摘要:

Determination of Arsenic and Bismuth in Biological Materials by Total Reflection X-ray Fluorescence After Separation and Collection of Their Hydrides JU� RGEN MESSERSCHMIDT, ALEX VON BOHLEN, FRIEDRICH ALT AND REINHOLD KLOCKENKA� MPER* Institut fu� r Spektrochemie und Angewandte Spektroskopie (ISAS), Bunsen-KirchhoV-Str. 11, D-44139 Dortmund, Germany A combined procedure was developed for the determination of EXPERIMENTAL As and Bi in biological materials by total reflection X-ray Accessories fluorescence (TXRF).The materials were first digested by an open wet decomposition method. The separation of both Equipment elements from the sample matrix was then achieved by For sample decomposition an open digestion device was used, generation of their volatile hydrides and subsequent trapping in consisting of an aluminium heating block with drill-holes for a collection solution. The quantitative determination of As and the insertion of 10 ml quartz glass tubes.The separation and Bi was performed simultaneously by TXRF using Y as collection of the element hydrides was carried out in a laborainternal standard. The procedure was applied to several tory-built hydride generation and collection system (Fig. 1), diVerent biological certified reference materials. The recovery consisting of a peristaltic pump (LKB 2132, Pharmacia, rate determined for mass fractions between 40 ng g-1 and Freiburg, Germany), a 10 ml quartz glass tube, a system for 10 mg g-1 was 90–100% after correction with aqueous the supply of NaBH4 solution and carrier gas (N2), glass standard solutions.The RSD for the whole procedure ranged capillary tubes (Blaubrand intra end 2 ml and intra mark 50 ml; from 8 to 14% and the detection limits were about 10 ng g-1 Brand, Wertheim, Germany), an angular tube, a collection for As and 20 ng g-1 for Bi. vessel (1.5 ml Eppendorf-type vessel ) and a heating device (Reacti-Therm 18800, Pierce, Rockford, IL, USA).Keywords: Arsenic; bismuth; hydride generation; total reflection X-ray fluorescence; biological materials An EXTRA II TXRF spectrometer, equipped with Mo and W tubes for X-ray excitation (Rich. Seifert & Co., Ahrensburg, Germany) and a QX 2000 Si(Li) detector and analyzer (Link; Oxford Instruments, High Wycombe, Buckinghamshire, UK) Sample solutions to be prepared for analysis by total reflection including a software package were used to record and process X-ray fluorescence (TXRF) have to fulfil certain conditions in the spectra.order to make use of the high sensitivity of this technique. The solutions after sample decomposition should contain few matrix residues and the sample solvent (e.g., water, nitric acid) Reagents should have high volatility. About 10 ml of the final solution All reagents used were of analytical-reagent or high-purity have to be pipetted onto clean quartz glass carriers and dried grade (Merck, Darmstadt, Germany).Water was de-ionized by evaporation. A dry residue of only a few micrograms or by a Milli-Q water purification system (Millipore, Bedford, less deposited as a thin layer several micrometres thick and MA, USA). High-purity grade nitric acid was prepared by several millimetres in diameter has to be applied for TXRF measurements. The detection limits of the method can deteriorate by up to three orders of magnitude if the matrix content is too high.1 However, such interference can be reduced by using suitable techniques of analyte/matrix separation.Apart from extraction or chromatographic separation, which often only reduces the problem, volatilization of the analyte and its collection in a suitable solvent can be a successful approach. In this paper, a hydride separation/collection technique and its adaptation to TXRF is described. It is well known that a number of elements (e.g., As, Se, Sb, Bi) can be reduced to their corresponding hydrides.These can be transferred via a carrier gas stream (nitrogen, argon) into a heated quartz tube, positioned in an atomic absorption spectrometer. Another possibility is the coupling of a hydride generation system to an inductively coupled plasma spectrometer. These techniques have been well known for many years2–5 and used successfully in routine analysis.6,7 A combination of hydride generation, collection of the volatile hydrides in a suitable solvent and Fig. 1 Hydride generation and collection system: (a) 10 ml quartz multi-element determination by TXRF was first proposed by glass tube (with digested sample material) and introduction system for HaVer et al.,8 but the method was only applied to aqueous NaBH4 solution and carrier gas, (b) angular tube, (c) glass capillary standard solutions.A similar technique is described in this (Blaubrand intra mark, 50 ml ), (d) glass capillary (Blaubrand intra paper and was applied to biological certified reference end, 2 ml ), (e) collection vessel (1.5 ml Eppendorf-type vessel ), (f ) heating device (105–110 °C) and (g) silicone rubber tubing.materials. Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12 (1251–1254) 1251sub-boiling distillation (H. Ku� rner, Rosenheim, Germany) of was placed in a collection vessel. The vessel was placed in the heating device of the hydride generation/collection apparatus analytical-reagent grade acid, and then stored in quartz glass bottles.shown in Fig. 1 and pre-heated for 30 s (block temperature 105–110 °C). The quartz glass tube with the digested sample For the hydride generation, a NaBH4 solution was prepared by dissolving 3 g of NaBH4 (Merck) in 100 ml of 0.01 M NaOH. solution was connected to the collection vessel by capillaries. By means of a peristaltic pump, the NaBH4 solution was This solution was prepared just before use and was stored in a polyethylene bottle. pumped into the quartz glass tube (60 ml h-1, 45 s).The volatile hydrides were transported to the collection vessel by Stock standard solutions (1000 mg l-1) of As, Se, Sb and Bi were prepared from Merck Titrisol solutions; a stock standard a stream of nitrogen which acted as a carrier gas (20 ml min-1). The nitrogen was allowed to flow through the apparatus for a solution of Y (1000 mg l-1) was purchased from Aldrich (Milwaukee, WI, USA). Working standard solutions of each further 90 s in order to complete the hydride collection.The collection vessel was then removed from the heating device element were prepared daily from the stock solutions. and closed. Samples TXRF measurements and quantitative determinations First, the procedure (hydride generation, collection and TXRF Aliquots of 30, 50 or 100 ml of the collection solution were measurement) was tested using aqueous standard solutions dried on quartz glass carriers by means of IR radiation at a prepared by dilution of stock standard solutions.Subsequently, temperature of about 80 °C. TXRF spectra were recorded in a standard or certified reference materials (SRMs or CRMs) preset live time of 300–1000 s depending on the analyte concen- were subjected to the whole procedure including their tration in the sample. Yttrium was used as an internal standard decomposition by an open digestion method. The following element for quantification. The mass of the analyte element materials were chosen: Bovine Liver (NIST SRM 1577a), present in the collection solution was determined from the Orchard Leaves (NIST SRM 1571), Tomato Leaves (NIST following equation: SRM 1573), Plankton (BCR CRM 414) and Tea Leaves (NIES No. 7). All the materials were supplied by Promochem (Wesel, Germany). mx= Nx/Sx NY/SY ·mY (1) where m indicates the mass, N the measured net intensity and Procedure S the relative sensitivity of either the analyte, x, or the internal The whole procedure consists of the following steps: (i) open standard, Y.9 The sensitivity values were known from previous wet decomposition of the samples, (ii ) generation of the investigations on aqueous standard solutions;10 the mass mY hydrides AsH3, SeH2, SbH3 and BiH3 and their collection in was chosen to be 4 ng.The mass fraction cx of the analyte in a small volume of a collection solution and (iii ) measurement the chosen sample material was then determinedby TXRF. quotient of mx and the weighed portion of the sample material (80–100 mg).A raw recovery rate was then calculated from Open wet decomposition the ratio of the value of cx to the certified value. The recoveries of As and Bi were high but mostly ,90%. Portions of 80–100 mg of a biological sample material were To correct for the incompleteness of hydride generation and weighed in 10 ml quartz glass tubes. To soak the dry samples, collection, aqueous standard samples of As and Bi in 1 M HCl 500 ml of de-ionized H2O were added to each tube.After a few were prepared by addition of these elements and measured in minutes, 500 ml of HNO3 (65%) and 40 ml HClO4 (70%) were parallel with the real samples (the whole procedure except pipetted into the tubes. In a first step, the tubes were heated decomposition). The addition of As and Bi was made so as to in the aluminium heating block from room temperature to give concentrations in the aqueous standard samples compar- 140 °C within 60–70 min.The tubes were then allowed to cool able to the mass fractions of these elements in the real samples. outside the block and 500 ml of HNO3 together with 100 ml of The recoveries of As and Bi in the aqueous standard samples HClO4 were added. In a second step, the tubes were again were finally used as scaling factors in order to correct the mass placed in the heating block and the temperature was increased fraction cx and the raw recovery rate for the real samples by from 140 to 180 °C within 60–70 min.If the digested sample normalization. solutions exhibited a dark colour (undigested material), 400 ml of HNO3 were added and the temperature was maintained at RESULTS AND DISCUSSION 180 °C until the solutions became nearly colourless. In a third step, the temperature was increased from 180 to 210–215 °C in Optimization of Hydride Generation and Collection 10 min. This temperature was maintained for a further 10 min In view of the simultaneous multi-element capability of TXRF, taking care that the solutions did not evaporate to dryness.an attempt was made to separate and collect as many elements During the described procedure the excess of nitric acid as possible via their hydrides. The eYciency of an open wet evaporated totally. The colourless residues were dissolved in decomposition method had already been tested in earlier the small volumes of HClO4 that remained but did not interfere experiments with tea leaves.11 Therefore, this type of digestion with the following hydride generation process.The temperature was adapted to the above-mentioned biological materials with and time schedules given above had to be followed exactly to only small changes. It was necessary to optimize the conditions ensure complete mineralization without losses of As and Bi. for hydride generation of the elements and for collection of Finally, the quartz glass tubes were removed from the the hydrides in a collection solution.The usual procedure of aluminium heating block and cooled. The digestion residues drying the solutions on TXRF quartz glass carriers and the were diluted with 400 ml of 5 M HCl after which 1.6 ml of H2O standard settings for TXRF measurements could then be were added. applied. The optimization was first performed with aqueous standard Generation and collection of volatile hydrides of As and Bi (Se solutions of the elements in question (10–100 ng ml-1 As, Se and Sb) and Sb in 1 M HCl) without any digestion step.Only those reagents that could be dried on TXRF quartz glass carriers A mixture of 250 ml of HNO3 (65%), 250 ml of HCl (37%) and 20 ml of Y standard solution (4 ng Y absolute in HNO3, 65%) without leaving any interfering residues were used as collection 1252 Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12Fig. 2 Influence of the temperature of the collection solution [HNO3 (65%)–HCl (37%)] on the recovery of As.solutions. The Se hydride was quantitatively collected in 0.1 M ammonia solution and 0.01 M NaOH. This reaction was con- Fig. 3 Determination of As (47 ng g-1) and Bi (50 ng g-1 spiked) in trolled by the determination of Se (electrothermal atomic Bovine Liver (SRM 1577a). (a) TXRF spectrum for direct determiabsorption spectrometry). However, drying the collection solu- nation in the digested solution; (b) TXRF spectrum for determination after hydride separation and collection.The preset live time for both tion on a quartz glass carrier hampered the quantitative and spectra was 300 s. reproducible recovery of the volatile Se by TXRF measurements. Pd and Hg salts were added to the collection solutions as to high recovery rates and low relative standard deviations chemical modifiers in order to reduce the volatility of Se. Such (RSDs) for As and Sb, but not for Se. additions, however, resulted in a new interfering matrix which reduced the sensitivity of TXRF determinations.Application to Biological Materials An increased recovery rate was achieved by using strong oxidizing agents such as a mixture of HNO3 and H2O2 or Reference materials (RMs) were used in order to test the HNO3 and HCl. The latter acidic mixture used as a collection separation/collection technique for biological samples. In solution provided high recovery rates combined with high addition to As, Se and Sb, the element Bi was included in the precision and accuracy.Fig. 2 demonstrates the influence of investigations. First, Bovine Liver (NIST SRM 1577a) was the temperature of this solution on the recovery of As. The chosen in order to demonstrate the advantages of the proposed highest value was found for a temperature of 110 °C in the sample pre-treatment. As (47 ng g-1) and Bi (50 ng g-1 spiked) heating block. The temperature could not be increased further were determined by TXRF (a) directly in the diluted decompobecause of the instability of the polypropylene collection vessels sition solution and (b) after the described hydride generation/ used.At 110 °C, the recovery rate for As was about 80%, for collection procedure. The diVerences between the correspond- Sb about 90%, but for Se only about 10%. The velocity at which the hydrides are transported to the Table 2 Raw recovery rates for the determination of As, Se, Sb and collection solution is just as important as the temperature.A Bi in diVerent biological RMs slow transport to the HNO3–HCl mixture increased the recovery rates of the elements. Capillaries with a small inner diameter Mass fraction/ Raw recovery produced small gas bubbles of the hydrides which were obvi- Sample material Element mg g-1 (%) ously collected in a more eVective way. Furthermore, the NIST SRM 1577a volume of the collection solution had a significant influence Bovine Liver As 0.04–0.6 60–83 on the collection eYciency.A minimum volume of 400–500 ml Se 0.7–2.0 ,10 was necessary and the greatest eYciency was achieved with a Sb 0.1–0.6 20–43 Bi 0.05 70 mixing ratio of 250 ml of HNO3 (65%) to 250 ml of HCl (37%). Yttrium was added to the collection solution as an internal NIST SRM 1571 standard before starting the hydride generation procedure so Orchard Leaves As 10 .95 that evaporation of the collection solution or even possible Se 0.1–1.0 ,10 spraying would not aVect the accuracy. The optimum eYciency Sb 1.0 30–40 was obtained with the conditions indicated under NIST SRM 1573 Experimental.As listed in Table 1, the described procedure led Tomato Leaves As 0.27 60 BCR CRM 414 Table 1 Recovery rates and relative standard deviations (RSDs) for Plankton As 6.82 60 the determination of As, Se and Sb in aqueous standard solutions of Se 1.75 ,10 1 M HCl Sb 1.0 30–40 Standard Absolute mass/ Recovery RSD NIES No. 7 solution Element ng n (%) (%) Tea Leaves As 0.04–0.6 80 Se 0.1–1.0 ,5 1 M HCl As 10 11 75 11 1 M HCl Se 10 12 6 64 Sb 0.1–0.6 30 Bi 0.05–0.7 70 1 M HCl Sb 10 12 91 9 Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12 1253Table 3 Results of the determination of As and Bi in diVerent RMs Certified Corrected mass fraction/ Mean found/ s/ RSD recovery Sample material Element mg g-1 n mg g-1 mg g-1 (%) (%) NIST SRM 1577a Bovine Liver As 0.047 7 0.046 0.004 9.3 98 Bi 0.05* 4 0.044 0.005 11 88 Bi 0.06* 2 0.053 88 NIST SRM 1571 Orchard Leaves As 10 8 9.8 0.9 9.4 98 NIST SRM 1573 Tomato Leaves As 0.27 9 0.26 0.02 7.7 96 BCR CRM 414 Plankton As 6.82 4 6.35 0.91 14 93 NIES No. 7 Tea Leaves As — 6 0.042 0.004 8.8 Bi — 6 0.26 0.024 9.2 * Element, spiked. ing two spectra are shown in Fig. 3. No signals of As and Bi detection limits. Following the analyte/matrix separation described in this paper, both elements can be determined by can be observed in the spectrum recorded directly [Fig. 3(a)], but both peaks are present in the spectrum recorded following TXRF down to the low ng g-1 level, which may be important for biological investigations.Unfortunately other hydride- the described procedure [Fig. 3(b)]. The given amount of about 5 ng of As and Bi in a sample mass of 100 mg may be forming elements such as Se and Sb could not be determined by this method. This was due to the incomplete separation estimated to be 3–4 times the detection limit (3s) of these elements. and trapping of the relevant hydrides under the described experimental conditions.More specific results were obtained for As, Se, Sb and Bi in the diVerent RMs. Increasing amounts of these elements were pipetted onto the samples in the form of aqueous standard This study was supported financially by the Ministerium fu� r Wissenschaft und Forschung des Landes Nordrhein-Westfalen solutions. Amounts of 4–1000 ng of each element were added depending on the concentration of the elements in the RMs.and the Bundesministerium fu� r Bildung, Wissenschaft, Forschung und Technologie (the Ministry for Science and The samples were then digested and the elements were measured by TXRF following the described hydride generation/ Research of Nordrhein-Westfalen and the Federal Ministry for Training, Science, Research and Technology). collection procedure. Raw recovery rates were calculated for the range of mass fractions given in Table 2. High recovery rates were found for As (60–95%) and Bi REFERENCES (70%). In contrast to the recoveries found for aqueous standard solutions in 1 M HCl, unsatisfactory results were obtained for 1 Prange, A., Spectrochim.Acta, Part B, 1989, 44, 437. 2 Chu, R. C., Barrons, G. P., and Baumgardner, P. A. W., Anal. Sb in the RMs (only 20–40%). This was probably caused by Chem., 1972, 44, 1476. the hampered hydride generation in the digested matrix solu- 3 Thompson, K. C., and Thomerson, D. R., Analyst, 1974, 99, 595.tion and by the incomplete collection of the hydrides in the 4 Thompson, M., Pahlavanpour, S., Walton, S. J., and Kirkbright, collection solution. The low recovery of Se (,10%) also found G. F., Analyst, 1978, 103, 568. for aqueous solutions can be explained by the high volatility 5 Nakahara, T., Prog. Anal. At. Spectrosc., 1983, 6, 163. of this element. 6 Alt, F., Messerschmidt, J., and Schaller, K.-H., in Analyses of Hazardous Substances in Biological Materials, ed. Augerer, J., and Because of their high recoveries, only As and Bi were Schaller, K.-H., VCH Verlagsgesellschaft, Weinheim, 1988, vol. 2, included in the following investigations. Individual results p. 231. found for the diVerent RMs are listed in Table 3. Since these 7 Schierling, P., Oefele, Ch., and Schaller, K.-H., A� rztl. L ab., 1982, materials are not certified for Bi, samples were spiked with 28, 21. this element. Both elements were determined in a wide range 8 HaV er, E., Schmidt, D., Freimann, P., and Gerwinski, W., of mass fractions (ng g-1 to mg g-1) with detection limits of Spectrochim. Acta, Part B, 1997, 52, 935. 9 Klockenka�mper, R., T otal-Reflection X-Ray Fluorescence 10 ng g-1 for As and 20 ng g-1 for Bi. The RSD for repeated Analysis, Wiley, New York, 1996. measurements (n=4–9) is about 10%. The mean values for 10 Klockenka�mper, R., and von Bohlen, A., Spectrochim. Acta, Part the mass fractions of As and Bi were found by normalisa- B, 1989, 44, 461. tion to the recoveries of aqueous standard solutions as de- 11 Xie, M. Y., Messerschmidt, J., von Bohlen, A., Ma, Y., scribed above (see under Procedure). By this means, corrected Pfeilsticker, K., and Gu� nther, K., Z. L ebensm.-Unters. Forsch., recoveries of 88–98% were found for the diVerent RMs. 1995, 201, 303. Paper 7/05093C CONCLUSION Received July 16, 1997 Accepted September 2, 1997 The developed method allows the simultaneous determination of As and Bi in biological materials with significantly improved 1254 Journal of Analytical Atomic Spectrometry, November 1

ISSN:0267-9477    

DOI:10.1039/a705093c

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Radiofrequency Glow Discharge as an Ion Source for Gas Chromatography With Mass Spectrometric Detection† M. A. BELKIN, L. K. OLSON AND J. A. CARUSO* Department of Chemistry, University of Cincinnati, Cincinnati, OH 45221, USA A radiofrequency (rf) powered glow discharge (GD) has been obtained, which would assist in analyte identification and provide quantitative information. used as an ion source for GC–MS. The improved GC–GD interface has allowed separation and detection of five organotin In a recent study from this laboratory an rf-GD has been applied successfully as an ion source for GC with MS detec- compounds with better figures of merit than previously reported.Limits of detection for four compounds were from tion.24 This initial development and evaluation of the GC–GD interface allowed the separation and determination of 0.6 to 13 picogram (as tin). Optimization of the discharge pressure and sampling distance with respect to the primary organometallic compounds with absolute detection limits of the metal in the low picogram range.That compares favorably analytical species of interest, elemental tin, has been performed. Characterization of the source has been made by with commercially available techniques.24 The rf-GD is suitable for element selective detection in GC and shows promise for studying the eVect of the cell pressure, applied rf power and sampling distance on analyte signal and organotin obtaining structural information since the molecular fragments were observed in the rf-GD plasma.Furthermore, rf-GD fragmentation patterns. The results obtained indicate that the mass spectra generated do not diVer significantly with the studies with Ar show appreciably higher electron energies than those for dc-GDs.25 These are also dependent on the cathode varying source pressure and rf power as they exhibit the same features in terms of parent-molecular peaks and fragment ions. conductivity suggesting a high potential for a variable energy MS source.However, the ionization mechanism and details of Keywords: Radiofrequency glow discharge; mass spectrometry; the fragmentation occurring in the rf-GD plasma were not ion source; capillary gas chromatography; organotin addressed and only the preliminary figures of merit were reported in the first study. In the present work the eVect of the cell pressure, applied rf power and cell geometry on the In recent years, GD devices have become popular excitation– ionization sources for various atomic spectrometric tech- analyte fragmentation is discussed using a representative separation of organometallic compounds.A time-resolved niques.1 Direct current (dc) GDMS is now established as a standard method for elemental analysis of bulk metals, alloys analysis mode was used to obtain total ion-beam chromatograms with subsequent extraction of the mass spectrum for each and semiconductors.2–4 Rf-GD devices have been developed, mostly for direct analysis of solid conducting and non- peak.The new GD cell was designed so that the low cell volume would be advantageous for GC with respect to dilution conducting samples with AE,5–7 AA,8,9 and MS10–15 detection. Such rf-GD sources have been interfaced to quadrupole,11,13,16 eVect and peak broadening. double focusing12 and ICP time-of-flight17 mass spectrometers. Determination of organic species by GDMS sources has also EXPERIMENTAL been performed.Dc-GD mass spectra of several organic compounds, including sucrose and tyrosine, deposited directly onto Glow Discharge Source a copper probe tip have been acquired.18 Sample introduction The rf-GD source is shown schematically in Fig. 1, and the into GDMS sources using GC or LC has also been utilized.19–23 overall schematic is shown in Fig. 2. The aluminium body of A GDMS source exhibited picogram range detection limits the GD cell was mounted on the front plate of the ICP mass with six orders of magnitude linear response for organic spectrometer with its regular sampling cone removed.The 1/2 compounds introduced via an LC pump.19 An atmospheric in od direct insertion probe used was based on the design by sampling GD ionization source has been used for analysis of Duckworth and Marcus10 and was fitted into the 3/4 in cell organic vapors sampled from ambient air.20 In these experibody opening via combination of 3/4 and 3/4–1/2 in ultra- ments, air served as the discharge support gas and ionization Torr vacuum fittings (Swagelock, Crawford Fitting, Salon, of target organic analytes occurred, as with conventional OH, USA).The rear part of the probe was mounted on a chemical ionization (CI), through ion–molecular reactions. An translational stage (Oriel, Stratford, CT, USA) to allow the atmospheric pressure dc-GD in helium has been used as an cathode to be positioned reproducibly at a certain distance ionization source for organic samples introduced by liquid from the orifice opening.High purity aluminium (6161 T6) injection.21 was used as the cathode material while the cell body served as Plasma source MS combines the ability to determine selected an anode. By using a graphite ferrule around the probe inside isotopes and perform multielement analysis at ultra-trace levels. the cell, the cell volume was limited to approximately 1.8 cm3. As an element specific technique it has high potential for UHP helium was used as the discharge gas.Cell pressure coupling with diVerent chromatographic methods to obtain was regulated by varying the helium flow rate which was speciation information along with the total trace element controlled by a needle valve (HP 5080 6710). The cell was profile. In the common operating modes, the plasma fully evacuated through the 1 mm sampling orifice by the roughing decomposes the analyte, leaving little chance for obtaining pump of the mass spectrometer expansion stage.The residual structural information. However, utilizing reduced-pressure, pressure in the cell was measured with a Type 0531 thermo- low-power plasmas should allow fragmentation spectra to be couple gauge (Varian, Palo Alto, CA, USA); the discharge pressure was measured with an EPS10 strain gauge (Edwards, † Presented in part at the 23rd FACSS Conference, September 29–October 4, 1996, Kansas City, MO, USA. Crawley, West Sussex, UK).Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12 (1255–1261) 1255Fig. 1 Schematic diagram of the rf-GD cell. Fig. 2 Schematic diagram of the GC–rf-GD interface. Radiofrequency power was supplied by a 13.56 MHz generator (ACG-3) and was impedance matched by an automated matching network (MV-5, ENI, Rochester, NY, USA). and was threaded through the 1/16 in stainless-steel tube into the GD cell. The tip of the capillary was positioned in front Mass Spectrometer of the cell sampling orifice.A 1/16 in Swagelock Tee was used to introduce the discharge support gas into the GD cell. The A Fisons (Loughborough, Leicestershire, UK) VG gas was heated to the transfer line temperature (250 °C) by PlasmaQuad II was modified to accept the GD cell. The ICP passing it through a copper heating coil heated by an electrical torchbox was removed and replaced by the GD system. The heating tape. This was done to prevent any analyte condenion lenses were tuned using m/z=120. Both single-ion monitorsation on the transfer line or cell interior.Past the Tee the ing and time-resolved acquisition modes were employed. transfer capillary was threaded through a 1/8 in copper tube. Typical operating conditions of the GDMS system are listed The stainless-steel tube, the Tee and the copper tube were in Table 1. heated by an electrical heating tape powered by a variable voltage supply. The temperature along the transfer line was Gas Chromatography monitored with four thermocouples equipped with digital thermometers (Fisher). An HP Model 5890 Series II gas chromatograph (Hewlett- Packard, Rockville, MD, USA) was used.A deactivated retention gap (1.5 m length×0.32 mm id, uncoated) was used as Reagents the interface between the cool on-column injection inlet and Individual stock solutions (1000 mg l-1 as Sn) of Me4Sn the DB5 (J&W Scientific, Austin, TX, USA) capillary column (TMT) (99% purity, Pfaltz & Bauer, Waterbury, CT, USA), (30 m length×0.32 mm id×0.25 mm film thickness). Sample Et4Sn (TET, 97% purity), Me3PhSn (TMPT, 98% purity), injection was achieved using a helium-activated W-type Et3SnBr (TETBr, 97% purity) and Bu4Sn (TBT, 93% purity, internal volume sampling valve (Valco, Houston, TX, USA) all Aldrich, Milwaukee, WI, USA) were prepared in HPLC fitted with a 0.2 ml rotor. grade methanol (Fisher).Mixed working solutions were pre- The column outlet was connected to a Valco HT four port pared daily by dilution of the stock solutions with methanol. 1/16 in zero volume fitting valve which allowed solvent venting The GD plasma gas and carrier gas used for GC was helium –flow rate measurements and connection to the transfer line. (99.999%, Wright Brothers, Cincinnati, OH, USA). The valve was inserted into a laboratory-built heated valve enclosure mounted on top of the GC oven. The temperature of the enclosure was maintained at the transfer line temperature RESULTS AND DISCUSSION (250 °C).The operating conditions for the GC–GDMS system Separation of Organotin Compounds are summarized in Table 2. The type of GC injection inlet is critical to achieving satisfactory separation of organotin compounds.26 The splitless injec- GC–GD Interface tor and chromatographic equipment used in the previous study A length (approximately 75 cm) of 0.32 mm id deactivated in combination with the GD cell design based on a six-way silica capillary served as the transfer line for the GC eZuents cube24 resulted in a nonoptimal separation and poor reproducibility.The cool on-column injection was then evaluated; a Table 1 GDMS operating conditions deactivated retention gap (1.5 m length×0.32 mm id, uncoated) was used as an interface between the 0.2 ml injector valve Incident rf power 10–40 W and the capillary GC column. Reflected power 0 W Both single-ion monitoring and time-resolved acquisition Discharge gas Helium modes of the mass spectrometer were employed.The total ion Cathode material Aluminium Source pressure 6–30 Torr* chromatogram of a mixture of organotin compounds obtained Residual cell pressure <35 mTorr with GDMS detection is shown in Fig. 3. Although the re- Analyser pressure 4×10-7 mbar† designed transfer line in this study was much shorter and Intermediate pressure <1×10-4 mbar better temperature controlled than in the previous experiments, Lens tuning Sn, m/z=120 the TBT peak shows significant broadening which could also be attributed to the poor GC column conditions (high bleeding, * 1 Torr=133.322 Pa.† 1 bar=105 Pa. etc.). Triethyltin bromide, the ionic compound, was not stable 1256 Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12Fig. 3 Total ion chromatogram of a mixture of organotin compounds: 65 ng (as Sn) injection; rf power, 30 W; cell pressure, 26 Torr; and sampling distance, 2.5 mm.under the conditions of these experiments and often eluted as two peaks. The chromatograms obtained for the same mixture of four organotin compounds with FID (a) and GDMS (b) detection are shown in Fig. 4. Since the passage of a solvent did not disrupt the discharge under the given experimental Fig. 4 Chromatogram of a mixture of organotin compounds (80 ng conditions and the early eluting peaks were not obscured by as Sn) obtained with (a) FID detection and (b) rf-GDMS detection.the solvent peak, solvent venting was not used in most of the experiments. The eVect of carrier gas flow rate on the peak integrals for separation distance (single-ion monitoring at m/z=120). The separation of organotin compounds is shown in Fig. 5. These profound eVect of both pressure and distance is illustrated in data were obtained with single-ion monitoring at m/z=120, Fig. 6. The analyte signal intensity increased as the helium corresponding to the major elemental tin isotope.Flow rates pressure increased, until a maximum occurred at 26 Torr, of up to 20 ml min-1 were still low compared with the discharge beyond which higher pressures resulted in lower intensities. At gas flow rate (up to 300 ml min-1) at the discharge pressures constant pressure, decreasing the cathode–anode separation employed in this study, so the changes in the carrier flow rate resulted in increasing signal intensity, with a fall-oV at closer did not appreciably aVect the cell pressure.A carrier flow rate spacing, probably owing to physical interference between the of 18–20 ml min-1 was chosen in order to maximize the transfer line capillary and the cathode and/or arcing. analyte signals without compromising on the quality of separa- The rf-GD source design considerations and parameter tion. In the study on the GC–AED determination of organo- optimization with respect to analyte signal from sputtered metallic compounds, carrier flow rate was shown not to aVect cathode material for both conducting and non-conducting the atomic emission signal responses for TBT and tributylphen- samples have been reported.10–17 The similar sharp maximum yltin while a 30–80% increase of the signal for organolead in sputtered 63Cu+ signal intensity was observed at a pressure and organomercury compounds was observed.26 of 0.42 Torr and a sampling distance of 5.3 mm.10 Optimum The figures of merit obtained for this separation (0.2 ml conditions for the aluminium signal were reported between injection volume, single-ion monitoring at m/z=120) are given 0.35 and 0.55 Torr argon pressure and 3–5 mm sampling in Table 3.The theoretical detection limits were calculated as distance for a pin-type ion source on a quadrupole instruthree times the standard deviation of the background; and the ment.14 For the studies employing a six-way cube or similar minimum detectable amounts were calculated based on the ion volume the optimum pressures in the region of 0.1–0.5 Torr concentration equivalent of twice the peak-to-peak noise.The were attributed to the critical positioning of the plasma such RSDs (%) were calculated for ten replicate 1.6 ng injections. that ions are sampled from the interface of the negative glow region and the cathode dark space.10 An inverse relationship between ion volume and actual volume of the luminous plasma Optimization of Discharge Parameters has been proposed,13 that is, a small cell requires that the plasma volume be drastically compressed in order to form Peak integrals for the eluting TET, TBT and TMPT were monitored as a function of cell pressure and cathode–anode properly, and thus higher gas pressures are required.(In fact, Table 2 GC operating conditions Instrument Hewlett-Packard HP 5890 Series II GC GC column and carrier gas conditions: Stationary phase DB-5 Column length 30 m Column id 0.32 mm Column film thickness 0.25 mm Carrier gas Helium Carrier gas flow rate 5–20 ml min-1 Injection Cool on-column Injection volume 0.2 ml Oven program: Initial temperature 50 °C (1 min) Ramp rate 20 °C min-1 Final temperature 220 °C (1 min) Transfer line temperature 250 °C Solvent venting Yes Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12 1257Fig. 5 EVect of carrier gas flow rate on peak integrals for separation of organotin compounds: 2, TET; &, TMPT; +, TBT. Injection of 8 ng (as Sn), rf power 30 W, cell pressure 26 Torr and sampling distance 2.5 mm.preliminary experiments with a concentric GD cell with a smaller volume than that employed in the present study confirmed that observation. With actual cell pressures not obtainable at this time, the stable discharge conditions were obtained at the discharge gas flow rate of 700 ml min-1.) As the relative size of the negative glow changes, decreasing with increasing pressure, both the actual sampling and the analyte introduction positions may be in more or less energetic regions of the discharge, thus explaining the observed signal intensity– Fig. 6 EVect of (a) cell pressure and (b) sampling distance on peak pressure/distance dependencies. integrals for separation of organotin compounds: 2, TET; &, TMPT; and +, TBT. Injection of 8 ng (as Sn), rf power 30 W, cell pressure 26 Torr and sampling distance 2.5 mm. Mass Spectra of Eluting Organotin Compounds Previous reports have shown that many ionization processes the high mass being determined by the limitations of the ICP occur simultaneously in the GD plasma, leading to diVerent mass spectrometer used.fragmentation pathways.27 Glow discharge ionization can pro- The mass spectrum obtained for an eluting TMT peak is duce fragments which are similar to those obtained by electron shown in Fig. 7(a). The obtained spectra resemble the EI impact (EI) and CI methods.19 spectra [NIST library, Fig. 7(b)] for which the fragment assign- The mass spectra of organotin compounds were extracted ments have been done28 very closely in terms of the structurally from the total-ion chromatograms using the time-resolved significant fragment ions observed.The prominent peaks can analysis mode of the MS instrument. The mass spectra did not be assigned as 120Sn+ at m/z=120, (Me)Sn+ at m/z=135, appear in the same abundance across the chromatographic (Me2)Sn+ at m/z=150 and (Me3)Sn+ at 165. The parent peaks, that is, not only absolute intensities, but also relative molecular ion peak at m/z=180 can also be seen.Clearly intensities were changing for the mass spectra obtained at 1 s evident is the decrease in intensity as a function of fragment spaced points at the peaks. This observation was initially thought to be an experimental artifact due to the relatively slow MS data acquisition system. However, preliminary experiments with a new GD cell design employing axial sample introduction lead us to believe that the observed phenomenon was due to the changing plasma characteristics as the organic components are added.With the axial sample introduction design this was not observed, possibly owing to the eliminated discharge asymmetry and increased analyte residence time in the plasma. The mass spectra of the eluting organotin compounds were then averaged across the chromatographic peak widths and are believed to be the representative GDMS spectra.The detection mass range used was 110–250 m/z with Table 3 Figures of merit Parameter TMT TET TMPT TBT Linear range studied (decades) 2.5 2.5 2.5 2.5 Slope/counts s-1 pg-1 681 615 379 262 Correlation coeYcient 0.9950 0.9874 0.9713 0.9991 Log–log slope 1.011 1.126 1.187 0.9196 MDA*/pg 1.2 2.0 2.6 87 Detection limit/pg 0.6 0.6 1.2 13 RSD (%) <5% <5% <5% <5% Fig. 7 Mass spectra of TMT: (a) mass spectrum of eluting TMT, * Minimum detectable amount (MDA) based on peak height, all others based on peak area.conditions as in Fig. 3; (b) NIST library mass spectrum of TMT. 1258 Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12Fig. 9 EVect of cell pressure on (a) TMT fragmentation pattern for signal ratios of m/z: 2, 1205135; &, 1205150; and +1205165. (b) Sn+ ion signal: 1.6 ng (as Sn) injection of TMT, rf power 20 W, cell pressure 26 Torr. Fig. 8 Mass spectra of eluting (a) TET, (b) TMPT and (c) TBT; conditions as in Fig. 4.which are ionized by the sputtering process itself cannot reach mass. Similar trends have been observed for molecular frag- the detection system because of the large negative bias of the ments of rf-GD sputtered PTFE-based polymers.15 The mass cathode. Actual measurements of the pertinent plasma species spectrum of eluting TET is shown in Fig. 8(a). All peaks can with Langmuir probe and AA techniques will be reported in be interpreted as fragments of the general composition a separate study.(Etn)Sn+. The TMPT, Fig. 8(b), yields characteristic fragment The ion signal ratios of the elemental tin isotope at m/z= peaks such as (Ph)Sn+ and PhSn+(Me2). The apparent change 120 to the structurally significant molecular fragment ions in the isotope ratio for tin at m/z=177 [TBT, Fig. 8(c)] is were chosen to assess the extent of molecular fragmentation probably due to the presence of the butyltin and diethyltin as a function of discharge parameters.The discharge pressures fragments in various amounts.24 were varied from 12 to 28 Torr, while the rf powers were varied between 10 and 40 W. The eVect of cell pressure on the extent of molecular fragmentation for TMT is shown in Fig. 9. While EVect of Discharge Parameters on Fragmentation of Organotin the 120Sn+ signal is seen to increase almost ten-fold as pressure Compounds increases from 12 to 26 Torr and rolls oV at higher pressures, the ion signal ratios do not show an appreciable trend.Higher Parameters such as the sampling distance (interelectrode spacing), source pressure and rf power have important eVects on discharge pressures have been shown to increase collisional dissociation of sputtered PTFE fragments CxFy+, yielding the signal intensity and quality. Previous reports have discussed the plasma parameter influence on the sputtered analyte inten- more 12C+.15 An analogous response is seen in atomic GDMS, where collision dissociation of metal dimers (M2+) has been sity for various rf-GDMS sources.10–16,29 Analyses of organic species by dc-GDMS with GC or LC type sample introduction implied for both rf- and dc-powered GDs with increases in discharge pressure.16,31 Similar behavior for Eu+ current as a have also been performed.Chapman and Pratt obtained mass spectra using a sector-based mass spectrometer with a thermo- function of pressure (although with a maximum achieved at 50–100 mTorr) has been observed for both argon and neon in spray interface and GD accessory, inducing fragmentation of caVeine in a controlled manner to obtain structural infor- an rf-GD.It was concluded that the shape of these curves is determined primarily by the density of metastable atoms of mation.23 Carazzato and Bertrand reported the eVect of discharge parameters on atomic and molecular species for several the sputtering gas. Specifically, the metastable atom population is particularly sensitive to discharge pressure with a clear classes of organic compounds.19 Fragmentation patterns for ionization using GD, EI and CI for organometallic, inorganic maximum in metastable density occurring at specific discharge pressures.29a If the ions observed in this study are generated and organic compounds have been characterized.30 These studies also clearly indicated that the ionization through a Penning process, the ion pressure profile should exhibit maxima corresponding to the peak metastable mechanisms observed in a particular discharge are highly dependent upon source parameters with no universal mechan- population.In the experimentally observed rf power–Sn+ intensity ism of ionization dominant for all GD configurations. To our knowledge, none of the parametric studies for the gas-phase dependence (Fig. 10) the Sn+ intensities are seen to decrease with increasing rf power. The ion signal ratio–power depen- analytes in rf-GD sources have been reported.It still seems possible to draw preliminary conclusions from the presented dence does not reveal any appreciable trend. The signal intensity dependence seems to contradict that reported in the parametric studies with the most pertinent comparison with the literature data on rf-GD sputtered ions. The ions in this literature. For a given discharge pressure, subsequent rf power increases have been shown to enhance analyte signal intensities case are produced from sputtered neutral atoms (as they would be from injected neutral gas-phase analytes) since the atoms in elemental10,11,13 and polymer15 analysis applications.Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12 1259Fig. 11 EVect of sampling distance on (a) Sn+ ion signal, (b) TMT Fig. 10 EVect of rf power on (a) Sn+ ion signal and (b) TMT fragmentation pattern for signal ratios of m/z:2, 1205135;&, 1205150; fragmentation pattern for signal ratios of m/z:2, 1205135;&, 1205150; +, 1205165.Injection of 1.6 ng of TMT as Sn, rf power 15 W, cell +, 1205165. Injection of 1.6 ng of TMT as Sn, rf power 20 W and cell pressure 26 Torr. pressure 26 Torr. at m/z 135, 150 and 165 are increased when the cathode is Increases in applied power result in proportional increases in moved close to the orifice. In the work of Carazzato,19 the dc bias values, while an inverse relationship exists between the fragment ions intensities of butylbenzene were observed to discharge pressure and the dc bias.32,33 However, for each increase when sampling occurred closer to the cathode.19 The fixed-pressure data set an rf power was observed where a butylbenzene molecular ion also followed this pattern, showing maximum Cu+ signal was detected.34 As the discharge pressure that the molecular species can be produced more eYciently in was increased, the position of the maximum signal shifted to this region, but with higher internal energy resulting from lower power settings with only decaying signals observed at collisions with electrons of higher velocities leading to fragmen- the maximum pressure used (0.35 Torr), similar to what was tation.Langmuir probe studies showed that the ion popu- observed in the present study. Decreases in discharge power lations are greatest in the area of the negative glow–dark space cause the cathode fall (potential drop) to occur over longer interface, decreasing as a function of sampling distance, while distances away from the cathode surface extending the plasma the electrons are more uniformly distributed throughout the negative glow–dark space interface toward the exit orifice.One glow.25 Average electron energies have been shown to decrease would expect this interfacial region to be the region of highest with decreasing cathode–anode separation distance at high helium metastable atom density. The experimentally observed discharge pressures.That could explain the observed inverse rf power–signal intensity dependence is then consistent with relationship between the anode–cathode separation distance the premise that Penning ionization is the dominant ionization and the degree of fragmentation [Fig. 11(b)]. process in this type of discharge. Eckstein et al.29b have shown that the ion signal of sputtered atoms in an rf-GD is proportional to the product of the sputtered atom density (copper) CONCLUSIONS and the discharge gas metastable atom density (neon).The dependence on metastable concentration indicates that inelas- The experimental data presented here justify the further development of an rf GD–GC interface. With the new discharge tic collisions with the neon metastables is the primary source of analyte ionization. Some studies have suggested that cell design and diVerent chromatographic system the quality of the organotin separation has been dramatically improved Penning ionization may not be the dominant ionization mechanism in other rf discharges.In a similar set of experiments compared with the previously reported data. The limits of detection for elemental tin even for the discharge conditions no correlation was observed between the analyte ion signal and discharge gas metastable ion densities.31 A changed contri- (sampling distance and rf power) not yet optimal, are in the sub-picogram and low picogram range. In terms of organomet- bution by EI ionization at the negative glow–cathode dark space interface may explain the lack of correlation between allic compound analysis, the mass spectra obtained for eluting organotin compounds yield both elemental and extensive MS and AA measurements.The lack of variability in molecular fragmentation with changing discharge pressure and rf power fragment information. While detailed ion distribution mapping is not feasible in this work, the ratios of tin ion signal to ion observed [Figs. 9(b) and 10(b)] would then be consistent with both of the ionization mechanisms. The large variations in the signals of major molecular fragments were chosen as indicators for the possibility of obtaining controllable fragmentation with ratios between the molecular ions and free metal ions could be explained by the changing plasma composition as the changing discharge parameters. The results obtained indicate that the mass spectra generated do not diVer significantly with organic components are added due to the discharge asymmetry in the present experimental configuration.the varying source pressure and rf power as they exhibit the same features in terms of parent-molecular peaks and fragment The eVect of sampling distance on Sn+ ion signal and fragmentation pattern (ion signal ratio) for TMT is shown in ions. The tin ion signal intensity was observed to decrease with increasing rf power. This dependence is diVerent from Fig. 11 (similar dependencies were obtained for TET and TMPT, not shown).Both the Sn+ ion signal and the signals that reported in the literature for the sputtered analyte signal 1260 Journal of Analytical Atomic Spectrometry, November 1997, Vol. 1214 De Gendt, S., Van Grieken, R. E., Ohorodnik, S. K., and intensities. This experimental observation is explained in Harrison, W. W., Anal. Chem., 1995, 67, 1026. terms of the changed contribution of Penning–EI ionization, 15 Shick, C. R., Jr., DePalma, P.A., and Marcus, R. K., Anal. Chem., although the relative contribution of diVerent ionization types 1996, 68, 2113. to the total ionization process cannot be quantitatively meas- 16 Shick, C. R., Jr., and Marcus, R. K., Appl. Spectrosc., 1996, 50, 454. ured in this work. Because the rf power–signal intensity 17 Myers, D. P., Heintz, M. J., Mahoney, P. P., Li, G., and Hieftje, G. M., Appl. Spectrosc., 1994, 48, 1337. relationship depends on factors such as the nature of the 18 Mason, R., and Milton, D., Int.J. Mass Spectrom. Ion Processes, cathode and the discharge gas, pressure, cell geometry and 1989, 91, 209. distance between electrodes, it is diYcult to directly compare 19 Carazzato, D., and Bertrand, M. J., J. Am. Soc. Mass Spectrom., one source with another and each design will have its own 1994, 5, 305. characteristics. Studies on the relative contributions of diVerent 20 McLuckey, S. A., Glish, G. L., Asano, K. G., and Grant, B.C., ionization mechanisms and experiments employing the GD Anal. Chem., 1988, 60, 2220. 21 Zhao, J., Zhu, J., and Lubman, D. M., Anal. Chem., 1992, 64, 1426. cell with axial sample introduction warrant future exploration. 22 Sofer, I., Zhu, J., Lee, H.-S., Antos, W., and Lubman, D. M., Appl. Spectrosc., 1990, 44, 1391. Financial support from the National Institute of En- 23 Chapman, J. R., and Pratt, J. A. E., J. Chromatogr., 1987, 394, 321. vironmental Health Sciences through grant # ESO4908 is 24 Olson, L.K., Belkin, M., and Caruso, J. A., J. Anal. At. Spectrom., greatly appreciated. 1996, 11, 491. 25 Ye, Y., and Marcus, R. K., Spectrochim. Acta, Part B, 1996, 51, 509. 26 Liu, Y., Lopez-Avila, V., Alcaraz, M., and Beckert, W. F., J. High REFERENCES Resolut. Chromatogr., 1994, 17, 527. 27 Chapman, B. N., Glow Discharge Processes, John Wiley, New 1 Marcus, R. K., Glow Discharge Spectroscopies, Plenum, New York, 1980. York, 1993. 28 Spalding, T. R., in Mass Spectrometry of Inorganic and 2 Hutton, R.C., and Raith, A., J. Anal. At. Spectrom., 1992, 7, 623. Organometallic Compounds, ed. Litzov, M. R., and Spalding, 3 Jakubowski, N., Feldmann, I., and Stuewer, D., Spectrochim. T. R., Elsevier, Amsterdam, 1973. Acta, Part B, 1991, 50, 639. 29 a Coburn, J. W., and Kay, E., Appl. Phys. L ett., 1971, 18, 435. 4 Shi, Z., Brewer, S., and Sacks, R., Appl. Spectrosc., 1995, 49, 1232. b Eckstein, E. W., Coburn, J. W., and Kay, Int. J. Mass Spectrom. 5 Winchester, M. R., Lazic, C., and Marcus, R. K., Spectrochim. Ion. Phys., 1975, 17, 129. Acta, Part B, 1991, 46, 483. 30 DePalma, P. A., Jr., You, J. Z., Marcus, R. K., and Willoughby, 6 Harville, T. R., and Marcus, R. K., Anal. Chem., 1995, 67, 1271. R. C., presented at the 43rd ASMS Conference on Mass 7 Bordel-Garcý�a, N., Pererio-Garcý�a, R., Ferna�ndez-Garcý�a, M., Spectrometry and Allied Topics, Atlanta, GA, USA, May 22–26, Sanz-Medel, A., Harville, T. R., and Marcus, R. K., J. Anal. At. 1995, poster TPB 094. Spectrom., 1995, 10, 671. 31 King, F. L., McCormack, A. L., and Harrison, W. W., J. Anal. 8 Parker, M., and Marcus, R. K., Appl. Spectrosc., 1994, 46, 623. At. Spectrom., 1988, 3, 8salan, G., Chakrabarti, C. L., Hutton, J. C., Back, M. H., 32 Myers, D. P., Heintz, M. J., Mahoney, P. P., Li, G., and Hieftje, Lazik, C., and Marcus, R. K., J. Anal. At. Spectrom., 1994, 9, 45. G. M., Appl. Spectrosc., 1995, 49, 945. 10 Duckworth, D. C., and Marcus, R. K., J. Anal. At. Spectrom., 33 Parker, M., and Marcus, R. K., Spectrochim. Acta, Part B, 1995, 1992, 7, 711. 50, 617. 11 Shick, C. R., Jr., Raith, A., and Marcus, R. K., J. Anal. At. 34 Duckworth, D. C., and Marcus, R. K., Anal. Chem., 1989, 61, 1879. Spectrom., 1993, 8, 1043. 12 Duckworth, D. C., Donohue, D. L., Smith, D. H., Lewis, T. A., Paper 7/00266A and Marcus, R. K., Anal. Chem., 1993, 65, 2478. Received January 10, 1997 13 Shick, C. R., Jr., Raith, A., and Marcus, R. K., J. Anal. At. Spectrom., 1994, 9, 1045. Accepted June 2, 1997 Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12 1261

ISSN:0267-9477    

DOI:10.1039/a700266a

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Low Pressure Inductively Coupled Plasma Ion Source for Molecular and Atomic Mass Spectrometry: The Effect of Reagent Gases GAVIN O’CONNOR, LES EBDON AND E. HYWEL EVANS* Department of Environmental Sciences, University of Plymouth, Drake Circus, Plymouth, UK PL 4 8AA A low pressure inductively coupled plasma (LP-ICP) source, covering the range in-between, has been equated to that for sustained at only 6 W and utilising 6 ml min-1 helium, has the holy grail. However, a more realistic approach of using been investigated as an ionisation source for molecular and one ionisation source to provide alternately molecular and atomic mass spectrometry.Iodobenzene and dibromobenzene atomic information, but not covering the whole range, has were introduced to the LP-ICP via gas chromatography and been achieved by a number of research groups.1 yielded purely atomic ion signals for the iodine and bromine Plasma sources for MS have generally been used for elemenpresent, with detection limits of 4 and 76 pg for iodobenzene tal analysis.This association can be mainly attributed to the and dibromobenzene, respectively. The addition of nitrogen to success of the atmospheric argon ICP, which combines a high a LP helium ICP increased the molecular ion signal for thermal temperature source with almost complete atomisation chlorobenzene, with a detection limit of 2 pg. However, the and ionisation. One disadvantage of the atmospheric ICP is addition of nitrogen did not aid the production of molecular that it is diYcult to sustain plasmas using gases other than ions of iodobenzene and dibromobenzene.A study of the eVect argon. An MIP can be formed using a variety of gases, of skimmer spacing and forward power revealed considerable including helium,1,2 which has a higher ionisation potential spatial separation of ionisation processes within the expansion than that of argon and so leads to a more ionising plasma. chamber of the molecular beam interface.On the addition of However, plasma sources have also been used to provide both 0.07 ml min-1 isobutane the LP-ICP yielded mass spectra molecular and atomic mass spectra. Shen and Satzger3 have similar to those obtained by an electron impact source. shown that it is possible to form molecular ions, indicative of However on the addition of more isobutane only the molecular the analyte compound, using an atmospheric pressure helium ions (M+) for chlorobenzene, iodobenzene and dibromobenzene MIP.In this work the analyte was introduced into the after were observed. The detection limits for the instrument plume of the plasma so did not experience the full force of the operating in the molecular mode were 100, 140 and 229 pg for source. Reduced pressure MIPs have also been investigated chlorobenzene, iodobenzene and dibromobenzene, respectively. for providing both atomic and molecular mass spectra.4–6 However, in these studies a pure compound or vapour was Keywords: L ow pressure inductively coupled plasma; helium generally introduced into the source, which provides little plasma; mass spectrometry; reagent gas; element selective information on how such a source would behave if used for detection; molecular ion trace level determinations.ICPs, operated at reduced pressure and sustained with Mass spectrometry (MS) is a continuously growing area of argon, have been used for the production of atomic mass analytical chemistry.Proof of this is the ever increasing number spectra, using gaseous and vapour sample introduction.7–9 of analytes being qualitatively and quantitatively determined, Evans et al.10 have investigated the use of a low pressure (LP) in a wide selection of matrices. However, the increased use of helium ICP, at powers between 4 and 40W and 1 mbar MS can be partly attributed to the proliferation of sources pressure, for the production of mass spectra similar to those now available.The use of soft ionisation techniques, such as obtained with an EI source, for a series of organometallic and chemical ionisation (CI), fast atom bombardment (FAB), halogenated species introduced by gas chromatography (GC). matrix-assisted laser desorption/ionisation (MALDI) and elec- On increasing the power and pressure of this source it was trospray ionisation (ESI), have allowed ionisation of fragile, possible to increase the degree of fragmentation until at 150 W long chain, high molecular weight hydrocarbons without the and 10 mbar pressure total fragmentation occurred.Kohler total destruction of the analyte molecular ion, hence allowing and Schlunegger11 used a Penning ionisation source to provide molecular weight determination. At the other end of the a tuneable degree of fragmentation for a series of gaseous ionisation source spectrum are the harsh ionisation sources. organic compounds. This source was investigated for both These include inductively coupled plasmas (ICP), microwave positive and negative ion formation and was said to give induced plasmas (MIP) and glow discharge (GD) sources.spectra similar to those obtained with EI, GD and ICP sources. These sources are generally used to totally atomise analyte Olson et al.12 have used an rf GD source, with GC sample compounds, in the case of ICP, MIP and GD sources, allowing introduction, for the speciation of a series of organotin and ultratrace elemental analysis.Electron impact (EI) sources are organolead compounds, and observed molecular fragment intermediate sources, used to fragment organic compounds, peaks from the analyte compounds. From these studies it has providing structural information on the analyte. This depenbecome obvious that an LP plasma source is capable of being dence on ion source has led to many laboratories purchasing operated in a tuneable mode. Recently a specially designed a selection of sources and employing the relevant experts to instrument has been assembled to further investigate the use operate them.These extra capital and employment costs on of an LP-ICP as a tuneable source.13 top of instrumental running costs have made MS a costly field To date, the analytical figures of merit for the LP plasma and hence unattractive to many potential users. sources have been determined using the source in its atomic The search for a universal ionisation source, capable of operating as both a harsh and soft ionisation source and mode.In order to obtain molecular spectra, large quantities Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12 (1263–1269) 1263of the analyte have been introduced into the source. Thus, the 20 °C min-1 with a helium carrier gas flow rate of 3 ml min-1. A diagram of the instrumental set-up is shown in Fig. 1. source has been used for trace elemental analysis but has not been used for providing molecular fragment ion information on trace level analytes.It has been suggested that the partial Data Acquisition Parameters pressure of the analyte in the LP-ICP is a contributing factor in molecular fragment ion formation.13 This would explain the Data were acquired on a Hewlett-Packard MS workstation, non-linear relationship between concentration and molecular with HP59970A (Version 3.1) software, which was interfaced ion signals in the LP-ICP. to the MSD.The ions were detected using two diVerent MS In the present study some of the problems associated with operating modes. For structural information on the analytes the non-linear nature of calibration, for molecular fragment the instrument was operated in scanning mode, where the ions, in an LP-ICP have been addressed. Initial studies on the mass range 60–800 m/z was monitored. For quantitative deteruse of reagent gases in the LP-ICP suggest that by altering mination of the analytes the instrument was operated in the composition of the plasma gas alone, it is possible to utilise selective ion monitoring (SIM) mode.In this mode of operation the LP-ICP as a soft ionisation source, yielding spectra similar the molecular ions, halogen ions and phenyl ions of the to that of a CI source, or as a harsh ionisation source which analytes were monitored. provides only elemental information, such as an atmospheric ICP. Furthermore the source can be operated in a tuneable Reagents and Standards mode between hard and soft ionisation regimes. Standards were diluted in pentane (HPLC grade, Rathburn Chemicals, Walkerburn, UK) to the required concentration.EXPERIMENTAL Chlorobenzene, iodobenzene and dibromobenzene were Low Pressure Plasma Mass Spectrometer obtained from Aldrich (Gillingham, UK). Nitrogen (99.9%) and isobutane (99%) were obtained from Air Products (Crewe, A detailed description of the design and optimisation of the Cheshire, UK).GC–LP-ICP-MS system used in this study has been given previously.13 In brief, a Hewlett-Packard (Stockport, Cheshire, UK) mass selective detector (MSD) was modified to enable it RESULTS AND DISCUSSION to analyse and detect ions from the LP-ICP. This was achieved To date LP-ICPs have been sustained with mainly argon or by using a custom made ion sampling interface. The LP plasma helium gas. The 1 1 min-1 argon LP-plasma has been utilised was sustained using a modified rf generator and matching for the production of atomic mass spectra, totally atomising network, in a 140 mm long quartz tube of 1/2 od, with a 1/4 analytes introduced to the source via a GC instrument.The od side arm to which a calibration vial containing perfluorohelium LP-ICP has been used as a dual mode ionisation tributylamine (PFTBA) was attached. The quartz plasma torch source producing both atomic and molecular ion mass spectra, was connected to the ion sampling interface via an LP sampling depending on the gas flow, plasma power and torch pressure cone (Machine shop, University of Plymouth), which was used.However, in the fragmentation studies relatively large machined from aluminium, had a 2 mm orifice and an Ultraamounts of analyte (>50 ng on-column) have been required torr fitting for a 1/2 pipe. This enabled a vacuum seal to be to facilitate the fragment ion formation. Also, the response formed between the LP torch and the ion sampling interface.obtained from the fragments produced by the LP-ICP was not The reagent gases were added to the plasma gas via the side linearly related to the analyte concentration. These two phenarm tube of the quartz torch. The amount of gas added was omena together are suggestive of the analyte playing a major controlled using a scaled needle valve (Edwards High Vacuum, role in the fragmentation and ionisation process, i.e., the Crawley, West Sussex, UK). Typical operating conditions are analytes were self ionising above a certain concentration.In shown in Table 1. order to suppress this phenomenon in LP-ICP-MS it was decided to investigate the eVect of reagent gases on analyte Gas Chromatography signal and molecular fragment formation. A gas chromatograph (PU 4550, Pye Unicam, Cambridge, UK) fitted with an on-column injector, was interfaced to the Nitrogen Addition LP-ICP-MS instrument by way of a heated transfer line Nitrogen was added to a 3 ml min-1 helium plasma via the maintained at a constant temperature of 250 °C. The GC side arm tube of the quartz torch.The amount of nitrogen capillary column used was a DB5 0.32 mm×30 m with a added was controlled using a scaled metering valve. The flow 0.1 mm film thickness (J & W, Fisons, Loughborough, UK). rate of the nitrogen gas through the valve was measured at The capillary column was passed through the heated transfer atmospheric pressure, for a series of needle valve settings, and line and into the LP torch, the vacuum seal being made using a combination of Ultra-torr and Swagelock fittings.This configuration has previously been described in more detail.10 One microlitre of a mixed standard was injected on-column and the GC programme was typically 40–110 °C at Table 1 LP-ICP-MS operating conditions Mass spectrometer Modified Hewlett-Packard MSD L ow pressure plasma— Forward power/W 6 Reflected power/W 0 Pressure/torr— Torch 0.2 Interface 0.03 Analyser <10-6 Fig. 1 Schematic diagram of the GC–LP-ICP-MS system. 1264 Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12the flow of gas through the orifice of the needle valve, while diVered greatly depending on the skimming distance and fragment ion studied. Fig. 3 shows the eVect of power on the operating under LP conditions, was then calculated using Poiseuille’s relationship.14 signal intensity for fragment ions for PFTBA with a sampler– skimmer distance of 7 mm.This shows that at this skimming It would be expected that the helium–nitrogen plasma would diVer in temperature from a helium only LP-ICP, due to the distance the optimum power for the 69 and 219 m/z ions was between 6 and 8 W. The 69 m/z fragment ion optimises at the diVering thermal conductivity of nitrogen compared with helium, but also because of the diVering ionisation potentials highest power, between 7 and 8 W, whilst the 502 m/z ion showed a rapid decrease in signal as the power was increased.of the gases. If a change in the plasma gas kinetic temperature occurred then the ion flux through the sampler and skimmer As the power was increased the 502 m/z fragment of PFTBA was quickly broken down. The increase in the 219 and 69 m/z would also change. The physical processes that describe this phenomenon have been described in detail elsewhere.1,10,15,16 signals for the PFTBA suggests that the 502 m/z ion was further fragmented, hence increasing the signals of the lighter It has been our experience that for an LP-helium ICP the experimental optimum pressure and flow conditions were fragments.However, above 8 W forward power even the smaller molecular fragments began to disintegrate, which diVerent to those calculated using theory,13 hence it was decided to experimentally optimise the ion sampling con- should add to the atomic ion signals, though these could not be monitored because of the high background signals between ditions.For the optimisation study PFTBA was introduced into a helium–nitrogen LP-ICP. Three fragment ions of 12 and 32 m/z. Optimisation of the helium carrier gas flow and the nitrogen PFTBA, at 69, 219 and 502 m/z, respectively, were continuously monitored while the plasma forward power and the sampler– reagent gas flow for the production of stable molecular ions was then performed by introducing 10 ng of chlorobenzene skimmer spacing were optimised.Fig. 2 (a)–(c) shows the resulting plots of the signal intensity versus skimmer–sampler into the plasma via the GC instrument. The molecular ion for chlorobenzene (112 m/z) and the phenyl ion (77 m/z) were spacing and forward power for these fragment ions. The points labelled ‘A’ correspond to maxima on each plot. The plots are continuously monitored for three repeat injections of the 10 ng ml-1 standard. The helium carrier gas flow had little eVect shown in two dimensions only to help reveal the pertinent features. If the plots were to be shown in three dimensions they would reveal three-peak plots with the central peaks being the most intense for the 69 and 219 m/z fragment ions.This phenomenon has been described previously for helium only LP-ICP-MS13 and suggests the formation of several ‘shock’ regions behind the sampler. For the helium–nitrogen plasma the fragment ions at 69 and 219 m/z [Fig. 2 (a) and (b)] gave rise to maximum signal intensity at a skimming distance of 7 mm, with less intense peaks at 4 and 10 mm.However, the fragment ion at 502 m/z [Fig. 2 (c)] yielded no maxima between 5–9 mm and instead yielded maxima at 3 and 10 mm. This may be because the higher mass fragment at 502 m/z underwent a diVerent ionisation process compared to the lower mass fragments or was ionised in a diVerent part of the plasma or interface. Alternatively, the helium–nitrogen plasma may simply cause the molecular ion at 502 m/z to Fig. 3 Plot of normalised signal intensity versus plasma power for further fragment into smaller molecular species. the fragment ions of PFTBA at 69, 219 and 502 m/z, at 7mm skimming distance. It is also evident that the optimum plasma operating power Fig. 2 Surface contour plots showing the eVect of plasma power and skimming distance on the signal intensity of PFTBA at: (a) 69 m/z; (b) 219 m/z; (c) 502 m/z. The points labelled ‘A’ indicate intensity maxima.Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12 1265on the chlorobenzene signal (Fig. 4), with both the phenyl and eVects, or that the decreased thermal conductivity of the nitrogen may have stabilised the LP-ICP. molecular ion remaining fairly constant in intensity between 2 and 5 ml min-1. However, as the carrier gas flow was increased Once the optimisations were completed, an investigation of the analytical figures of merit (given in Table 2) was performed. the signals became increasingly unstable, indicated by the increased standard deviations of the chlorobenzene signals, The figures of merit were obtained by SIM for the molecular ion of chlorobenzene (112 m/z).The detection limit of 2 pg shown in Fig. 4. The optimum carrier flow rate was found to be 3 ml min-1. The signal for the molecular ion decreased suggests that the LP helium–nitrogen plasma is capable of providing structural information on the analyte even at ultra above 6 ml min-1 helium.This could be due to the extra gas increasing the electron and ion number density and thermalis- trace levels. Also the nitrogen addition improved the linear range of calibration, with calibration over three orders of ing the plasma. This would lead to a greater amount of collisions between the analyte and electrons, which in turn magnitude possible. This is a vast improvement compared to helium only LP-ICP-MS for which calibration was not poss- would lead to increased fragmentation of the analyte causing a reduction in the molecular signals.This theory would seem ible. Fig. 6 shows a SIM chromatogram at 112 m/z for a 100 pg on-column injection of chlorobenzene illustrating the excellent to be confirmed by the addition of more helium gas. On the addition of 7 ml min-1 of helium the molecular ion decreased signal to noise obtained. These results suggest that addition of nitrogen to the whilst the phenyl ion signal increased.This may be indicative of the molecular ion fragmenting and adding to the phenyl LP-ICP-MS instrument would cure the problems observed previously.10,13 However, on the addition of analytes with ion signal, however, the precision was poor so it was not possible to draw a firm conclusion. Above 7 ml min-1 helium retention times greater than 2 min, only the atomic signals were observed. This was thought to be due to the influence of the phenyl and molecular signals disappeared, which could be because the ions became totally atomised, yielding only atomic the tail of the solvent peak on chlorobenzene due to the short retention time of the latter.In order to minimise this eVect it information. Alternatively, these eVects could be due to changing the chromatographic conditions. was decided to investigate the use of isobutane as the reagent gas. The eVect of the nitrogen gas added to a 3 ml min-1 helium plasma on the chlorobenzene signal is shown in Fig. 5. The signal for the molecular ion peak at 112 m/z for chlorobenzene Isobutane Addition was relatively unaVected by the nitrogen as the signal remained fairly constant up to 2.1 ml min-1. With nitrogen flows above Isobutane was added to the LP helium plasma in a similar 2.1 ml min-1 the signals for both the molecular ion and the manner to nitrogen and its eVect on the molecular, phenyl and phenyl ion were reduced by over 50%. Again this may be due atomic ion signals for a series of halobenzenes was investigated.to the increased electron and ion density in the plasma further The analytes were injected, approximately 10 ng each fragmenting the analyte. A point of interest is the stability of on-column, as a mixed standard. Fig. 7 (a) shows the eVect of the analyte signals, even above a combined gas flow of the isobutane on the signals for 10 ng on-column injection of 7 ml min-1. This suggests that the unstable signals obtained chlorobenzene.The atomic signal for chlorine has not been on adding helium carrier gas were due to chromatographic shown because fragment ions from the reagent gas interfered with ion signals below 58 m/z. With a 3 ml min-1 helium only plasma, the molecular ion of chorobenzene was the parent ion. On the addition of the reagent gas both molecular ion and phenyl ion signals were greatly increased. However, as the reagent gas partial pressure was further increased the phenyl ion peak disappeared, leaving only the molecular ion.This is Table 2 Analytical figures of merit for chlorobenzene using a 0.43 ml min-1 nitrogen, 3 ml min-1 helium LP-ICP Single ion monitoring, mass monitored 112 m/z Linear range studied/decades 3 Slope/counts pg-1 99 r2 (regression coeYcient) 0.985 Slope of log–log plot 1.012 Detection limit*/pg 2 Fig. 4 EVect of helium carrier gas flow rate on the signal intensity of RSD† (%) 8.5 a 10 ng on-column injection of chlorobenzene, in a nitrogen–helium LP-ICP.* LOD=3s/slope. † RSD (%) for five replicate 10 pg injections. Fig. 5 EVect of nitrogen reagent gas flow rate on the signal intensity Fig. 6 Chromatogram of a 100 pg on-column injection of chlorobenzene for a helium–nitrogen (3.0 and 0.43 ml min-1) LP-ICP using of a 10 ng on-column injection of chlorobenzene, in a 3 ml min-1 helium LP-ICP. selected ion monitoring at 112 m/z. 1266 Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12Fig. 8 Total ion chromatogram for 10 ng on-column injection of chlorobenzene, iodobenzene and dibromobenzene for a helium–isobutane (3.0 and 0.07 ml min-1) LP-ICP.Fig. 7 EVect of isobutane reagent gas flow rate on the signal intensity of the molecular and fragment ions of (a) chlorobenzene, (b) iodobenzene and (c) dibromobenzene in a 3 ml min-1 helium LP-ICP. consistent with the isobutane–helium plasma acting as a conventional CI source where the partial pressure of the reagent gas often determines the analyte spectra obtained.Unlike the nitrogen reagent gas, the eVects of the isobutane were consistent throughout the chromatographic run. Fig. 7 (b) and (c) show the eVect of the isobutane on iodobenzene and dibromobenzene which had retention times of 1.1 and 2.0 min, respectively. For the 3 ml min-1 helium plasma, with a 10 ng on-column injection, the only ions that were observed were the atomic signals Fig. 9 Mass spectra scans obtained from an isobutane–helium (0.07 and 3.0 ml min-1) LP-ICP for 10 ng on-column injection of (a) chlorob- for the halogens.When 0.07 ml min-1 of isobutane reagent enzene, (b) iodobenzene and (c) dibromobenzene. gas was added the phenyl and molecular ions were observed. This yielded spectra very similar to those obtainable by EI source MS. On the addition of more isobutane the phenyl and and in the case of iodobenzene no MH+ peak was visible. This, along with the reduction in molecular and fragment ion atomic halogen signals were no longer observed and only the compound molecular ion remained, yielding spectra similar to signals on increasing the isobutane partial pressure, suggests that the isobutane was not behaving as a proton transfer those expected from CI source MS.However, on the addition of greater than 1 ml min-1 isobutane the molecular ion signals reagent gas. Also, no quasimolecular ions were observed. On the addition of 0.07 ml min-1 of isobutane the major reagent started to reduce in intensity.The halogen and phenyl ions for the analytes did not increase on the reduction of the molecular ion was 57 m/z. This is consistent with the loss of a proton from the isobutane, however, it has already been shown that ions. This suggests a reduction in the ionisation power of the source, because if the ionisation power increased one would protonation of the analytes was not the dominant ionisation process. As the reagent gas concentration was increased the expect to see a corresponding increase in the signal of the lower mass fragment ions as the molecular ion decomposed.most abundant reagent ion changed from 57 to 43 m/z, which is consistent with the loss of a methyl group from isobutane. Fig. 8 shows a total ion chromatogram for a 10 ng on-column injection of chlorobenzene, iodobenzene and This suggests that as more isobutane was added the plasma ionisation processes were getting harsher, because greater dibromobenzene, obtained using a 0.07 ml min-1 isobutane–3 ml min-1 helium LP-ICP.The resulting mass fragmentation was observed, however, analyte fragmentation exhibited the opposite trend. An alternative explanation may spectra for each compound are shown in Fig. 9 (a)–(c). The predominant ionisation mechanism for isobutane in CI is be that the ionisation process of the helium plasma was suppressed by the presence of the isobutane, and that as more proton transfer. However, the mass spectra of the halobenzenes studied [Fig. 9 (a)–(c)] show little sign of protonation with the isobutane was added a greater amount of energy was required to form the reagent ions, thereby leaving less energy to ionise MH+ peak being less than one third the intensity of the M+, Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12 1267Table 3 Analytical figures of merit for chlorobenzene, iodobenzene and dibromobenzene, using a 0.25 ml min-1 isobutane, 3 ml min-1 helium LP-ICP Analyte Chlorobenzene Iodobenzene Dibromobenzene Selected ion monitoring, mass monitored 112 m/z 204 m/z 236 m/z Linear range studied/decades 3 3 3 Slope/counts ng-1 84 645 28 405 4905 r2 (regression coeYcient) 0.9925 0.9848 0.9925 Log–log slope 0.85 0.74 0.70 Detection limit*/pg 100 140 229 RSD† (%) 12 6 5 * LOD=3s/slope.† RSD (%) for five replicate 380 pg injections. the analyte and resulting in molecular ion production of the analyte. This suggests that the source was not acting as a conventional CI source and that a number of ionisation mechanisms may be taking place.This is consistent with other plasma sources where a number of non-equilibrium properties are used to describe the plasma ionisation characteristics. This is a well known phenomenon because most plasmas do not exhibit thermal equilibrium even at atmospheric pressure, let alone at reduced pressure. Charge transfer is a well known ionisation mechanism in helium plasmas, and if ionisation was occurring via charge transfer in a helium–isobutane plasma one would expect a small degree of fragmentation and ionisation due to the low ionisation potential of isobutane (10.57 eV).17 The figures of merit for the helium–isobutane plasma operating in the molecular mode are shown in Table 3.Helium Addition In the initial studies performed using an LP helium ICP the dependence of molecular ion formation on the analyte concen- Fig. 10 Mass spectra scans obtained from a 6 ml min-1 helium tration was suggestive of chemical ionisation processes pre- LP-ICP for a 50 ng on-column injection of (a) iodobenzene and dominating in the plasma.If the helium was acting as a reagent (b) dibromobenzene. gas for conventional CI the expected predominant ionisation process would be charge transfer. The rate of charge transfer Dublin, Ireland) in place of the needle valve. A 6 ml min-1 is dependent on the partial pressure of reagent gas and analyte helium LP-ICP was then studied for the production of elemen- in the source.The survival of molecular ions in a charge tal mass spectra for iodobenzene and dibromobenzene. Fig. 10 transfer source is also dependent on the internal energy of the (a) and (b) show the resulting mass spectra scans (60–240 m/z) ion. If the internal energy is large (>5 eV) a great deal of of a 50 ng on-column injection of the standards and shows the fragmentation would be expected. The internal energy of a existence of only the atomic signals for the iodine (127 m/z) molecular ion can be calculated using eqn.(1):18 and bromine (79 and 81 m/z) even at this relatively high Eint=RE(X+)-IP(M) (1) concentration. This shows that the 6 ml min-1 helium plasma where RE(X+) is the recombination energy of the reagent ion (24.6 eV for helium) and IP(M) is the ionisation potential of the analyte molecule. This would lead to an internal energy of over 13 eV for the molecular ions of the halobenzene series studied, with ionisation potentials between 9–11 eV.Hence, extensive fragmentation of the analyte molecules would be predicted using a dense helium plasma. However, in conventional CI MS it is not unusual to observe molecular ions for organic molecules, with ionisation potentials less than 10 eV, when using helium as the reagent gas. Therefore, the presence of the rf magnetic field may induce collisional energy exchange between excited electrons and the analyte, increasing the ionisation power of the plasma.Hence, by increasing the helium partial pressure in the LP-ICP, the rate of charge exchange would increase. This should lead to greater fragmentation, and eventually atomisation, of the analyte molecules, leaving only the atomic ions to be detected. This suggests the possibility of utilising a low flow helium LP-ICP-MS for atomic MS. To test this hypothesis a helium make up gas was added to Fig. 11 Extracted ion chromatograms for a 50 ng on-column injection the plasma gas, via the side arm of the LP torch.The helium of (a) iodobenzene at 127 m/z and (b) dibromobenzene at 81 m/z, using a 6 ml min-1 6W helium LP-ICP-MS instrument. was introduced using a mass flow controller (Unit Instruments, 1268 Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12Table 4 Analytical figures of merit for iodobenzene and dibromobenzene, using a 6 ml min-1 helium LP-ICP Analyte Iodobenzene Dibromobenzene Selected ion monitoring, mass monitored 127 m/z 81 m/z Linear range studied/decades 3 3 Slope/counts pg-1 235 129 r2 (regression coeYcient) 0.999 0.999 Log–log slope 0.890 0.881 Detection limit*/pg 4 76 RSD† (%) 8 12 * LOD=3s/slope.† RSD (%) for five replicate 100 pg injections. operating at only 6 W forward power can atomise and ionise only molecular ions of the analytes, as it would in CI source MS, at low level concentrations. the halobenzenes.Fig. 11 (a) and (b) show extracted ion chromatograms for 50 ng on-column of iodobenzene and dibromoben- An LP plasma sustained at 6 W and utilising only 6 ml min-1 of helium has been used to totally atomise both zene at 127 and 81 m/z, respectively. The chromatograms show extensive peak tailing which is thought to be due to the analyte iodobenzene and dibromobenzene, producing atomic mass spectra. This proves that a GC–LP-ICP-MS system is capable atomic ion interacting with the wall of the plasma torch, whereas this does not occur for the molecular ion signals using of providing diVerent degrees of fragmentation for a series of halobenzenes. the same chromatographic conditions.The figures of merit for the 6 ml min-1 helium only The authors would like to thank: BP International (Sunbury LP-ICP-MS are shown in Table 4. The detection limits for the Group) for their kind donation of the HP 5970 MSD; the instrument operating in the atomic mode, reported in this NuYeld Foundation for the provision of an instrument devel- study, are comparable to those obtained by GC–LP-MIP-MS, opment grant; and the University of Plymouth for continuing namely 22, 0.1 and 3.5 pg for chlorotoluene, iodobenzene and financial support of G.O’C.bromononane, respectively.1 Studies of a GC–LP-ICP-MS system sustained with 0.5 l min-1 of helium have yielded element selective detection limits of 2.9 and 3.8 pg for chlorob- REFERENCES enzene and bromobenzene.1 In comparison, detection limits 1 Evans, E.H., Giglio, J. J., Castillano, T. M., and Caruso, J. A., given for the Hewlett-Packard MS instrument operating with Inductively Coupled and Microwave Induced Plasma Sources for an EI source are typically 10 pg, for SIM of the molecular ion Mass Spectrometry, ed. Barnett, N. W., Royal Society of of methyl stearate at 298 m/z. It is interesting to note that in Chemistry, Cambridge, 1995. 2 Chambers, D. M., Carnahan, J. W., Jin, Q., and Hieftje, G., the previous study10 no peak tailing for the atomic species was Spectrochim.Acta, Part B, 1991, 46, 1745. observed. With a 1 l min-1 argon LP-ICP there is a distinct 3 Shen, W., and Satzger, R. D., Anal. Chem., 1991, 63, 1960. central channel evident, much like a conventional atmospheric 4 Heppner, R. A., Anal. Chem., 1983, 55, 2170. pressure ICP. However, unlike an atmospheric pressure ICP 5 Poussel, E., Mermet, J. M., Deruaz, D., and Beaugrand, C., Anal.this is thought to be formed by the pressure drop at the 2 mm Chem., 1988, 60, 923. diameter sampler orifice pulling the central portion out of the 6 Olson, L. K., Story, W. C., Creed, J. T., Shen, W., and Caruso, J. A., J. Anal. At. Spectrom., 1990, 5, 471. plasma. This eVectively pulls the analyte ions into the centre 7 Evans, E. H., and Caruso, J. A., J. Anal. At. Spectrom., 1993, 8, 427. of the plasma and away from the torch walls. At very low gas 8 Castillano, T. M., Giglio, J. J., Evans, E. H., and Caruso, J. A., flows and pressures this eVect is not observed so it is likely J. Anal. At. Spectrom., 1994, 9, 1335. that the analyte interacts more with the walls of the torch. 9 Yan, X., Tanaka, T., and Kawaguchi, H., Appl. Spectrosc., 1996, 50, 2, 182. 10 Evans, E. H., Pretotius, W., Ebdon, L., and Rowland, S., Anal. Chem., 1994, 66, 3400. CONCLUSIONS 11 Kohler, M., and Schlunegger, U. P., J. Mass Spectrom., 1995, 30, 134. The GC–LP-ICP-MS system has been shown to be capable 12 Olson, L. K., Belkin, M., and Caruso, J. A., J. Anal. At. Spectrom., of providing a tuneable degree of fragmentation for a series of 1996, 11, 491. halobenzene compounds. The problems associated with poor 13 O’Connor, G., Ebdon, L., Evans, E. H., Ding, H., Olson, L. K., linear calibration range and high detection limits for the and Caruso, J. A., J. Anal. At. Spectrom., 1996, 11, 1151. molecular ions have been addressed and alleviated by the use 14 Physical Chemistry, Oxford University Press, Oxford, 4th edn., of reagent gases. 1990. 15 Niu, H., and Houk, R. S., Spectrochim. Acta, Part B, 1996, 51, 779. The addition of small amounts of nitrogen to the LP-ICP 16 Douglas, D. J., and French, J. B., J. Anal. At. Spectrom., 1988, increased the stability of the plasma and the detection limits 3, 743. for the molecular fragments of chlorobenzene were greatly 17 Handbook of Chemistry and Physics, CRC Press, Boca Raton, FL, improved. However, this eVect did not extend to the other 65th edn., 1984–1985. halobenzenes studied. 18 Chapman, J. R., Practical Organic Mass Spectrometry, Wiley, The addition of isobutane enhanced all the analyte molecular Chichester, 2nd edn., 1993. and fragment ion signals. The isobutane did not seem to be acting as it would in a conventional CI source as proton Paper 7/03733C transfer reactions were minimal. However, the isobutane ReceivedMay 29, 1997 Accepted August 20, 1997 seemed to reduce the ionisation energy of the plasma, yielding Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12 1269

ISSN:0267-9477    

DOI:10.1039/a703733c

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Air Segmented Discrete Introduction in Inductively Coupled Plasma Mass Spectrometry VIOLETA STEFANOVA, VESELIN KMETOV AND LUBOMIR FUTEKOV* Center of Analytical Chemistry and Applied Spectroscopy, University of Plovdiv, 24 T sar Assen, 4000 Plovdiv, Bulgaria An assessment of some advantages and disadvantages of shape and maximum depend on the dispersion produced by the particular FI introduction device.8 When small volumes ICP-MS analysis using air segmented discrete sample are injected, a considerable diVusion through the sample– introduction was performed.Air segmented discrete carrier boundary layer occurs. This is usually beneficial, introduction (ASDI) was eVected with two small air bubbles because the amount of the matrix introduced into the spec- placed between the sample and the washing solvent and was trometer is limited. On the other hand, the time available to carried out by an in-house built device. Sampling by ASDI was make analytical measurements is restricted to the duration of compared with continuous introduction and flow injection, the transient21 and the strong dispersion eVect results in testing 12 isotopes simultaneously (27Al, 55Mn, 63Cu, 64Zn, considerable loss of sensitivity.This is the reason why many 75As, 103Rh, 114Cd, 115In, 130Te, 133Cs, 205Tl, 208Pb). The main workers1–3,8 use relatively large sample volumes. For trace benefit of the ASDI mode is the elimination of dispersion, analysis close to detection limits, the minimum dispersion is leading to sensitivity enhancement at the same injection usually required. An eVective way of decreasing the dispersion frequency and long-term stability compared with flow injection.is to inject the sample into a gas as a carrier. The use of Although small volumes (104–300 ml ) were injected, slightly FI-ICP-MS with an air carrier has been reported by increased signals (‘hyper signals’) were observed with ASDI Beauchemin.22 On substituting the water carrier with air, an compared with continuous nebulization.Hyper signals up to enhancement of sensitivity by a factor of 2–6 was observed 15% were observed for isotopes with low and medium (100 ml injections). Although the background increases when ionisation potentials (<7.7 eV) and a variety of masses. For using air, a reduction of detection limits for 14 elements was the elements tested at a 104 ml injection, the ASDI-ICP-MS reported. On the other hand, large and persistent memory detection limits were improved by a factor of #3 (peak eVects are observed.maximum) compared with the same volume introduced by flow The advantages of sampling into a liquid carrier could be injection and were similar to those of continuous sample combined with those into a gas carrier using a segmented introduction. continuous flow. The injection into a water flow with air Keywords: Discrete sample introduction; air segmentation; bubbles is not an innovation.This approach has been applied flow injection; inductively coupled plasma mass spectrometry by Houk and Thompson.23 A detailed characterisation of continuous segmented-flow ICP-MS is presented in ref. 24. No matter what type of inductively coupled plasma mass The flow was produced by pumping air and water in parallel spectrometry (ICP-MS) instrument is used, the analytical and merging the two streams through a T-piece. The segmented mode characteristics were compared with those of water and outcome of the determination still depends on the sample air carriers only.The air–water flow mixture was found to be introduction method applied. Continuous sample introduction more beneficial because of the memory eVect reduction together (CSI) of solutions is commonly used in ICP studies because with the avoidance of dispersion. The influence of several of its simplicity and robustness. The permanent introduction parameters (flow rates, water-to-air ratio, rf power and sample of the sample aerosol into the ICP allows energy transfer to injection volume) on the signals of 17 isotopes have been the plasma central channel to reach equilibrium and produces studied.25 The results show that: (i) the gain of sensitivity using a relatively constant concentration of the components of the an air segmented carrier is not as high as with air only; (ii) the plasma.The signal measurement starts after stabilisation of memory eVect increases with the air5water ratio.the sample loading process when equilibrium between ionis- An FI-ICP-MS method based on a segmented air–water ation and excitation is reached. Thus, the generated signals carrier was applied to the determination of 44Ca in steel.26 are constant at the time of registration. In general, ICP-MS is The objective of this work was to evaluate the analytical a flexible detection method, compatible with various sample figures of merit of a new system for air segmented introduction introduction systems.1 Alternatives to CSI are introduction in ICP-MS.The system establishes a method in which the methods in which small discrete sample volumes are used: flow sample plug is injected between two small air bubbles, which injection2,3 (FI), electrothermal vaporisation4 (ETV) and laser isolate the sample from the water carrier. This sampling mode ablation5,6 (LA). In recent years, FI has been applied instead is carried out by an in-house built device, and is termed ASDI of CSI in combination with ICP-MS.The principle of FI, viz., (air segmented discrete introduction). A similar device has been the injection of a fixed sample volume into a continuous carrier previously applied in FAAS successfully.27 The device generates flow, given by Ru° z¡ ic¡ka and Hansen,7 has been used in almost a mode of introduction in the following order: solvent–air– all branches of atomic spectrometry.8 The advantages of FI sample–air–solvent.A comparison between both ASDI and FI analysis are widely discussed in the literature: high eYciency,9 and their relation to CSI was performed and a critical evalu- low sample and reagent consumption, easy automation,10,11 ation of the methods was made. good tolerance of dissolved solids,12–14 and reduction of matrix interferences15 and matrix induced drift. The hyphenated FI-ICP-MS technique allows contamination control, on-line EXPERIMENTAL sample preparation (e.g., automated dilution or preconcen- Instrumentation tration, matrix isolation16–18) and speciation analysis.19,20 An important aspect of discrete sampling in ICP-MS is that the A Perkin-Elmer SCIEX ELAN 500 ICP mass spectrometer with an ELAN 5000 software modification was used. Data measured ion signal varies as a function of time.21 The signal Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12 (1271–1276) 1271acquisition was achieved in two ways: on-line, using ELAN air reaches the T-piece, the VT opens the vent, and the solvent flows again, following the second air bubble.The solvent 5000 transient signal software; and oV-line, the raw count rates were transferred and processed for calculation and graphical washes out the sample before the next cycle. The mass spectrometer transient mode measurement is display using the MS EXCEL 7.0 spreadsheet program. A standard double-pass Scott spray chamber and cross-flow started from the VT and was optimised by selecting the appropriate ‘read delay’ time in the ELAN software.nebulizer were used. The liquid flow was controlled by a peristaltic pump (Gilson, Minipuls 2). The sample and solvent amounts are modified by selecting the time programme, as shown in Fig. 2. The injection volume Three sampling modes were investigated: CSI, FI and ASDI. The connection capillaries for the systems tested were kept at depends on the flow rate (regulated by the peristaltic pump) and the sample aspiration time.The time programmes were the same length (#70 cm) and internal diameter (0.76 mm) in all instances. adjusted to ensure that the injection volumes were equivalent to those of the loop volumes of the applied FI system. The sample volume was checked by weighing the mass of ten water Flow Injection injections. A conventional single-line FI manifold (laboratoryconstructed) was connected to the ICP mass spectrometer. A Instrumental Conditions six-channel manually controlled vent was equipped with a number of loops with volumes of 56, 104, 180, 270, 300, 500 The instrument was optimised for routine multi-element analysis.Signals for 7Li and 209Bi (standard solution, 10 mg l-1 Li, and 1000 ml. The sample loop was filled manually using a disposable plastic syringe. Rh and Bi) were equalised for a similar sensitivity, while the signal for 103Rh (middle mass range) was kept as high as possible. The parameters liquid flow (Ql) and nebulizer argon ASDI Device flow (Qg) were optimised with respect to higher sample transport eYciency and minimum dispersion for FI introduction.The air segmented discrete sample introduction device was assembled by modification of the commercial Perkin-Elmer AS-50 autosampler. A schematic diagram of the ASDI system Table 1 Operating conditions is shown in Fig. 1. Plasma conditions— The probe of the autosampler is connected by a T-piece to Torch Long quartz a solvent tank, mounted at a fixed height.The flow is continu- Forward power 1.3 kW ously advanced to the nebulizer by a peristaltic pump. An Reflected power <10 W electromagnetic vent, controlled by a vent timer (VT), is Argon plasma gas flow 16 l min-1 mounted on the solvent capillary branch from the tank. rate Argon auxiliary gas flow 1.7 l min-1 Coordination between the autosampler, solvent vent and the rate ‘read’ command of the ICP-MS software is provided by the Argon nebulizer gas flow 1.28 l min-1 VT. The time programme includes two steps: rate (Qg) (1) Washing step.The vent is open and the solvent stream Liquid flow rate (Ql) 1.14 ml min-1 passes through the system. The probe is in the upper position Mass spectrometer settings— and the probe capillary loop is filled with air because of the Cones: lower position of the solvent tank. While the solvent flows nickel sampler 1.0 mm orifice through the nebulizer, the autosampler probe moves and is nickel skimmer 0.75 mm orifice inserted into the selected sample vessel (numbered position Detector CEM, pulse count Acquisition Transient peak hopping from the carousel of the autosampler).Points per spectral peak 1 (2) Injection step. The VT closes the solvent vent and thus Dwell time 40 ms the pump starts to suck from the sample capillary branch. The Number of measured 12 air in the probe loop passes through the system, followed by masses the sample solution.After finishing the time for sample aspir- Readings per replicate 50 ation, the autosampler probe moves up and its end opens to Isotopes monitored: 27Al+, 55Mn+,63Cu+, 64Zn+, 75As+, 103Rh+, 114Cd+, 115In+, 130Te+, the atmosphere to take in the second air segment. After the 133Cs+, 205Tl+, 208Pb+ Fig. 1 Schematic diagram of the ASDI-ICP-MS sampling system. Fig. 2 Time programme of ASDI sampling. 1272 Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12In order to make precise comparison between both FI and ASDI sampling systems and CSI, the instrument operating conditions given in Table 1 were constant for all measurements. The number of registered masses for each sample injection and the dwell time of each mass was optimised to fit the shortest transient signal at 56 ml.Twelve isotopes were selected, covering a wide range of masses. Reagents and Standards A 50 mg l-1 multi-element (26 elements) standard solution was prepared in doubly distilled water with 1% v/v HNO3 (suprapur; Merck) using a 1000 mg l-1 ‘ICP IV multi-element’ stock solution (Merck; 23 elements in 1% v/v HNO3) and 1000 mg l-1 single element standard solutions (As, Rh, Te) (Fluka). Nitric acid (0.5% v/v) in doubly distilled water was Fig. 3 Signals for 50 mg l-1 Pb obtained by FI and ASDI introduction used as the solvent carrier stream for both discrete sampling of a 104 ml sample volume and CSI signal from continuous systems. FI introduction was performed manually, while samnebulization. pling by ASDI was automated.In both cases, the ‘sample delay time’ of the Elan 5000 software was 50 s. RESULTS AND DISCUSSION Volume Injected A number of injection volumes between 56 and 1000 ml were tested with both systems, viz., FI and ASDI, and compared with CSI. In FI, the sample volume depends on the particular loop connected to the vent, whereas using ASDI, the injection could easily be changed by selecting the time programme.Another distinction is that FI needs an extra sample volume to clean and fill the injection loop (2 or 3 times higher than the loop volume), whereas no additional sample consumption is needed for ASDI. Working at a fixed flow rate (Ql), the ASDI volume was equalised to the FI loop volume. As mentioned above, the ASDI sampling pattern results in the sample plug being packed between two air bubbles. This air Fig. 4 Signal profiles for diVerent volumes in ml of 50mg l-1 Pb border eliminates direct contact between both liquid phases, introduced by ASDI.(A, 56; B, 104; C, 180; D, 270; E, 350; and F, 500). viz., sample and solvent, and removes their mutual diVusion during transport. In contrast to FI introduction with a continuous segmented air–water carrier, ASDI sampling uses only two Furthermore, the ASDI profile tends to a plateau shape, as constant air bubbles and they are precisely located. This the sample volume increases. This plateau eVect becomes more contributes to a better system wash-out and improves sam- pronounced for volumes over 180 ml (Fig. 4) and demonstrates pling stability. the absence of sample–solvent diVusion. The ASDI-ICP-MS transient signals are much higher (#6 The dispersion of the FI and ASDI systems was estimated times for 56 ml ) than those for FI (Table 2). at diVerent volumes. Reciprocal dispersion coeYcients (Dr) The signal profiles obtained with both systems diVer con- were calculated using eqn.(1): siderably, particularly when small volumes are used. The distinction is clear in Fig. 3 which presents the peaks for Dr= IDS ICSI . (1) 50 mg l-1 Pb with a 104 ml injection. The ASDI profile is narrow, symmetrical and more than three times higher than where IDS is the transient signal maximum from discrete the FI profile. The full width at half maximum (FWHM) of sampling and ICSI the steady-state signal from continuous the ASDI signal is approximately 2-fold less than for FI.sample introduction. The calculated results for Dr (from five replicates of a 50 mg l-1 Pb standard) are illustrated in Fig. 5 where the Table 2 ASDI5FI peak maximum ratio observed at equal volumes injected by both techniques (mean of three injections for each device) reciprocal dispersion coeYcient is plotted versus the volume injected. 56 ml 104 ml 180 ml 300 ml 500 ml Even when sampling relatively high volumes (500–300 ml ) 27Al+ 6.0 3.5 2.5 1.7 1.3 by FI, Dr decreases by 20–40%.The lowest volume, 56 ml, 55Mn+ 6.2 3.7 2.4 1.7 1.3 reduces Dr by up to one order of magnitude. In contrast, Dr is 63Cu+ 5.6 3.4 2.3 1.7 1.3 not changed drastically using the ASDI system. A comparison 64Zn+ 3.6 2.3 1.8 1.5 1.2 between both plots in Fig. 5 shows that diVusion in the 75As+ 4.5 2.8 2.1 1.8 1.3 capillary is the predominant process causing the dispersion 103Rh+ 6.3 3.6 2.5 1.7 1.3 eVect. For ASDI using a 56 ml volume, dispersion occurs into 114Cd+ 5.0 3.0 2.1 1.8 1.3 115In+ 6.5 3.8 2.6 1.7 1.3 the spray chamber; nevertheless, Dr decreases by no more 130Te+ 3.8 2.3 1.8 1.7 1.2 than 10%. 133Cs+ 6.7 3.8 2.5 1.7 1.3 The absence of dispersion is not the only eVect registered. 205Tl+ 6.7 4.0 2.6 1.8 1.3 As can be seen in Figs. 3 and 5, the ASDI maximum is slightly 208Pb+ 6.4 3.7 2.6 1.8 1.3 increased compared with the CSI signal, using small injections. Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12 1273Fig. 7 Calculated relative signal change [calculated from eqn. (2)] Fig. 5 Average reciprocal dispersion coeYcient versus volume injected for 12 elements (ASDI introduction into a standard solution identical by FI and ASDI from five replicates of a 50 mg l-1 Pb standard. with the sample) versus injection volume. The elements were [the first ionisation potential (eV) is given in parentheses next to the element]: A, As (9.81); B, Rh (7.46); C, In (5.77); D, Te (9.0); E, Cs (3.89); F, Pb (7.42); G, Al (5.97); H, Mn (7.44); I, Cu (7.73); J, Zn (9.39); K, Cd (8.99) and I, Tl (6.11).ASDI produces constant small air bubbles (<100 ml ) which are suYcient to break the flow, so that the signal reaches the background level before sample injection. The second air bubble is shorter, which means that the signal does not completely reach the background level again. It is preferable to keep the second air bubble as small as possible, in order to start washing immediately after the injection.The amount of air introduced into the plasma is negligible compared with the nebulizer argon flow. Nevertheless, the results show that during the passage of the dry argon–air flow the energy balance of the plasma changes. The signals plotted in Fig. 6 for Rh and As have diVerent shapes. The Rh signal has a plateau shape while the As signal gradually increases and reaches a maximum at the end of the sample loading. This presumes that the Fig. 6 ASDI signals for 50 mg l-1 As and Rh obtained by introduction optimum ionisation conditions for As (IP=9.81 eV) are of 180 ml of sample into a standard solution with the same concenachieved after stabilisation of the process of aerosol loading tration of the measured elements.and compensation for plasma disturbance. The disturbance eVect increases for small volumes (<300 ml ). The distinction between the transient signal maximum (SINJ) and the average Similar behaviour was found for 103Rh, 115In, 133Cs, 27Al and continuous signals before and after injection (SAVER) indicates 55Mn.The results in Fig. 5 show Dr>1 for volumes between the change in plasma ionisation eYciency. The relative signal 104 and 500 ml. This eVect cannot be explained by dispersion. change (RSC) was calculated from eqn. (2): It seems that preconcentration has been eVected in order to obtain such ‘hyper signals’. The same phenomenon was RSC(%)= SINJ-SAVER SAVER ×100 (2) observed by Craig and Beauchemin24 during studies of a water–air carrier in FI–ICP-MS.A possible explanation might be that the air segmentation impacts the nebulization process, Fig. 7 presents the RSC for 12 elements versus the injection volume. The plots show that the elements tested could be increasing the transport eYciency through the spray chamber. Additional experiments are needed to clarify the hyper signals divided into three groups according to their behaviour. Most elements, viz., Rh, In, Cs, Pb, Al, Mn and Tl, have up to 15% eVect, and this will be considered further in future work.As long as the transport eYciency of the aerosol to the hyper signals. This group includes isotopes with a variety of masses (from 27Al to 208Pb) but all of them have medium and plasma is common for the total sample plug, the eVect should play an equal role for all the elements tested. On the other low ionisation potentials (3.9–7.7 eV). Hence, ASDI introduction has a positive eVect on such elements.A significant hand, for ASDI sampling, there is a change in the plasma conditions which aVects the behaviour of the signal of diVerent negative eVect was found for the other two groups, viz., Te+Zn and As+Cd, which have high ionisation potentials (over 9 eV). elements in a specific way. The signals of elements with diVerent ionisation potentials (IP) were compared. In order to study Obviously, the plasma equilibrium disturbance is critical for these elements.The trend becomes stronger with decreasing the influence of air bubbles on ASDI-ICP-MS measurements the washing solution was replaced by a standard solution, sample volume. The diVerent behaviour of the last two groups cannot be explained simply by the dependence of the ionisation identical with the sample injected. Hence, the influence of the air bubbles could be precisely recorded by eliminating the signal eYciency on the ionisation potentials of the elements tested.The observed trends have to be taken into consideration for time drift. Plots of the signals obtained using sampling into a standard flow by ASDI are presented in Fig. 6. internal standard selection using ASDI-ICP-MS. 1274 Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12pooled relative standard deviation, i.e., the sum of relative standard deviations obtained from five sets of five injections. The results are presented in Fig. 8. In all instances the pooled relative standard deviation for FI was worse than that for ASDI, with the exception of the Mn peak maximum.Thus, segmentation of the flow stream does not aVect significantly the reproducibility of the signals; however, taking into account the gain in sensitivity with ASDI, better relative standard deviations are obtained. The long-term stability of ICP-MS measurements was tested applying FI and ASDI introduction systems. The experiment was limited to 100 injections.Fig. 9 summarises the long-term test results for the injection of 104 ml of a 50 mg l-1 Pb standard including the first and last five replicates. The introduction of small volumes reduces the drift of the mass spectrometry signals owing to the limitation of sample loading. Reliable working conditions were observed for over 1 h microsampling by both techniques. After each sample injection, the solvent stream washes the transport pathway and removes the memory eVect.As shown in Fig. 3 the ASDI signal has steep slopes, but a second small peak in the tail of the main signal is recorded, which is due to the sample film formed on the tube wall. The appearance of the tail peak is due to the solvent front cleaning the transport pathway. The additional peak is insignificant and does not aVect the reproducibility. The wash-out time is similar for both sampling modes. With an introduction frequency of less than 80 injections per hour, the washing time between samples is suYcient to avoid the memory eVect.Sensitivity and Detection Limits The sensitivity of ICP-MS analysis with ASDI and FI was assessed using peak maximum and peak area signal treatments. Both systems were compared by means of the ratio of cali- Fig. 8 Reproducibility of 104 ml injections of a 50 mg l-1 standard by bration line slopes obtained from four standard solutions: 0, ASDI and FI expressed as the pooled relative standard deviation 10, 50 and 200 mg l-1, for a 104 ml injection volume.The ratio (PRSD) from five sets of five injections. (A) Peak maximum and slopeASDI5slopeFI was calculated. The increase in sensitivity, (B) peak area. with ASDI, is more evident for the peak maximum, the removal of dispersion being the predominant reason; the improvement Reproducibility and Long-term Stability factor is between 2 and 4. The peak area ratio is less than 1.5, and is due to the better transport eYciency and compact signal Signal reproducibility was assessed by statistical evaluation of 25 successive injections (104 ml, 50 mg l-1 standard) profile of ASDI.Only for two elements, viz., Zn and Te, does FI show higher sensitivity for peak area. As was pointed out accomplished using ASDI and FI. The data were collected using the peak transient mode (ELAN 5000 software) and earlier, the ASDI signals for these elements are strongly suppressed. Generally, the gain in sensitivity for ASDI over FI were smoothed (five-point Savitsky–Golay). The signals were processed by both options, viz., ‘signal profile maximum’ and is lower for elements that are diYcult to ionise.The higher sensitivity, at similar reproducibility, of the ASDI signals ‘signal profile area’. The reproducibility was calculated as the Fig. 9 Long-term stability for ASDI and FI of 104 ml injections containing 50 mg l-1 Pb over a period of 1 h. (A) Replicates 1–5 and (B) replicates 95–100. Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12 1275Table 3 Detection limits in mg l-1 obtained using three times the The ASDI detection limits are better than those obtained standard deviation of the first set of blank injections (n=10) for ASDI, with FI at the same sample volumes, and are comparable to FI (104 ml injection) and continuous nebulization those of CSI. This makes the ASDI–ICP-MS technique attractive as an alternative to CSI for the analysis of small volumes ASDI FI or diYcult samples.Peak Peak Peak Peak Continuous Element maximum area maximum area nebulization The authors thank the National Science Fund of Bulgaria (Grant No. X-467). The donation of the ELAN 5000 software 27Al+ 0.90 0.46 2.8 0.65 0.88 55Mn+ 0.46 0.06 1.4 0.09 0.41 by Perkin-Elmer is also gratefully acknowledged. The technical 63Cu+ 0.73 0.21 2.2 0.34 0.72 assistance of S. Tenev and V. Gerdjikov is greatly appreciated. 64Zn+ 3.10 1.29 5.4 1.08 2.49 75As+ 1.28 0.60 2.7 1.44 1.28 103Rh+ 0.42 0.07 1.4 0.13 0.44 REFERENCES 114Cd+ 0.48 0.18 1.5 0.39 0.45 1 Stefanova, V., Kmetov, V., and Futekov, L., Anal.L ab., 1993, 115In+ 0.09 0.04 0.26 0.09 0.09 2, 159. 130Te+ 2.10 0.55 3.8 0.67 1.72 2 Gine, M. F., Reis, B. F., Krug, F. J., Jacintho, A. O., Bergamin, H., 133Cs+ 0.30 0.07 0.98 0.12 0.34 Fo., and Zagatto, E. A. G., ICP Inf. Newsl., 1994, 20, 413. 205Tl+ 0.41 0.14 1.3 0.22 0.46 3 Luque de Castro, M. D., and Tena, M., T alanta, 1995, 42, 151. 208Pb+ 0.69 0.27 2.1 0.51 0.78 4 Gregoire, D. C., Lamoureux, M., Chakrabarti, C. L., Al Maawali, S., and Byrne, J. P., J. Anal. At. Spectrom., 1992, 7, 579. 5 Rommers, P., and Boumans, P., Fresenius’ J. Anal. Chem., 1996, 355, 763. compared with those for FI is reflected in an improvement of 6 Longerich, H. P., Jackson, S. E., and Guenther, D., J. Anal. At. the ICP-MS detection limits. The diVerence between the detec- Spectrom., 1996, 11, 899. tion limits for the ASDI and FI systems increases with a 7 Ru° z¡ ic¡ka, J., and Hansen, E.H., Flow Injection Analysis, Wiley, decrease in the sample volume injected. However, owing to New York, 2nd edn., 1988. the lower reproducibility and the loss of sensitivity when 8 Tyson, J. F., Spectrochim. Acta Rev., 1991, 14, 169. 9 Denoyer, E., Stroh, A., and Lu, Q., At. Spectrosc., 1993, 14, 55. working with very small volumes, viz., 56 ml (Fig. 5), the 10 Longerich, H. P., J. Anal. At. Spectrom., 1993, 8, 371. detection limits obtained by ASDI are worse than those found 11 Woller, A., Garraud, H., Martin, F., Donard, O.F. X., and for CSI. For these reasons, a 104 ml sample volume is preferable. Fodor, P., J. Anal. At. Spectrom., 1997, 12, 53. Table 3 presents the detection limits achieved by CSI, ASDI 12 Coedo, A. G., and Dorado, M. T., Appl. Spectrosc., 1995, 49, 115. and FI for the 12 elements tested. They were calculated using 13 Vollkopf, U., Gusler, A., and Janssen, A., At. Spectrosc., 1990, three times the blank standard deviation (3sblank) of peak 11, 135. 14 Stroh, A., Vollkopf, U., and Denoyer, E., J. Anal. At. Spectrom., maximum and peak area signals for discrete sampling and 1992, 7, 1201. steady-state signals for CSI. Even when working with a sample 15 Wang, J., Shen, W. L., Sheppard, B. S., Evans, E. H., Caruso, volume of 104 ml, the ASDI detection limits obtained via the J. A., and Fricke, F. L., J. Anal. At. Spectrom., 1990, 5, 445. peak maximum are very close to those of CSI and lower when 16 Ebdon, L., Fisher, A.S., and Worsfold, P. J., J. Anal. At. peak area is applied. The FI peak maximum detection limits Spectrom., 1994, 9, 611. are higher by a factor corresponding to the dispersion demon- 17 Coedo, A. G., Dorado, M. T., Padilla, I., and Alguacil, F., Anal. Chim. Acta, 1997, 340, 31. strated above. 18 Becottehaigh, P., Tyson, J. F., Denoyer, E., and Hinds, M. W., Spectrochim. Acta, Part B, 1996, 51, 1823. 19 Thompson, J. J., and Houk, R. S., Anal. Chem., 1986, 58, 2541. CONCLUSIONS 20 Powell, M. J., Boomer, D. W., and Wiederin, D. R., Anal. Chem., ASDI can be successfully coupled with ICP-MS. The ASDI 1995, 67, 2474. 21 Denoyer, E. R., At. Spectrosc., 1994, 15, 7. sampling oVers several advantages over conventional FI tech- 22 Beauchemin, D., Analyst, 1993, 118, 815. niques. In contrast to FI, where the use of small volumes 23 Houk, R. S., and Thompson, J. J., Biomed. Mass Spectrom., 1983, reduces the signal intensity, the ASDI signals have the same 10, 107. or even higher intensity compared with continuous introduc- 24 Craig, J. M., and Beauchemin, D., Analyst, 1994, 119, 1677. tion. Furthermore, ASDI allows easy change of the injection 25 Craig, M. J., and Beauchemin, D., J. Anal. At. Spectrom., 1994, volume by choosing the time programme, low sample con- 9, 1341. 26 Coedo, A. G., Dorado, M. T., Padilla, I., and Alguacil, F. J., sumption and analysis automation. The plateau profile of the J. Anal. At. Spectrom., 1996, 11, 1037. transient signals promotes the multi-element capabilities of 27 Kmetov, V., and Futekov, L., Fresenius’ J. Anal. Chem., 1990, ICP-MS. The most suitable sample volume for ASDI–ICP-MS 338, 895. analysis was found to be between 100 and 300 ml, where ‘hyper signals’ were observed. A relative standard deviation lower Paper 7/04472K than 5% can be achieved during multi-element analysis using Received June 25, 1997 injection volumes of 104 ml. A long-term stability test shows Accepted August 5, 1997 that reliable sampling by both discrete techniques without instrument drift is possible for up to 1 h. 1276 Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12

ISSN:0267-9477    

DOI:10.1039/a704472k

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Determination of Trace Metals in Seawater by Inductively Coupled Plasma Mass Spectrometry After Off-line Dithiocarbamate Solvent Extraction GRANT J. BATTERHAM, NIELS C. MUNKSGAARD AND DAVID L. PARRY* School of Mathematical and Physical Sciences, Northern T erritory University, Darwin 0909, NT , Australia An oV-line solvent extraction procedure with inductively technique that utilises the inherent advantages of ICP-MS analysis is yet to be reported. The development of such coupled plasma mass spectrometric (ICP-MS) determination was developed for the determination of heavy metals in sea- a technique would significantly reduce analysis times and improve detection limits owing to the simultaneous water at ultra-trace levels.The procedure was adapted from a dithiocarbamate–diisobutyl ketone solvent extraction system multi-element capability and high sensitivity of ICP-MS. Recently, we developed a rapid solvent extraction technique with Hg back-extraction, which was previously validated using electrothermal atomic absorption spectrometry.The to determine Cd, Co, Cu, Fe, Ni, Pb and Zn in sea-water with ETAAS analysis.13 The technique complexes trace metals in simultaneous ICP-MS determination significantly reduced the analysis times and improved the detection limits (Cd 0.2, Co sea-water with DTC, extracts these complexes into a retained diisobutyl ketone phase and then back-extracts the complexed 0.3, Cu 3, Fe 21, Ni 2, Pb 0.5 and Zn 2 ppt).The rapid single extraction procedure was quantitative and external standards metals into aqueous solution by exchange with Hg, which has a much greater DTC stability constant (of the order of 1040).14 were used for ICP-MS calibration. The method gave good reproducibility with precisions at the 100 ppt level better than In this work, we examined whether this solvent extraction system could be modified and coupled with ICP-MS for the 5% (n=4), except for Fe (7%).The method was validated by accurate analysis of CASS-3 Nearshore Sea-water and determination of trace metals in sea-water. NASS-4 Open Ocean Sea-water Standard Reference Materials. EXPERIMENTAL Keywords: Heavy metals; sea-water; preconcentration; solvent Reagents extraction; dithiocarbamate; inductively coupled plasma mass Ultrapure water was obtained from a Permutit HI-PURE spectrometry system (Sydney, Australia) fed with reverse osmosis water and was used for the blank and to make up all solutions.Chemicals In the field of trace metal analysis of natural waters, inductively were of analytical-reagent grade unless indicated otherwise. coupled plasma mass spectrometry (ICP-MS) has become The solvent phase was diisobutyl ketone (DIBK) (95% accepted as one of the most sensitive, reliable and accurate 2,4-dimethylheptan-6-one; Ajax, Auburn, Australia) and was techniques. Regulatory agencies have adopted this method for dispensed from a Fortuna 10 Optifix solvent dispenser (Walter compliance monitoring of natural waters in relation to their Graf u.Co, Main, Germany). The DTC complexing agent water quality criteria.1 However, for sea-water, the high salt was 0.5% each of sodium diethyldithiocarbamate (Ajax) content and low levels of trace metals invariably preclude and ammonium pyrrolidinedithiocarbamate (Ajax). The comdirect analysis by ICP-MS. The salt matrix produces lower plexing agent was made up in a 10 ml calibrated flask immedianalyte signals due to ionisation suppression2 and even with ately prior to use.The solution was cleaned once by adding sample dilution, polyatomic isobaric interferences still aVect a 1 ml of DIBK, shaking for 1 min, then discarding the upper number of analytes such as 40Ar23Na on 63Cu and 44Ca16O on DIBK phase. Stock ammonium acetate buVer solution (3 M 60Ni. Hence the analysis of sea-water by ICP-MS generally ammonia–2 M acetic acid) was prepared from Suprapur 25% requires a preliminary matrix separation. ammonia solution (Merck, Darmstadt, Germany) and Aristar Traditionally, matrix separation was performed using either acetic acid (BDH, Poole, Dorset, UK). A stock 100 ppm Hg organic complexing agents with solvent extraction3 or chelating back-extracting solution was prepared from HgNO3 (Ajax) ion-exchange resins.4 These techniques all utilised electrother- and was acidified to 0.02 M HNO3 with Suprapur acid (Merck) mal (graphite furnace) atomic absorption spectrometry and spiked (10 ppb) with Ga, In and Tb (APS solutions, Alpha (ETAAS) for analysis.ICP-MS was first introduced for sea- Resources, Stevensville, MI, USA) to act as internal standards. water analysis by McLaren et al.,5 who used an oV-line column of silica immobilised 8-hydroxyquinoline to chelate trace Extraction Procedure metals, prior to elution and analysis in dilute HCl–HNO3 solution. Chelating ion-exchange techniques have evolved The solvent extraction procedure was described in detail by Batterham and Parry.13 The extraction procedure was under- considerably in recent times, with many researchers developing sophisticated on-line ICP-MS systems using both taken in a class-100 laminar flow cabinet.All laboratory ware was acid-washed to an ultraclean standard. In addition, a 8-hydroxyquinoline and iminodiacetate columns.6–10 These techniques simultaneously separate the salt matrix and precon- small acid bath (5% HNO3) and a high purity water bath were maintained in the laminar flow cabinet to rinse polyethyl- centrate trace metals by a factor of 5–10, achieving low ppt detection limits with an analysis time of 10–15 min per sample.ene automatic pipette tips immediately prior to use. These baths and other dispensing glassware were maintained under Many researchers still employ dithiocarbamate (DTC) solvent extraction techniques based on the work of Danielsson plastic wrap (Gladwrap) when not in use.For the extraction procedure, 80 ml of acidified filtered and co-workers3,11 and Magnusson and Westerlund12 to determine trace metals in sea-water by ETAAS. A solvent extraction (<0.45 mm) sea-water were placed in a 125 ml screw-capped Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12 (1277–1280) 1277polypropylene separating funnel (Nalgene, Rochester, NY, Table 2. The 111Cd analyte mass includes a correction for a possible MoO interference as recommended by the USEPA.1 USA) and adjusted to approximately pH 4.5 with ammonium acetate buVer.The amount of buVer required was determined External standards (1, 10 and 100 ppb) were prepared using the stock Hg solution (APS solutions, Alpha Resources). The on a separate representative non-extracted sample. A 100 ml volume of DTC complexing solution and 5 ml of DIBK were internal standards in the Hg back-extraction solution were used to correct for ionisation suppression and instrument drift; added (Optifix dispenser).The solution was shaken for 10 min with a mechanical wrist-shaker (Lab-Line, Melrose Park, IL, In was used for Cd, Tb was used for Pb and Ga was used for the remaining elements. USA), followed by a 5 min rest for phase separation. The lower sea-water phase was drained and 4.5 ml of the DIBK were removed by an adjustable pipette (Gilson, Worthington, OH, Method Validation USA) fitted with a long tip, and placed in a 10 ml screwcapped polypropylene, tapered centrifuge tube.A 1 ml volume CASS-3 Nearshore Sea-water and NASS-4 Open Ocean Seaof Hg back-extraction solution was then added and the tube water Standard Reference Materials (SRMs) were obtained was shaken by hand for 2 min for back-extraction. The upper from the National Research Council of Canada (Ottawa, DIBK phase was completely removed from the centrifuge tube Canada) to validate the method. CASS-3 was extracted and using an adjustable pipette fitted with a polypropylene tube analysed by both ICP-MS and ETAAS for comparison, and extension on the pipette tip.NASS-4, which contains lower levels of trace metals, was For analysis by ETAAS, about 0.9 ml of the aqueous phase analysed only by ICP-MS. was transferred into an autosampler cup and analysed immediately as described by Batterham and Parry.13 For analysis by RESULTS AND DISCUSSION ICP-MS, the centrifuge tube was re-capped and refrigerated at 4 °C prior to analysis.These refrigerated solutions were On-line column ICP-MS techniques that have involved inadfound to be stable for at least 3 d. equate washing of the column to remove residual salts or To examine whether the relatively high concentration of Hg co-elute retained alkaline metals have been reported to be significantly suppressed the ionisation of the lighter elements subject to isobaric interferences such as 40Ca16OH on 57Fe, with ICP-MS analysis, a series of solutions were prepared 42Ca16OH and 43Ca16O on 59Co, 44Ca16O on 60Ni and containing Hg concentrations from 0 to 200 ppm and equival- 40Ar23Na on 63Cu.6,15–17 Similarly, the use of a solvent extracent concentrations of trace metals (32 ppb Zn, Fe and Cu; 16 tion technique with ICP-MS requires the eVective elimination ppb Pb, Ni and Co; 10 ppb Ga, Tb and In; and 3.2 ppb Cd).of the salt matrix. The solvent DIBK meets an important The respective ICP-MS count rates were then recorded for the criterion in this regard, having a low water miscibility trace metals at each diVerent Hg concentration.(0.06±0.02 ml per 100 ml).18 This and the non-complexing of alkali metals by DTC provide a relatively interference free matrix for the determination of trace metals by ICP-MS. Instrumentation Solvent extraction techniques generally back-extract the The ETAAS instrument was a Varian (Mulgrave, Australia) DTC complexed metals retained in the solvent into an aqueous SpectrAA40 with a GTA-95 graphite tube atomiser and auto- phase using nitric acid decomposition of DTC.We have found sampler. Pyrolytic graphite-coated platform ETAAS tubes and the acid decomposition to be very ineYcient for Cu and Co, deuterium background correction were used. Ammonium dihy- which form very strong and stable DTC complexes, respectdrogenphosphate (0.5%) was used as a chemical modifier and ively. Acid back-extraction also entails a large final backthe operating parameters and furnace programmes were as extraction volume to bring the acid concentration to 2% for specified by Batterham and Parry.13 ICP-MS analysis.Metal-exchange back-extraction, however, The ICP-MS system was an Elan 6000 with an AS90 displaces DTC complexed metals with a metal of greater autosampler (Perkin-Elmer, Norwalk, CT, USA) fitted with complex aYnity. Mercury has a very high DTC stability an MCN-100 microconcentric nebuliser (CETAC, Omaha, NE, constant and gives quantitative back-extraction of all trace USA).The AS90 autosampler was enclosed inside a purpose metals examined in this study in 2 min, into a 1 ml volume. built, sealed acrylic housing, and the sample uptake tubing This enables a high preconcentration factor (72) to be achieved was replaced with the MCN-100 capillary tubing. The instru- from a small volume of sea-water (80 ml ). Metal-exchange ment was operated in accordance with the manufacturer’s back-extraction is essentially non-destructive, with the specifications and the operating conditions are given in Table 1.DTC–Hg complex retained by the solvent, and hence not The nebuliser gas flow and ion lens voltage were optimised for contributing to any polyatomic interferences. The typical conmaximum sensitivity prior to analysis. centration of Hg remaining in an aqueous back-extract after The analyte masses and elemental corrections are given in back-extraction was approximately 50 ppm.The ionisation suppression of the relatively high Hg concentration on the lighter metals was examined and found to be very small over Table 1 Instrument conditions for Elan 6000 ICP-MS system with an MCN-100 microconcentric nebuliser Power 1000 W Table 2 Recommended elemental equations Argon plasma gas flow rate 17 l min-1 Argon auxiliary gas flow rate 1.2 l min-1 Element Elemental equation Argon nebuliser gas flow rate 1.0 l min-1 Sample rinse 20 s at 48 rpm Fe 57M Co 59M Sample uptake 20 s at 48 rpm 60 s at 24 rpm Ni 60M Cu 63M Scan mode Peak hopping Sweeps per reading 15 Zn (64M)-(0.035313) (60M) Ga 69M Replicates 3 Dwell time 100 ms Cd (111M)-(1.073) ((108M)-(0.712) (106M)) In (115M)-(0.016) (118M) Integration time 1500 ms Analysis time per sample 65 s Tb 159M Pb (206M)+(207M)+(208M) Total volume used per sample 300 ml 1278 Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12Table 3 EVect of Hg concentration in the back-extraction solution on the ionisation suppression of other metals Metal/103 counts s-1 Hg, ppm 57Fe 59Co 60Ni 63Cu 64Zn 69Ga 111Cd 115In 159Tb 208Pb 0 23 224 49 226 185 140 11 345 490 395 50 20 209 52 233 178 150 12 376 490 420 100 21 213 48 230 185 126 13 365 530 420 150 20 204 42 200 160 132 13 370 504 410 200 20 200 48 190 150 133 13 345 462 426 the range 0–100 ppm Hg (Table 3) and could be compensated masses for Cu (63 and 65) and Zn (64 and 66) and found no interferences, with identical results being obtained for NASS-4 for by the use of the internal standards.The results obtained for CASS-3 and NASS-4 SRMs are at both analyte masses for each metal. Only the results for the most abundant isotope in each case are presented in Table 4. given in Table 4. The results were within the 95% confidence limits of the certified values for all metals examined using To investigate the versatility of our technique, we quantitatively extracted sea-water samples spiked with 25–200 ppb of either ETAAS or ICP-MS analysis.The blanks and detection limits achieved with both methods are given in Table 5. The Zn or Cu. Of the metals we examined, Zn forms one of the weakest DTC complexes and Cu forms the strongest (after use of ICP-MS analysis improved the detection limits by up to an order of magnitude (Co, Pb), with all metals 3 ppt, Hg). This indicates that our procedure can determine total dissolved metal concentrations to the stoichiometric equivalent except Fe (21 ppt), compared with ETAAS analysis.The detection limit of 2 ppt for Zn (Table 5) is diYcult to achieve on a of 200 ppb of Zn or Cu. However, one exception was an upper limit of approximately 3 ppb for the quantitative back- routine basis given the relatively high Zn blank (61 ppt), but a detection limit of better than 10 ppt is typically attained. extraction of Co. Although Co has a lower extraction constant than Cu, it is transformed into a Co(DTC)3 complex which is These detection limits are lower than those of other solvent extraction techniques that until now have relied upon ETAAS kinetically stable20 and therefore diYcult to back-extract.Generally, even contaminated sea-water contains sub-ppb analysis. The detection limits are also generally lower than those previously reported for on-line column ICP-MS analy- levels of Co. For higher Co levels, additional Hg solution and/or an extended back-extraction time can be used.sis.6–10 On-line techniques generally only utilise a preconcentration factor of 5–10 to limit analysis times, whereas our oV- We previously reported that a spike correction may be necessary for Zn determined by ETAAS using our extraction line preconcentration technique uses a factor of 72. Ultrapure water for the blanks was extremely important with such a high method,13 attributing this to incomplete extraction with aged NaDDC. ICP-MS analysis has shown that the Zn recoveries preconcentration factor when determining low ppt metal levels.The precision obtained for our technique with ICP-MS was are indeed quantitative and that our ETAAS spike correction has been acting as a standard additions calibration, compensat- better than 5% at the 100 ppt level, except for Fe (7%). This precision is also generally better than that previously reported ing for an unknown matrix interference. This has only occurred since changing sources of NaDDC and it was likely that an for on-line column ICP-MS analysis.6–10 Few on-line column ICP-MS procedures determine Fe and unknown contaminant or breakdown product in the original NaDDC source acted as a chemical modifier in the ETAAS those which do have very poor precision owing to high Fe blanks and ArO isobaric interferences.6,19 Our method gives analysis.The interference does not aVect ICP-MS determination and can be decreased in ETAAS analysis by greater better precision, but still has a relatively high detection limit (21 ppt) in comparison with the other metals (3 ppt). This sample dilution.One advantage of on-line column ICP-MS systems is that was due to the high background isobaric interferences on 54Fe and 57Fe from 40Ar14N and 40Ar16OH, respectively. Even the potential for exposure and airborne contamination of samples is reduced. This must be balanced by the meticulous though the low concentration of nitric acid (0.02 M) used in our Hg back-extraction minimises the 40Ar14N interference, we attention necessary to limit contamination from the column assembly and reagents, particularly with regard to poor Fe found that this interference had a greater fluctuation than 40Ar16OH. This fluctuation can become significant when meas- and Zn blanks.6,8,10,15 Our solvent extraction system requires only a single quantitative extraction that entails minimal uring Fe at low ppt levels.We also examined both analyte Table 4 Analysis of CASS-3 and NASS-4 SRMs (ppt) Method and sample Cd Co Cu Fe Ni Pb Zn ETAAS, CASS-3* 33.2±0.8 47±6 532±7 1210±20 390±14 11±4 1200±40 ICP-MS, CASS-3* 31.0±0.2 43±2 537±5 1160±20 384±11 9.1±0.9 1250±20 Certified† 30±5 41±9 517±62 1260±170 386±62 12±4 1250±240 ICP-MS, NASS-4* 17.3±0.1 8.6±0.2 223±2 121±8 231±1 9.0±0.5 106±4 Certified† 16±3 9±1 228±9 105±16 228±11 13±5 115±18 * Precision expressed as standard deviation of four replicates.† Precision expressed as 95% confidence interval.Table 5 Comparison of blanks and detection limits (DL) obtained with ETAAS and ICP-MS analysis (ppt) Parameters Cd Co Cu Fe Ni Pb Zn Blank, ETAAS BDL* BDL 19 BDL BDL BDL 69 Blank, ICP-MS 0.2 0.3 24 38 5 6 61 DL (3s), ETAAS 1.0 15 6 30 18 8 10 DL (3s), ICP-MS 0.2 0.3 3 21 2 0.5 2 * BDL=below detection limit. Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12 1279exposure. Of the reagents used in our procedure, only the Council for a grant allowing the purchase of the ICP-MS system. DTC solution requires minimal pre-cleaning.Most of our blank (Table 5) was attributed to technical grade DIBK. These blank levels are generally lower than that obtained with on-line REFERENCES column ICP-MS procedures.6–10 1 Method 1638: Determination of T race Metals in Ambient Waters Unlike on-line techniques, where the signal is transitory, by Inductively Coupled Plasma-Mass Spectrometry, USEPA, OYce longer analysis times can be used with oV-line techniques to of Water, Washington, DC, 1995, EPA 821-R-95–031.allow isotopic analyses to be undertaken. We used this tech- 2 Beauchemin, D., McLaren, J. W., Mykytiuk, A. P., and Berman, nique to study Pb isotope ratios at low ppt levels in sea-water, S. S., Anal. Chem., 1987, 59, 778. 3 Danielsson, L.-G., Magnusson, B., and Westerlund, S., Anal. using sample analysis times up to 3 min to determine the Chim. Acta, 1978, 98, 47. potential impact of Pb–Zn ore concentrates in a marine 4 Kingston, H.M., Barnes, I. L., Brady, T. J., Rains, T. C., and environment. Champ, M. A., Anal. Chem., 1978, 50, 2064. With our system, a dozen samples can be extracted simul- 5 McLaren, J. W., Mykytiuk, A. P., Willie, S. N., and Berman, S. S., taneously in about 1 h and back-extracts can be stored prior Anal. Chem., 1985, 57, 2907. to analysis. The CASS-3 ICP-MS results (Table 4) were 6 McLaren, J. W., Lam, J. W. H., Berman, S. S., Akatsuka, K., and Azeredo, M.A., J. Anal. At. Spectrom., 1993, 8, 279. obtained on back-extracts that had been stored at 4 °C for 3 d. 7 Bloxham, M. J., Hill, S. J., and Worsfold, P. J., J. Anal. At. This allows many extraction runs to be undertaken prior to Spectrom., 1994, 9, 935. analysis, with the internal standards compensating for any 8 Bettinelli, M., and Spezia, S., J. Chromatogr., 1995, 709, 275. changes in the sample viscosity and instrument drift. Taylor 9 Nelms, S. M., Greenway, G.M., and Koller, D., J. Anal. At. et al.10 employed internal standards in the elution acid for Spectrom., 1996, 11, 907. on-line column ICP-MS. This would be particularly important 10 Taylor, D. B., Kingston, H. M., Nogay, D. J., Koller, D., and Hutton, R., J. Anal. At. Spectrom., 1996, 11, 187. for on-line column techniques given that the ICP-MS instru- 11 Danielsson, L.-G., Magnusson, B., Westerlund, S., and Zhang, K., ment is in continual operation, generally taking 10–15 min per Anal.Chim. Acta, 1982, 144, 183. sample. Our overall instrumental analysis time per sample was 12 Magnusson, B., and Westerlund, S., Anal. Chim. Acta, 1981, 2.75 min (including rinse cycles), giving a major reduction in 131, 63. the ICP-MS running time. The overall preparation and analysis 13 Batterham, G. J., and Parry, D. L., Mar. Chem., 1996, 55, 381. time in our procedure, for at least 10 samples, would be 14 Lo, J. M., Yu, J. C., Hutchison, F. I., and Wal, C. M., Anal. Chem., 1982, 54, 2536. comparable to those in existing on-line procedures. Our pro- 15 Beauchemin, D., and Berman, S. S., Anal. Chem., 1989, 61, 1857. cedure is relatively simple and cheap to set up and run in 16 Heithmar, E. M., Hinners, T. A., Rowan, J. T., and Riviello, J. M., comparison with existing on-line column ICP-MS pro- Anal. Chem., 1990, 62, 857. cedures.6–10 The microconcentric nebuliser uses about 300 ml 17 Huang, K. S., and Jiang, S. J., Fresenius’ J. Anal. Chem., 1993, of back-extract per analysis, allowing for the repeated analysis 347, 238. 18 Bone, K. M., and Hibbert, W. D., Anal. Chim. Acta, 1979, 107, 219. of samples if required. In addition to the inherently better 19 Seubert, A., Petzold, G., and McLaren, J. W., J. Anal. At. detection limits obtained using ICP-MS analysis, this has also Spectrom., 1995, 10, 371. shortened the analysis times by at least 6 h in comparison with 20 Stary, J., and Kratzer, K., Anal. Chim. Acta, 1968, 40, 93. analysis by ETAAS. Paper 7/04309K The authors acknowledge the financial support of McArthur Received June 19, 1997 Accepted July 22, 1997 River Mining Pty. Ltd. and thank the Australian Research 1280 Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12

ISSN:0267-9477    

DOI:10.1039/a704309k

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Flow Injection On-line Reductive Precipitation Preconcentration With Magnetic Collection for Electrothermal Atomic Absorption Spectrometry S. SELLAb , R. E. STURGEON* a , S. N. WILLIEa AND R. C. CAMPOSc aInstitute for NationalMeasurement Standards, National Research Council of Canada, Ottawa, Ontario, Canada K1A 0R9 bDepartment of Analytical Chemistry, Universidade Federal Fluminense, Niteroi/RJ, Brazil cDepartimento De Quimica da PUC/Rio, 22453 Rio de Janeiro, Brazil A flow injection on-line preconcentration system coupled to an forming elements.15 The presence of palladium, used as a coprecipitant, is also attractive in that it serves as a matrix electrothermal atomic absorption spectrometer for the determination of trace metals in saline media is described.The modifier for subsequent ETAAS determinations of the more volatile analytes. Detection limits in the ng l-1 range have filterless, magnetic collection of coprecipitated analytes is based on the tetrahydroborate reductive precipitation of added been reported following the processing of large volumes of sample (i.e., 900 ml), suYcient for the determination of elements iron and palladium in alkaline medium at a sample flow rate of 1.7 ml min-1.The precipitate is dissolved in a 20 ml volume of interest in uncontaminated seawater. It is desirable to implement this procedure on-line using FI technology, thereby of mixed acid and transported direct to the graphite furnace.With the exception of Cr (33% recovery in seawater), reducing sample and reagent consumption while enhancing throughput. FI manifolds for collection of inorganic precipi- recoveries of Ag, As, Bi, Cd, Co, Cu, Mn, Ni, Pb, Sb and Tl averaged 84% from deionized water and 67% from coastal tates invariably utilize some form of in-line filter, often a stainless steel HPLC screen18 or membrane filter.19 To date, seawater with a frequency of 12 measurements h-1. The sensitivity of the graphite furnace technique can be enhanced filterless on-line coprecipitation systems have exclusively relied on the use of knotted reactors and organic precipitants.To over 400-fold (for an 11 ml sample volume) compared to a standard 20 ml injection volume. Detection limits in the low pg the best of our knowledge, this is the first report on the implementation of an on-line filterless system based on the (absolute) or pg ml-1 (relative) range can be reached. Results, presented for the determination of As, Cd, Cr, Cu, Mn and Ni collection of an inorganic precipitate. in 1 ml volumes of CASS-3 NRCC Certified Reference Material seawater, are not statistically diVerent (t-test, 95% EXPERIMENTAL confidence limit) from the certified values for these elements.Apparatus Keywords: Flow injection preconcentration; magnetic collection; reductive precipitation; seawater; electrothermal A Perkin-Elmer (Norwalk, USA) Model 5000 atomic absorpatomic absorption spectrometry tion spectrometer fitted with continuum source background correction, an HGA 500 furnace and an AS 60 autosampler was used in combination with a Perkin Elmer Model FIASFlow injection (FI) approaches for electrothermal atomic 400 flow injection unit (equipped with 2 software controlled absorption (ETAAS) have traditionally been more diYcult to pumps and an injection valve).Perkin-Elmer hollow cathode implement than those associated with continuous sample intro- or electrodeless discharge lamps (As, Se, Sb) were used as line duction atomic spectrometric techniques.The advent of com- sources under the manufacturers’ recommended conditions of mercialized dedicated hardware and software, however, has current and power. The FIAS manifold is schematically illusserved to alleviate this problem and most applications of trated in Fig. 1. Teflon separatory funnels served as reagent FI–ETAAS are associated with on-line trace element precon- and sample reservoirs.The unit was also fitted with a 12 V 3- centration.1,2 Pursuit of this objective is attractive, as significant enhancement factors can be realized at sample throughputs approaching that of conventional ETAAS procedures. Notable examples make use of solution phase chelation with extraction systems based on adsorption onto reversed-phase C18 substrates3 –5 and collection in knotted reactors,6,7 sequestration with immobilized chelating agents8,9 and coprecipitation (organic carrier) with either collection on a membrane filter10 or filterless collection in a knotted reactor.11,12 Precipitation is one of the more commonly used techniques for the enrichment of inorganic analytes,13 trace preconcentration generally being accomplished by co-precipitation of an added carrier, since direct precipitation often yields amounts of precipitate too small to conveniently recover.Both inorganic and organic carriers have been utilized for this purpose in oV- line techniques.14–16 Particularly attractive has been the relatively non-selective multi-element preconcentration technique based on tetrahydroborate reductive precipitation.15,17 This Fig. 1 FIAS manifold. P1 and P2 are pumps; S is a solenoid valve; methodology permits a wide range of analytes to be collected, V is the FIAS valve; W, waste. Note: as illustrated, the manifold is in the sample elution stage (see Table 1, step 5). including transition and noble metals as well as hydride Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12 (1281–1285) 1281way solenoid valve (Cole Parmer, Vernon Hills, IL, USA) (Johnson Matthey).National Research Council of Canada Certified Reference Material Coastal Seawater CASS-3 (salinity having zero dead volume with all wetted parts made of PTFE. This valve which, in a normally open configuration, was 30.2‰) was used for method evaluation. plumbed to pass a flow of air, was briefly activated in step 3 of the FIAS program (Table 1), using a remote switch on the Procedure rear of the FIAS-400, to permit uptake of an approximately 60 ml volume of distilled deionized water (DDW).Initial Although all sample and reagent preparation was undertaken in a class-100 clean room environment, all FIAS manipulations optimization of experimental parameters was done by maximizing analyte recovery. Multielement response was achieved by were subsequently accomplished in a regular chemical laboratory.A schematic of the FIAS manifold is shown in Fig. 1 and analysing processed test solutions using a Perkin-Elmer SCIEX ELAN 5000 inductively coupled plasma–mass spectrometer the software controlled sequence of operations is summarized in Table 1. Note that the position of the sampling valve in the (P.-E. Sciex, Thornhill, Canada). Tygon peristaltic pump tubing (Cole Parmer) propelled all reagents (black–black 0.76 mm id manifold illustrated corresponds to the sample elution stage, not loading. Time-based processing of the sample was for sodium borohydride and air, yellow–blue 1.52 mm id for sample and ammonium hydroxide, orange–green 0.38 mm id implemented.Samples were prepared to contain 30 mg ml-1 each of added PdII and FeIII as coprecipitation reagents. During for elution acid). One to 2 m lengths of silicone (Cole Parmer, 1.27 mm id), microbore tygon (Cole Parmer, 1.27 mm id) and the priming step (P, in Table 1), the sample and reagent uptake pump, P1, is on so as to fill the lines with the (next) sample.Teflon (Cole Parmer, 1.27 mm id) tubing were all examined in an eVort to determine the best material for use as a collection In step 1 the FIAS valve is switched (precipitation–collection stage), sample (1.7 ml min-1), ammonium hydroxide (diluted coil. All lines were interconnected using 1/4–28 Tefzel flangeless nuts and ferrules (UpChurch Scientific, Concord, Ontario, to 0.09 M, 1.7 ml min-1) and sodium tetrahydroborate (2% m/v, 0.5 ml min-1) solutions delivered via pump 1 are merged Canada).A 45 cm length of 1 mm id Teflon transfer line separated the collection coil from the last reagent–sample prior to the collection coil in which the precipitate is subsequently collected. One ml sample volumes were processed merge point. The filterless collection coil was wrapped around a (5.7×1.4×1.5 cm) rare earth cobalt ceramic magnet with the program presented in Table 1.During step 2, pump 2 is activated and air (0.25 ml min-1) is used to drive any (Edmund Scientific, Toronto, Canada) of 8.58 kG field strength. This unit was placed in the cavity of a heated aluminium liquid remaining in the collection coil line to waste while at the same time the dissolution acid (2 M HCl and 2 M HNO3) block. The block was maintained at a pre-set temperature of 98 °C using a VICI temperature controller (Model ITCK fills a 20 ml loop. During step 3, a remote switch at the rear of the FIAS unit activates the solenoid for a brief time period 10399; Valco Instrument, Houston, Texas, USA) connected to a 75 W immersion heater placed in a recess in the wall of (2 s), permitting uptake of approximately 60 ml DDW.This plug of DDW is propelled by a flow of air during step 4, the block. thereby washing any residual salt matrix from within the transfer and collection lines with the air serving to leave the Reagents system as free as possible of any moisture that may dilute the minimum volume of acid used in the subsequent dissolution All reagents were purified prior to use.Deionized, distilled step. The position of the valve then changes from the fill mode water (DDW) was produced with a commercial mixed-bed to inject mode in step 5 and 20 ml of acid is driven by air NanoPure ion exchange system (Barnstead/Thermolyne, (0.25 ml min-1) to the collection coil. Both pumps are briefly Boston, MA, USA) fed with distilled water.Concentrated stopped (20 s) during step 6 when the major portion of the HNO3 and HCl were prepared by sub-boiling distillation of precipitate is submerged in acid in order to permit a more feedstocks. A saturated ammonia solution (Environmental complete dissolution of the precipitate. The exit end of the Grade, 28% v/v) was obtained from Anachemia Science collection coil was fixed to the arm of the AS 60 autosampler (Montreal, Quebec, Canada). Stock solutions (1000 mg l-1) of such that in step 7 (elution stage) the eluent is directed into all analytes were obtained by dissolution of high-purity metals the graphite furnace by manually turning the sampling arm or their salts (Spex Industries, Edison, NJ, USA) and working into its injection position and holding it there until transfer of standards were prepared by serial dilution of the stocks with the acid was complete. The entire sequence requires approxi- DDW containing 1% HNO3.Solutions of various concenmately 5 min for a throughput of 12 measurements h-1.This trations of sodium tetrahydroborate (Alfa Aesar, Johnson is fully compatible with the operation of the graphite furnace. Matthey, Ward Hill, MA, USA) were prepared immediately Following dissolution, the furnace program was manually prior to use by dissolution of the material in DDW and initiated and the absorbance transient recorded using in-house stabilized by the addition of sodium hydroxide (0.5% m/v for software which calculated both peak height and area.The 2% NaBH4 concentration). Stabilization was particularly presence of palladium in the furnace served as a matrix important at high concentrations of tetrahydroborate as repromodifier, permitting sample pyrolysis temperatures to be set ducible addition of this reagent to the flowing stream could from 900 (Cd) to 1200 °C for all elements and atomization only be made if bubbles were excluded from the line.Stock with maximum power heating from 1600 (Cd) to 2600 °C. solutions (10 000 mg l-1) of FeIII and PdII were obtained by Quantitation of analyte recovery was implemented by cali- acidic dissolution of the 99.997% Puratronic grade metal wires bration, using integrated absorbance, against a matrix matched standard manually injected into the furnace and subjected to Table 1 FIAS program an identical atomization program. Optimization of such experimental parameters as reagent Step Time P1 P2 Valve Remote concentrations, tubing type and sample pH was achieved by P 30 50 0 Inject OFF processing 1 ml volumes of DDW spiked to contain 1 ng ml-1 1 35 20 0 Fill OFF of all elements of interest.Following elution with acid, the 2 10 0 30 Fill OFF 3 2 0 30 Fill ON solution was diluted to 1.0 ml with DDW and analysed by 4 99 0 30 Fill OFF ICP-MS. Blanks were prepared using DDW in a manner 5 40 0 30 Inject OFF identical to that of the samples. 6 20 0 0 Inject OFF Final optimization of sample and reagent flow rates was 7 99 0 30 Inject OFF done using 1 ml volumes of DDW spiked to contain 1 ng ml-1 1282 Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12Table 2 Experimental variables Solution Flow rate/ml min-1 Volume/ml Concentration Tube type Sample 1.7 1.0 — Yellow–Blue NH4OH 1.7 1.0 0.09 M Yellow–Blue NaBH4 0.5 0.3 2% (m/v) Black–Black Acid 0.25 20 ml 2M HCl; 2 M HNO3 Orange–Green DDW 0.25 60 ml — Black–Black Air 0.25 — — Black–Black Cd as the test element followed by analysis by ETAAS.Appropriate blanks were carried through all procedures. Table 2 summarizes all experimental variables relating to solution flow rates, volumes and pump tubing used. NRCC reference material CASS-3 (Coastal Seawater) was analysed by the method of additions using on-line processing of 1 ml volumes of sample and spiked samples. The relatively high concentration of Mn in this material (2.5 ng ml-1) necessitated use of an internal purge gas flow during atomization in order to reduce response to maintain linearity.RESULTS AND DISCUSSION Implementation of the reductive precipitation methodology in Fig. 2 EVect of total mass of FeIII and PdII carrier added to a DDW an on-line FI format proved to be a non-trivial exercise in sample (in equal amounts) on recovery of: A, MnII and B, BiIII present downsizing of the oV-line process. EVorts to scale the procedure at 1 ng ml-1. down by the sample volume ratio (i.e., 900) proved totally inappropriate.A minimum amount of carrier was necessary to eVect recovery. In part, this may arise as a result of the nature ensure any collection of analyte. All variables were thus of the collection technique in that a critical mass of material systematically optimized to achieve maximum analyte recovery is necessary for either sedimentation or magnetic attraction from DDW solutions and then individual parameters were re- against the force of the flowing stream; as well, because no evaluated to determine their overall impact on response.time was permitted for extensive crystal digestion or aging, the Optimization of experimental parameters was done in an eVort initial size of particles is more critical and this is predominantly to achieve the best compromise amongst the various factors influenced by higher solution concentrations of the aVecting throughput (flow rates), element recoveries (pH, acid coprecipitants.composition) and blank (Fe and Pd concentrations) so as to Fig. 3 illustrates the eVect of tetrahydroborate concentration achieve rewarding limits of detection. on the recovery of added CuII. A 2% (m/v) solution was selected for use, based on the lack of further signal enhancement caused by higher concentrations of this reagent. Accounting Parametric optimization for the relative flow rates of sample and reductant (Table 2), Reductive precipitation oV-line is performed at pH 8–9 follow- this solution concentration of BH4- is 10-fold greater than the ing the addition of Fe and Pd coprecipitants.Hydrolysis of oV-line condition, again reflecting the need for higher reagent the iron occurs, creating a colloidal precipitate of large surface concentrations in a kinetically controlled environment. area capable of adsorbing or scavenging trace elements from Collection coils, consisting of 2 m lengths of either Teflon, solution. Addition of the tetrahydroborate reductant serves to microbore tygon or silicone tubing (all of the same 1.27 mm reduce these metals and the presence of Pd inhibits loss of id) were wrapped around the collection magnet and evaluated volatile hydride forming elements such as As, Se and Sb.15 for multielement recovery eYciency.Best performance was Although precipitation can also occur eYciently at pH 6.5, it obtained with the use of silicone tubing. Although silicone was does so more slowly.Since the kinetics of events in a flowing only marginally better than tygon (14% on average), it was system are of significance, a pH of 8.5 was therefore selected. rejected in favour of the latter as the blanks were significantly The pH was conveniently adjusted on-line by merging the higher with this material. The collection eYciency with Teflon sample stream with that of a 0.09 M solution of NH4OH (see tubing was only 60% of that of microbore tygon. Performance Fig. 1). OV-line pH adjustment was not feasible since precipitation reactions resulted in significant losses of material in the sample reservoir itself. With equal flow rates of both sample and base, a 151 dilution of the sample was accomplished online. This had the advantage of reducing the back pressure in the system which arose due to the concurrent precipitation of large amounts of magnesium and calcium hydroxides present when seawater was processed. Fig. 2 illustrates the eVect of the mass of PdII and FeIII added to the sample.Optimum concentrations, varied in unison, were found to be 30 mg ml-1 of each carrier. Manganese and bismuth were selected as representative elements for illustrative purposes since their characteristics are clearly diVerent. Excessive amounts of coprecipitants were avoided in an eVort to both minimize the blank and the mass of material introduced into the furnace. In comparison with the oV-line procedure,15 Fig. 3 EVect of sodium tetrahydroborate concentration on recovery of 1 ng ml-1 CuII from 1.0 ml volumes of seawater.considerably higher concentrations of carriers are required to Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12 1283Table 3 Recovery of trace elements from deionized distilled water may be related to the nature of the surface roughness or and seawater ‘stickiness’ of the material, but this is entirely unclear at the moment and beyond the scope of this study.Using microbore Recovery (%)* tygon, the minimum length of tubing comprising the collection Element DDW CASS-3 coil was determined to be 1 m; use of a 0.5 m length resulted in a 55% relative loss of recovery. A 45 cm Teflon transfer Ag 89±2 73±2 AsV 71±1 60±1 line, inserted between the final merge point and the collection Bi 79±2 58±1 coil, permitted some time for the development of the precipi- Cd 86±1 87±1 tation process (while minimizing any losses to the tube wall) CrVI 55±4 33±3 prior to collection. Chemical processing on-line may be kin- Co 89±3 85±5 etically limited if relatively slow reactions are involved.In Cu 92±4 54±3 addition, collection eYciency, whether by magnetic assistance Mn 90±5 67±8 Ni 100±5 83±4 or in a knotted reactor, will be influenced by the hydrodyn- Pb 79±6 65±3 amics of the flowing stream. In the present case, multielement SbV 74±3 57±2 recovery was degraded if sample flow rates in excess of Tl 78±4 52±2 1.8 ml min-1 were used (for a total solution flow rate of sample and ammonium hydroxide of 3.6 ml min-1).At 2.0 ml min-1 * Mean and standard deviation (n=10); 1 ml sample volumes prorecovery decreased by 20% and at 3 ml min-1 by 60%. cessed. All analytes spiked to contain 1 ng ml-1. Although the system can be operated at elevated flow rates in the interest of enhancing sample throughput, the degradation of sensitivity is significant. has been deduced in an earlier study.15 Losses of trace elements from samples of seawater may occur by occlusion in precipi- The relative collection eYciency of a 1 m length of microline tygon tubing was investigated when used in a variety of tated calcium and magnesium hydroxides,21 thereby reducing the eYciency of the system for this sample matrix.In all cases, configurations: in the form of a coil, as a coil wound around the magnet and when configured as a knotted reactor (with recoveries of analytes from seawater are lower with this manifold than that obtained oV-line using a filter for collec- no magnet present).Although knotted reactors have been widely utilized in FI manifolds for the collection of organic tion15 but, with the exception of Cr, never by more than 2- fold. This is remarkable in that samples are processed within based precipitates,2,11 that used in this study exhibited only a 60% collection eYciency relative to a coil wound around a minutes of the precipitation event in the FI manifold, whereas 15 h is typical for precipitate aging before collection in oV-line magnet (magnetic assisted collection) for the elements studied.Magnetic assisted recovery of the precipitate enhanced procedures. Additionally, the highly reproducible nature of the manipulations available with the FI manifold has resulted in eYciency 3-fold over that obtained with the coil alone. In this respect, advantage was taken of the high magnetic permeability good precision of recovery, similar to that which can be obtained oV-line.15 Although not shown in Table 3, recoveries of iron, added as a coprecipitant. Towler et al.20 utilized magnetite, added to seawater samples, to enhance the magnetic of the FeIII and PdII coprecipitants, determined by flame-AAS, were greater than 95% in all cases.recovery of several elements adsorbed onto a manganese dioxide carrier. Use of Co and Ni (in place of iron) as Table 4 summarizes the absolute and relative methodological blanks determined by processing 1.0 ml volumes of DDW coprecipitants for this purpose may also prove attractive as they also have relatively high magnetic permeabilities.through the procedure. Absolute blanks are all in the low pg regime and range from 10–1000-fold lower than their oV-line One of the diYculties experienced with the use of reductive precipitation oV-line is the subsequent acidic dissolution of the counterparts.15 This is a consequence of the closed system used and the significant reduction in reagent volumes.Relative precipitate, this process demanding use of concentrated nitric and hydrochloric acid mixtures.15 Direct transfer of such blanks (ng ml-1), are typically only 10-fold worse than their oV-line counterparts,15 despite the 900-fold smaller sample concentrated acids to the graphite furnace would result in rapid oxidation of the tube and severely shorten its lifetime. volume processed. There is thus an overall enhancement in performance with FI.The major source of the blanks proved Thus, dilute acid mixtures were required and this necessitated raising the reaction temperature to enhance the kinetics of to be contamination from the added PdII in the case of Ag, Pb and Sb, and contamination from the added FeIII for dissolution. A study of various combinations of HCl and HNO3 (in the ratios: 6 M52 M; 3 M52 M; 2 M52 M; 1 M52 M; Mn. No single dominant source of contamination could be identified for the remaining elements. 0 M52 M) revealed that a 20 ml volume of a 2 M52 M mixture of HCl5HNO3 at a temperature of 98 °C was as eVective as the Estimated absolute limits of detection (LODs) based on a 3s(blank) criterion are also summarized in Table 4. Con- more concentrated acid mixtures. A ratio of 1 M52 M produced a precipitous drop in recovery for all elements, suggesting that centration LODs (based on a 1 ml sample volume) are comparable to their oV-line counterparts,15 despite the fact that sample Cl- is a key element in the solubilization process.Operation of the system at elevated temperatures for extended periods of volumes processed are 900-fold smaller; in the worst cases, these data are degraded by factors of 20 (for Cr), 10 (for Pb), time did not visibly deteriorate the collection coil. The system was cycled several hundred times without degradation. 6 (for Ag and Ni) and 5 (for Co and Sb). Detection limits for As, Bi, Co, Mn and Sb are likely all limited by instrument noise. However, it is important that the magnet be isolated from the tygon coil by a polyethylene sheet in order to minimize Throughput is approximately 12 h-1, which is fully compatible with the operation of the graphite furnace.However, since corrosion by acidic vapours diVusing through the tygon line. the method of additions is required, actual sample throughput is somewhat less, typically 9 h-1 when a blank and two spikes Figures of Merit are processed.Linear calibration curves could be constructed for all Table 3 summarizes the average recoveries of analytes from both spiked DDW and coastal seawater (CASS-3) matrices. elements by varying the volume processed at constant concentration and also by varying the concentration at constant Typically,15 recoveries are somewhat higher from DDW and reflect the need for analysis of samples by the method of volume. Linearity was limited by the upper range of instrument response in that curvature in the absorbance–mass relationship additions.No attempt was made to investigate the influence of oxidation state of elements such as As, Sb and Cr, as this set in. In a similar manner, a linear relationship was observed 1284 Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12Table 4 Absolute and relative methodological blanks and absolute limit of detection* Blank Element Relative/ng ml-1 Absolute/pg Absolute detection limit†/pg Ag 0.012±0.001 12±1 4.2 As 0.031±0.007 32±7 21 Bi 0.029±0.004 29.0±0.5 1.5 Cd 0.012±0.001 12.0±0.9 2.8 Cr 0.071±0.007 70±7 21 Co 0.017±0.007 17±7 21 Cu 0.020±0.005 20±5 15 Mn 0.002±0.001 2±1 3 Ni 0.07±0.01 65±13 38 Pb 0.11±0.01 110±11 32 Sb 0.030±0.007 30±7 22 * Mean and standard deviation (n=10); 1 ml sample volume.† Defined as the mass of analyte which gives a response equivalent to 3 times the standard deviation of the methodological blank, based on the processing of a 1 ml sample volume.Table 5 Analysis of CASS-3 Coastal Seawater reference material achieved. Complete automation of the system (including control of the autosampler arm during acid elution into the Concentration/ng ml-1 furnace) is feasible with current FIFU software technology (Perkin-Elmer). The methodology should also prove useful for Element Determined* Certified value† the preparation of samples for analysis by other atomic spectro- As 1.06±0.02 (±0.05) 1.09±0.07 metric means, including ICP-AES and ICP-MS.For the latter, Cd 0.032±0.003 (±0.007) 0.030±0.005 avoidance of hydrochloric acid for minimization of isobaric Cr 0.087±0.013 (±0.032) 0.092±0.006 Cu 0.48±0.04 (±0.10) 0.517±0.062 interferences may be necessary. Mn 2.7±0.3 (±0.7) 2.51±0.36 Ni 0.37±0.02 (±0.05) 0.386±0.062 The authors thank Perkin-Elmer Bodenseewerk for loan of the FIAS-400 used in this study. Dr. J. Lam of this laboratory * Mean and standard deviation (n=3); 1 ml sample volume.Values is thanked for his invaluable help in the ICP-MS analysis of in parentheses are corresponding 95% confidence limits. samples as is Dr. N. Panichev for helpful and stimulating † Precision expressed as 95% confidence limits. discussion. S.S. thanks the CNPq of Brazil for a fellowship which made this study possible. between the blank response and the volume of sample processed, as expected. Although sample throughput is degraded, REFERENCES it is possible to process larger sample volumes without diY- 1 Fang, Z., Spectrochim.Acta Rev., 1991, 14, 235. culty. Experiments to determine the upper limit to the volume 2 Fang, Z., Xu, S., and Tao, G., J. Anal. At. Spectrom., 1996, 11, 1. of sample which could be processed were constrained by the 3 Fang, Z., Sperling, M., and Welz, B., J. Anal. At. Spectrom., 1990, maximum pumping time available with the current FIAS 5, 639. software. Using multiple 99 s repeats of step 1 (Table 1), 4 Sperling, M., Yin, X., and Welz, B., J.Anal. At. Spectrom., 1991, 6, 295. 11.3 ml of sample were processed and linear response was still 5 Porta, V., Abollino, O., Mentasti, E., and Sarzanini, C., J. Anal. obtained with Ag as the test element. High concentrations of At. Spectrom,, 1991, 6, 119. analyte introduced into the furnace necessitated operation of 6 Yan, X.-P., Van Mol, W., and Adams, F., Analyst, 1996, 121, 1061. the device with high internal purge gas flows during atomiz- 7 Sperling, M., Yan, X.-p., and Welz, B., Spectrochim.Acta, Part B, ation in an eVort to maintain absorbance within the linear 1996, 51, 1891. 8 Beinrohr, E., Cakrt, M., Rapta, M., and Tarapci, P., Fresenius’ Z. range. Nevertheless, the sensitivity of the graphite furnace Anal. Chem., 1989, 335, 1005. technique can be enhanced nearly 50-fold (when processing 9 Azeredo, L. C., Sturgeon, R. E., and Curtius, A. J., Spectrochim. only a 1 ml sample volume) as compared to the standard 20 ml Acta, Part B, 1993, 48, 91.injection volume. 10 Zhuang, Z.-X., Wang, X.-R., Yang, P.-Y., Yang, C.-L., and Huang, To assess the performance of this analytical system, the near- B.-L., Can. J. Appl. Spectrosc., 1994, 39, 101. 11 Fang, Z., and Dong, L., J. Anal. At. Spectrom., 1992, 7, 439. shore seawater reference material CASS-3 was analysed. Table 12 Chen, H., Xu, S., and Fang, Z., J. Anal. At. Spectrom., 1995, 10, 533. 5 summarizes the results for a number of elements. Measured 13 Mizuike, A., Enrichment T echniques for Inorganic T race Analysis, concentrations are in good agreement with certified values in Springer Verlag, Berlin, 1983. all cases. Application of a simple t-test indicates no significant 14 Akatsuka, K., and Atsuya, I., Fresenius’ Z. Anal. Chem., 1987, diVerence between the results and the certified values at the 329, 453. 15 Nakashima, S., Sturgeon, R. E., Willie, S. N., and Berman, S. S., 95% confidence level. Of the elements given in Table 4, Ag, Bi Anal. Chim. Acta, 1988, 207, 291. and Sb are not certified in CASS-3 and Pb and Co are present 16 Niskavaara, H., and Kontas, E., Anal. Chim. Acta, 1990, 231, 273. at concentrations below the method detection limit. One ml 17 Skogerboe, R. K., Hanagan, W. A., and Taylor, H. E., Anal. sample volumes were processed with resulting precision of Chem., 1985, 57, 2815. replicate determination averaging about 8% for the six 18 Santelli, R. E., Gallego, M., and Valca�rcel, M., J. Anal. At. Spectrom., 1989, 4, 547. elements that could be examined. 19 Debrah, E., Adeeyinwo, C. E., Bysouth, S. R., and Tyson, J. F., Analyst, 1990, 115, 1543. 20 Towler, P. H., Smith, J. D., and Dixon, D. R., Anal. Chim. Acta, CONCLUSION 1996, 328, 53. A FI manifold for reductive precipitation concentration of 21 Lin, C.-R., Analyst, 1993, 118, 189. samples has been successfully implemented based on the filterless magnetically assisted collection of precipitate. Despite Paper 7/02608K the substantial decrease in sample volume processed (900- Received April 16, 1997 Accepted July 22, 1997 fold), performance comparable to oV-line methodology can be Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12 12

ISSN:0267-9477    

DOI:10.1039/a702608k

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Methods for Improving the Sensitivity in Atom Trapping Flame Atomic Absorption Spectrometry: Analytical Scheme for the Direct Determination of Trace Elements in Beer HENRYK MATUSIEWICZ* AND MARIUSZ KOPRAS Politechnika Poznan�ska, Department of Analytical Chemistry, 60–965 Poznan� , Poland. E-mail: Henryk.Matusiewicz@fct.put.poznan.pl A method is described for the atomic absorption (AA) cally result in a further improvement in both sensitivity and determination of Ag, Cd, Cu, Fe, In, Mn, Pb, Tl and Zn in detection limit.Surprisingly, the possibility of combining these beer using an ‘integrated atom trap’ system mounted on a techniques has only recently been independently investigated.8,9 standard AA air–acetylene flame burner. A new design of The investigation by Turner and Roberts8 into the use of a atom trapping technique that would exceed the operational single silica tube atom trap at high tube obscuration resulted capabilities of existing arrangements (a water-cooled single or in the introduction of a new type of hybrid atom trap, the dual silica tube or a double-slotted quartz tube) and permit slotted tube water-cooled atom trap (STWCAT), which conconstruction of an ‘integrated trap’ was investigated.A sists of a commercially available slotted tube atom trap (STAT) significant improvement in detection limit was achieved and a single silica tube water-cooled atom trap (WCAT), compared with that obtained using either of the above atom fabricated in-house. It was found that the STWCAT performed trapping techniques separately.Rapid, accurate analyses can very well in comparison with the other two tubes, showing be achieved using continuous aspiration. The concentration very high sensitivity with excellent precision. The analytical detection limits were 3.0, 5.0, 1.2, 4.0 and 0.1 ng ml-1 for Cu, sensitivities were discussed for Cd and Pb in river water and Fe, Mn, Pb and Zn, respectively, using a 2 min in situ sewage eZuent samples.In a previous paper,9 we described preconcentration time. The relative standard deviations are of the use of a laboratory-made quartz ‘integrated atom trap’ the order of 3.0–6.0% for this technique. Basic analytical (IAT) structure to enhance the sensitivity in flame atomic performance characteristics are also given for Ag, Cd, In and absorption spectrometry (FAAS). Tl using various designs of atom trap. The designs studied ‘Conventional’ atom trapping AAS involves the collection include both slotted tube and single silica tube water-cooled and in situ preconcentration of the analyte in a flame system atom traps.containing a silica tube cooled by water circulation followed by an atomization period when gas (air, argon, nitrogen, etc.) Keywords: T race element determination; integrated atom trap is forced into the silica tube to remove the water and thus system; flame atomic absorption spectrometry; sensitivity cause heating of its surface.In this study, an alternative means increase; beer of atomization for some elements is suggested, involving a change in flame composition by altering the gas5acetylene The most common form of atom trapping technique employs ratio prior to re-atomization of the analyte, so that the hot a tube with an aperture, often a slot to admit the vaporized and reducing tip of the blue zone touches the silica surface, sample (analyte), mounted above the burner of a flame atomic eVecting atomization of the analyte. This is termed the ‘flame absorption spectrometer and on its optical axis.The sample is alteration technique’. nebulized into the flame. There are two modes of operation of The importance of several metals (As, Cd, Cu, Fe, Pb, Zn) atom trapping systems where the analyte is concentrated ‘onin beer is now well recognized. Many governmental organiza- line’. In the most commonly used mode, the vaporized sample tions have established maximum concentration limits for many flows continuously through an atom trapping slotted quartz trace elements. Several atomic spectrometric techniques have tube.1 The gain in sensitivity over a conventional long-path been studied and recommended for the determination of trace burner is of the order of 3- to 5-fold for easily atomized elements in beer samples, including FAAS,10–13 electrothermal elements.This improvement is attributable to the increased atomic absorption spectrometry (ETAAS)12–18 and inductively residence time of the atoms in the optical path.The alternative coupled plasma atomic emission spectrometry (ICP-AES).19 approach depends on first collecting the analyte atoms on the Trace amounts of Cd, Cu, Pb and Zn have been determined exterior wall of a water-cooled single or dual silica tube by in beer by AAS with the use of slotted quartz tubes.20 The condensation over a period of time; this is then followed by same technique has been used to determine Pb in beer.21 The their rapid release when the tube temperature is raised.2 In the main advantage of ICP-AES is its multi-element capability. latter mode, the gain in sensitivity can be of the order of ETAAS has the advantage of lower detection limits for many 10- to 50-fold.elements in comparison with the other techniques mentioned, This technique has attracted considerable interest over the and thus preconcentration steps may not be required.However, years and the enhancement eVect using two main types of ICP-AES and ETAAS equipment is more costly to purchase atom trapping was subsequently confirmed by several workand to maintain for many laboratories than FAAS systems. A ers.3,4 More recently, Turner et al.5 optimized parameters for convenient and inexpensive method of preconcentration of determining Pb and Cd by a dual ‘bent’ water-cooled silica trace elements with minimal analyst intervention would enable tube.This was further developed by Ellis and Roberts6,7 for trace elements in beer liquids to be measured fairly rapidly by As, Sb, Cu, Mn and Hg. means of FAAS. A combination of the advantages of the water-cooled silica tube with those of a double-slotted quartz tube should theoreti- This paper reports on the results from a number of experi- Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12 (1287–1291) 1287ments using the IAT system with the flame alteration technique for solutions containing several metals.For comparison, the same solutions were also analyzed without the IAT system, but with various designs of atom trap. The purpose of this work was to develop a practical and sensitive method for the direct determination of trace amounts of Ag, Cd, Cu, Fe, In, Mn, Pb, Tl and Zn in beer by atom trapping FAAS using an IAT. The sensitivities obtained with the use of this system coupled with FAAS were similar to those obtained by ETAAS.EXPERIMENTAL Apparatus A Carl Zeiss Jena (Jena, Germany) Model AAS 3 flame atomic absorption spectrometer was used with an IBM-PC compatible computer. The sampling rate for the PMT signal was 10 Hz. Signals were processed with in-house software (Turbo Pascal Version 7.0) to extract the transient peak heights, areas and Fig. 1 Integrated atom trap (IAT) structure attachment on the burner peak times. The operating conditions for the analysis are listed head: 1, slotted tube atom trap (STAT); 2, single silica tube; 3, water– in Table 1.Hollow cathode lamp currents and slit-widths argon control system; 4, burner; 5, modified holder (holder arm); 6, were as recommended by the manufacturer. Deuterium lamp handle; 7, clamping screw; 8, mounting peg; and 9, burner clamp background correction was used throughout. (bracket). Three designs of atom trap were investigated. A watercooled single or dual silica tube atom trap was arranged as previously described3 and mounted on a 10 cm burner in such dimensions (8 cm×2 mm wide).The length of the quartz tube is limited by the atomizer compartment. To prevent the AAS a manner as to permit the system to be vertically and laterally adjusted in the flame. The tubes had an od of 3 mm and an id quartz optics from contacting hot gases escaping from the quartz tube, the quartz tubTAT) has two slots on both of 1 mm for water cooling. The distance between the two tubes was 1 mm.A four-way rotary glass valve allowed water or sides so that the hot gases are vented vertically instead of horizontally towards the quartz windows. With both clamps argon (or nitrogen) to pass through the silica tubes. A double-slotted quartz tube (modified metal holder–clamp- in position, the observation (analytical) section of the single or dual silica tube can be held inside the slotted quartz tube. ing system Model ACT-80 atom concentrator tube, Varian Associates, Palo Alto, CA, USA)3 was installed on a standard The height of the silica tube(s) could be raised or lowered inside the STAT.The IAT system was mounted over an air– 10 cm air–acetylene burner. The design permits changeover from analysis with the modified ACT-80 to that for a acetylene burner on a mounting bracket, which permitted calibrated movement both vertically and horizontally. conventional flame in a few seconds. An IAT was designed and constructed in this laboratory; it The automatic flame ignition system was disabled for this study. The fixed position of the modified ACT-80, single consisted of a combination of a single or dual silica tube and a double-slotted quartz tube atom trap (STAT) (Fig. 1). A or dual silica tube trap and IAT interfered with the autoignition probe. 10 cm pathlength burner head was slightly modified and fitted with two rigid metal clamps. One clamp was capable of holding A cooling thermostat was used for cooling the water, its internal pump circulating water via PVC tubes connected to the modified holder of the ACT-80 within the flame used for the STAT, the other was designed to hold a single or dual the single or dual silica tube atom traps.The continuously flowing cooling water kept the surface of the silica tube(s) at silica tube on to the burner head. The ACT-80 had two lengthwise cuts on opposite sides of the tube angled at 120° below 100 °C, which allowed the analyte atoms to condense on the surface of the tube.to each other, relative to the axis of the tube. The longitudinal slot under the tube (flame entrance slot) was enlarged The single or dual silica and slotted quartz tubes were coated with lanthanum to prevent devitrification and to (12.5 cm×8 mm wide) and the upper slot had the original Table 1 Standard operating conditions for the integrated atom trap (IAT) system Parameter Ag Cd Cu Fe In Mn Pb Tl Zn Wavelength/nm 328.1 228.8 324.8 248.3 304.0 279.5 217.0 278.8 213.9 Spectral bandpass/nm 0.3 0.3 0.3 0.2 0.3 0.2 0.3 0.2 0.2 Background correction Yes Yes Yes Yes Yes Yes Yes Yes Yes Lamp current/mA 4 4 4 5 5 5 5 5 5 Flame type* Air–C2H2 Air–C2H2 Air–C2H2 Air–C2H2 Air–C2H2 Air–C2H2 Air–C2H2 Air–C2H2 Air–C2H2 Flame conditions† Preconcentration Stoichiometric Lean Lean Stoichiometric Stoichiometric Stoichiometric Lean Lean Lean in situ Atomization Stoichiometric Rich Rich Stoichiometric Stoichiometric Stoichiometric Rich Rich Rich Water-cooled silica tube atom trap position Above the burner/mm 5 5 5 5 5 5 5 5 5 Tube obscuration (%) 30 30 30 30 30 30 30 30 30 STAT tube–burner 5 5 5 5 5 5 5 5 5 gap/mm * Nebulizer uptake rate for all elements, 5 ml min-1.† Air flow rate 475 l h-1, acetylene flow rate 50 l h-1 (fuel-lean flame); air flow rate 475 l h-1, acetylene flow rate 80 l h-1 (fuel-rich flame); air flow rate 475 l h-1, acetylene flow rate 70 l h-1 (stoichiometric flame); 10 cm slot burner. 1288 Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12improve the silica surface properties (increasing the surface Determination of Ag, Cd, Cu, Fe, In, Mn, Pb, Tl and Zn Levels Present in Beer Samples area) by continuous aspiration via the burner nebulizer of 0.5 and 1% m/v lanthanum solutions, respectively, for 15 min. All samples were analyzed for levels of Ag, Cd, Cu, Fe, In, The tubes were then conditioned by allowing them to heat up Mn, Pb, Tl and Zn using the STAT, the single and dual silica for 15 s.The tubes were re-coated after approximately 50 runs. tubes discussed in earlier work3 and the IAT apparatus. The results obtained from the three diVerent atom traps were then compared and contrasted. The accuracy of the method was Reagents checked by calculating the percentage recovery of elements added to the beer sample solution. Standard solutions of the elements were prepared from Titrisol solutions (Merck, Darmstadt, Germany), containing 2 g l-1 of the element.Serial dilutions were made with high-purity dis- RESULTS AND DISCUSSION tilled water from a Heraeus Bi18 (Heraeus, Hanau, Germany) system in order to prepare working solutions. The lanthanum The results of this study relate to two distinct aspects of the chloride solution used to coat the quartz tubes (0.5 and 1% overall investigation: atom trapping techniques to increase the m/v) was prepared from a 10% m/v lanthanum chloride sensitivity of FAAS, and its application to practical analysis.solution supplied by Alfa Inorganics (Ward Hill, MA, USA) Although these are related, there are developmental details for as a releasing agent for use in atomic absorption. each that are independent. Hence, it is convenient to discuss the development of the atom trap and analysis technique separately. Sample Preparation Samples of commercial beers were purchased at local stores Evaluation of Atom Trap Designs and were de-gassed by filtration (Whatman No.4 filter-paper) The initial study consisted of varying some parameters to find and analyzed undiluted.The samples were treated with the optimum sensitivity for each of the three designs of atom HNO3 and H2O2 to avoid clogging the burner by direct trap. However, substantial optimization of the preconcen- aspiration of the samples into the flame. Beer solutions were tration parameters was not undertaken, as this information subjected to UV irradiation for 4 h in the presence of HNO3 was readily available from a review paper4 (and references and H2O2, which were added to the beer solutions prior to cited therein) pertaining to trace metal detection by atom UV digestion.22 A UV digester, Model R-6 (Mineral, Warsaw, trapping and in situ preconcentration for FAAS. Poland), was used for all digestions.The parameters used for the optimization of the IAT for the Four diVerent commercial brands of beer were investigated. determination of trace elements are shown in Table 1.The height of the silica tube above the burner, the percentage obscuration of the light beam and fuel flow rate were found Principle of the Atom Trapping Procedure to have significant eVects on the sensitivities of the elements studied. One general conclusion was that, while the conditions In order to collect (trap) the analyte, each sample solution was conventionally aspirated via the nebulizer/burner directly into for diVerent elements were optimized at diVerent heights of the silica tube above the primary reaction zone within the flame, the flame for a pre-determined collection time (1–5 min) in which an IAT system had been installed above the burner slot.in all cases the highest signals were obtained when the light path was close to the tube surface. Increased sensitivity was The lower edge of the silica tubes (both the water-cooled and STAT) was fixed 5 mm above the burner top. The height of also achieved by lowering the tube closer to the burner slot.In this work, 5 mm was the lowest height that could be used, the burner was adjusted so that the tubes obscured 10–50% of the incident light beam. Sample solutions were aspirated given the present tube clamping device on the burner for the single silica tube and STAT. In addition, despite the higher into the flame using the flame alteration technique. During the trapping process, a fuel-lean flame was used; flow rates of air enhancement factors for the smaller gaps, distances closer than 5 mm cannot be recommended, at least when the burner system (475 l h-1) and acetylene (50 l h-1) were adjusted.During this collection period, a continuous flow of cooling water permitted used in this study was allowed to heat the tube continuously, owing to the danger of overheating the burner head. In analyte atoms to condense (adsorb) on the surface of the tubes during solution aspiration. After this collection period, argon agreement with Turner and Roberts,8 we also observed that the sensitivity of all the elements studied using the single silica was passed through the tubes to remove the cooling water rapidly and a pulsed fuel-rich flame was immediately used for tube was similar to that obtained using the dual silica tube system. No analytically useful results were obtained with the releasing; flow rates of acetylene (80 l h-1) were adjusted (the hot and reducing tip of the blue zone touches the silica surface dual tube design and, consequently, it was not investigated further.The single silica tube was therefore chosen for further eVecting volatilization/atomization of the analyte), so that no soot deposited on the cooled silica tubes during the atomization study. It was demonstrated that although the relationship is not, in general, particularly linear, a longer aspiration time period (a rich flame at flow rates of acetylene above 80 l h-1 caused soot formation, wherein the signal-to-noise ratio increased the analytical signal.A reasonable aspiration time per sample in a routine laboratory would be about 2 min. This became unacceptably high). The tubes rapidly heated in the rich flame, generating a transient atomic absorption signal as time was chosen to investigate the analytical performance of the IAT system with respect to linearity, sensitivity, precision the analyte atoms were released (volatilized) from the surface.Peak height absorbance was measured during a 15 s read and detection limit. No diVerent tube coating materials were examined, but the lanthanum coating was applied primarily period. Peak area measurements gave poor precision, which could be attributed to the relatively large width of the signals as a result of practical laboratory experience. A comparison of detection limits is given in Table 2 for (2–8 s). Deuterium lamp background correction was applied to eliminate non-atomic absorption and scattering.conventional FAAS, ETAAS and with three atom trap designs. For the elements studied, the IAT technique improved detection Under the same conditions, a series of standard solutions were aspirated and analyzed. The concentrations of metals in limits considerably when a 2 min collection time was used. Detection limits obtained with a 2 min collection time the samples were obtained by the regression equation of the calibration graphs. are low enough to suggest that this procedure provides a Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12 1289Table 2 Comparison of detection limits (DL)* for elements obtained using various atom traps (ng ml-1) Conventional FAAS STAT Silica tube atom trap† IAT‡ ETAAS§ Precision,|| Precision, Precision, Precision, Element DL X¶ % RSD DL X % RSD DL X % RSD DL X % RSD DL X Ag 30 1 5.8 10 3 5.0 2.0 15 6.3 0.6 50 5.5 0.2 150 Cd 15 1 6.1 5 3 6.2 1.0 15 6.3 0.5 30 5.1 0.08 187 Cu 40 1 4.3 16 2.5 3.3 7.0 5.7 4.5 3 13.5 3.9 0.6 67 Fe 55 1 4.9 22 2.5 4.1 11 5 5.0 5 11 4.7 0.4 137 In 350 1 7.3 110 3.1 6.5 50 7 7.8 10 35 5.9 6 58 Mn 20 1 3.2 10 2 3.0 2.5 8 3.9 1.2 16.5 3.0 0.3 67 Pb 150 1 6.3 55 2.7 5.9 15 10 6.6 4.0 37.5 6.0 1 150 Tl 300 1 8.1 150 2 7.1 20 15 8.5 10 30 6.1 5 60 Zn 12 1 5.4 3.0 4 4.9 0.5 25 6.1 0.1 120 5.0 0.05 240 * Detection limit defined by 3 blank criterion (n=9).† 2 min collection time for water-cooled single silica tube atom trap.‡ 2 min collection time for IAT technique. § 10 ml. ¶ Enhancement (improvement) factor. || RSD of nine replicate aspirations at analyte concentrations 20-fold above calculated detection limit. viable alternative to the use of ETAAS methodology for the Analysis of Beer Samples determination of these (volatile) trace metals. To illustrate the use of the IAT system in practical application and to assess the quality of the results obtained with the Flame Alteration Technique developed methodology, real beer samples were analyzed.Samples of beer of diVerent brands from Poland purchased in The eVect of flame conditions on the trapping and release of glass bottles in the retail market were analyzed in duplicate. the elements was studied by varying the fuel flow rate. In these The accuracy of this method of analysis can only be assessed experiments, the richness of the flame was restricted to a level by an examination of the recovery data, as no certified reference such that no carbon deposited on the tube.In the lean flame, materials were available. For this purpose, calibration was the fuel supply was reduced until the primary reaction zone achieved using the method of standard additions. The results became small and was bright blue, as opposed to the situation for the beer samples analyzed using the IAT are given in where the blue zone touches the silica surface in the fuel-rich Table 3. Comparison of standard additions with the slope of experiments.The same air flow rate (475 l h-1) was maintained the calibration graphs showed that there were no significant in both series of experiments. The sensitivities of atom-trapping interferences from the sample matrices, and element spikes AAS for Cd, Cu, Pb, Tl and Zn appeared to be dependent on added before the UV digestion procedure were quantitatively the flame conditions; therefore, a marked increase in signals recovered. Therefore, analyses can be performed using a direct was obtained when the flame was altered.The best sensitivity calibration graph. Although no interference study was under- was obtained when a lean flame (acetylene flow rate of taken, to eliminate possible matrix eVects, only the method of 50 l h-1) was used for trapping (collecting) these elements and standard additions was used to obtain accurate results. The a fuel-rich flame (acetylene flow rate of 80 l h-1) was used for precision of replicate determinations is typically better than releasing (atomizing).In this novel case the signal is sharp 10% RSD (n=2). and rapid. The detection limits obtained with a 2 min collection time The sensitivity of the atom-trapping AAS technique for Ag, are low enough to suggest that the procedure for the determi- Fe, In and Mn appeared to be virtually independent of the nation of trace elements in beer using the IAT is a viable flame conditions. The use of a fuel-rich flame when analyte alternative to the standard ETAAS method for this determi- species are being released is apparently not very important.nation. The usual advantage of the high sample throughput of The results for collection in a lean flame and release in a rich the conventional FAAS technique cannot be claimed for the flame were not very diVerent from those observed when both IAT procedure; the time per sample is similar to that of the trapping and release were carried out in a stoichiometric flame (acetylene flow rate of 70 l h-1).ETAAS procedure. In a 3 month period of intensive work, not Table 3 Analysis of commercial beer samples* Concentration†/ng ml-1 Description Ag Cd Cu Fe In Mn Pb Tl Zn Pils Premium ND‡ ND 80±3 16±1 ND 164±6 95±6 ND 70±4 Premium ND ND 90±4 45±2 ND 110±4 101±6 ND 92±5 10,5 ND ND 105±5 30±2 ND 100±4 105±6 ND 105±6 BCC ND ND 70±3 15±1 ND 135±5 65±5 ND 65±4 * Polish beer: LECH Browary Wielkopolski S.A. † Mean±standard deviation (n=2).‡ ND=Not detected. 1290 Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12Table 4 Polish legislation, literature values and results of this work Financial support by the Poznan� University of Technology is for the levels of elements studied gratefully acknowledged (Grant No. BW31–524/97). Polish legislation*/ Literature This work/ Element ng ml-1 values/ng ml-1 ng ml-1 REFERENCES Cd 4 ND† 1 Watling, R. J., Water SA, 1977, 3, 218. Cu <100 11–130 70–105 2 Lau, C., Held, A., and Stephens, R., Can.JSpectrosc., 1976, Fe <5000 70–700 15–45 21, 100. Mn 100–164 3 Matusiewicz, H., Sturgeon, R., Luong, V., and MoVatt, K., Pb <200 9–68 65–105 Fresenius’ J. Anal. Chem., 1991, 340, 35, and references cited Zn <1000 7–70 65–105 therein. 4 Matusiewicz, H., Spectrochim. Acta Rev., 1997, 52B, in the press. * Polish Norm, Beer, No. PN-89 A-79098. 5 Turner, A. D., Roberts, D. J., and Le Cor, Y., J. Anal. At. † ND=Not detected.Spectrom., 1995, 10, 721. 6 Ellis, L. A., and Roberts, D. J., J. Anal. At. Spectrom., 1996, 11, 259. 7 Ellis, L. A., and Roberts, D. J., J. Anal. At. Spectrom., 1996, one tube failed, so the inference is that the apparatus is 11, 1063. 8 Turner, A. D., and Roberts, D. J., J. Anal. At. Spectrom., 1996, reasonably robust. 11, 231. Table 4 compares the results obtained in this work with 9 Matusiewicz, H., and Kopras, M., paper presented at XXIX literature values. The Cu, Fe, Pb and Zn levels in the Polish Colloquium Spectroscopicum Internationale, Leipzig, Germany, beers are comparable to other published values.All of the 1995, p. 297 (Tu-B 239). samples are well below the maximum levels allowed by Polish 10 Borriello, R., and Sciaudone, G., At. Spectrosc., 1980, 1, 131. legislation. 11 Hergenreder, R. L., At. Spectrosc., 1991, 12, 74. 12 Skurikhin, I. M., J. Assoc. OV. Anal. Chem. Int., 1993, 76, 257. 13 Li, Y., Van Loon, J. C., and Barefoot, R.R., Fresenius’ J. Anal. CONCLUSIONS Chem., 1993, 345, 467. 14 Wagner, H. P., Dalglish, K., and McGarrity, M. J., J. Am. Soc. The IAT system has been evaluated for analytical applications Brew. Chem., 1991, 49, 28. to real beer samples. This approach is considerably more 15 Srikanth, R., Ramana, D., and Rao, V., Bull. Environ. Contam. sensitive than conventional FAAS for the determination of T oxicol., 1995, 54, 783. 16 Cervera, M. L., Navarro, A., Montoro, R., and Catala, R., trace elements and provides a viable alternative to the conven- J. Assoc. OV. Anal. Chem., 1989, 72, 282. tional ETAAS technique. Therefore, this method can be 17 Cervera, M. L., Navarro, A., Montoro, R., de la Guardia, M., recommended and widely used when ETAAS is not available. and Salvador, A., J. Anal. At. Spectrom., 1991, 6, 477. The types of sample most suited to analysis by this procedure 18 Wagner, H., J. Am. Soc. Brew. Chem., 1995, 53, 141. are relatively simple aqueous matrices and those where a 19 Matsushige, I., and de Oliveira, E., Food Chem., 1993, 47, 205. reasonable volume (5–10 ml ) of solution is available. The 20 Li, H., Lian, J., and Du, J., Fenxi Ceshi T ongbao, 1991, 10, 51. 21 Lee, M., and Brown, A., Brewers’ Guardian, 1985, 13. method is restricted to elements atomized in an air–acetylene 22 Wahdat, F., and Neeb, R., Fresenius’ Z. Anal. Chem., 1989, flame and, in particular, to the more volatile elements such as 335, 748. Cd, Pb or Zn. The major advantages of this procedure are its relative simplicity, low cost and that the IAT system is also Paper 7/04407K readily decoupled. The main disadvantages of the IAT system Received June 23, 1997 and, in general, the atom trapping FAAS technique, are Accepted August 5, 1997 that they are limited to relatively few elements and there is significant consumption of sample during the trapping step. Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12 1291

ISSN:0267-9477    

DOI:10.1039/a704407k

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Effects of Conditions for Pyrolysis of Ascorbic Acid as a Chemical Modifier on the Vaporization Mechanism of Gold in Electrothermal Atomic Absorption Spectrometry ETSURO IWAMOTO*a , MIHO ITAMOTOa , KAZUE NISHIOKAa , SHOJI IMAIb , YASUHISA HAYASHIc AND TAKAHIRO KUMAMARUd aDepartment of Health Science, Hiroshima Women’s University, Hiroshima 734, Japan bDepartment of Chemistry, Faculty of Integrated Arts and Science, T he University of T okushima, T okushima 770, Japan cDepartment of Chemistry, Joetsu University of Education, Joetsu 943, Japan dDepartment of Chemistry, Faculty of Science, Hiroshima University, Higashi-hiroshima 739, Japan The eVect of the ramp and hold times and the temperature of 580 K; (ii ) active carbon species between 600 and 1100 K; and (iii ) thermally stable carbon species between 1200 and 2400 K.the pyrolysis step of ascorbic acid as a chemical modifier on AA signals for gold have been investigated. Three methods of Raman spectrometry17 also indicated that a disordered structure was observed for the wall surface when treated with pyrolysis for ascorbic acid were used: (1) pyrolysis of ascorbic acid before deposition of a gold solution on the platform ascorbic acid at high temperatures (1200–2150 K).Mechanistic studies of the adsorption and desorption of surface; (2) pyrolysis of ascorbic acid after ashing a gold solution; and (3) charring a solution containing both ascorbic gold on graphite by McNally and Holcombe18 and Fonseca et al.19 showed that evaporation of gold could occur from acid and gold.Although pyrolysis methods (2) and (3) gave a delayed single absorption peak for gold compared with that in microdroplets of various sizes or adsorbed atoms, depending on the analytical conditions, and that the tendency of the the absence of ascorbic acid, in pyrolysis method (1) a double peak appeared even at a high pyrolysis temperature of microdroplets to disperse on the graphite was lowered by mechanical roughing of the furnace surface, which may aVect 1500 °C.However, as the ramp time increased from 5 to 80 s from a drying temperature of 120 up to 1500 °C, the first peak the number of active sites on the graphite. A double peak or a peak with a shoulder for the atomization of gold has often increased and concomitantly the second peak decreased with an isosbestic point; the integrated absorbance remained been observed when the matrix composition of a sample was complex, containing organic compounds8,10 and heavy metals,9 constant. The concentration dependence of the gold signals indicates that a fractional-order of release is shown for the and when old tubes were used.11 Although the higher-temperature shift of the gold signal in first peak and a first-order process is obtained for the second, indicative of gold atoms adsorbed onto the active carbon ETAAS has been related to the active carbon or carbon residue, characterization of the ascorbic acid residue and the surface.From inspection of a scanning electron micrograph and Raman spectra of the pyrolysed ascorbic acid, it was clear mechanism of its interactions with gold atoms are still unclear with respect to the AA signal for gold. The optimal use of that a carbon film formed on the platform surface. It was concluded that use of a short ramp and hold time, even at ascorbic acid as a chemical modifier requires knowledge of the atomization mechanism of gold with ascorbic acid. 1500 °C, for the pyrolysis of ascorbic acid leads to the formation of active amorphous carbon enriched in micro-sized In the present work, an extensive examination of the eVects of temperature and ramp time for the pyrolysis of ascorbic pores (r<25 nm), where adsorption of gold atoms that give rise to the second absorption signal occurs. Furthermore, the acid on the AA signal for gold in ETAAS was carried out. Furthermore, the extent of formation of amorphous carbon, micro-sized pores are almost destroyed by treatment at temperatures higher than 1800 °C, resulting in graphitization which was determined by the pyrolysis conditions for the ascorbic acid, is shown to be related to the double peak for gold. of the carbon residue.Keywords: Ascorbic acid; chemical modifier; electrothermal EXPERIMENTAL atomic absorption spectrometry; gold; amorphous carbon; carbon film; Raman spectrometry Apparatus A Perkin-Elmer Model 4100ZL AA spectrometer equipped Ascorbic acid has been used as a chemical modifier for some with a longitudinal Zeeman-eVect background corrector system elements such as lead,1–6 tin6,7 and gold8–11 in ETAAS.An and an AS-71 autosampler was used. The graphite furnace increase in the pyrolysis temperature and enhancement in the used was a transversely heated graphite tube with an integral sensitivity for gold in the presence of ascorbic acid6,8–10 and L’vov platform. A Perkin-Elmer hollow-cathode lamp for gold proteins12 were ascribed to the formation of a carbon residue was used as the light source.as a result of thermal decomposition during the pyrolysis step. Instrumental operating conditions and a typical furnace The eVectiveness of ascorbic acid has also been discussed from programme are given in Table 1. the viewpoints of formation of active carbon species and reductive gases.13–16 Reagents Imai et al.17 reported that the decomposition of pyrolysed ascorbic acid, when investigated by ETV-ICP-MS, indicated An aliquot of a commercially available stock solution (1000 mg l-1 HAuCl4 in 1 mol dm-3 HCl; Hayashi Pure three signals in the atomization cycle which correspond to: (i ) gaseous compounds (hydrocarbons, CO and CO2) below Chemical Industries) was diluted with deionized water to a Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12 (1293–1296) 1293Table 1 Instrumental operating conditions and temperature programmes for pyrolysis of ascorbic acid and atomization of gold Spectrometer— Wavelength 242.8 nm Bandpass 0.7 nm Lamp current 15 mA Signal type Zeeman AA Purge gas Ar THGA— Ar flow Ramp Hold rate/ml Step Temp./°C time/s time/s min-1 Read 1 80 1 20 250 — 2 120 5 30 250 — Fig. 1 Absorbance–time profiles for Au, varying the ramp time for 3 Various Various 5 250 — pyrolysis of ascorbic acid at 1500 °C according to the heating pro- 4 80 1 20 250 — gramme in Table 1; Au, 1.0 ng.Broken line, with ramp time 80 s in 5 120 5 30 250 — the absence of ascorbic acid; solid lines, in the presence of ascorbic 6 500 10 20 250 — acid. Ramp time: a, 5; b, 10; c, 20; d, 25; e, 40; f, 50; and g, 80 s. 7 2300 1 5 0 On 8 2500 3 1 250 — Thus, the second peak is a characteristic of gold with ascorbic Sequence of acid as a chemical modifier. It should be noted in Fig. 1 that actions— the first peak increases with increasing the ramp time and the 1 Inject 20 ml of 1%m/v ascorbic acid solution second peak concomitantly decreases with an isosbestic point, 2 Run THGA steps 1–3 with the peak area remaining constant.The eVect of pyrolysis 3 Inject 20 ml of 50 mg l-1 gold solution temperature for a long ramp time of 80 s with two diVerent 4 Run THGA steps 4 to end hold times, 5 and 60 s, on the first and second peak heights is indicated in Fig. 2. The second peak is stable below a pyrolysis temperature of 1000 °C, even for the longer hold time of 60 s.suitable concentration for use as a working standard solution. The above results indicate that the longer ramp and hold times Ascorbic acid was of analytical-reagent grade (Katayama for a pyrolysis temperature above 1200 °C suppress the appear- Chemical Industries). ance of the second peak, although rapid pyrolysis of ascorbic acid gives the second peak even at 1500 °C. It was found that Procedure no second peak was obtained at a pyrolysis temperature above 1800 °C. Three pyrolysis methods for ascorbic acid were used.(1) An aliquot of 20 ml of ascorbic acid (1% m/v) was deposited in the graphite furnace by the autosampler. It was pyrolysed Carbon Residue and Active Sites according to a heating programme consisting of the first three When ascorbic acid is heated at 400 °C or above on the steps of the THGA programme shown in Table 1. The temperagraphite surface, most of the carbon and hydrogen are dissi- ture and ramp time for step 3 was varied, e.g., 1500 °C with a pated as CH and CO by thermal decomposition of the ascorbic ramp time of 5 s.After cooling, 20 ml of the gold standard acid.8,17 Some residues of pyrolysed carbon remain on the tube solution (50 mg l-1) were injected by the autosampler followed surface.17 In Fig. 3 is shown a micrograph produced by SEM by atomization. In atomization step 7, a 1 s ramp time was of the surface of the platform on which ascorbic acid had been used because it gave a double peak with a deeper bottom pyrolysed at a temperature of 1500 °C with a 5 s ramp time between the two peaks than did the maximum power heating and a 5 s hold time, conditions which mainly give the second mode with a 0 s ramp.(2) The gold solution (20 ml ) was peak (Fig. 1). It can be seen that a carbon film is formed. The injected and ashed according to a heating programme conplatform with the carbon film was again installed in the sisting of 500 °C (ramp time 10 s, hold time 20 s) for step 3 in graphite tube and heated at 2300 °C for 3 s.Then the gold Table 1. After cooling, the ascorbic acid solution (20 ml ) was pyrolysed according to a heating programme at, for example, 1500 °C (ramp time 5 s, hold time 5 s) for step 6 in Table 1 followed by atomization step 7. (3) An aliquot of 20 ml of the solution containing gold and ascorbic acid was deposited in the furnace, ashed at 1500 °C (ramp time 5 s, hold time 5 s) and atomized according to the heating programme, skipping over steps 4–6 in Table 1.RESULTS AND DISCUSSION Pyrolysis Conditions The eVects of ramp time and temperature for the pyrolysis of ascorbic acid on the absorption signals for 1.0 ng of gold were investigated using pyrolysis method (1). Typical results at a constant temperature of 1500 °C and varying the ramp time between 5 and 80 s for step 3 are shown in Fig. 1. Double peaks appeared in the presence of ascorbic acid but not in the Fig. 2 EVects of temperature with a constant ramp time of 80 s for absence of ascorbic acid (broken line in Fig. 1). Hereafter the the pyrolysis of ascorbic acid on the first and the second peak heights; peak which appears initially will be referred to as the first peak Au, 1.0 ng. Hold time (60 s): %, first signal; #, second signal. Hold time (5 s): &, first signal; $, second signal. and the one which appears later is described as the second. 1294 Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12into the transport pore (macropore, radius >25 nm) and the adsorbing pore (mesopore 1–25 nm, micropore 0.4–1 nm and submicropore <0.4 nm).21 A thermal dependence of the microscopic surface structure of carbon prepared from the pyrolysis of cyclodextrin and amylose was examined by Hosokawa and Yamaguchi.22 They reported that the pore size radius in cyclodextrin carbon treated at 1473K is distributed in the range 0.5–4.5 nm with a peak distribution at 0.7–1 nm, for which the pore volume is more than 1×10-3 cm3 g-1.However, when treated at 1873 K the distribution of pore sizes from 0.7 to 1 nm falls to a level of 0.2×10-3 cm3 g-1 in the case of the size for radii of 2.5–4.5 nm. This shows that higher temperature treatment results in a marked decrease in the number of micropores. This is in good agreement with a change in the microstructure of the carbon film formed with ascorbic acid in the platform furnace by heating at 2300 °C.Fig. 3 SEM micrograph of the carbon film obtained by pyrolysis of ascorbic acid (5% m/v, 20 ml ) at 1500 °C (ramp time 5 s and hold time Concentration Dependence of Absorption Profiles 5 s) on the platform. According to McNally and Holcombe18 and Fonseca et al.19 the order of atom release can be predicted from the concensolution (20 ml ) was deposited on it followed by atomization tration dependence of absorbance–time profile characteristics. according to the heating programme of steps 4–8 in Table 1.In the first-order process, analyte desorption takes place from It was found that no second peak was given but SEM a monolayer of analyte atoms which are physically sorbed or apparently gave an unchanged image of the carbon film, chemisorbed onto the wall surface, and in the fractional-order showing that it is thermally stable. process the analyte atoms are released from microdroplets. Information from Raman spectra is very useful for the They showed that for a zero- or fractional-order of release the discrimination of amorphous and graphite carbon.Raman peak maximum shifts to a later time with an increase in the spectra in the macro-mode (100 mm diameter laser beam) are amount of gold, while in a first-order signal process the peak shown in Fig. 4 for three types of platform surfaces which were maximum appears at the same time, independent of the amount prepared in the same way as for SEM, using the 5% m/v of gold deposited. Gold atoms in the gas phase are known to ascorbic acid solution (20 ml ).As shown in Fig. 4a for a new be produced by evaporation of the pure metal or microdropyrolytic graphite (PG) surface of the platform, two Raman plets.18,19 The microdroplets interact with the active sites on bands, a sharp band (E2g mode) near to 1580 cm-1 and the graphite surface and their mobility is reduced, thereby a broad band (disorder mode)20 with a weak intensity of preserving the larger sized droplets.19 The presence of a chemi- 1350 cm-1 were observed, showing that its graphite structure cal modifier can alter the release order of an analyte in a is almost complete.It is illustrated in Fig. 4c that for the graphite atomizer.9,11,23 Thomaidis et al.11 found that addition surface of a platform treated with ascorbic acid at 1500 °C of 5 mg of rhenium as a chemical modifier to the gold solution (ramp time 5 s, hold time 5 s) which gives the second peak, was suYcient to convert the fractional-order of release into a two broad Raman bands were observed at the same shifts as first-order process. The chemical modification eVect was those for the non-treated PG surface. However, once the explained by rhenium reacting with carbon atoms at the active surface had been heated at 2300 °C for 5 s, the two bands sites on the graphite surface, which decreases the number of (Fig. 4b) became sharper than those (in Fig. 4c) for the surface active sites, leading to easy dispersion of gold atoms; thus the treated with ascorbic acid at 1500 °C.Thus, it was shown that formation of highly dispersed atoms rather than microdroplets the graphite structure of the carbon residue becomes more is expected. ordered by pyrolysis at 2300 °C, and as a result no second Concentration dependence, in the present system, of the first peak is produced by this platform. and second peaks is shown in Figs. 5 and 6, respectively. In It is known that the adsorbing capacity of active carbon is dependent upon the pore size in the carbon, which is classified Fig. 5 Absorbance–time profiles for the first Au signal with masses Fig. 4 Raman spectra of pyrolysed ascorbic acid. a, Platform PG surface without the ascorbic acid residue; b, ascorbic acid pyrolysed varying between 0.1 and 1.0 ng, obtained using a heating programme with a temperature of 1700 °C (ramp time 60 s and hold time 5 s) for at 1500 °C (ramp time 5 s and hold time 5 s) followed by heating at 2300 °C for 5 s; and c, ascorbic acid pyrolysed at 1500 °C (ramp time step 3 in Table 1.Mass of Au: a, 1.0; b, 0.8; c, 0.6; d, 0.4; e, 0.2; and f, 0.1 ng. 5 s and hold time 5 s). Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12 1295Fig. 6 Absorbance–time profiles for the second Au signal with masses varying between 0.2 and 1.0 ng, obtained using a heating programme with a temperature of 1500 °C (ramp time 5 s and hold time 5 s) for step 3 in Table 1.Mass of Au: a, 1.0; b, 0.8; c, 0.6; d, 0.4; and e, 0.2 ng. Fig. 7 Absorbance–time profiles for Au for various ascorbic acid pyrolysis methods. (1) Pyrolysis method (2) (see the Experimental); and (2), pyrolysis method (3). Broken line, in the absence of ascorbic Fig. 5 the eVect of injecting various masses of gold between acid (mass of Au, 1.25 ng); solid lines, in the presence of ascorbic acid 0.1 and 1.0 ng is shown, after pyrolysing ascorbic acid at a (mass of Au: a, 1.25; b, 1.0; c, 0.75; d, 0.5; and e, 0.25 ng).temperature of 1700 °C, with a ramp time of 60 s and a hold time of 5 s, which gives the first peak. However, in Fig. 6 the which gives no second absorption signal, although the carbon eVect of injecting various masses of gold between 0.2 and film residue itself is thermally stable at that temperature. 1.0 ng is shown, after pyrolysing ascorbic acid at a temperature of 1500 °C, with a ramp time of 5 s and a hold time of 5 s, This work was supported by a Grant-in-Aid for the Scientific which gives the second peak.The results indicate that the Research No. 07404055 from the Ministry of Education, profiles of the first peaks correspond to the fractional-order Science and Culture in Japan. process as expected, and the second peak, for which the maximum is independent of concentration, is due to the first- REFERENCES order of release. Therefore, it can be deduced that in the presence of ascorbic acid the first peak signal is due to 1 Regan, J.G. T., and Warren, J., Analyst, 1976, 101, 220. atomization from microdroplets of gold and the second peak 2 McLaren, J. W., and Wheeler, R. C., Analyst, 1977, 102, 542. to atomization from highly dispersed gold atoms adsorbed 3 Tominaga, M., and Umezaki, Y., Anal. Chim. Acta, 1982, 139, 279. onto the carbon residue on the graphite wall. Thus, it is 4 Gilchrist, G. F. R., Chakrabarti, C. L., and Byrne, J. P., J.Anal. At. Spectrom., 1989, 4, 533. interesting to note that an active form of carbon, which adsorbs 5 Imai, S., and Hayashi, Y., Anal. Chem., 1991, 63, 772. monomeric gold atoms, is formed by the rapid pyrolysis of 6 Volynsky, A. B., Tikhomirov, S. V., Senin, V. G., and Kashin, ascorbic acid. A. N., Anal. Chim. Acta, 1993, 284, 367. Pyrolysis method (2) in which ascorbic acid was pyrolysed 7 Tominaga, M., and Umezaki, Y., Anal. Chim. Acta, 1979, 110, 55. after pyrolysing gold solutions gave the results shown in 8 Imai, S., and Hayashi, Y., Bull.Chem. Soc. Jpn., 1992, 65, 871. Fig. 7(1). It is seen that the use of ascorbic acid delays the 9 Aller, A. J., Anal. Chim. Acta, 1994, 292, 317. 10 Imai, S., Okuhara, K., Tanaka, T., Hayashi, Y., and Saito, K., gold signal compared with the case [broken line in Fig. 7(1)] J. Anal. At. Spectrom., 1995, 10, 37. in the absence of ascorbic acid. Although microdroplets of 11 Thomaidis, N., Piperaki, E. A., and Efstathiou, C.E., J. Anal. At. gold are formed during ashing of the gold solution, the Spectrom., 1995, 10, 221. subsequent presence of the ascorbic acid residue converts 12 Matthews, D. O., and McGahan, M. C., Spectrochim. Acta, Part the fractional-order of release into a first-order process B, 1987, 42, 909. for the delayed signal, showing the gold atoms dispersed and 13 Hageman, L. R., Nichols, J. A., Viswanadham, P., andWoodriV, R., Anal. Chem., 1979, 51, 1406. adsorbed into the amorphous carbon as monomeric atoms. 14 Sturgen, R. E., and Berman, S. S., Anal. Chem., 1985, 57, 1268. Pyrolysis method (3) in which gold and ascorbic acid were 15 Volynsky, A. B., Sedykh, E. M., and Spivakov, B. Ya, Anal. Chim. pyrolysing at the same time gave results fairly similar to those Acta, 1985, 174, 173. for pyrolysis method (2), as shown in Fig. 7(2). 16 Gilchrist, G. F. R., Chakrabarti, C. L., Byrne, J. P., and Lamoureux, M., J. Anal. At. Spectrom., 1990, 5, 175. 17 Imai, S., Nishiyama, Y., Tanaka, T., and Hayashi, Y., J. Anal. At. CONCLUSIONS Spectrom., 1995, 10, 439. 18 McNally, J., and Holcombe, J. A., Anal. Chem, 1987, 59, 1105. The microstructure of the pyrolysed carbon residue from 19 Fonseca, R. W., McNally, J., and Holcombe, J. A., Spectrochim. ascorbic acid on the graphite surface is dependent upon Acta, Part B, 1993, 48, 79. thermal conditions of the pyrolysis, such as temperature, 20 Yoshikawa, M., Mater. Sci. Forum, 1989, 52 & 53, 365. heating rate and hold time. A rapid heating rate for the 21 IUPAC Manual of Symbols and T erminology for Physicochemical pyrolysis of ascorbic acid, for example, 276 °C s-1 even at Quantities and Units, Butterworths, London, 1972. 22 Hosokawa, K., and Yamaguchi, M., T anso (Carbons), 1988, 1500 °C (hold time 5 s), gives an amorphous carbon film with 132, 17. micro- or submicro-pores during the thermal decomposition, 23 Qiao, H., and Jackson, K. W., Spectrochim Acta, Part B, 1991, which is when the gold atoms are adsorbed, leading to the 46, 1841. second signal with a first-order release of atoms. The gold– carbon interaction in micro-size pores is stronger than the Paper 7/01659J gold–gold interaction, resulting in dispersion of the gold atoms. ReceivedMarch 10, 1997 Thermal treatment of the amorphous carbon film at tempera- Accepted June 30, 1997 tures higher than 1800 °C gives rise to well-ordered graphite, 1296 Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12

ISSN:0267-9477    

DOI:10.1039/a701659j

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

On the Determination of Carbon, Phosphorus, Sulfur and Silicon in Grey Cast Irons Using Glow Discharge Optical Emission Spectrometry MICHAEL R. WINCHESTER*a AND CHARLES MAULb aAnalytical Chemistry Division, Chemical Science and T echnology L aboratory, National Institute of Standards and T echnology, Gaithersburg, MD 20899, USA bL ECO Corporation, 3000 L akeview Avenue, St. Joseph,MI 49085, USA Glow discharge optical emission spectrometry is used to with the spark. When graphitic C is present, P and Si are usually aVected as well.This is why the metalloid, Si, was determine C, P, S and Si in grey cast irons. White cast irons included in the set of analytes. are used as calibrants, and data are acquired while sputtering In a recent paper,Weiss discussed the grey cast iron problem is non-stoichiometric. Type standardization is employed to at length and elegantly demonstrated the potential of GD-OES correct the resulting matrix eVect. For comparison, analytical to alleviate fully these matrix eVects.4 The success of GD-OES mass fractions of P, S and Si are also calculated from the in this regard depends on the use of a judiciously chosen data while omitting the matrix eVect correction.A statistical presputtering routine that assures that stoichiometric sputter- evaluation of the data shows that while all except one of the ing5 is established prior to data acquisition. Using such a experimental mass fractions, computed with or without type routine, Weiss was able to cause both grey (graphitic) and standardization, are in agreement with corresponding certified white (non-graphitic) cast irons to fall along the same cali- values at the P=0.05 level of significance, the use of the type bration line for each element investigated.It was shown that standard is indispensible for the unbiased determination of C if data are acquired before sputtering becomes stoichiometric, and S and may be helpful in the case of P.In contrast, the white and grey cast irons may tend to fall along diVerent data indicate that type standardization may in some cases calibration lines. This is especially true for C, owing to the introduce analytical bias into the determination of Si. relatively low sputtering rate of graphite. No analytical deter- Keywords: Analytical bias; cast iron; glow discharge optical minations were performed in that particular work. emission spectrometry; metalloids; non-metallic elements The temporal behavior of emission intensities during the approach to stoichiometric sputtering is highly predictable within a given matrix.By making use of this fact, it is possible Quantitative determination of non-metallic elements in solids to analyse samples of one matrix with calibration equations is an important analytical problem in a wide range of technol- developed using calibrants of a diVerent matrix while ogies. As an example of its importance, manufacturers of jet employing a presputtering routine that does not allow stoichioengine turbine blades have learned that sub-mg kg-1 mass metric sputtering to be attained prior to the acquisition of fractions of S in the alloys used to make the blades can lead data.For example, grey cast irons may be analysed using to engine failure. Similar findings are likely to be forthcoming calibration equations based on white cast irons only. In this for P as well.1 The successful control of S and P necessitates approach, one or more grey cast iron reference materials are the use of reliable, quantitative analytical methods. Given the used to determine correction factors that can be applied to the widespread need to quantify non-metals in solids, it is surpris- analytical mass fractions for the unknown samples.This ing that this analytical problem has historically been somewhat method of correcting the matrix eVect after data acquisition is ignored by the analytical community.Although methods exist known as ‘type standardization’. for the determination of these elements, there is a need for The studies discussed in the present paper build upon the better methods to be developed. work reported by Weiss. They were conducted at the LECO At NIST, GD-OES is being investigated for its potential for World Headquarters in St. Joseph, MI, USA, as part of a quantitative determination of non-metals. Although other ana- collaboration between NIST and LECO.Prior to undertaking lytical figures of merit such as sensitivity and detection limit the studies, it was anticipated that a presputtering routine that are important, the emphasis is on accuracy. This is because it would eliminate the matrix eVect associated with nonis hoped that GD-OES may eventually prove useful for the stoichiometric sputtering would be used. In this way, the plan certification of non-metallic elements in reference materials. Of was to analyse grey cast irons using only white cast iron calibrants, without the need for any sort of matrix eVect the figures of merit, accuracy has historically been one of the correction. Although this routine would have required only an least discussed among GD practitioners.extra 2 min per burn, a shorter presputtering routine was used The present paper reports the initial studies in this project. to save analysis time, and the resulting matrix eVect was The work discussed involves the use of GD-OES for the corrected with a type standard.For comparison, analytical determination of several non-metallic elements (C, P and S) mass fractions for P, S and Si were also calculated from the and a metalloid (Si) in grey cast irons. In the past, GD data without matrix eVect correction. A statistical evaluation techniques have occasionally been used for this sort of analysis, of the data is presented. though very little has been published. The existing publications, for example, refs. 2 and 3, do not focus on accuracy, and no statistical evaluation has been reported. EXPERIMENTAL The analysis of grey cast irons is particularly problematic Instrument for spark-based spectrometries, owing to severe matrix eVects associated with the presence of graphitic C. It is very diYcult, These studies were performed on a GDS-750A glow discharge optical emission spectrometer (LECO, St. Joseph, MI, if not impossible, to determine C accurately in grey cast irons Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12 (1297–1305) 1297Table 1 Wavelengths, PMT models and PMT voltages employed for are strictly matrix matched, not only in terms of elemental the elements of interest composition, but also in terms of metallurgical structure. As seen in Table 2, the four grey cast irons are very similar in Element Wavelength/nm PMT Model* PMT voltage/V terms of elemental composition. Considering metallurgical C 165.701 R306 -850 structure, the important characteristics in this case are the P 177.499 R306 -900 proportion of total C that exists as graphite and the form of S 180.731 R306 -900 that graphite (i.e., nodules, flakes, etc.).The four grey cast irons Si 288.158 R300 -900 should be very similar in this regard as well, because they were Fe† 249.318 R300 -900 all manufactured from white cast irons through annealing. * All PMTs manufactured by Hamamatsu Photonics. Finally, type standardization should be most eVective when † Used as an internal standard.the analyte mass fraction does not vary appreciably from sample to sample and from sample to type standard. Table 2 shows that this criterion is at least approximately met by the USA).† Briefly, the instrument incorporates a Grimm-type GD set of grey cast irons. For these reasons, the grey cast irons lamp that can be operated in either dc or rf mode. Using all used in these studies present an ideal case for the use of type reflective optics, optical emission is focused onto the entrance standardization. slit of a 750 mm focal length, f/10, Paschen–Runge polychromator.The polychromator provides a nominal spectral reso- Data Reduction lution of <25 pm over the full spectral range from 120 nm to 800 nm. The spectrometer chamber is pumped with a turbo- Even though the software accompanying the instrument promolecular pump for operation at wavelengths below 200 nm. vides data reduction capabilities, the raw data were transferred Emission intensities are measured and quantified by means of to a separate computer at NIST for processing.This is because PMTs and associated electronics. The entire instrument is the available time at LECO was insuYcient to allow a complete monitored and controlled with a software package running on evaluation of the data. All data reduction and analysis were a PC. Full data reduction routines are also provided within performed in the Excel (Microsoft, Redmond, WA, USA) and the software.SigmaPlot (Jandel, San Rafael, CA, USA) software packages. For the present studies, the Grimm lamp was operated in the dc mode, and the anode diameter was 4 mm. The power RESULTS AND DISCUSSION supply was operated in voltage regulation, while the discharge EVect of Uncertainty in the Independent Variable on Calibration current was maintained by adjusting the source pressure in Equations real time. In this way, the lamp was actually operated in constant discharge power. The argon support gas was obtained Before calibration equations for the four analytes could be from the house supply.For all samples, each burn consisted calculated from the raw data with confidence, it was necessary of a 5 s evacuation, a 55 s presputtering period at 1000 V and to determine whether least squares regression would be an 35 mA, and a 10 s integration under the same conditions. acceptable method. As noted earlier, internal standardization Elemental analytes were C, P, S and Si.Emission from the was used in these studies. This means that the calibration major matrix element, Fe, was also monitored and used as an graphs consist of mean intensity ratios plotted against ratios internal standard. The wavelengths employed for the elements, of mass fractions, with the latter being the independent variable. along with the PMT models and voltages, are listed in Table 1. The issue at hand is the uncertainties associated with the mass Though the instrument is capable of background correction, fraction ratios for the calibration points.As seen in Fig. 1, it was not used for these studies. these uncertainties are generally comparable in magnitude to the uncertainties in the intensity ratios. Least squares regression assumes that all uncertainty is in the dependent variable. Samples Because of this assumption, it will tend to underestimate the Samples used for the preparation of calibration lines consisted slope and overestimate the intercept if significant uncertainty of nine white cast iron reference materials (CRM Nos. 241–249, is associated with the independent variable. CKD Research Institute, Prague, Czech Republic). Prior to There are variations on least squares regression that are analysis, these calibrants were dry sanded using 120 grit ZrO2 designed to accommodate uncertainties in the independent paper. The grey cast irons consisted of four reference materials variable.6 These methods are fairly eVective, so long as the (CRM Nos. 21G, 22G, 23G and 24G, Brammer Standard, magnitudes of these uncertainties are the same at each cali- Houston, TX, USA). These samples were wet sanded using bration point, or can at least be modelled. For these experi- 600 grit paper prior to analysis. Known mass fractions for the ments, the uncertainties in the mass fraction ratios meet neither four analytes and Fe in both the white and grey irons are of these criteria.Under such circumstances, the eVectiveness of presented in Table 2. these methods for reliably estimating regression coeYcients Samples 21G, 22G and 23G served as unknowns, while 24G tends to be compromised. Moreover, it is diYcult to estimate served as the type standard from which correction factors were uncertainties associated with unknown values predicted with derived. The choice of which of the four grey irons to use as equations computed with these methods when such circumthe type standard was made randomly.The randomness of the stances are present. Uncertainties in predicted values are choice is important, because, with certified values for the four essential for the statistical analyses reported herein. For these analytes in hand, there is a natural tendency to choose the one reasons, the use of these variant methods was not considered that will provide the most eVective corrections. further. Type standardization should be most eVective for matrix The validity of using least squares regression in these experie Vect correction when the type standard and unknown samples ments was tested by simulating the actual calibration data in Excel using the Visual Basic programming language.The † In order to describe experimental procedures adequately, it is simulation was done four times, once for each analyte. This occasionally necessary to identify commercial products by manufac- was necessary, because the noise associated with the calibration turer’s name or label.In no instance does such identification imply points is diVerent for each analyte. endorsement by the National Institute of Standards and Technology, The simulation for a particular analyte consisted of nine nor does it imply that the particular products or equipment are necessarily the best available for that purpose. ‘dummy’ calibration points with means positioned exactly 1298 Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12Table 2 Elemental mass fractions and uncertainties, expressed as 95% confidence intervals, for the four analytes and the matrix element, Fe, in the white and grey cast irons Certified mass fractions (%) Sample C P S Si Fe* White cast irons: 241 1.71±0.007 0.006±0.0016 0.14±0.004 3.20±0.016 93.23±0.52 242 2.21±0.022 0.040±0.0015 0.033±0.0014 2.88±0.020 92.55±0.52 243 2.23±0.026 0.162±0.0054 0.085±0.0016 2.44±0.020 93.03±0.52 244 2.60±0.017 0.024±0.0015 0.019±0.0015 2.11±0.024 92.95±0.52 245 2.78±0.028 0.40±0.008 0.049±0.0018 1.60±0.017 92.89±0.52 246 2.82±0.028 0.60±0.006 0.022±0.0024 0.62±0.016 92.92±0.52 247 3.12±0.028 0.095±0.0048 0.005±0.0015 1.16±0.010 92.82±0.52 248 3.41±0.018 0.050±0.0014 0.006±0.0009 1.81±0.028 92.96±0.52 249 3.75±0.025 0.25±0.009 0.008±0.0011 0.34±0.016 92.94±0.52 Grey cast irons: 21G 3.98±0.030 0.057±0.0020 0.028±0.0024 1.61±0.038 92.60±0.29 22G 3.70±0.013 0.102±0.0025 0.032±0.0012 1.96±0.037 92.61±0.28 23G 3.17±0.019 0.047±0.0017 0.024±0.0013 2.61±0.032 92.49±0.28 24G 2.42±0.019 0.022±0.0019 0.019±0.0019 2.93±0.019 92.65±0.28 * Uncertified, with uncertainties estimated from statistical information provided on the Certificates of Analysis and/or general knowledge of grey cast irons.Fig. 1 Internally standardized calibration lines obtained from triplicate burns on each of the nine white cast irons ($) for (a) C, (b) P, (c) S and (d) Si. Points for the 24G grey cast iron type standard (#), also based on triplicate burns, are plotted as well.All error bars are 95% confidence intervals. Weighted least squares regression was employed for the calibration lines. along a straight line of known slope and intercept, but with the mean slope and intercept was estimated with simulated experimental data. noise in both the x and y coordinates that mimicked as closely as possible the observed noise associated with the experimental As expected, the slopes were underestimated and the intercepts were overestimated for all four analytes.For all analytes calibration points. With the use of random number generation, 10 000 sets of dummy calibration data were computed, each except C, their mean values diVered from the corresponding design values at the P=0.05 level of significance. Neither the set diVering from every other set in terms of the (x, y) coordinates of the points. Linear least squares regression was per- slope nor the intercept diVered significantly in the case of C.Fortunately, in terms of predicting an unknown x-value from formed on each set, thus producing 10 000 slopes and 10 000 intercepts. The t-test was then used to determine if the mean a known y-value, the biases in the slope and intercept being in opposite directions has a partially compensating eVect. As slope and mean intercept diVered significantly from their corresponding design values. Finally, the magnitude of bias a result, the biases expected to be introduced into the final elemental determinations in these experiments were found to expected to be introduced into unknown values predicted with Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12 1299be negligible for all four analytes. Therefore, it was decided to on which the data in the figure are based, S=s/2. Corresponding 95% confidence intervals are extremely broad use least squares regression to compute the calibration equations.(0.52s<s<6.28s).8 In the figure, the standard deviations of the means of the observations, rather than the SDs of the observations, are plotted. As a result, the uncertainties associated Heteroscedasticity in the Dependent Variable with the points may be quantitatively somewhat diVerent from the example. Nevertheless, it should be obvious that the Next, it was necessary to determine whether weighted or standard deviations of the means are very uncertain.This fact unweighted least squares regression should be employed. accounts for the considerable scatter in the points. Owing to Weighted regression is usually more appropriate when there is these large uncertainties, the data for all four analytes are significant heteroscedasticity associated with the dependent better treated as an ensemble, rather than separately. This is variable (i.e., when the magnitudes of the uncertainties associwhy the data for the four analytes have been plotted on the ated with the ordinate values of the calibration points are not same graph. equivalent).Therefore, the mean intensity ratios for the cali- The heteroscedasticity of the mean intensity ratios is clearly bration points were evaluated for heteroscedasticity. revealed in Fig. 2(a). From this, it was concluded that weighted The first means of evaluation was a close visual examination least squares regression should be employed to calculate the of the error bars associated with the mean intensity ratios in calibration equations for the various analytes.Weighted the various plots of Fig. 1. Though somewhat inconclusive, an regression is typically performed using the reciprocals of the examination of this sort seems to reveal that the error bars variances in the ordinate values of the calibration points as generally increase in magnitude with increasing mean intensity weighting parameters for those points. In this work, the ratio.This is most clearly evident in the P and Si data in experimentally observed standard deviations of the mean inten- Fig. 1(b) and (d), respectively. In order to assess this apparent sity ratios were not used to generate these weighting param- heteroscedasticity more eVectively, the standard deviations of eters, because of the large uncertainties associated with them. the mean intensity ratios were plotted against the mean Instead, estimates of these standard deviations derived by intensity ratios themselves [see Fig. 2(a)]. plugging the experimental mean intensity ratios into the equa- In looking at these data, it is important to remember that tion for the regression line in Fig. 2(a) were used. This line, standard deviations usually have very large uncertainties. For which approximates the noise behavior of the mean intensity example, the standard deviation, S, of the SD, s, of n obserratios, was calculated using all 36 points and was forced vations is: through the origin, since, in the ideal case, if there is no signal, there should also be no noise.Weighting parameters derived S= s Ó2(n-1) (1) in this way should be more meaningful. Finally, it should be noted that assuming a linear dependence assuming that the observations are drawn from a normally between the mean intensity ratios and their standard deviations distributed population.7 For n=3, the number of observations is synonymous with assuming that the data are flicker noise limited.One way to provide some justification for this assumption is to plot the mean intensity ratios against their relative standard deviations (RSDs) [see Fig. 2(b)]. If flicker noise is dominant, there should be no dependence of the RSD on the mean. As shown in the figure, this appears to be at least approximately true. Given the assumption of flicker noise dominance, the slope of the regression line in Fig. 2(a) is essentially an estimate of the flicker factor.Calibration and Determinations Employing Type Standardization Weighted least squares regression lines were calculated for each of the four analytes using the data obtained from the nine white cast iron calibrants (see Fig. 1). The resulting calibration equations and correlation coeYcients accompany the graphs. All four calibrations are acceptably linear, though the linearity of the S data is somewhat undesirable. The linearities of the P and Si data are exceptionally good. All four intercepts are reasonably close to the origin.Before proceeding to the analytical determinations using the calibration equations, it is helpful to discuss type standardization in a little more detail. As noted before, type standardization is a method by which the matrix eVect associated with non-stoichiometric sputtering can be corrected after data acquisition. The method assumes that the bias introduced by the matrix eVect is additive and consistent from sample to sample within a given matrix.Type standardization is performed as follows. First, the mass fraction of the analyte in a known sample that is matrix matched to the samples to be analysed (i.e., the type standard) Fig. 2 Standard deviations (a) and relative standard deviations (b) of is determined experimentally from the calibration equation. the mean intensity ratios plotted against the mean intensity ratios Since the calibration equation is based on calibrants of a themselves for the C ($), P (#), S (&) and Si (%) calibration data.diVerent matrix, the mass fraction determined in this way may The regression line in Fig. 2(a) was computed using all 36 points as an ensemble and was forced through the origin. disagree considerably with the known value. The diVerence 1300 Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12between the experimental and known mass fractions for the corresponding certified values. Statistical tests are performed in the next section to evaluate whether the remaining determi- type standard is a measure of the bias introduced by the matrix eVect.This diVerence serves as the correction factor for the nations agree with the corresponding certified values on a statistical basis. analysis of the unknowns. Next, the mass fractions of the analyte in the unknown samples are determined experimentally The relatively large errors associated with the S determinations for samples 21G and 22G may be attributable in part from the calibration equation.These experimental mass fractions are then adjusted using the correction factor. to digitization noise. The same is true for the determination of P in sample 23G. The digitization noise results from the fact Points for the 24G grey cast iron type standard used in this work are plotted on the graphs in Fig. 1. Considering where that only two significant figures are reported for these mass fractions.Digitization noise should play a lesser role in the they each fall in relation to their respective calibration line, it seems that type standardization is only necessary for the remaining determinations, since all of these are reported with three significant figures. determination of C. The grey iron points for P, S and Si are essentially indistinguishable from the white iron calibration The relatively large errors associated with the C determinations are probably caused by the fact that C required the points.However, on a theoretical basis, if type standardization is necessary for even one element, it should be necessary for largest matrix eVect correction. The relative magnitudes of the corrections for the various analytes are indicated by the every element, since stoichiometric sputtering was not attained prior to data acquisition. Given this fact, type standardization proximities of the points for the 24G type standard to their respective calibration lines in the graphs in Fig. 1. was initially used for all four analytes. As a matter of interest, P, S and Si mass fractions computed without matrix eVect correction are reported in a subsequent section. Statistical Evaluation of the Results Analytical determinations for the three grey cast iron unknowns were based on triplicate burns. Mathematically, As alluded to above, 9 of the 12 analytical mass fractions in analytical mass fractions were calculated using the following Table 3 do not agree numerically with the corresponding equation: certified values.For each of these determinations, the t-test was performed to determine if the observed diVerence between the experimental and certified values can be attributed to [A]unk,exp=GA[A] [Fe]Bunk,exp [Fe]unk,certH random noise. Since the precisions associated with the certified mass fractions and the mass fractions determined in this work +G[A]type,cert- A[A] [Fe]Btype,exp [Fe]type,certH cannot be assumed to be equivalent, it was necessary to employ a version of the t-test that does not pool the uncertainties from the two sets of data.With this constraint in mind, the exper- (2) imental t-statistic is defined as: In this equation, square brackets symbolize mass fraction, and A refers to the analyte. The subscripts unk and type refer to texp= |xE-xC| ÓuE2+uC2 (3) the unknown sample and type standard, respectively, while the subscripts exp and cert refer to experimentally determined and where xE and xC are the means for the experimental and certified values, respectively.In this regard, it is important to certified data sets, respectively, and uE2 and uC2 are the note that the mass fraction ratios are the experimentally variances associated with those means. The eVective degrees determined values calculated from the mean intensity ratios of freedom for this t-test are given by: and the calibration equations. The remaining mass fractions on the right side of eqn. (2) are the known values from Table 2. 1 veff = 1 vE A uE2 uE2+uC2B2 + 1 vC A uC2 uE2+uC2B2 (4) Examination of the equation reveals that the experimentally determined mass fraction for the unknown sample prior to matrix eVect correction is given by the first bracketed term, where vE and vC are the degrees of freedom associated with the data sets.9 while type standardization is performed with the addition of the second bracketed term. In order to perform these t-tests, it is obviously necessary to know the means, variances and degrees of freedom associ- The determinations of the various analytes in the unknowns are presented in Table 3.As seen in the table, only 3 of the 12 ated with both the experimental and certified data. All of the necessary statistics for the certified mass fractions were avail- analytical mass fractions are numerically identical to the Table 3 C, P, S and Si determinations in three grey cast irons based on triplicate burns.Brammer CRM 24G was used as the type standard Sample Certified Mass fraction determined from Analyte No. mass fraction (%) this work (%) Relative error (%) C 21G 3.98 3.88 2.51 22G 3.70 3.62 2.16 23G 3.17 3.20 0.95 Average relative error: 1.87 P 21G 0.057 0.057 0 22G 0.102 0.102 0 23G 0.047 0.048 2.1 Average relative error: 0.7 S 21G 0.028 0.027 3.6 22G 0.032 0.031 3.1 23G 0.024 0.024 0 Average relative error: 2.2 Si 21G 1.61 1.59 1.24 22G 1.96 1.94 1.02 23G 2.61 2.60 0.38 Average relative error: 0.88 Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12 1301able from the Certificates of Analysis, whereas the statistics for experimental t-statistic is: the experimentally determined mass fractions had to be calculated. The variances and degrees of freedom associated with tpaired= xdÓn Sd (5) the experimental values were computed by applying the law of the propogation of error and the Welch–Satterthwaite where n=3 (i.e., the number of grey cast iron samples), xd is formula, respectively, to eqn.(2). These methods are well the average of the signed diVerences between the experimental known in the statistical literature, for example, reference 10, and corresponding certified mass fractions, and Sd is the SD and so they will not be reproduced here. associated with these diVerences. For the Si data, the value of After these preliminary computations were completed, the tpaired calculated using eqn.(5) is 4.18. (In calculating tpaired, t-tests using eqns. (3) and (4) were performed (see Table 4). As all available significant figures for the various mass fractions seen in the table, at a level of significance of P=0.05, the null were employed, rather than the rounded-oV values in Tables 3 hypothesis, H0, is rejected in only one of the nine cases, the and 4.) Given that the degrees of freedom for the paired t-test determination of C in sample 21G. Note that for this determi- are n-1, or 2 in this case, the corresponding critical value of nation, texp is just barely larger than the corresponding critical t at the P=0.05 level of significance is 4.30.Therefore, at the value of t. This means that the observed diVerence between 95% confidence level, H0 is retained. However, since P=0.053 the experimental and certified C mass fractions for this sample for tpaired=4.18 and 2 degrees of freedom, H0 would be rejected is significant at the 95% confidence level, but just barely so.at a confidence of 94%. Therefore, it was concluded that there In fact, given that P=0.043 for 18 degrees of freedom and is statistical evidence for systematic bias in these Si determitexp= 2.18, H0 would have been retained at the 96% confidence nations, but this evidence is somewhat weak. The cause of this level. In view of these observations, it was concluded that, on apparent systematic bias will be explained in a later section. the basis of these t-tests, there is evidence of analytical bias associated with this one determination, but that the evidence P, S and Si Determinations Without Type Standardization is weak.As discussed earlier, the points for the 24G type standard are Another way to assess the presence or absence of analytical essentially indistinguishable from the calibration points in the bias is to employ the paired t-test.11 In contrast to the t-tests cases of P, S and Si. Nevertheless, on the basis of theoretical just performed, the paired t-test does not compare an individual considerations, type standardization was used for all four experimental mass fraction with its corresponding certified analytes. It is informative at this point to recalculate the value.Instead, it compares the full set of experimental mass analytical mass fractions for these three analytes without fractions for a particular analyte with the corresponding set of making use of the matrix eVect correction.certified mass fractions. While the paired t-test provides no Omitting type standardization, eqn. (2) simplifies to: information on any single determination, it can be especially helpful for discerning whether the set of experimental values [A]unk,exp= GA[A] [Fe]Bunk,exp [Fe]unk,certH (6) for a particular analyte as a whole is systematically biased relative to the set of certified values. Such systematic bias may sometimes remain hidden with the use of the non-paired t-test.where all symbols and subscripts are used exactly as before. The determinations using eqn. (6) are presented in Table 5. The validity of the paired t-test rests on the assumption that the uncertainties associated with the mass fractions for the As seen in the table, only one of the nine determinations agrees numerically with the corresponding certified mass frac- various samples under study are independent of mass fraction. Certainly, this is not strictly true for any of the four analytes tion.Non-paired t-tests were performed for the remaining determinations to ascertain whether the observed diVerences in this study. However, as shown in Table 4, the variances in general do not vary greatly with mass fraction, and so this between the experimental and certified mass fractions can be attributed to random noise. Eqns. (3) and (4) were again assumption was considered to be eVectively satisfied. Obviously, the paired t-test may not need to be performed employed for these tests.The necessary statistics for the data sets were computed exactly as before. for all four analytes, but only for those whose sets of analytical mass fractions appear to be biased. Referring to the data in The results of the non-paired t-tests (see Table 6) indicate that all of the analytical mass fractions are in agreement with Table 3, the only one of the four analytes for which the set of analytical determinations as a whole appears to be biased the corresponding certified values at the 95% confidence level.This is surprising for S, because the S determinations have relative to the corresponding set of certified mass fractions is Si. The analytical Si mass fractions for all three grey cast iron large relative errors. The agreement in this case can be attributed to the fairly large uncertainties in the experimentally samples are underestimated. Furthermore, they are each underestimated by approximately the same amount.Therefore, the determined values. These large uncertainties are probably a result of the relatively low mass fractions of S in the samples. paired t-test was used to evaluate the Si data only. For the application of the paired t-test to the data, the Next, in order to assess the presence or absence of systematic Table 4 Results of t-tests for the determinations in Table 3 that did not agree exactly with certified mass fractions Statistics from Certificates of Statistics from this work Analysis Results of t-test Sample Analyte No.x� E (%) uE2 (%2) vE x� C (%) uC2 (%2) vC veff texp tcrit,double-sided,P=0.05 H0 rejected? C 21G 3.876 2.349×10-3 16 3.985 1.402×10-4 5 18 2.18 2.10 Yes 22G 3.624 2.064×10-3 17 3.700 2.400×10-5 5 17 1.66 2.11 No 23G 3.204 1.694×10-3 18 3.170 4.500×10-5 4 19 0.81 2.09 No P 23G 0.0483 4.13×10-6 16 0.0467 4.27×10-7 5 18 0.75 2.10 No S 21G 0.0271 1.25×10-5 18 0.0281 9.66×10-7 6 21 0.28 2.08 No 22G 0.0314 1.47×10-5 17 0.0319 2.41×10-7 6 17 0.14 2.11 No Si 21G 1.588 1.604×10-3 15 1.609 2.401×10-4 6 18 0.49 2.10 No 22G 1.939 1.777×10-3 17 1.959 2.042×10-4 5 20 0.45 2.09 No 23G 2.599 2.204×10-3 19 2.608 1.352×10-4 4 21 0.18 2.08 No 1302 Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12Table 5 P, S and Si determinations in the three grey cast irons without the use of type standardization Sample Certified Mass fraction determined from Analyte No. mass fraction (%) this work (%) Relative error (%) P 21G 0.057 0.058 1.8 22G 0.102 0.103 1.0 23G 0.047 0.049 4.3 Average relative error: 2.4 S 21G 0.028 0.024 14 22G 0.032 0.028 13 23G 0.024 0.021 13 Average relative error: 13 Si 21G 1.61 1.59 1.24 22G 1.96 1.94 1.02 23G 2.61 2.61 0 Average relative error: 0.75 Table 6 Results of t-tests for the determinations in Table 5 that did not agree exactly with certified mass fractions Statistics from Certificates of Statistics from this work Analysis Results of t-test Sample Analyte No.x� E (%) uE2 (%2) vE x� C (%) uC2 (%2) vC veff texp tcrit,double-sided,P=0.05 H0 rejected? P 21G 0.0577 4.14×10-6 9 0.0567 6.91×10-7 6 12 0.46 2.18 No 22G 0.1029 1.294×10-5 9 0.1020 1.041×10-6 6 10 0.24 2.23 No 23G 0.0488 2.98×10-6 9 0.0467 4.27×10-7 5 11 1.12 2.20 No S 21G 0.0241 7.58×10-6 9 0.0281 9.66×10-7 6 11 1.37 2.20 No 22G 0.0284 9.75×10-6 9 0.0319 2.41×10-7 6 9 1.11 2.26 No 23G 0.0211 6.25×10-6 9 0.0245 2.40×10-7 5 10 1.34 2.23 No Si 21G 1.594 3.417×10-4 9 1.609 2.401×10-4 6 15 0.63 2.13 No 22G 1.945 5.150×10-4 9 1.959 2.042×10-4 5 14 0.54 2.14 No analytical bias more completely, the paired t-test was per- Final Consideration of Type Standardization formed for those analytes in Table 5 whose sets of analytical Before drawing this discussion to a close, it is helpful to plot mass fractions appear to be systematically biased relative to the points for all four grey cast irons, using known mass the corresponding certified values.An examination of the data fractions, on each of the calibration graphs. Doing so provides in the table indicates that such systematic bias almost certainly a clearer understanding of type standardization and the results exists for S and may exist for P. However, no such systematic reported in this paper. Plots of this sort, focusing on the bias is implied for Si. Therefore, the paired t-test was applied domains of the grey iron points, are presented in Fig. 3. to the P and S data only.The values of tpaired for the P and S As seen in the plots in Fig. 3(a) and (c), the points for the data were calculated using eqn. (5) and all available significant greys and the points for the whites clearly fall along diVerent figures for the various mass fractions. The critical value of t calibration lines for C and S. This behavior, which is a result that tpaired must exceed for H0 to be rejected at the P=0.05 of the matrix eVect associated with non-stoichiometric sputter- level of significance is 4.30.ing, is known from previous work.4 The wide separation For the P data, tpaired=3.58, indicating that H0 should be between the S calibration lines for the whites and greys in retained at the 95% confidence level. However, since P=0.070 Fig. 3(c) explains the systematic analytical bias in the determi- for tpaired=3.58 and two degrees of freedom, H0 would be nations performed without type standardization demonstrated rejected at the 92% confidence level.Therefore, it was conin the preceeding section. cluded that there is statistical evidence for systematic bias in Referring now to the P data in Fig. 3(b), all four grey cast the P determinations performed without matrix eVect correciron points are indistinguishable from the white cast iron tion, but that the evidence is somewhat weak. Comparing the points. The error bars associated with each grey iron point non-corrected P determinations in Table 5 with the P mass fully overlap the white iron calibration line.This is why fractions determined with type standardization in Table 3, it reasonably acceptable determinations of P could be done was found that the use of matrix eVect correction improved without type standardization. However, a closer examination the relative errors by about a factor of three on average. The of the data indicates that all four grey iron points lie slightly apparent systematic bias in the non-corrected P mass fractions above the white cast iron calibration line.This means that the accounts for this improvement. greys may in fact lie along a calibration line of their own. It For the S data, tpaired=19.5, indicating sound rejection of also accounts for the systematic bias in the non-corrected H0 at the 95% confidence level. In fact, given such a large determinations demonstrated in the preceeding section. value of tpaired, H0 would be rejected even at the 99% confidence Referring to the Si data in Fig. 3(d), it is apparent that, as level. Therefore, there is strong statistical evidence for systemfor the P data, all four grey cast iron points are indistinguish- atic bias in the S determinations performed without type able from the white cast iron points. Again, the error bars standardization. Comparing the non-corrected S determicharacterizing each grey iron point fully overlap the calibration nations with those performed with matrix eVect correction in line developed with the white cast irons.This accounts for the Table 3, it was found that the use of the type standard improved low relative errors associated with the Si determinations the relative errors dramatically. The systematic bias in the non-corrected mass fractions accounts fimprovement. obtained without type standardization. Unlike P, however, all Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12 1303Fig. 3 Points for the four grey cast irons (#), using known mass fractions, plotted with the calibration data from Fig. 1 ($), for (a) C, (b) P, (c) S and (d) Si.For clarity, the graphs focus on the domains of the grey iron points for P, S and Si. All error bars are 95% confidence intervals. As for the white cast iron calibration lines, weighted least squares regression was used to generate best-fit lines through the grey cast iron points. four grey iron points do not lie on one side of the white cast helpful in the case of P.However, in some cases it may introduce analytical bias into the determination of Si. iron calibration line. In fact, the grey irons seem to lie along a calibration line of their own that intersects the white iron For all four analytes, the relative errors observed for the determinations performed with matrix eVect correction are calibration line. This is why the Si determinations performed without type standardization appeared to be free of system- acceptably small for many applications.As discussed earlier, the set of grey cast irons employed in these studies constitute atic bias. A more careful examination of the figure reveals that while an ideal case for the application of type standardization. Poorer results would be expected in less ideal cases. the 24G type standard point lies slightly above the white cast iron calibration line, the three remaining grey cast iron points Among the determinations for which matrix eVect correction was performed, the largest relative errors were associated with lie either directly on the line or slightly below it. This is why matrix eVect correction using the 24G sample as the type the C determinations, with one of the three analytical C mass fractions disagreeing with the corresponding certified mass standard seemed to introduce systematic bias into the Si determinations.If one of the other grey cast irons had been fraction at the P=0.05 level of significance.The relatively large errors associated with C were attributed to the fact that this chosen for the type standard, or if a diVerent set of grey irons altogether had been used, the results may have been diVerent. element required the largest matrix eVect correction. Certainly, better results for C would be expected if data were acquired Finally, as discussed earlier, type standardization assumes that the error introduced by the matrix eVect associated with after sputtering becomes stoichiometric.This may constitute a future study. non-stoichiometric sputtering is additive and consistent from sample to sample within a given matrix. If this assumption is Regarding the determinations performed without type standardization, it is important to note that the relative errors justified for the work reported herein, the calibration lines for the whites and greys should be parallel. The plots in Fig. 3 associated with the analytical P and Si mass fractions were also small enough for many applications.This can be attributed show that this is at least approximately true. It is important to note that, theoretically, they should not be exactly parallel, to the fact that these two elements required little or no matrix eVect correction. The fact that P and Si can be determined in since this would mean that one or the other would have an intercept far removed from the origin, possibly even negative, this way while sputtering is still non-stoichiometric demonstrates the relative lack of matrix eVects, compared with most in the case of an element such as C.This inherent aparallelism is why type standardization should be most eVective over small other analytical methodologies, that characterizes GD-OES. ranges of analyte mass fraction. The authors thank David Valensi (Product Manager— CONCLUSIONS Material Characterization) and Joel Mitchell (Manager— Atomic Spectrometry) of LECO for their kind hospitality and Three non-metals (C, P and S) and a metalloid (Si ) have been determined in grey cast irons using GD-OES. The results the use of the LECO facilities. Robert Watters (Analytical Chemistry Division, Chemical Science and Technology reported in this paper demonstrate that, under the circumstances of these experiments, type standardization is indispens- Laboratory, NIST) is acknowledged for many helpful discussions. ible for the unbiased determination of C and S and may be 1304 Journal of Analytical Atomic Spectrometry, November 1997, Vol. 127 Statistical Methods in Research and Production, ed. Davies, O. L., REFERENCES and Goldsmith, P. L., Hafner, New York, 4th edn., 1972, pp. 51–52. 1 Measurement of Sulfur in Superalloys Workshop, held at the 8 Statistical Methods in Research and Production, ed. Davies, O. L., National Institute of Standards and Technology, Gaithersburg, and Goldsmith, P. L., Hafner, New York, 4th edn., 1972, p. 66. MD, USA, March 22, 1996. 9 Statistical Methods in Research and Production, ed. Davies, O. L., 2 Radmacher, H. W., and De Swardt, M. C., Spectrochim. Acta, and Goldsmith, P. L., Hafner, New York, 4th edn., 1972, pp. 61–62. Part B, 1975, 30, 353. 10 Taylor, B. N., and Kuyatt, C. E., Guidelines for Evaluating and 3 Fujita, M., Kashima, J., and Naganuma, K., Anal. Chim. Acta, Expressing the Uncertainty of NIST Measurement Results (NIST 1981, 124, 267. T echnical Note 1297), US Government Printing OYce, 4 Weiss, Z., Spectrochim. Acta, Part B, 1996, 51, 863. Washington, DC, September, 1994, pp. 7–8. 5 Fang, D., and Marcus, R. K., in Glow Discharge Spectroscopies, 11 Miller, J. C., and Miller, J. N., Statistics for Analytical Chemistry, ed. Marcus, R. K., Plenum Press, New York, 1993, ch. 2. Ellis Horwood, Chichester, 1984, pp. 56–57. 6 Bevington, P. R., and Robinson, K. D., Data Reduction and Error Analysis for the Physical Sciences, McGraw-Hill, New York, 2nd Paper 7/02873C edn., 1992, pp. 100–101. Received April 28, 1997 Accepted June 20, 1997 Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12 1305

ISSN:0267-9477    

DOI:10.1039/a702873c

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

System for Automatic Selection of Operating Conditions for Inductively Coupled Plasma Atomic Emission Spectroscopy WAYNE BRANAGH†, CIARA WHELAN AND ERIC D. SALIN* Department of Chemistry, McGill University,Montreal, Quebec, Canada H3A 2K6 The design of a system for automatic parameter and that all the data collected was being analyzed properly, then post-analysis processing might be satisfactory; however, new methodology selection for inductively coupled plasma atomic emission spectroscopy (ICP-AES) is presented.The ICP is an calibration methodologies may have to be selected or operation conditions may have to be changed to provide accurate and excellent excitation source that suVers less from matrix eVects than other sources but is not altogether problem-free. To precise analytical results. As a reflection of this need for real time data analysis and control, a number of laboratories have compensate, a variety of methodologies and approaches can be used to minimize matrix-induced error present in an analysis.been working in the area of atomic spectrometry with techniques ranging from ‘traditional’ expert systems1–3 to pattern For example, one can prepare matrix-matched standards, use standard additions or select instrumental parameters (rf power, recognition4 and data processing.5,6 The Autonomous Instrument Project1 has been underway pump rate and viewing height) specifically for the analytes of interest. Ideally, an Autonomous Instrument, operating without in our laboratory for some time.Its goal is the development of software that will allow an instrument to operate as indepen- human intervention, needs to have the ability to determine the analytical parameters and methodology to apply to a sample. dently of the operator as possible. The top level of the system provides the highest level of cognition and consequently is the A measure of expected analytical error for a sample to be used by an Autonomous Instrument, the total interference level most complex.Its control process is very similar to the list of actions performed by an analyst when analyzing a sample. (TIL), is introduced. The TIL applies the results from a semiquantitative scan to a ‘lookup’ table to determine what the During its development, we have also found that it is the most dynamic segment of the software. As greater sophistication has approximate expected error will be. It relies on a ‘conservative’ approach and assumes the eVect of interferences been required to deal with diYcult conditions, major restructuring has been necessary.A flow chart of the basic components on a particular analyte are not only linear, but additive. This approach was applied to samples with high concentrations of of the thought pattern is presented in Fig. 1. The flow has interferents and works well, providing acceptable estimates in 90% of analyte–interferent combinations (at 10% error tolerance).The emphasis of the discussion is placed on how well this system deals with ‘problematic’ samples, i.e. those samples where matrix eVects cause considerable analytical error. Keywords: Matrix eVects; analysis parameters; inductively coupled plasma atomic emission spectroscopy; expert systems In the analytical laboratory, eVorts at automation have been very successful. In the last decade auto-diluters and sample changers have matured to the point where it is now possible to make standards or spike solutions automatically. This capability also has the potential for allowing liquid standards to be prepared automatically with a wide variety of calibration methodologies including standard additions, internal standards, buVering and dilution.The next level in automation will go beyond mechanical automation to automation of the thought process. This level of automation is desirable not only from an economic standpoint, but for practical reasons as well.One must appreciate that many modern instruments can generate far more information than a human operator can handle in a reasonable amount of time. For example, a modern semiconductor-based inductively coupled plasma (ICP) atomic emission spectrometer can generate 200 000 data points per minute. In this data, there may be only 500 points that are critical to the analysis at hand. Recent developments in sample introduction methodologies will soon increase the rate by an order of magnitude or more. If the analyst could be assured † Present address: CETAC Technologies, Inc., 5600 South 42nd St., Fig. 1 Autonomous Instrument flow chart. Omaha, NE 68107, USA. Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12 (1307–1315) 1307been discussed in detail previously.1 For the purposes of this the TIL, all elements present in the sample at a concentration of greater than 100 ppm are considered interferents. If the TIL discussion, the most important path is through the center for an unknown sample.In this case there is no prior knowledge exceeds a predefined ‘maximum error’ threshold set by the user (a TILT condition), then the system would ‘know’ that about sample composition, as there would be on the left hand path, optimizing accuracy using a reference material, nor are the sample is problematic and that the analysis is probably not possible using external standards. As Fig. 1 demonstrates, there the built-in constraints of a quality control test.The ‘unknown’ path is just that, because there is no feedback on the TIL sits at a major decision nexus in the entire thought pattern and dictates whether the sample is to be treated as a the quality of the results. The first step down this path involves estimating what will probably be the most diYcult obstacle conventional or a problematic sample. One of our basic assumptions is that any modern instrument for Autonomous Instruments, matrix eVects.Matrix eVects are a complex problem and a great deal of can quickly collect an SQS by rapidly scanning or integrating the spectrum. The SQS provides a rough estimate of the research has been done on single families of interfering elements (e.g., easily ionizable elements, EIEs7,8) or with the eVect of analyte concentrations and can be used as the basis for pattern recognition, if the sample is to be identified, or as a first step specific interferents on several analytes.9,10 Sophisticated models for predicting single matrix eVects have been pro- in an evaluation process, if the eVect of a matrix on an analyte is to be determined.After the SQS has been performed, the posed,11 but few papers discuss the potentially complex eVect of several interferences on individual analytes and the eVect of TIL is computed. At this point a decision is made regarding the complexity of the sample. In many real samples, the TIL multiple interferences in combination.This is not surprising since the eVect that some relatively simple matrices exert on value will be below the operator specified accuracy threshold, and the sample can be analyzed using external standards with analytes can be quite complex.10 For example, diVerent mechanisms exist, including electron enhanced collisional excitation standard conditions. When the sample has a problematic matrix and analyzing it under standard conditions will yield of the analyte, hindrance of atomization and excitation higher up in the plasma, and ion–electron recombination even further poor results, pattern recognition is applied to the sample to see whether the sample type has been run previously.If the up in the plasma. Based on the nonlinear and often unpredictable ways inter- sample is similar to previously analyzed samples, analysis parameters and/or the calibration approach can be read from fering species can combine, it becomes almost impossible to predict accurately the analytical error that one will encounter the sample database and the sample can be analyzed using those settings.for a combination of several analytes and multiple interferences. It has been demonstrated that there is a complex correlation In the more general case when the TIL threshold is exceeded, suggesting that interferences may be important, the system between observed interference eVects and almost all known variable operating parameters of the ICP.12 In the absence of also considers four constraints: time (which we consider to be the equivalent of cost), the amount of sample that can be used, a solid theoretical model, one might consider exhaustively analyzing all possible combinations of interferences at diVerent the accuracy desired and whether the sample is the first in a batch of similar composition.The SQS data is used first to concentrations at all possible wavelengths of interest to obtain a good empirical matrix eVect model.This is obviously imprac- make an ordered list of applicable methodologies ranging through standard additions, internal standards, matrix match- tical. The best one can do is characterize the eVect of ‘typical’ matrices on diVerent analytes and apply simplifying assump- ing, buVering and two types of external standards calibration. The constraints are then considered, starting with the most tions if multiple interferences are encountered. Given the almost infinite number of combinations of analytes ‘desirable’ methodology.Each methodology is considered until a match is found for all the constraints. This methodology is and matrices that are present in real samples, an Autonomous Instrument needs the capability to calculate the problematic then used. The reasoning of the system can sometimes be quite interesting. For example, if the SQS determines that analyte nature of a sample, i.e., is this sample matrix problematic for the analytes? One way to determine this is to rely on the concentrations are high, then dilution becomes a viable option.If analyte concentrations are low and the amount of sample is information from a semiquantitative scan (SQS) and from calculating the total interference level (TIL) [eqn. (1)]. In limited (thereby giving a limited amount of time for the sample analysis), then internal standards becomes a more attractive order to compare the results from a SQS with previously obtained values, it is important that each SQS is performed methodology.When analyzing a sample of unknown composition, the under a standard set of conditions with a single calibration standard having been run at those same settings. requirements are quite diVerent from ‘learning’ to run a known composition sample type because there is no feedback on In the calculation of the TIL, a series of interference coeYcients (ICs) are used. These are rough indicators of the accuracy.The system must make its best estimate of the calibration methodology and associated parameters by using suppression or enhancement eVect that an interfering element has on analytes in an ICP when standard operating conditions logic and pattern recognition. In essence, it is a ‘one shot deal’ rather than an iterative learning process. The ‘one shot’ process are used. The ICs are determined experimentally by measuring analyte intensities in the presence of diVerent interferences. of selecting a methodology becomes more diYcult when one considers the problems that may be encountered.When a sample is to be analyzed, the ICs and the estimated elemental concentrations from the SQS can be used to calculate In the process of developing the TIL, a table of ‘best’ instrumental settings was developed. The ‘best’ instrumental the expected TIL as presented in eqn. (1), settings are those settings which result in a minimum error for a particular analyte in the presence of a single major interfering TILx= .n interferences m=1 fm,x(Cm)ICm,x (1) element. These ‘best’ parameters were tested, and the results are presented in the second part of the paper. The ‘best’ where TILx is the total interference level of analyte x, Cm is the SQS concentration estimate for interferent m, ICm,x is the parameters to use for an analysis should be qualified. Normally, an investigation of analysis parameters would be done in an interference coeYcient for interferent m acting on analyte x and fm,x( ) is the function describing how the analyte is aVected exhaustive examination of combinations of diVerent parameters.All of the instrumental parameters: gas flows; power by diVerent concentrations of interferent m. In reality, the eVect of the diVerent concentrations of a particular interferent on an settings; viewing height, are under computer control on our instrument and are varied in discrete steps. This results in only analyte is often nonlinear,10 however, for simplicity, it is assumed here that the function is linear.For the purpose of a finite number of settings for each parameter. In the case of 1308 Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12Table 2 Parameter combinations used in matrix eVect study power, the settings are valid from 750 to 1750 W, in increments of 200 W. A further handicap present is the speed of the Parameter Pump rate/ instrument.Adjusting parameters and analyzing multiple anacombination Rf power/W Viewing height/mm ml min-1 lytes on this instrument is a time-consuming process, especially 1 750 10 0.5 compared with more modern, CCD-detector based systems. 2 750 20 1.0 For this reason, an exhaustive examination of all instrument 3 1550 10 1.0 setting combinations was not possible. Combinations of three 4 1550 15 0.5 values for each parameter, rf power, viewing height and sample 5 1550 20 1.5 6 750 15 1.5 solution feed rate, were used.The ‘best’ settings are actually 7 1150 10 1.5 the best setting combinations examined. 8 1150 20 0.5 9 1150 15 1.0 EXPERIMENTAL To measure the eVect of a matrix on several analytes, solutions Model-25 instrument (Thermo-Jarrell Ash, Franklin, MA, were prepared containing a variety of commonly determined USA) using a TJA high-salt nebulizer after a 30 min instrumen- elements (Table 1). Solutions with 10 ppm of these analytes tal warm up. were prepared in a variety of single interferent matrices.The In order to verify the TIL values generated, solutions with analytes were added in the form of a multi-element standard 10 ppm of the same collection of analytes were prepared in a (SCP Science, St. Laurent, Canada). In the generation of the variety of single and double interferent matrices with concen- interferent coeYcients, all possible analyte–interferent combitrations ranging from 100 to 10 000 ppm. Again, matrix- nations should be explored; however, for practical reasons, we matched blanks were prepared for each sample.The analytes simplified our universe to only common analytes or internal in the samples, blanks and standards were analyzed by external standards: Y, Pt, Cu, Ca, Fe, Pb, Mg, Ni, Zn, Al, K and Na. standards under ‘standard conditions’ (1150 W, 15 mm viewing To simplify experiments further, the number of possible interheight, 1.0 ml min-1). ferents was also limited to: Na, K, C, Ca, Cu, Mg, Fe, Zn and In addition, the validity of the best instrumental parameters Al.These particular interferences were chosen because they obtained was tested. Samples were prepared in Na, K, Fe, Al, are commonly found in many analytical matrices (biological Mg and C matrices with interferent concentrations ranging samples, geological samples, etc.). from 500 to 5000 ppm. Each of these samples was analyzed at The interferents were employed at a concentration of the ‘optimum’ parameter settings for the combination of ana- 3000 ppm.Half of the analytes were analyzed using atomic lyte and interferent. The accuracies of determinations were lines and the others were analyzed using ionic lines as shown compared with those obtained when the sample was analyzed in Table 1. All of the interferents were prepared from reagentunder standard conditions. grade nitrates (Merck, Poole, UK) with the exception of C, which was added in the form of glacial acetic acid (Fisher Scientific). A matrix-matched blank was also prepared for each RESULTS AND DISCUSSION sample in order to restrict the sources of error to those due Initially experiments were undertaken in order to characterize strictly to the presence of the interferent. Because of the high the eVect of diVerent matrices on the selected analytes and to concentration of interferents added, a matrix-matched blank determine which instrumental setting combinations minimized eliminates concern over reagent contamination of the interthe matrix eVects.The matrices chosen consisted of commonly ferent with trace levels of analyte. It also eliminates spectral available elements, both EIEs and non-EIEs, and analytes in interferences, but does not compensate for viscosity eVects each of the matrices were analyzed at a variety of instrumental such as the situation when less analyte reaches the plasma settings. The eVects of most of these matrices have been well because of reduced nebulization eYciency.In some respects, characterized in the literature already, though not for all of the error measured for each analyte is the ‘best’ (i.e., least) the analytes presented here. The eVects of the matrices on each error possible. In actual analyses, a matched blank would analyte are displayed in Figs. 2–10. Each figure presents the normally not be available and the errors encountered might matrix eVects present for a particular set of analysis parameters.be higher. It can be seen that some parameter combinations (1, 2 and 3, The analytes in the samples, blanks and standards were analyzed by external standards using diVerent combinations of viewing heights, rf power settings and pump rates (Table 2). These parameter combinations were chosen using a partial factorial experimental design in order to achieve the greatest variety of combinations. All experiments were run on a TJA Table 1 Analyte data Excitation Ionization Analyte potential/eV potential/eV Wavelength/nm YII — 6.22 371.0 PtI — 9.00 265.9 CuI 3.82 7.72 324.7 CaII 7.05 6.11 317.9 FeII 4.77 7.87 259.9 PbII 7.3 7.4 220.3 MgII 8.86 7.65 279.0 NiII 6.39 7.64 231.6 ZnI 5.80 9.39 213.8 AlI 4.01 5.99 309.2 KI 1.62 4.34 766.4 NaI 2.10 5.14 589.5 Fig. 2 Analytical error for parameter combination 1. Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12 1309Fig. 6 Analytical error for parameter combination 5. Fig. 3 Analytical error for parameter combination 2. Fig. 4 Analytical error for parameter combination 3. Fig. 7 Analytical error for parameter combination 6. Fig. 5 Analytical error for parameter combination 4. Fig. 8 Analytical error for parameter combination 7. Figs. 2–4) are clearly less eYcient at reducing the eVects of matrices than others (combinations 5, 7 and 9, Figs. 6, 8 and (Fig. 7) uses low power and is relatively problem free, and parameter combination 3 (Fig. 4) uses high power and some 10).One could assert that the primary cause of the matrix eVect is the low power used (750 W). Unfortunately, this analytes still suVer serious eVects. One can take the coarse view of averaging the eVect of each assertion is inconsistent with two observations: combination 6 1310 Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12Fig. 12 Susceptibility of analytes to matrix eVects. potentials (1.62 to 4.01 eV), Pb was analyzed at an ionic line that had a high excitation potential (7.3 eV).In contrast, other Fig. 9 Analytical error for parameter combination 8. low excitation potential atomic lines, such as Cu, were not aVected appreciably by these matrices nor were high excitation potential ionic lines like Mg. This lack of readily definable trends suggests that interferent–analyte combinations will need to be considered on an individual basis. The eVect individual matrices had on the analytes present is summarized below.EVect of K and Na Matrices The eVects of EIE matrices, especially Na, have been thoroughly studied. These elements have been reported to cause a vertical shift in the emission intensity profiles for analytes9 with a general enhancement at lower viewing heights.10 Since, in the current work, the viewing height, pump rate and rf power are changed, a direct comparison can not be made with previous published work. When samples were prepared with Na or K present as an interferent, the most adversely aVected analytes were EIEs and Al. The eVect was especially pronounced at lower power settings.As discussed above, these analytes were measured at atomic lines with low excitation potentials and this trend was not indicative of lines with similar Fig. 10 Analytical error for parameter combination 9. characteristics like Cu. At lower power settings and lower viewing heights, Na and of the matrices over all of the analytes to see, in general, which K matrices did enhance the emission of the analytes at both matrices are most problematic.In Fig. 11, one can see that of ionic and atomic lines (Fig. 2). At a higher viewing height of the matrices examined, Ca, C, Mg, Na and Al are the most 20 mm and the same power setting, suppression of the analytes problematic, yielding the highest average absolute error for was noted (Fig. 3). This was probably due to the presence of the analytes. Errors encountered for the analytes varied from a crossover point.13 At this point in the plasma, the enhance- 9 to 13%.The most aVected analytes, in order of decreasing ment at the edge of the plasma and the emission depression sensitivity, are K, Na, Al and Pb (Fig. 12). There is no in the central channel balance each other. For the analytes explanation for why Pb was aVected so dramatically. While examined, high power settings were generally favorable for K, Na and Al were examined at atomic lines with low excitation minimizing error in the presence of K.EVect of C Matrix Carbon, in the form of acetic acid (HAc), was also examined as a matrix element. It has been reported that in the presence of acetic acid, there is a decrease in emission intensity due to loading of the plasma.9 In our case, though, the concentrations of HAc used were much lower and this same decrease in intensity was not present. On the contrary, an enhancement of emission was noted at most power settings. At these concentrations, acetic acid may behave like the structurally similar ethanol.Ethanol in small concentrations increases the temperature of the plasma14 and enhances emission by approximately the amount reported (12% versus 14% reported). If there was an increase in plasma temperature, it seems reasonable that the profile of the plasma would be raised, resulting Fig. 11 Average absolute matrix eVect caused by each interferent. in a higher optimal viewing height. Interestingly, the enhance- Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12 1311ment was most prominent for parameter combinations 5, 6 ments and suppressions were analyte dependent.Some elements, like Y, were always suppressed while others were and 8, which use relatively high viewing heights (15–20 mm). McCrindle14 has suggested that the presence of ethanol can either suppressed or enhanced a great deal. The eVect on K, for instance, ranged from +93 to -14%, depending on the be used to increase detection limits.It would seem that this is possible, even using small amounts of HAc, providing that analysis parameters used. If an analysis of K and another analyte is performed by ICP spectrometry in an Al matrix, some problematic instrumental settings are avoided (combinations 2 and 4). variability like this could cause great problems for the selection of compromise conditions. Fortunately, analytes other than K and Pb were not aVected appreciably by 3000 ppm Al.EVect of Ca and Mg Matrices If one considers a single analyte and the related interferent, Due to their ubiquitous nature, any instrument capable of a table of best analysis conditions can be generated for the automatic analysis must be able to deal with analytes in a Ca constrained universe (Table 3). This table could then be used or Mg matrix. Ca has been widely studied, even to the point to counter the eVect of an interference. Initially, an SQS could where an automated correction system has been suggested to be performed and the appropriate conditions could simply be reduce interferences by interactive power adjustment.15 In our looked up.This approach could also be applied to determine samples, the presence of Ca most adversely aVected K and, to how badly aVected analytes are by single interferences. In the a lesser extent, Al and Na. This is a similar trend to that seen presence of multiple interferences, though, determining how with the EIEs.Most analytes were suppressed from -139 to badly an analyte is aVected becomes less straightforward. The -3%, depending on the parameters used. use of an objective measure, the TIL, allows one to easily As it is in the same elemental family as Ca, one would classify samples as problematic or not. expect Mg to cause similar eVects and it does, aVecting K, Na and Al most noticeably. The eVect of magnesium has a Validation of the TIL crossover point for analytes with a high ionization potential The TIL was developed to provide a fast, approximate measure (IP), with an enhancement low in the plasma and a suppression of the worst analytical error likely to appear in an analysis further up.Analytes with high IP values like Zn and Pt can with a given sample. In the best case, the TIL would be a be enhanced or suppressed by Mg. direct measure of the amount of error that would be encountered if the sample were analyzed using external standards EVect of Fe and Zn Matrices under standard operating conditions. If the TIL is not an accurate measure of the error encountered, it is still considered In the Fe and Zn matrices, the most adversely aVected analytes acceptable, although less desirable, if the TIL is pessimistic were K, Pb and Na, with the best analysis parameters being and predicts that the sample is problematic when it is not (i.e., analyte specific.Generally, the eVect of these interferences was TIL value.actual error).In the worst cases, the TIL value not significant for most of the analytes examined. would be lower than the actual error, in eVect indicating the sample would not be problematic when really it is. EVect of Al Matrices Samples with a variety of problematic matrices were prepared containing the same analyte as before (Table 1). The When dealing with Al matrices, researchers have noticed inconsistent interference eVects.10 In the current work, enhance- TIL values were calculated for each of the analytes in the Table 3 Best parameter combinations for given analytes and interferents at 3000 ppm and expected percentage error (in parentheses)* Na Mg Al Zn Fe K Ca C Cu Y 6 3 5 6 4 9 9 4 6 (0.1) (1.5) (-0.4) (-0.4) (-0.2) (-0.5) (0.9) (-1.8) (0.3) Pt 6 8 6 8 9 3 4 7 1 (0.5) (-1.7) (-0.5) (-0.5) (-0.2) (-3.8) (0.6) (-0.8) (1.0) Cu 2 1 8 5 5 8 1 2 — (-0.4) (0.8) (-0.4) (-0.1) (-0.4) (1.2) (-0.4) (3.0) Ca 6 3 1 9 7 4 — 1 1 (0.5) (-1.7) (-0.0) (0.2) (0.6) (0.1) (0.3) (-0.7) Fe 5 7 9 6 — 8 8 4 6 (-0.2) (-0.1) (-0.3) (-0.4) (0.0) (-1.5) (-1.0) (-1.8) Pb 2 3 9 6 3 7 9 7 9 (-0.3) (1.5) (3.9) (-0.1) (0.3) (-0.4) (0.5) (1.8) (1.2) Mg 1 — 7 5 1 1 1 8 4 (0.6) (0.3) (-0.2) (0.4) (0.7) (0.0) (2.2) (-1.0) Ni 8 7 5 4 1 7 1 4 9 (0.7) (1.1) (-0.5) (0.1) (0.5) (-0.4) (-0.3) (1.4) (0.1) Zn 3 3 5 — 8 3 7 2 7 (1.7) (2.1) (0.0) (1.0) (-1.6) (-0.5) (-0.4) (-1.5) Al 2 1 — 7 1 7 4 2 3 (-1.6) (-6.6) (0.1) (0.4) (-5.3) (0.0) (-2.9) (-0.9) K 8 9 1 5 6 — 7 7 6 (-9.4) (0.9) (0.8) (1.5) (1.7) (7.8) (2.5) (3.0) Na — 5 7 7 5 4 2 2 9 (-0.8) (5.6) (0.6) (0.2) (-5.0) (7.9) (-0.8) (0.1) * Analytes are listed vertically, interferences are listed horizontally. 1312 Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12samples according to the formula presented previously. The where the TIL overestimates the error in the sample and cases where the TIL underestimates the error present in the sample. percentages of analytes that agreed with the TIL value within diVerent tolerances are presented in Figs. 13–15. The number Of the three instances, only the last is unacceptable. When a tolerance of 5% for agreement between the TIL and the actual of analytes that fell into each of three diVerent categories are shown: cases where the TIL predicts the error correctly, cases error is used, 73% of the TIL values are acceptable (Fig. 13). Fig. 13 Prediction of analytical error by TIL (5% tolerance).Fig. 14 Prediction of analytical error by TIL (10% tolerance). Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12 1313Fig. 15 Prediction of analytical error by TIL (20% tolerance). Relaxing the tolerance to 10% means that 90% of the analytes in the samples would have acceptable TIL values calculated for them (Fig. 14). At a generous tolerance of 20% (Fig. 15), 98% of the analytes’ errors are predicted or overestimated by the TIL. Given the non-linear nature of the matrix eVects and the rather simplistic approach adopted by the TIL calculation, it is not unreasonable that a few underestimations occur.The TIL assumes a linear increase of the matrix eVect with increasing concentration. A more realistic approach would be to replace the linear function, fm,x( ), with a function that more accurately reflects the nature of the matrix eVect. Considerably more work would be required to obtain the data required to generate these curves, but would undoubtedly result in more accurate TIL values for single interferences.Alternatively, the function describing the matrix eVect for a particular interferent operating on an analyte could be refined in an iterative Fig. 16 Capacity of instrumental parameters to overcome matrix eVects. approach. Whenever a sample with known analyte concentrations, such as a reference material, was analyzed, additional information about the eVects of a matrix on an analyte would be obtained and could be applied to generating a more to use to minimize the error present in the analysis (Table 3).accurate TIL. If only the external standards methodology is selectable due In the case of matrices containing two interferences, the TIL to cost or sample size limitations, one can analyze the sample generally performed well, but it did underestimate the error using the ‘best’ settings, taking advantage of the relatively present for 3000 ppm K and 3000 ppm Mg.The worst failure rapid methodology of external standards, yet still suVering was an error underestimated by 50%. Given the complexity through minimum error. of these systems, it was expected that some would behave Using the results of the TIL work, the best instrumental unpredictably. Generally, the TIL is quite usable as a quick conditions were tabulated. Samples containing diVerent conmeasure of the problematic nature of a sample. centrations of the most problematic interferences and several analytes were prepared and analyzed at the ‘optimum’ conditions previously determined.The results are presented in Application of Best Parameter Combination Table Table 4. There was an overall improvement in the accuracy for the It is comforting to note that the application of standard conditions (combination 9, Fig. 16) usually results in minimal majority of the analyte–interferent combinations. In some cases, the improvement was quite dramatic (e.g., 5000 ppm Al error compared to the other setting combinations (Fig. 16). In the case where standard conditions can not be used to perform acting on K and Na, improved from 17.9% error to -3.7% error and from 37.5% error to 10.5% error) when optimal the analyses, Figs. 2–10 can be condensed to provide the instrument with a table of ‘optimum’ parameter combinations conditions were used. In other cases, however, there was a 1314 Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12Table 4 Improvement in error by running at optimally determined conditions % Experimental % Error % Experimental Expected Actual error (standard expected error (optimum % % Matrix/ppm Analyte conditions) (optimum conditions) conditions) improvement improvement 5000 Al K 17.9 0.8 -3.7 17.1 14.2 5000 Al Pt -8.3 -0.5 -5.2 7.9 3.1 5000 Al Na 11.1 5.6 6.7 5.5 4.4 5000 Al Mg -5.1 0.3 -5.4 4.8 -0.2 1000 C Pt 5.3 -0.8 -0.4 4.5 4.9 1000 C Pb 3.8 1.8 2.5 2.0 1.3 1000 C K 4.4 2.5 1.3 1.9 3.2 1000 C Cu 6.3 3.0 0.0 3.3 6.3 1000 C Zn 5.6 -0.4 1.9 5.2 3.6 1000 C Al 4.4 -2.9 4.2 1.5 0.2 1000 C Na 3.5 -0.8 3.6 2.7 -0.1 1000 C Ca 3.4 0.3 -0.4 3.2 3.1 5000 K Mg -2.4 0.7 -4.5 1.7 -2.1 5000 K Pt 6.7 -3.8 11.7 2.8 -5.0 5000 K Zn 4.8 -1.6 5.6 3.2 -0.8 5000 K Pb 11.6 -0.4 16.8 11.2 -5.2 5000 K Ni 2.0 -0.4 7.9 1.6 -5.9 500 Mg Cu -4.5 0.8 -7.7 3.7 -3.2 500 Mg Fe -5.8 0.1 -4.7 5.7 1.1 5000 Na Mg -1.2 0.6 19.0 0.6 -17.8 5000 Na Pb 4.0 -0.3 -2.8 3.7 1.2 5000 Na Al 37.5 -1.6 10.2 35.9 27.3 5000 Na Zn 5.2 1.7 8.1 3.5 -2.9 5000 Na Y 6.1 0.1 6.5 6.1 -0.4 5000 Na Pt 2.9 0.5 2.9 2.4 0.1 5000 Na Ca 2.5 0.5 2.2 2.0 0.3 2000 Fe Mg -2.2 0.4 -4.5 1.8 -2.3 2000 Fe Ni -1.0 0.5 -1.4 0.5 -0.4 2000 Fe K 2.0 1.7 -0.3 0.2 1.7 2000 Fe Ca -6.8 0.6 -5.8 6.2 1.0 noticeable degradation of the analytical results compared to 2 Webb, D.P., Hamier, J., and Salin, E. D., T rends Anal. Chem., 1994, 13, 44. standard conditions (e.g., the eVect of 5000 ppm Na on Mg, 3 Sharbari, L., and Stillman, M. J., Anal. Chem., 1994, 66, 2954. and the eVect of 5000 ppm K on Pt). These instances may be 4 Branagh, W., Yu, H., and Salin, E. D., Appl. Spectrosc., 1995, caused by too many simplifying assumptions in describing 49, 964. matrix eVects. It was assumed that the eVects of matrices were 5 Karanassios, V., presented at the 78th Canadian Society for linear and diVerent species did not interfere with each other. Chemistry Conference and Exhibition, Guelph, Ontario, Canada, This may be too simplistic an approach to take for real world May 1995. 6 Catasus, M., Branagh, W., and Salin, E. D., Appl. Spectrosc., samples, especially when these eVects are known to be 1995, 49, 798. non-linear. 7 Blades, M. W., and Horlick, G., Spectrochim. Acta Part B, 1981, In conclusion, with a minimal number of experiments, it is 36, 881. possible to obtain suYcient information to enable an 8 Galley, P. J., Glick, M., and Hieftje, G. M., Spectrochim. Acta, Autonomous ICP-AES Instrument to determine approximately Part B, 1993, 48, 769. how problematic a sample is using the TIL, as well as to 9 Sager, M., ICP Inf. Newsl., 1991, 17, 435. 10 Dahai, S., Zhanxia, Z., Haowen, Q., and Mingxiang, C., generate a list of instrumental conditions that can be used to Spectrochim. Acta Part B, 1988, 43, 391. minimize matrix eVects. 11 Ramsey, M. H., and Thompson, M., J. Anal. At. Spectrom., 1986, 1, 185. The authors wish to thank NSERC for operating funding and 12 Tripkovic, M. R., and Holclajtner-Antunovic, I. D., J. Anal. At. Thermo Jarrell Ash for ongoing support. Spectrom., 1993, 8, 349. 13 Wu, M., and Hieftje, G. M., Spectrochim. Acta Part B, 1994, 49, 149. 14 McCrindle, R. I., and Rademeyer, C. J., J. Anal. At. Spectrom., 1995, 10, 399. 15 Thompson, M., Ramsey, M. H., Coles, B. J., and Du, C. M., J. Anal. At. Spectrom., 1987, 2, 185. REFERENCES Paper 7/04601D 1 Branagh, W., and Salin, E. D., Spectroscopy (Eugene, Oreg.), 1995, 10, 20. Received July 1, 1997 Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12 1315

ISSN:0267-9477    

DOI:10.1039/a704601d

出版商:RSC    

年代:1997

数据来源: RSC