element. and commercial pyrolytic, as well as laboratory-made standard, graphite platforms, inserted into pyrolytically coated graphite Several studies have been performed to find the best modifier for phosphorus determination. Among the compounds tested, tubes were used.Eppendorf (Westbury, NY, USA) micropipettes with plastic disposable tips were used to manually inject palladium nitrate, alone or mixed with calcium nitrate5 and lanthanum nitrate,6 provides high sensitivity and allows for the phosphorus solutions onto the dual cavity platforms. A Philips Analytical (Eindhoven, The Netherlands) Model high-temperature pretreatment.Unfortunately these compounds are relatively expensive, and lanthanum has been XL 30 scanning electron microscope was used to study the morphology of the material accumulated onto pyrolytic graph- shown to considerably reduce the useful lifetime of the atomisers and platforms.7 ite platforms. In the dark region below the photomicrographs the following information is displayed from left to right: Recently, the use of fluoride compounds was proposed as an alternative for phosphorus chemical modification.8 It was acceleration voltage (kV), magnification, detector (SE), working distance (WD) and scale (mm).EDS was performed found that some fluoride compounds grant phosphorus a thermal stability comparable to that provided by Pd and La, with an ultra-thin windows (UTW) detector attached to the XL 30 instrument, allowing element detection analysis from as well as oVering the advantages of being less expensive, presenting no memory eVects and causing less deterioration of boron onwards.A Leybold Heraeus (Export, PA, USA) Model LH-11 X-ray the atomiser. Among the fluoride compounds studied, NaF Journal of Analytical Atomic Spectrometry, December 1997, Vol. 12 (1391–1396) 1391fluorescence analyser was used for identification of the com- obtained after heating, the platforms served as sample supports so that disturbance of the residue after its collection was pounds formed at some pretreatment temperatures.avoided. The platform, with the material deposited on it, was placed directly into the corresponding spectrometer analysing Reagents and Standards chamber and the analysis was carried out. No Au or Au–Pt sputtered coatings, usually applied to non-conductive materials Ammonium dihydrogen phosphate (Merck, Elmsford, NY, for SEM or EDS studies, were used in these experiments in USA, analytical-reagent grade), diammonium hydrogen phosorder to avoid any chemical disturbance of the material phate (Merck, analytical-reagent grade), sodium hydrogen deposited on the platform.phosphate, potassium dihydrogen phosphate (Mallinckrodt, St. Louis, MO, USA, analytical-reagent grade), ammonium hexafluorophosphate (Aldrich, Milwaukee, WI, USA, analyt- RESULTS AND DISCUSSION ical-reagent grade) and sodium hexafluorophosphate (Aldrich, analytical-reagent grade) were used to prepare phosphorus Experiments with Dual-cavity Platforms solutions.Sodium fluoride (Mallinckrodt, analytical-reagent The use of dual-cavity platforms, which allows separate injecgrade) was used as a chemical modifier. Distilled, de-ionised tions of the modifier and the analyte, is a suitable way to study water (18MV cm) was used throughout for sample preparation some processes that take place within the graphite furnace and dilution purposes. Nitric acid (J. T. Baker, Phillipsburg, during heating. If care is taken not to mix the solutions, NJ, USA, ULTRATREX II Ultrapure reagent) was used for information about either gas or condensed phase reactions can stabilising the phosphorus solutions.be obtained.7, 20–25 Fig. 1 shows the charring plots for 5 ml of a 100 mg ml-1 phosphorus solution atomised from dual-cavity platforms, using 5 ml of a 0.08% m/v NaF solution injected Procedure either in the same cavity as the or in a Preparation of standards diVerent cavity. This amount of modifier was found to be optimal for stabilisation of the amount of phosphorus con- Phosphorus solutions (1000 mg l-1) were prepared by direct tained in the test solutions used here.Greater amounts did weighing of the corresponding phosphorus compound. Nitric not favour either the stabilisation of the analyte or the sensi- acid was added to give a final HNO3 concentration of 0.2% tivity of the measurements. Smaller amounts were not suYcient v/v for stabilisation purposes. A 0.08% m/v NaF solution was for stabilising all the phosphorus present in the test solutions.prepared by direct weighing and dissolution. No acid was It is clearly seen that the maximum stabilising eVect is added to this solution. achieved when both the analyte and the modifier are placed in the same spot of the dual-cavity platform. From this, it is Experiments with dual-cavity platforms inferred that stabilisation is more eYcient in the condensed phase. However, it is worth noticing that even though a poorer Dual-cavity pyrolytic graphite platforms were used to detersensitivity is obtained when phosphorus and the modifier are mine if stabilisation occurs in the condensed or in the vapour atomised from diVerent cavities, stabilisation is still present phase.Charring plots for phosphorus and NaF placed either under these conditions. Bearing in mind that modifiers in P in the same or in a diVerent cavity were recorded. Five determinations by ETAAS are needed mainly to inhibit forma- microlitres of a 1000 mg ml-1 phosphorus solution and 5 ml of tion of volatile phosphorus species, which are lost even at a 0.08% m/v solution of NaF were atomised under the heating comparatively low ashing temperatures, it is possible that mass programme given in Table 1.transport processes, promoted by heating, allow these volatile species to reach the cavity where the modifier was placed. Study of the material accumulated onto graphite platforms Formation of stable phosphorus species will then occur in the bulk of the modifier.The possibility that these reactions occur Solutions of phosphorus and the modifier were injected onto with both reactants being in the gas phase is ruled out by the pyrolytic graphite platforms and heated up to one of the high boiling point of NaF (1695 °C)26 and by the fact that, following charring temperatures: 200, 400, 800, 1000 and according to Fig. 1, stabilisation occurs at much lower tempera- 1350 °C.These temperatures were chosen to be within the tures. The lower eYciency of stabilisation when both reagents range of charring temperatures where phosphorus analytical are placed in diVerent cavities could then be due to the fact signals exhibit maximum sensitivity, i.e. within the plateau range of the charring curve for this element. The procedure was repeated until suYcient material had been accumulated, #3 mg, at each temperature. Each platform was stored in a desiccator to minimise water adsorption.The analysis of the material was performed as soon as possible after accumulation. For the analysis by SEM, EDS and XPS of the residue Table 1 Heating programme for the determination of phosphorus by ETAAS Temperature/ Ramp/ Hold/ Gas flow/ Step °C s s ml min-1 Dry 90 5 10*/30† 300 Dry 120 10 10*/30† 300 Charring Variable 5 20 300 Cool-down 50 5 10 300 Fig. 1 Charring plots for phosphorus atomised from dual-cavity Atomisation 2650 0 5 0 Cool-down 20 1 8 300 platforms.Five microlitres of a 100 mg ml-1 NH4H2PO4 solution and 5 ml of a 0.08% m/v NaF solution. A, Analyte without modifier; B, analyte and modifier in diVerent cavities; and C, analyte and modifier * Holding time for dual-cavity platform experiments. † Holding time for conventional platform experiments. in same cavity. 1392 Journal of Analytical Atomic Spectrometry, December 1997, Vol. 12that only the portion of analyte that can, as volatile species, tubes made of the same material, any diVerence in the analytical signals should be related exclusively to the diVerent platform reach the modifier’s cavity is stabilised; the rest would be lost surfaces. during heating.As can be seen from Fig. 2, the use of NaF as a modifier does not eliminate the dependence of phosphorus sensitivity Dependence of the Phosphorus Absorption Signal on the Surface on the material of the atomisation surface. This is also true of the Atomiser when using Pd and La, as reported by Curtius et al.27 In fact, when phosphorus is atomised from a standard graphite plat- Another interesting feature in Fig. 1 is that, in the absence of form its sensitivity is slightly enhanced as compared to atomis- modifiers, the phosphorus analytical signal does not fall to ation from a pyrolytic surface. Moreover, although analyte zero even at charring temperatures as high as 1800 °C. losses start at the same charring temperature (ca. 1350 °C) However, in the presence of a modifier, the phosphorus signals regardless of whether a standard or a pyrolytic graphite completely disappear at charring temperatures equal to or platform is used, phosphorus signals can still be recorded at higher than 1700 °C. A similar behaviour was observed by charring temperatures as high as 1800 °C when the analyte is Curtius et al.,27 who attributed it to the formation of stable atomised from the standard platform. This behaviour of the phosphorus–carbon species, such as P2C6, or to an interstitial analytical signal, which contrasts with the one observed when compound.According to Oh and Rodriguez,28 phosphorus is atomising from a pyrolytic graphite platform in the presence capable of attacking the arm-chair sites of the graphite strucof a modifier, can be explained on the basis of the model ture to form stable compounds at temperatures higher than proposed by L’vov et al.,31 in which the atomisation of an 1050 °C.29 This might explain the stability of the analytical element from a standard graphite surface can be envisioned as signal of phosphorus at such high charring temperatures, when taking place from the bulk of the graphite. Thus, part of the no modifier is present, as shown in Fig. 1. In the presence of analyte, on injection, penetrates the pores of the surface and, a modifier, the possibility of having free phosphorus species to on heating, is converted into a phosphorus–carbon species,27,28 interact with graphite is greatly reduced in the range of which is retained more strongly and remains in the atomiser temperature in which the modifier is eVective.At higher at temperatures higher than when it is atomised from pyrolytic charring temperatures and/or at the beginning of the atomisgraphite surfaces in which penetration is considerably limited. ation stage, the modifier releases the analyte to an environment Formation of the stable phosphorus–carbon species after reac- in which its interaction with graphite is not favoured due to tion of the phosphate species and the carbon at the active sites the fact that gas expansion promotes losses of the analyte.of graphite, when atomisation occurs from a standard graphite Since no previous, i.e. at lower temperatures, interaction of surface, minimises formation of volatile molecular species phosphorus with graphite to form stable phosphorus–carbon which would be lost on heating.2 The increased sensitivity species occurred, due to the sequestering of the analyte by the observed when atomisation is done from standard graphite modifier, phosphorus is lost and therefore the P atomic absorpsurfaces can be attributed to this behaviour. tion signals do not exist at temperatures equal or higher than Retention of part of the analyte in the porous graphite 1700 °C, as shown in Fig. 1. surface is also indicated by the rising portions of the atomic Several workers have shown that the sensitivity of phosabsorption profiles of phosphorus shown in Fig. 3, which phorus measurements depends on the material of the atomisdenotes faster kinetics for the vaporisation of atoms from the ation surface, either in the presence5,30 or in the absence27 of pyrolytic graphite surface than from the standard graphite chemical modification. The best sensitivity is achieved when surface. Fig. 3 also shows that sweeping of phosphorus atoms the analyte is atomised from standard graphite surfaces (platfrom the observation volume of the atomiser is faster from the form or wall atomisation), because partial stabilisation of pyrolytic surface than from the standard one, in agreement phosphorus by the material of the surface is easier in these with the assumption that less interaction should be expected atomisers.27 In order to determine the eVect of the atomisation between the analyte and the pyrolytic surface. surface on the sensitivity of the phosphorus signal, when NaF is being used as a chemical modifier, ashing plots for phos- Study of the Material Accumulated Onto Graphite Platforms phorus, using phosphate solutions atomised from laboratorymade standard platforms and from commercially available Fig. 4 shows a micrograph of part of the residue left after pyrolytic graphite platforms, inserted into pyrolytically coated successive injections of solutions of NH4H2PO4 and NaF onto graphite tubes, were recorded.These plots are depicted in Fig. 2. Since the diVerent platforms were placed within graphite Fig. 2 Charring plots for the atomisation of a 100 mg ml-1 phos- Fig. 3 Peak profiles of the atomisation of 10 ml of a 100mg ml-1 phorus solution in the presence of 10 ml of a 0.08% m/v NaF solution in standard and pyrolytic graphite platforms. A, Atomisation from a phosphorus solution in the presence of 10 ml of a 0.08% m/v NaF solution. Atomisation from (a) pyrolytic graphite platform, and (b) stan- pyrolytic graphite platform; and B, atomisation from a standard graphite platform. dard graphite platform.Journal of Analytical Atomic Spectrometry, December 1997, Vol. 12 1393Table 2 Results of the EDS analysis of the particles formed at 200 °C (a) Marked with the ‘a’ arrow in Fig. 5 Element Mass (%) Atom (%) K ratio Z A F C K 0.29 0.48 0.0011 1.0715 0.2563 1.0005 O K 38.53 48.08 0.3948 1.0474 0.7021 1.0010 F K 11.26 11.84 0.0738 0.9817 0.4789 1.0013 Na K 33.18 28.81 0.3287 0.9718 0.7317 1.0007 P K 16.73 10.78 0.2016 0.9509 0.9102 1.0000 Total 100.00 100.00 (b) Marked with the ‘b’ arrow in Fig. 5 Element Mass (%) Atom (%) K ratio Z A F C K 0.24 0.41 0.0010 1.0942 0.2978 1.0004 O K 3.96 5.16 0.0417 1.0695 0.7666 1.0039 F K 40.72 44.73 0.4440 1.0025 0.8480 1.0025 Na K 53.85 48.85 0.4997 0.9922 0.8678 1.0000 P K 1.26 0.85 0.0136 0.9709 0.8678 1.0000 Total 100.00 100.00 Fig. 4 Micrograph of the residue left on a graphite platform after injection of NH4H2PO4 and NaF solutions and heating at 200 °C.maximum charring temperature for phosphorus using NaF as a platform that was later heated up to 200 °C. According to a modifier, the accumulated material adopts the shape of this figure, at this low pretreatment temperature the spherical particles dispersed through the platform (see Fig. 6). NH4H2PO4 melts (mp 190 °C)26 and penetrates into the bulk This phenomenon is probably caused by melting of the modifier of the modifier, which has a higher melting point (988 °C).26 at temperatures around 1000 °C during charring and the The rest of the NH4H2PO4, which does not find its way into sudden reduction of temperature from the end of the charring the bulk of the modifier, deposits above and around it. The step, #1350 °C, to the cool-down step, #50 °C, prior to ‘hole’ in the centre of the picture reveals the graphite substrate.atomisation (see Table 1). Melting of the modifier promotes a Fig. 5 shows isolated particles of NaF covered by molten more eYcient mixing, which in turn could lead to a more NH4H2PO4, as shown by EDS analysis of material deposited eYcient trapping, of that part of NH4H2PO4 which at the onto the same platform. The results of the EDS analysis of beginning of the heating programme penetrated its matrix. At these particles, Table 2 (a) and (b), indicate that at 200 °C and these temperatures, that part of the phosphorus which did not at lower temperatures there are no chemical reactions between penetrate the matrix of the modifier must have already been the analyte and the modifier, i.e., there is no indication of lost as some molecular, volatile species.2 formation of any stable P–F species in this range of temperature.Since part of the analyte is just deposited on and around EVect of the Presence of Fluoride as a Modifier on the the modifier and there is no evidence to show formation of a Sensitivity of DiVerent Inorganic Phosphorus Compounds P–F species which could stabilise phosphorus, it is reasonable to expect losses of this portion of the analyte in some molecular Ediger6 showed that in the absence of chemical modifiers the form as the heating programme proceeds.2 On the other hand, sensitivity of phosphorus determinations depended upon the that portion of the analyte which penetrates the bulk of the type of phosphorus compound introduced into the atomiser, modifier could remain trapped, and therefore stabilised, inside but in the presence of a 1% lanthanum solution the diVerences the modifier.This could explain stabilisation of the analyte in sensitivity were eliminated. In order to test the eVect of the and the lower sensitivity of phosphorus determinations with fluoride modifier on the sensitivity of measurements of diVerent NaF as a modifier compared to the sensitivity achieved with phosphorus compounds, solutions of diVerent inorganic phospalladium, as reported by Alvarado et al.8 These assumptions phorus salts were heated in the presence of NaF, and the will be further verified later in this paper.ashing plots were recorded (Fig. 7). At a temperature of 1350 °C, which is quite close to the The results show that the thermal stability of phosphorus Fig. 5 Micrograph showing molten NH4H2PO4; indicated by the ‘a’ Fig. 6 Micrograph showing spherical particles of material deposited on a graphite platform after heating at 1350 °C and sudden cooling arrow: covering the NaF modifier; indicated by the ‘b’ arrow: after heating at 200 °C.at 50 °C. 1394 Journal of Analytical Atomic Spectrometry, December 1997, Vol. 12Fig. 7 Charring plots for 10 ml of 100 mg ml-1 phosphorus solutions Fig. 8 Charring and atomisation curves for phosphorus: A and D, in introduced as diVerent phosphorus compounds, in the presence of NaF. the presence of NaF; B and C in the presence of NaOH. signals in the presence of NaF is practically independent of when using NaF, the presence of NaOH stabilises phosphorus.the type of phosphorus compound being heated. The phos- The maximum charring temperature with no loss of analyte is phates studied behave in a similar way. A definite diVerence is in the vicinity of 1100 °C. This is in agreement with the fact observed for the hexafluorophosphates in the temperature that Na2O sublimes at #1275 °C25 and that trapping is range between room temperature and #500 °C.Otherwise, the possible only when the modifier exists in the condensed phase. trend of the curves is similar to those of the phosphates. Interpretation of the results obtained after the NaOH tests However, the sensitivity of the phosphorus absorbance signals shows that trapping of phosphorus by Na2O formed after is drastically influenced by the type of phosphorus compound oxidation of NaF is not a possible mechanism, given that being heated. Phosphorus signals obtained after heating diVer- retention of phosphorus using NaF is eVective up to, at least, ent phosphates in the presence of NaF are up to four times 1350 °C, a temperature at which Na2O, if formed, should have more sensitive than for the PF6- salts. This behaviour can be completely sublimed.This surmise also finds support in that explained on the basis of the findings already discussed. Part the chemical analysis of the material accumulated onto graphite of the phosphate compound, on melting at temperatures platforms does not reveal the presence of Na2O, at any of the around 200 °C, penetrates the matrix of the modifier and, on diVerent temperatures tested.On the other hand, since NaF further heating, intimately mixes with the modifier, being has a reported boiling point of 1695 °C, trapping of phosphorus trapped in it. On the other hand, NaPF6 does not melt but in this compound at temperatures as high as 1350 °C is decomposes according to reaction (1), which starts at about possible.For this kind of stabilisation mechanism, the fact that 270 °C and is complete at around 650 °C: the release of the analyte occurs at temperatures lower than the boiling points of the trapping compounds, 1275 °C for NaPF6(s)�NaF(s)+PF5(g) (1) Na2O, 1695 °C for NaF and 3600 °C for MgO, could be Formation of the volatile PF5 molecule leads to losses of explained assuming that, at the charring temperatures at which phosphorus at relatively low temperatures, which is reflected losses of the analyte are first observed, the kinetic energy of in the relatively low sensitivity of the measurements for hexa- the trapped compound is such that it is able to free itself from fluorophosphate compounds shown in Fig. 7. trapping at that temperature even though the change in phase of the modifier has yet to occur. In the particular case of phosphorus, the analyte could be released as some molecular, Possibility of Formation of Na2O After Heating NaF rather than atomic, form at temperatures around 300 °C lower Stabilisation of an analyte by physical trapping in the structure than the boiling point of NaF.This assumption finds support of the modifier has been shown to operate for the use of in Fig. 9. The blank space observed between #1700 and Mg(NO3)2.32 This compound, on heating, transforms itself into MgOand the analyte is retained, trapped, in the bulk of the oxide. The possibility that a similar mechanism, i.e.conversion of NaF into Na2O and trapping of phosphorus in the bulk of the oxide, operates for stabilisation of phosphorus by NaF was considered and later discarded by the following tests. Aqueous phosphorus solutions, of the same phosphorus concentration as used in the previous tests, 100 mg ml-1, obtained by dissolution of NH4H2PO4, were atomised in the presence of NaOH solutions having the same concentration (0.08% m/v) as that of the NaF solutions previously used. Sodium hydroxide was chosen since this compound, at relatively low temperatures, transforms into Na2O.Formation of Na2O in the early stages of the heating will favour trapping of phosphorus in the structure of this molecule. Fig. 8 shows charring and atomisation curves obtained during this test. The corresponding curves using NaF as modifier are also shown for Fig. 9 Charring (A) and atomisation (B) curves for phosphorus in comparison purposes. According to Fig. 8, although the sensi- the presence of NaF as a modifier showing a blank space between #1700 °C and #2100 °C.tivity of the measurements is around 1.7 times lower than Journal of Analytical Atomic Spectrometry, December 1997, Vol. 12 13952100 °C in Fig. 9 has been found in every charring and REFERENCES atomisation graph for phosphorus plotted during this work. A 1 Manning, D. C., and Slavin, S., At. Absorpt. Newsl., 1969, 8, 132. similar behaviour was observed by Persson and Frech,2 in the 2 Persson, J.A., and Frech, W., Anal. Chim. Acta, 1980, 119, 75. temperature range of ca. 2000–2100 °C. Usually one expects 3 Prevo� t, A., and Gente-Jauniaux, M., At. Absorpt. Newsl., 1978, the atomisation curve to begin at temperatures slightly higher 17, 1. than the maximum charring temperature allowed without 4 Havezov, I., Russeva, E., and Jordanov, N., Fresenius’ Z. Anal. Chem., 1979, 296, 125. losing analyte. The atypical feature shown in Fig. 9 could be 5 Curtius, A.J., Schlemmer, G., and Welz, B., J. Anal. At. Spectrom., indicative of losses of analyte in this range of temperature, in 1987, 2, 115. the presence of this particular modifier. This blank space could 6 Ediger, R. D., At. Absorpt. Newsl., 1976, 15, 145. mean that at temperatures slightly higher than the maximum 7 Welz, B., Curtius, A. J., Schlemmer, G., Ortner, H. M., and charring temperature, phosphorus is released not as an atomic Birzer, W., Spectrochim. Acta, Part B, 1986, 41, 1175.species, as deduced from the lack of atomic absorption signals, 8 Alvarado, J., Cristiano, A. R., and Curtius, A. J., J. Anal. At. but most likely as a molecule. These molecules will dissociate Spectrom., 1995, 10, 483. 9 Ortner, H. M., Schlemmer, G., Welz, B., and Wegsheider, W., to produce the atomic species at temperatures close to 2100 °C, Spectrochim. Acta, Part B, 1985, 40, 959. at which atomic absorption is first observed, according to 10 Welz, B., Curtius, A., Schlemmer, G., and Ortner, H.M., Fig. 9. According to Persson and Frech,2 the main phosphorus Spectrochim. Acta, Part B, 1986, 41, 567. species existing in the temperature range #1200–2200 °C, with 11 Ortner, H. M., Birzer, W., Welz, B., Schlemmer, G., Curtius, A. J., the minimum value depending upon the type of phosphorus Wegsheider, W., and Sychra, V., Fresenius’ Z. Anal. Chem., 1986, compound originally heated, is the gaseous P2 molecular 323, 681. species.It is reasonable to expect, on the basis of the findings 12 Mu� ller-Vogt, G., and Wendl, W., Anal. Chem., 1981, 53, 651. 13 Bendicho, C., and de Loos-Vollebregt, M. T. C., Spectrochim. of these authors, that this could be the main species responsible Acta, Part B, 1990, 45, 679. for phosphorus losses during thermal pre-treatment. 14 Jianping, W., and Bo, D., Spectrochim. Acta, Part B, 1992, 47, 711. 15 Qiao, H., and Jackson, K. W., Spectrochim. Acta, Part B, 1992, 47, 1267.CONCLUSIONS 16 Shan, X. Q., and Wang, D. X., Anal. Chim. Acta, 1985, 173, 315. 17 Jianping, W., and Bo, D., Spectrochim. Acta, Part B, 1992, 47, 711. Stabilisation of phosphorus in the presence of NaF as a 18 Peng-yuan, Y., Zhe-ming, N., Zhi-xia, Z., Fu-chun, X., and modifier proceeds via physical trapping of the analyte in the An-bei, J., J. Anal. At. Spectrom., 1992, 7, 515. bulk of the modifier. Losses of phosphorus in some molecular 19 Xuan, W., Spectrochim. Acta, Part B, 1992, 47, 545.form, mainly as P2, are likely to be responsible for the lower 20 Welz, B., Akman, S., and Schlemmer, G., Analyst, 1985, 110, 459. sensitivity of the measurements as compared to that obtained 21 Dedina, J., Frech, W., Cedergren, A., Lindberg, I., and using Pd. The proposed mechanism for stabilisation of phos- Lundberg, E., J. Anal. At. Spectrom., 1987, 2, 435. phorus by NaF could also be applicable to other fluoride salts 22 Akman, S., and Doner, G., Spectrochim.Acta, Part B, 1994, 49, 665. 23 Akman, S., and Doner, G., Spectrochim. Acta, Part B, 1995, 50, 975. such as LiF (bp 1670 °C), KF (bp 1498 °C) and CsF (bp 24 Akman, S., and Doner, G., Spectrochim. Acta, Part B, 1996, 1253 °C), which have been previously shown to stabilise this 51, 1163. analyte.8 However, HF and NH4F, which do not leave solid 25 Doner, G., and Akman, S., Spectrochim. Acta, Part B, 1996, 51, 181. residues after heating and which also exert a stabilisation eVect 26 Handbook of Chemistry and Physics, ed. Weast, R. C., CRC Press, on phosphorus,8 must act in a diVerent way. Tests are being Boca Raton, FL, 53rd edn., 1972. carried out to check the possibility of stabilisation via chemical 27 Curtius, A. J., Schlemmer, G., and Welz, B., J. Anal. At. Spectrom., reaction of the analyte and these fluoride compounds. In spite 1986, 1, 421. of the lower sensitivity of phosphorus measurements, the use 28 Oh, S. G., and Rodriguez, N. M., J. Mater. Res., 1993, 8, 2879. 29 Holcombe, J. A., and Droessler, M. S., Fresenius’ Z Anal. Chem., of NaF as a modifier for phosphorus determination by ETAAS 1986, 323, 689. is an economical and convenient alternative which could be 30 Ediger, R. D., Knott, A. R., Peterson, G. E., and Beaty, R. D., At. useful when analysing samples with phosphorus concentration Absorpt. Newsl., 1978, 17, 28. levels which do not demand the highest sensitivity from this 31 L’vov, B. V., Bayunov, P. A., and Ryabchuck, G. N., Spectrochim. technique. Acta, Part B, 1981, 36, 397. 32 Slavin, W., Carnrick, G. R., and Manning,, Anal. Chem., 1982, 54, 621. The authors thank Dr. Bernhard Welz, from Perkin-Elmer Corporation, U� berlingen, Germany, for donating the dual- Paper 7/04689H cavity platforms. Thanks are also due to CONICIT for Grant Received July 3, 1997 MPS-RP-VII260076 and to the Decanato de Investigaciones at Universidad Simo�n Bolý�var. Accepted August 16, 1997 1396 Journal of Analytical Atomic Spectrometry, December 1997, V

ISSN:0267-9477    

DOI:10.1039/a704689h

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Tandem Preconcentration of Cobalt by On-line Ion Exchange and Gas Phase Chelates Generated by Merging-zones Flow Injection Analysis With Electrothermal Atomic Absorption Spectrometric Determination MARIA S. JIME� NEZ AND JUAN R. CASTILLO* Analytical Spectroscopy and Sensors Group, Department of Analytical Chemistry, Faculty of Sciences, University of Zaragoza, E-50009 Zaragoza, Spain An automated combined method of on-line preconcentration of exploited, is the volatilization of the metal after the formation of volatile organometallic compounds, such as ethyl Co on Amberlite IR-120 resin was developed followed by the derivatives7 or chelates of transition metals.8 generation of volatile cobalt chelates (diethyldithiocarbamate A large number of chelates have been used in studies by GC9 and acetylacetonate) which are then vaporized with a and to develop analytical procedures for the determination of thermospray device to obtain a second preconcentration stage some metals in biological materials.10 The introduction of the with decomposition of the gas phase generated on the surface analyte in a gaseous form has certain advantages: viz., lower of a pre-heated graphite platform with subsequent detection limits and separation of matrix interferences.determination by ETAAS. The use of a by-pass valve between The chelate complexes normally used in GC11–17 have a low the thermospray device and the graphite chamber makes it sublimation temperature and high vapour pressure. Various possible to overcome the classical problems caused by methods based on determination by atomic spectrometric condensation of the eluates.The sensitivity (0.0044 A s) techniques, involving direct introduction of the gaseous sample achieved by the proposed tandem preconcentration–ETAAS into the flame18–26, or into a graphite chamber26 or the use of method is 0.8 and 1.1 ng l-1 with a detection limit (3s) of 2 ICP-OES27 have been employed to determine several metals.and 3 ng l-1 for cobalt diethyldithiocarbamate and cobalt In previous papers, fundamental studies of the on-line gener- acetylacetonate, respectively. The method was evaluated ation of volatile chelates followed by atomization in a quartz against unpolluted certified reference natural waters and tube28 or in the flame, directly from the nebulization chamber,29 satisfactory results were obtained. were described. The best detection limit obtained was 4 ng for Keywords: Ion-exchange column preconcentration; flow cobalt hexafluoroacetylacetonate and 25 ng for cobalt acetylainjection; cobalt; thermospray; electrothermal atomic cetonate using a horizontal reactor.The thermogravimetric absorption spectrometry; unpolluted natural water; ultratrace and volatilization characteristics of Co chelates are well known analysis and were reported in a previous paper.29 On the other hand, the characteristic mass of Co (0.0044 A s) obtained by conven- The determination of Co in certain samples of environmental tional ETAAS from a platform is 17 pg and the detection limit interest (mainly natural waters) in which it is found at very low by ICP-MS is between 1 and 10 ppt.It is clear that atomization concentrations requires the use of preconcentration methods in a quartz tube is less eYcient than in a graphite furnace, and coupled to highly sensitive techniques, such as ETAAS or less sensitive than ICP-MS.ICP-MS, to achieve the necessary detection limit levels ( lower More recently,30 the continuous on-line thermospray than 10 ppb).However, with complex samples, such as sea-water, volatilization of volatile Co, Al and Cr chelates and their both analytical techniques suVer from problems caused by the determination by heated quartz tube AAS has been described. high salinity of the sample. The saline content, mainly composed The use of on-line systems of synthesis, volatilization of the of halides and alkalines, causes interferences in the condensed complexes with a thermospray device (provided that the and gas phases during the atomization processes in the graphite operating temperature is lower than the sublimation temperachamber or serious problems in the connection systems (cones ture of the complex) and atomization in a quartz tube (despite and skimmers) between the ionization systems in the ICP and its low operating temperature) makes it possible to obtain the mass spectrometer, regardless of whether the spectrometer is better detection limits than those achieved with atomization a quadrupole or time-of-flight instrument of high resolution and in the flame.The best values for Co, Cr and Al were obtained great sensitivity. These problems are usually avoided by means with diethyldithiocarbamate as ligand. Detection limits of of long separation processes. 0.13 ppm for Co, 0.3 ppm for Cr and 0.25 ppm for Al were Several methods, such as coprecipitation, cocrystallization, obtained.These values are too high to be applied to determine solvent extraction, column extraction and electrolysis, have been Co in natural waters. Furthermore, Cu, Fe, Ni and Zn at devised for the preconcentration of trace metals in sea-water and concentration ratios of 155 seriously interfere with the determiwith some of these methods1–5 the detection limits reported were nation. However, the feasibility of the method as an on-line suYciently low for the determination of Co in unpolluted open synthesis system was shown, although an improvement in the ocean water and sea-water by atomic spectrometry although detection system used appears to be necessary. ICP-MS was always used as the detection technique.In recent papers, we studied the synthesis of cobalt The volatilization of metals as a preconcentration technique acetylacetonate using flow injection analysis (FIA) for the has received little attention, mainly because of the problems determination of Co in steel by flame AAS31 and the extraction involved with the high temperatures required to volatilize most of cobalt diethyldithiocarbamate to determine Co in vitamins by coupling FIA to flame AAS.32 With both methods the metal compounds.6 One possibility, which has been little Journal of Analytical Atomic Spectrometry, December 1997, Vol. 12 (1397–1402) 1397detection limits were higher and the sensitivities lower than (1000 mg l-1) and acetylacetone were used as complexing agents.BuVer solutions of acetic acid and sodium acetate those required for the determination of Co in unpolluted natural waters. (0.1 M) and of sodium hydrogenphthalate (0.1 M in 0.1 M HCl) were used to adjust the pH. The resins used were Chelex-100 However, the synthesis of the corresponding chelate and suitably automated volatilization, combined with a detection (100–200 mesh, sodium form) and Amberlite IR-120 (20–50 mesh, ASTM).system which fulfils certain preconcentration and sensitivity requirements in the measurement (as occurs with The ion-exchange columns were made with glass tubing of diVerent lengths and an id of 5 mm. The ends of the tube were decomposition–retention systems in a graphite chamber and determination by ETAAS) may give rise to a simple, useful plugged with glass wool and joined to the PTFE tube of the FIA manifold by means of Tygon tubing with a suitable method of determination in samples of unpolluted natural water.diameter and diVerent fittings and connectors. The resin was introduced into the column using a syringe. This paper describes the results obtained with simple on-line preconcentration using ion-exchange resins followed by automated on-line synthesis of volatile Co chelates, sample RESULTS AND DISCUSSION volatilization by means of a thermospray device, reten- The analytical process followed to develop a simple automated tion–preconcentration on the surface of a L’vov platform method with suYcient sensitivity to determine Co in unpolluted and ETAAS determination.The method was applied to the natural water samples using a tandem system of preconcen- determination of Co in samples of unpolluted natural water. tration on ion-exchange resins, on-line synthesis of volatile Co chelates, preconcentration–retention of the gaseous Co species EXPERIMENL on the surface of a graphite platform with subsequent electrothermal atomization and measurement of the atomic absorp- Apparatus tion was carried out by optimizing each of the stages separately A Perkin-Elmer 2380 atomic absorption spectrometer with and finally combining them in a single analytical system.The deuterium arc background correction, equipped with a Co optimum manifold is shown in Fig. 1. hollow cathode lamp operated at 35 mA was used with both First, the conditions for the preconcentration of CoII on the the conventional nebulization system and air–acetylene flame ion-exchange mini-columns were optimized.Two diVerent and a quartz tube heated by a flame. The wavelength was set types of resin were tested: Chelex-100 and Amberlite IR-120. at 240.7 nm with a spectral slit-width of 0.2 nm. Integrated The detection system used in this case was aspiration of the absorbance was used for quantitative applications. The eluted phases into an air–acetylene flame (identical with the gathering and treatment of data was carried out with a PC system used in ref. 29) in order to find the best response ACER 911 computer. An automation program was developed conditions. Second, experiments were carried out to optimize with the Lab Windows application (National Instruments). all the parameters of the merging-zones FIA system for the This program controls both the diVerent components of the on-line synthesis of the cobalt diethyldithiocarbamate and FIA system used and the storing of data.The data, recorded cobalt acetylacetonate complexes. The detection system used at a rate of 10 data per second, appear on the monitor in in this case was the same as that used in ref. 30. Third, the graphic form and are simultaneously stored in a file for in-line connection of the ion-exchange preconcentration subsequent treatment with the calculus program Excel 3.0. system, the merging-zones FIA system for synthesis of the In the final step, a Perkin-Elmer Model 4110 spectrometer volatile cobalt chelates and the detection system, viz., thermoswith Zeeman-eVect background correction and a transversely pray nebulization–vaporization with atomization in a quartz heated graphite platform furnace was used.tube (cf. ref. 30), was optimized. Finally, the coupling of the The diVerent FIA manifolds employed were assembled from thermospray device to the graphite chamber by means of a Gilson Minipuls-3 peristaltic pumps with Tygon pumping by-pass valve was optimized with a system which allows the tubes of 0.100 and 0.125 in diameter (Gilson), automated bolus of the synthesized complex to enter the graphite chamber injection valves (Eurosas EPS-130) as injection systems and when the complex reaches the thermospray device while the various connectors, end fittings and PTFE tubes of 0.3 mm id eluent is diverted to waste.(line tubes) and 0.8 mm id (reactor tube) (Omnifit). The thermospray vaporizer used was similar to that used Optimization of the Cobalt Preconcentration Step With an previously,30 and was constructed in-house.Ion-exchange Column and Detection by AAS in an The thermospray device is placed on a mechanical piston Air–Acetylene Flame controlled by a PC. The piston can go back and forth rapidly in 0.5 mm steps. The total stroke of the piston is 3 cm. The On-line preconcentration in columns with ion-exchange resins piston is also synchronized with the FIA manifold and the by FIA systems has some advantages over conventional techgraphite furnace operating system by the computer.niques (coprecipitation, cocrystallization, solvent extraction Reagents All solutions were prepared using de-ionized, distilled water. All glassware was washed with soap, soaked in nitric acid and rinsed with de-ionized, distilled water prior to use. The solutions, once prepared, were stored in PTFE containers. All standard solutions of lower concentration were prepared by diluting the standard solutions with de-ionized, distilled Fig. 1 Schematic diagram of the tandem on-line column preconcen- water. tration–automated merging-zones FIA chelate formation–thermospray Analytical-reagent grade reagents (Merck and J. T. Baker) vaporization and atomization system: 1, CoII solution; 2 and 4, buVer were used for the preparation of all solutions: HCl and solution; 3, reagent; 5, automated injection valve; 6, waste; 7, reactor; HNO3 of diVerent concentrations and methanol were used. 8, computer; 9, thermospray device; 10, heating system; 11, ETAAS A standard solution of Co (1000 mg l-1) was prepared instrument; 12, automated by-pass valve; 13, T-connection; and 14, preconcentration column. from Co(NO3)2 6H2O. Sodium diethyldithiocarbamate 1398 Journal of Analytical Atomic Spectrometry, December 1997, Vol. 12and electrolysis): better sampling frequencies, lower sample From the results obtained experimentally, it can be seen that the best enrichment and transfer factors are obtained with consumption, greater precision and less risk of analyte contamination.Amberlite IR-120 and with a column with a length of 13 mm and an id of 5 mm. Furthermore, the volume changes associated The procedure is based on the continuous introduction of the CoII solution by means of a peristaltic pump into the loop with modification of the pH which occur with Chelex-100 and limit sampling frequency, do not take place with Amberlite of an automated injection valve for a specific period of time.The column containing the ion-exchange resin is located in IR-120. As an example, typical signals obtained for a 10-fold repetition of the preconcentration of 6 mg of Co under the the loop. The Co is retained on the resin and when the valve injection takes place, the eluent, which acts as a carrier, enters optimum conditions are given in Fig. 2. Although the only aim of this part of the work was to obtain the resin and elutes the analyte, transporting it through the conventional nebulization chamber to the air–acetylene flame the optimum conditions and the verification of the best ionexchange preconcentration system, a sensitivity of 0.28 mg ml-1 atomic absorption detector.Analyte loading in the resin was time-based so that it could (for 0.0044 A s) was obtained with the corresponding signals (similar to those in Fig. 2) and the calibration graphs obtained be carried out in an eYcient, precise way.From the atomic absorption signals obtained and by modifying the loading under similar conditions. These data are only given for comparison with those reported at a later stage. conditions, flow rate, etc., the conditioning of the Chelex-100 and Amberlite IR-120 resins and the various parameters, viz. In accordance with the results obtained, preconcentration of Co was henceforth carried out with Amberlite IR-120, which capacity of the resin, loading flow, elution flow, transference factor, enrichment factor and reproducibility of the entire gives excellent transfer and enrichment factors.process, were optimized. The term ‘phase transference factor’ refers to the ratio of the analyte mass in the eluate to that in Optimization of Automated Synthesis of Cobalt the original sample. Diethyldithiocarbamate [Co(DTC)3] by Merging-zones FIA, The enrichment factor is one of the criteria most frequently Followed by Thermospray Nebulization–Vaporization and used to evaluate preconcentration systems33 and is theoretically Detection by Quartz Tube AAS defined as the ratio of the analyte concentration after preconcentration to the original concentration in the sample.In a previous paper,30 we developed the continuous on-line However, as the enrichment factor is strongly influenced by thermospray volatilization of volatile Co chelates and their factors such as the geometrical design, kinetic characteristics, determination by heated quartz tube AAS and established the the physical processes involved and the rate of introduction of conditions for the automated synthesis and for volatilization the eluate, the eVect of which is diYcult to calculate, the with a thermospray device.Here, the insertion of a by-pass concentration peak was estimated on the basis of the volume with two valves (one towards the thermospray device and the of eluent used until the moment the analyte reaches the other towards waste) between the reactor and the thermospray detector.This parameter is closely related to the factors device was studied. Suitable calibration of the synthesis resipreviously mentioned and after the corresponding geometrical dence time makes it possible to automate the directionality of considerations, the expression EF=(2PVs)/Ve is obtained,34 the by-pass towards either waste or the thermospray device in where EF is the enrichment factor, P the transfer factor, Vs the such a way that only the zone in which the synthesis reaction volume of the original sample and Ve the volume of eluent occurs reaches the thermospray device and the quartz tube.used. As sample loading is time-based, Ve=qte and Vs=Qsts), With the merging-zones system it is very important for the where q is the elution flow, te the elution time, Qs the sample two valves to inject simultaneously. For this reason, control of flow and ts the sampling time, we can write the equation as the injection pump is carried out using the two digital outputs EF=(2PQsts)/qte.By exact measurement of the elution times and one of the analogue outputs of the PCL 711. One of the of the peaks it was possible to calculate the enrichment factors digital signals regulates the starting-up of the pump while the shown in Table 1. other controls the direction of rotation. The analogue signal In order to calculate the capacity of the column (micrograms serves to establish the rotation speed of the pump. Each of the of Co retained by the column) experimentally, a 50 mg l-1 CoII injection valves is controlled by a digital output.The software solution was introduced at a flow rate of 9 ml min-1 until a used makes it possible to control the pump and the injection Co signal was obtained at the detector, indicating that the Co valves and hence the loading times, injections and the was no longer being retained on the resin. The Co retained number of complete cycles of the program can be established was then eluted and its concentration measured. It was diluted independently.to a known volume and its concentration interpolated from a Another advantage of the merging-zones system is that once calibration graph. an aliquot of sample has been injected into a suitable carrier and an aliquot of reagent into a diVerent channel, the convergence of both aliquots can be regulated in such a way that the Table 1 Preconcentration parameters for the tested systems in the on-line ion-exchange column preconcentration of Co with air– acetylene flame AAS detection Amberlite IR-120* Parameter Chelex 100 Column id/mm 5 5 5 Column length/mm 100 100 13 Capacity/mg 1600 6595 1700 Loading flow/ml min-1 7 5 3 Eluent flow/ml min-1 5 3 7 Phase transfer factor 0.95 1.02 1.02 Enrichment factor 14.78 6.09 15.78 Throughput/samples h-1 20 20 25 * Several column lengths between 100 and 10 mm were tested with Amberlite IR-120 resin, but the best results and less dispersion were Fig. 2 Peaks obtained from the measurement of ten replicates of the obtained experimentally with a column length of 13 mm. Sample volume, 500 ml. on-line column preconcentration of 6 mg of Co. Journal of Analytical Atomic Spectrometry, December 1997, Vol. 12 1399Table 2 Optimum parameters and figures of merit for the automated two can be suitably mixed to give rise to the synthesis reaction. synthesis of Co(DTC)3 and Co(ACA)2 by merging-zones FIA using The simultaneous convergence can be achieved with high thermospray nebulization–vaporization and quartz tube AAS detection precision by means of synchronized multi-injection.A reactor with a suitable length allows ideal residence time with satisfac- Parameter— Co(DTC)3 Co(ACA)2 tory dispersion and an exact knowledge of the flow rate makes Synthesis flow rate/ml min-1 7 3 Reactor length/cm 75 75 it possible to know ‘where the sample is’ at any given moment.Injection volume/ml 500 500 Hence, the opening of the by-pass to either the thermospray Figures of merit— device or waste can be synchronised. Sensitivity/mg ml-1 per 0.0044 A 0.03 0.02 The manifold used consists of a peristaltic pump which Reproducibility (RSD%)* 7.67 5.43 pumps CoII solution to the loop of the injection valve, com- Detection limit†/mg ml-1 0.13 0.06 plexing reagent solution [diethyldithiocarbamate dissolved in a mixture of acetone and water (1+3) or acetylacetone, * Sample concentration, 1.5 mg ml-1.† Based on the 3s criterion. depending on the complex formed] to the loop of the second injection valve and acetic acid–sodium acetate buVer solution better than those obtained using on-line synthesis by analyte for the acetylacetone or hydrogenphthalate for the diethyldithi- injection into a ligand flow,30 owing to the smaller dispersion ocarbamate which act as carriers in each case. When the obtained with the merging-zones system.The improvement in simultaneous injection from the two valves is eVected, the CoII the detection limit is due to decreased noise in the blank. solution and the solution of complexing reagent are mixed by means of a T-connection and flow to a reactor (0.8 mm id) Development of the Tandem System of Column where the formation of the cobalt diethyldithiocarbamate and Preconcentration of CoII Coupled to Chelate Formation by cobalt acetylacetonate complexes takes place.The atomic Merging-zones FIA With Thermospray Nebulization– absorption signals obtained in the quartz tube make it possible Vaporization and Detection by Quartz Tube AAS to find the optimum values of the hydrodynamic parameters and the analytical figures of merit, although our main aim was The optimum hydrodynamic characteristics of the two systems to develop the procedure so that it could be coupled to the are slightly diVerent. The elution rate of the preconcentration ion-exchange preconcentration system and to detection in the system with Amberlite IR-120 ion-exchange resin is graphite chamber after thermospray nebulization–vaporization 7 ml min-1; the optimum synthesis rate is similar for and retention on the surface of the graphite platform.Typical diethyldithiocarbamate but 3 ml min-1 for acetylacetonate. signals obtained for cobalt diethyldithiocarbamate and cobalt The first experiments were carried out with diethyldithioacetylacetonate Co(ACA)2 are presented in Fig. 3. It can be carbamate, using the manifold shown in Fig. 1. seen that the dispersion is small, and is considerably smaller The column containing Amberlite IR-120 (13×5 mm id), for cobalt acetylacetonate than for cobalt diethyldithiocarba- on which the CoII solution was loaded at an optimum rate of mate; this should be taken into account when connecting the 3 ml min-1, was placed in the loop of the first valve. HCl (2 M FIA system to the thermospray device and for subsequent was used as the carrier.Diethyldithiocarbamate, dissolved in retention of the complex on the surface of the graphite platform. water–acetone (1+4), was introduced into the loop of the The greater dispersion obtained with the cobalt diethyldithio- second valve and sodium hydrogenphthalate buVer was used carbamate complex could be because its volatility and synthesis as the carrier. kinetics are lower than those of cobalt acetylacetonate. In order to optimize the coupling of the elution of CoII from The optimum parameters and figures of merit achieved with the Amberlite IR-120 column to the merging-zones FIA synthis system are summarized in Table 2.The analytical figures thesis of cobalt acetylacetonate, the same set-up was employed. of merit, sensitivity, reproducibility and detection limits are A solution of acetylacetone in ethanol was used as the ligand reagent in the loop of the second valve and acetic acid–sodium acetate buVer as the carrier.In both cases the first experiments were carried out with direct access to the thermospray device and atomization in the quartz tube. The by-pass was only included in the manifold when the optimum hydrodynamic values of the system for the two complexes were known. The optimum values for the two cases are given in Table 3 together with the analytical figures of merit, sensitivity, reproducibility and detection limits. The values were calculated from the quartz tube AAS signals without taking into account the possible preconcentration factor with the column.With regard to the values shown in Table 3, it should be noted that Table 3 Optimum parameters and figures of merit for the tandem system: on-line column preconcentration of Co and automated synthesis of Co(DTC)3 and Co(ACA)2 by merging-zones FIA, using thermospray nebulization–vaporization and quartz tube AAS detection Parameter— Co(DTC)3 Co(ACA)2 Elution–synthesis flow rate/ 3 3 ml min-1 Reactor length/cm 75 75 Injection volume/ml 500 1000 Figures of merit— Sensitivity/mg ml-1 per 0.0044A 0.03 0.02 Reproducibility (RSD%)* 3.43 1.67 Fig. 3 Peaks obtained in the automated merging-zones FIA chelate Detection limit†/mg ml-1 0.09 0.05 formation–thermospray vaporization of 5 mg of Co with quartz tube AAS detection: (a) as Co(DTC)3 and (b) as Co(ACA)2. * Sample concentration, 1.0 mg ml-1. † Based on the 3s criterion. 1400 Journal of Analytical Atomic Spectrometry, December 1997, Vol. 12compared with those obtained without preconcentration on thermospray deposition starts and with the merging-zones Amberlite IR-120, modifications occur in the optimum values system synchronously optimized no solvent vapour enters the of the elution synthesis rate, dominating the elution process, furnace with the valve to the thermospray device open. undoubtedly owing to the desorption kinetics in the column. As before, the solution of the complex synthesized by the It is also noteworthy that the optimum value of the ligand merging-zones method enters the thermospray system through injection volume for acetylacetonate is twice that for diethyl- an automated by-pass which is open to the thermospray device dithiocarbamate.A greater dispersion can also be seen for the when the reaction zone reaches the connection. Vaporization cobalt diethyldithiocarbamate complex owing to its lower takes place and the gas phase reaches the graphite furnace volatility compared with cobalt acetylacetonate (Fig. 4). pre-heated to the decomposition temperature after which The improvement in the reproducibility of the method using atomization occurs. acetylacetone cannot be attributed to any specific cause. It was proved experimentally that the top position of the thermospray device in the dosing hole of the graphite furnace has a considerable influence on the reproducibility of the Development of the Coupling of Ion-exchange Column absorbance values obtained, undoubtedly owing to the diVerent Preconcentration to Volatile Chelates Synthesis by Mergingvapour distribution on the graphite surface.Hence, the capil- zones FIA, Thermospray Vaporization, Graphite Platform lary outlet was placed exactly 3 mm from the opposite graphite Surface Deposition and ETAAS. Validation of the Method With platform wall because that was the point where the maximum Unpolluted Natural Water Certified Reference Materials absorbance signal was obtained.With the optimized values of the chemical and hydrodynamic The outlet of the vaporizer is positioned to waste to remove parameters and instrumental set-up of the manifold described the solvent during the period when no sample (reaction zone) so far, the only problems in the last stage seem to be those reaches the by-pass valve. When a sample is to be introduced posed by replacing the quartz tube with a standard graphite into the furnace several steps are implemented which are tube platform which, maintained at a suitable temperature, controlled by a timer. The furnace is heated to the desired allows the decomposition of the corresponding complex and deposition temperature.During this heating step and the its quantitative retention on the surface of the platform before sample deposition step the water-cooling of the furnace housing proceeding to an atomization cycle in the tube and obtaining is switched oV to prevent vapour condensation on the quartz the corresponding atomic absorption signal.windows of the furnace and at the exact moment when the However, a few points are worthy of consideration. Sample reaction zone reaches the thermospray device the by-pass valve introduction in ETAAS by aerosol deposition certainly has is set to allow the sample to enter the thermospray system. some disadvantages as the conventional nebulizer system only Some preliminary experiments with Co indicated the influtransfers a small fraction of the sample solution, sometimes ence of both vaporizer and deposition temperature.The best needs a separate desolvation system, and it provides low results were obtained with a vaporizer temperature of 350 °C; transfer eYciency. In our case, deposition through a thermosthis is slightly higher than that used by Bank et al.35 and is pray system with the sole access of the sample load containing probably due to diVerences in the composition of the solution the synthesized chelate overcomes the use of a desolvation which reaches the thermospray system or the diVerent dimensystem, and all the analyte, as well as a minimum amount of sions of the thermospray device.However, the temperature is solvent carrier, in the gaseous form, comes into contact with in the same margin below 375 °C, which, according to Bank the surface of the graphite platform inside the tube. et al.,35 does not aVect the signal. In order to obtain a stable introduction temperature the From the corresponding studies of the thermogravimetric furnace is pre-heated to a constant temperature before the characteristics of the two complexes by either thermogravimetry or diVerential thermal analysis, it is known that the best results are obtained with a deposition temperature in the furnace of 475 °C, although for cobalt acetylacetonate it would be possible to work at temperatures of 300 °C with a cobalt signal trace similar to that observed with conventional sample introduction in ETAAS.The evolution of the atomic absorption signal as a function of the graphite furnace platform temperature for the deposition of 10 ng of Co was used to optimize the temperature. The best results were obtained by working at 350 °C for cobalt acetylacetonate and at 475 °C for the diethyldithiocarbamate complex. In Table 4 the temperature programme used is given and in Table 5 the analytical performance of the proposed tandem system is presented.The sensitivity values, detection limits and reproducibility obtained are excellent in both cases and suYcient for the determination of Co in samples of unpolluted natural waters. Apart from the preconcentration process on the resin and the preconcentration process that occurs during retention on the graphite platform surface, these values may be influenced by the fact that, in accordance with the atomization routes described for Co in a graphite chamber,36 intermediate carbides do not appear to be formed either in the condensed phase or the vapour phase during the atomization of Co, regardless of whether the Co has an environment with a high carbon content, as with diethyldithiocarbamate.However, carbides which sublime with- Fig. 4 Peaks obtained by ETAAS with the tandem system: on-line out decomposition favour the formation of gaseous Co by direct column preconcentration, automated merging-zones FIA chelate dissociation. The higher volatility of cobalt acetylacetonate com- synthesis and thermospray vaporization–deposition on the graphite platform of 5 ng of Co as: (a) Co(DTC)3 and (b) Co(ACA)2.pared with cobalt diethyldithiocarbamate may account for the Journal of Analytical Atomic Spectrometry, December 1997, Vol. 12 1401Table 4 Graphite furnace temperature programme for the determination of Co with the tandem system: on-line column preconcentration, volatile chelate formation and thermospray vaporization–deposition on the platform surface using pyrocoated graphite tubes Temperature/°C Ramp time/s Hold time/s Ar Flow rate/ml min-1 Step Co(ACA)2 Co(DTC)3 Co(ACA)2 Co(DTC)3 Co(ACA)2 Co(DTC)3 Co(ACA)2 Co(DTC)3 Deposition 350 475 0 45 0 1 100 4 15 400 2 900 4 30 20 400 3(read) 2500 0 5 0 4 2650 1 5 400 Table 5 Figures of merit for the determination of Co with the REFERENCES proposed tandem system: on-line column preconcentration, volatile chelate formation, thermospray vaporization–deposition on a graphite 1 Boniforti, R., Ferraroli, R., Frigieri, P., Heltai, D., and platform and ETAAS determination Queirazza, Q., Anal.Chim. Acta, 1984, 162, 33. 2 Nakashima, S., Sturgeon, R. E., Willie, S. N., and Berman, S. S., Co(ACA)2 Co(DTC)3 Fresenius’ Z. Anal. Chem., 1988, 330, 592. 3 McLaren, J. W., Mykytiuk A. P., Willie S. N., and Berman S. S., Characteristic concentration/ng l-1 0.8 1.1 Anal. Chem., 1985, 57, 2907.Detection limit*/ng l-1 2 3 4 Isshiki, K., and Nakayama, E., Anal. Chem., 1987, 59, 291. Reproducibility (RSD%)† 3.4 4 5 Sperling, M., Yin, X., and Welz, B., J. Anal. At. Spectrom., 1991, 6, 615. * Based on the 3s criterion. † Sample concentration, 10 ng l-1. 6 Ba� chmann, K., T alanta, 1982, 29, 1. 7 Ashby, J., and Craig, P. J., Sci. T otal Environ., 1989, 78, 219. Table 6 Determination of Co in natural unpolluted water certified 8 Wolf, W. R., Anal. Chem., 1976 48, 1713. reference materials (CRMs) by on-line column preconcentration, vol- 9 Black, M.S., Thomas, M. B., and Browner, R. F., Anal. Chem., atile chelate formation, thermospray vaporization, deposition on the 1981, 53, 2224. graphite platform surface and ETAAS determination 10 Black, M. S., and Browner, R. F., Anal. Chem., 1981, 53, 249. 11 Moshier, R. W., and Sievers, R. E., Gas Chromatography of Metal Found value/mg l-1 Chelates, Pergamon Press, New York, 1965. Certified value/ 12 Tavlaridis, A., and Need, R., Fresenius’ Z.Anal. Chem., 1978, CRM Replicates mg l-1 Co(DTC)3 Co(ACA)2 292, 135. 13 Sievers, R. E., and Sadlowski, J. E., Science, 1978, 201, 217. SLEW-2* 7 0.055 0.057±0.002 0.055±0.001 14 Schaler, H., and Neeb, R., Fresenius’ Z. Anal. Chem., 1986, 322, 473. SLRS-3* 7 0.027 0.029±0.004 0.026±0.002 15 Meierer, H., and Neeb, R., Fresenius’ Z. Anal. Chem., 1983, TM-28† 7 3.3 3.3±0.005 3.2±0.002 315, 422. TM-27† 7 4.3 4.2±0.003 4.4±0.001 16 Gemmer-Colos, V., and Neeb, R., Fresenius’ Z.Anal. Chem., 1982, TM-26† 7 4.1 4.3±0.001 4.1±0.001 311, 496. 17 Wolf, W. R., Sievers, R. E., Brown, G. H., Inorg. Chem., 1972, * Using 50 ml in the ion-exchange column preconcentration step. 11, 1995. † Using 5 ml in the ion-exchange column preconcentration step. 18 de la Guardia, M., Mauri, A. R., and Mongay, C., J. Anal. At. Spectrom., 1988, 3, 1035. better sensitivity and detection limits obtained for the former, 19 Chau, Y. K., Wong, P.T. S., and Saitoh, H., Anal. Chem., 1975, owing to amore eYcient vaporization process in the thermospray 47, 2279. 20 Alary, J., Vandaele, J., and Escrieut, C., T alanta, 1986, 33, 748. device and decomposition–deposition process on the surface of 21 Lee, D. S., Anal. Chem., 1982, 54, 1182. the graphite platform. 22 Bailey, B. W., and Lo, F. C., Anal. Chem., 1972, 44, 1304. Under the previously mentioned conditions, calibration 23 Hilderbrand, D. C., and Pickett, E. E., Anal. Chem., 1975, 47, 424.graphs were obtained from the corresponding signals (Table 5) 24 Wolf, W. R., Anal. Chem., 1976, 48, 1717. and the analytical performance of the method was evaluated. 25 Wolf, W. R., J. Chromatogr., 1977, 134, 159. In order to validate the analytical properties of the proposed 26 Segar, D. A., Anal. L ett., 1974, 7, 89. 27 Black, M. S., Anal. Chem., 1981, 53, 249. method, aliquots of natural unpolluted water certified reference 28 Mir, J. M., Orea, S., Jimenez, M. S., and Castillo, J. R., Quim. samples were analyzed. The method was tested for the two Anal., 1992, 11, 45. proposed complexes and the results are summarized in Table 6. 29 Castillo, J. R., Delfa, J., Mir, J. M., Bendicho, C., de la The results obtained are in reasonable agreement with the Guardia, M., Mauri, A. R., Mongay, C., and Martinez, E., J. Anal. certified values. At. Spectrom., 1990, 5, 325 30 Jime�nez, M. S., Mir, J. M., and Castillo, J. R., J. Anal. At. Spectrom., 1993, 8, 665. CONCLUSIONS 31 Mir, J. M., Jime�nez, M. S., and Castillo, J. R., At. Spectrosc., 1995, 16, 127. The combination of a microcolumn ion-exchange system and 32 Mir, J. M., Jimenez, M. S., and Castillo, J. R., Quim. Anal., 1996, an automated merging-zones FIA system for volatile chelate 15, 255. formation with a thermospray device to deposit the analyte 33 Fang, Z. L., Ruzicka, J., and Hansen, E. H., Anal. Chim. Acta, on a graphite furnace platform without solvent intake and 1984, 164, 23. 34 Fang, Z., Flow Injection Separation and Preconcentration, VCH, ETAAS determination of Co is described. New York, 1993, pp. 15–18. The main advantages are the detection limit obtained and 35 Bank, P. C., de Loos-Vollebregt, M. T. C., and de Galan, L., the separation of the matrix. In addition, the instrumentation Spectrochim. Acta, Part B, 1989, 44, 571. used is simple and within the reach of laboratories that are 36 Styris, D. L., and Redfield, D. A., Spectrochim. Acta Rev., 1993, not highly specialized. Compared with other preconcentration 15, 71. systems, such as extraction, coprecipitation and cocrystallization, the method has numerous advantages. It is rapid, needs little reagent and the precision is excellent. Paper 7/03190D Received May 9, 1997 This work was sponsored by the DGICYT of the Spanish Accepted September 8, 1997 Ministry of Education and Science, project No. PB93/0306. 1402 Journal of Analytical Atomic Spectrometry, December 1997, V

ISSN:0267-9477    

DOI:10.1039/a703190d

出版商:RSC    

年代:1997

数据来源: RSC