JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 193 Speciation Analysis of Chromium by lnductively Coupled Plasma Mass Spectrometry With Hydraulic High Pressure Nebulization* Norbert Jakubowski Brigitte Jepkens Dietmar Stuewer and Harald Berndt lnstitut fur Spektrochemie und angewandte Spektroskopie Postfach 10 13 52 0-440 13 Dortmund Germany The speciation analysis of Cr was studied using hydraulic high pressure nebulization in combination with inductively coupled plasma mass spectrometry. Ion-pair chromatography with tetrabutylammonium acetate as the ion-pairing reagent and 25% methanol in the eluent was applied for the separation of Cr"' and Cr"'. Interferences from carbon could be reduced by the addition of oxygen to the aerosol gas. With careful optimization of the operating conditions a procedure has been established that results in detection limits of down to 1 ng ml-' for the Cr species.Keywords lnductively coupled plasma mass spectrometry; nebulization; speciation; chromium Speciation analysis is becoming of greater interest mainly being focused on selected elements the species of which appear in environmental samples and are known or at least suspected to be toxic. Among these elements Cr is obviously of particular interest as it is widely distributed in the environment because of many industrial applications e.g. in galvanization and the steel industry. It exists in several species of which Cr"' is considered to be essential whereas CrV' is thought to be strongly toxic owing to its high oxidation potential and the ease with which it penetrates biological membranes.This is the reason why speciation analysis of Cr in particular in drinking water has already been investigated extensively by application of different techniques such as precipita- tion adsorption solvent extraction volatilization and chromatography. Not only does speciation analysis require selectivity but also high sensitivity because in real samples the elements in ques- tion must be determined at ultratrace levels. Inductively coupled plasma atomic emission spectrometry (ICP-AES) and mass spectrometry (ICP-MS) together provide these require- ments,' and in particular ICP-MS excels by offering not only extremely low detection limits but also real multi-element capabilities and isotope information the latter enabling the application of isotope dilution techniques which are unsur- passed for accuracy of quantificati~n.~,~ Considerable work has already been devoted to the perform- ance enhancement of ICP-MS for speciation analysis with respect to the detection power and also to the ease of operation.Detection limits could be improved by preconcentrati~n~.~ or by application of new sample introduction techniques such as electrothermal vaporization thermospray (TS),6 direct injec- tion nebulization ( ultrasonic nebulization ( USN),9 hydraulic high pressure nebulization ( HHPN)1° and direct coupling of gas chromatography to ICP-MS.ll Most profitable as regards the ease of operation is an analytical procedure that enables the determination of the species in question to be made in a single step.The analysis time can be reduced considerably by application of on-line techniques which have recently been reviewed by Sperling et aL4 for speciation analysis of Cr. In particular high-performance liquid chromatography (HPLC) on-line separation which combines high chromato- graphic resolution with a short analysis time is favourable. Procedures for speciation analysis by plasma source mass spectrometry have been developed for several important elements.12 However it is somewhat strange that so far there are only a few results available for the speciation analysis of Cr by plasma source mass ~pectrometry.'~ With organic liquids plasma source spectrometry is generally impeded by inter- * Presented in part at the XXVIII Colloquium Spectroscopicum Internationale (CSI) York UK June 29-July 4 1993.ferences owing to the presence of carbon and in particular this is the case for Cr because the main isotope 52Cr+ is obscured by the abundant molecule 40Ar'2C+. Clogging of the sampling orifice is a further problem that arises as a conse- quence of the high carbon content. Addition of oxygen to the aerosol gas could be considered as a means of reducing any problems due to interferences from carbides and clogging. In a previous in~estigation,'~ it has been shown that HHPN as a sample introduction technique in ICP-MS simply com- bined with an effective method of desolvation can increase the sensitivity for the majority of elements in comparison with conventional pneumatic nebulization. In principle it is extremely well suited for direct coupling with HPLC.Thus it was clear that an investigation should be carried out of the capabilities of HHPN for the speciation analysis of Cr because with respect to the above mentioned requirements the tech- nique seems particularly appropriate for this type of appli- cation. For chromatographic separation of the Cr"' and CrV' species ion-pair chromatography with the reagent tetrabu- tylammonium acetate (TBAA) was utilized according to a procedure which was developed by Syty et aE.15 for on-line separation of Cr species and that was later modified by Berndt and co-workers for atomic absorption spectrometric (AAS)16 and ICP-AES dete~tion.'~ Experimental A schematic diagram of the whole experimental arrangement is shown in Fig.1; operating details are compiled in Table 1 t 6 -7 Ar II "V 5a C ' 4 7 I 5b - 7 3 1 / \ Fig. 1 Schematic diagram of experimental arrangement 1 HPLC pump; 2 sample loop; 3 chromatographic column; 4 HHPN nebuliz- ation chamber with impact bead; 5 desolvation system with heating (5a) and two-stage cooling (5b and 5c); 6 ICP torch; and 7 drain194 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 Table 1 Compilation of operating conditions HH PN system- Sample uptake rate Sample loop volume Pressure Nozzle diameter Desolvation system- Heating temperature Cooling temperature ICP-MS system- Power Reflected power Aerosol gas flow rate Intermediate gas flow rate Outer gas flow rate Oxygen flow rate Sampling distance Data acquisition system- Dwell time per data point Total acquisition time 1.0 ml min-' 0.1 ml 9.0 MPa 15 pm 120°C First stage 0°C Second stage -20°C 1500 W 25 W 0.75 1 min-' 0.5 1 min-' 15 1 min-' 0.06 1 min - ' 10 mm 327.68 ms ~ 3 0 0 s including the standard parameters derived from optimization experiments.The dual piston HPLC pump (Knauer Berlin Germany) enables flow control and is specifically for elemental analysis equipped with an inert Ti pumping head. The sample is introduced by a metal-free syringe to the system via an inert sample introduction valve (Knauer) connected to a 100 pl loop made from poly ether ether ketone ( 5 cm x 4.6 mm i.d.) (PEEK). The chromatographic column is filled with particles with a diameter of 5 pm (Eurospher 100 CI8 Knauer). All these components as well as the subsequent HHPN system with the desolvation unit and an inlet for addition of oxygen are operated outside the cabinet of the commercial ICP-MS system (PlasmaQuad PQ2 Turbo Plus Fisons Winsford UK).The design and function of the HHPN unit has been described in more detail e1~ewhere.l~ Specific details for these investigations are a nozzle with a diameter of 15 pm made from Pt-Ir in a nozzle holder made from Ti and fixed in the poly( tetrafluoroethylene) (PTFE) ground plate of the spray chamber an impact bead used as a droplet converter and a spray chamber designed in this laboratory. A Ti filter with a pore diameter of 3 pm protects the nozzle from clogging. It should be mentioned that HHPN is a functional part of the HPLC system; the high pressure of the pump is not only used to overcome the pressure drop of the column but also for nebulization so that a pressure of 9MPa is required with respect to the nozzle diameter and a sample uptake rate of 1 ml min-'.For reasons of transportation only an aerosol gas with an optimized flow rate of 0.75 1 min-' is added tangen- tially to the flow in the PTFE holder. The spray chamber is coupled to a quarz tube (0.8cm i.d. x 30 cm) which is heated by a temperature controlled heating device to about 120 "C. Cooling was performed in two stages. The first uses a glycol-water mixture to achieve a temperature of 0 "C. The second is made up by a water-cooled Peltier element (Type CP1 4-127-45L AMS Thermotech Miinchen Germany) providing a temperature of about - 20 "C. The whole HHPN system including the desolvation equip- ment as described is now commercially available in an inte- grated unit.The solutions draining from the spray chamber and the desolvation system are removed by use of a peristaltic pump. After desolvation oxygen is added to the gas flow which is then fed to the ICP torch uia a flexible tubing with a length of about 1 m. A calibrated flow controller (Type 246 MKS Miinchen Germany) is used to stabilize the flow of oxygen. Stock solutions of 0.1 moll-' TBAA were prepared by dissolution of the solid reagent (Fluka Chemie Neu Ulm Germany) in doubly distilled water. These were used to add the ion-pairing reagent to the sample and acetic acid for adjustment of the pH value to within an operating range of 3.0-3.3. It should be mentioned that 1 x 10-4mo11-1 of ammonium acetate solution (Merck Darmstadt Germany) was always added to the sample which is necessary to improve ion-pairing reactions in samples with a high salt load.Although this is not the case in drinking water samples it was also used here for investigations of different water samples. In the experiments for optimizing performance of the procedure doubly distilled water was used for blank measurements and a solution with a concentration of lOOngml-' of both Cr species was used to obtain the analytical signals. A tap-water sample from this Institute and a mineral-water sample were used in a test analysis. Standards for Cr"' and CrV' as Cr042- were prepared from two stock solutions (Merck) with a total Cr concentration of 1000 mg 1-' each. Diluted standards for calibration or standard additions were prepared daily because of the limited stability particularly of CrV'.18 For optimization of ion intensities in the organic eluent solution a standard solution was prepared containing In at a concentration of 20ngml-I and 25% methanol.A mixture of methanol (Fluka Chemie) doubly distilled water and ammonium acetate (1 x moll-') was used as the eluent the pH value of which was adjusted to within the range 3.0-3.3 by addition of acetic acid (96% Suprapur Merck). Ion-monitoring profiles were recorded at m/z 50 and 53 using the peak-jumping mode with a dwell time of 327.68 ms per data point. The total scan time was limited to about 300 s. Scanning was started just before injection of the sample. The peak area of the analytical signal was used as the intensity value.For correction of the blank value the intensity value of a blank solution was determined in the same way as for the analytical signals. No blank value correction was applied in the optimization experiments. Results and Discussion Optimization In the case of organic solutions independent optimization of instrumental parameters is necessary because standard working conditions differ considerably from those obtained for non- organic solutions. In comparison with the direct determination of Cr from standards diluted in doubly distilled water the detection limits are higher in the case of an organic solution mainly owing to interferences. On injecting the sample via a sample loop Cr"' is not retained by the column and is measured in the more or less aqueous environment of the sample whereas the retained species Cr"" is measured in the organic environment of the eluent mixture. In order to obtain the best detection limits for the more hazardous species optimization was always carried out with the organic eluent containing In as an internal standard.Because optimization of transient signals is difficult a 5 ml loop was used instead of the 100 pl loop so that injection of the organic standard resulted in a signal that remained constant for more than 5 min. Considering the analytical signals of the two Cr species it should be mentioned that it was always possible to achieve identical sensitivity for both species by careful tuning but with respect to the effort required this was not always considered a primary aim in the general optimization experiments.In contrast to previous e~perience,'~ desolvation required a two-stage cooling procedure in the present investigation to remove the methanol-water eluent effectively. In the first stage more than 95% of the water was removed by condensation while the methanol was mainly removed in the second stage. This two-step operation was indispensible because otherwise the aerosol tubing with a diameter of 8 mm would be clogged by ice formation after about 15 min of operation. For operation of the ICP when nebulizing organic solutions the power had to be increased from the usual value of 1.35 toJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRYy MARCH 1994 VOL. 9 195 1.5 kW. Even with the addition of oxygen the reflected power could not be tuned to less than 25 W.This increases with increasing methanol concentration in the eluent and also with increasing oxygen addition. A very high methanol content also leads to unsatisfactory stability. Improvements in the matching unit with respect to applications with organic solutions should provide a challenge for a manufacturer. Problems due to Carbon Several drawbacks to the operation of ICP-MS instrumen- tation can arise from the presence of an organic solvent in the aerosol spectral and non-spectral interferences clogging of the sampling orifice and the skimmer and a loss of sensitivities for certain elements. Utilization of HHPN instead of pneumatic nebulization can fully balance this loss for most elements by the gain in intensity achieved with this technique but only with effective desolvation.The desolvation system with two- stage cooling was therefore an essential pre-requisite of this work. The efficiency of operation is high enough to reduce the organic load to the ICP drastically about 94% can be con- densed from the aerosol before introduction to the ICP torch.20 In order to avoid the sampler and skimmer from becoming clogged addition of oxygen is known to be helpful. Normally this is associated with the disadvantage of the appearance of greater amounts of oxide ions in particular those of Ar. However in the case of Cr this is not a severe limitation. Although expected no increase in erosion of the skimmer or sampler material was observed. Furthermore the addition of oxygen has proved to be an effective means of reducing interferences.In general addition of oxygen leads to reduction of interferences induced by carbon by up to an order of magnitude with the only exception being m/z 56 for which the signal is increased owing to ArO'. Concerning the Cr isotopes almost no reduction of the interfer- ing molecules appears for the main isotope at m/z 52 owing to the high signal of Arc' whereas a significant reduction is observed for the minor isotopes at m/z 50 and 53 which can now be considered for quantitative evaluation. For the two isotopes "Cr and 53Cr considered for quantitat- ive evaluation the influence of oxygen addition has been studied in more detail. The ion-monitoring profiles of the two isotopes for a blank measurement with varying amounts of oxygen added are shown in Fig.2(a). The blank value for 53Crf is about one order of magnitude higher than for "Cr'. From this result higher oxygen addition seems generally to be preferable but this does not hold true when the analytical signals are taken into consideration. This is demonstrated in Fig. 2(b) which shows measurements as before but now with both species of Cr at a concentration of 100 ng ml-' each. For both species a pronounced maximum is observed at a flow rate of about 60 ml min-'. According to its higher natural abundance 53Cr' exhibits the higher intensity but despite this analytical interest was mainly focused on "Cr + because of its lower blank value. Chromatographic Working Conditions For improvement in the optimum chromatographic operating conditions not only was the addition of the ion-pairing agent to the eluent considered as Syty et a/.' did but alternatively addition of TBAA to the sample as has been done previously by Posta et a1.,16 was also investigated.The sample was injected by switching the sample loop into the high-pressure flow. No additional mixing loops were necessary. The results of these optimization experiments are shown in Fig. 3(a) and (b). In the measurements represented in Fig. 3(a) the concentration of TBAA in the eluent was kept fixed at a comparatively high value while the concentration in the sample was varied. Without TBAA in the sample the highest signal intensity was obtained for both species and they are clearly separated. 0.80 4- .- C 3 >.0.60 c g m 0 0.40 2 Y m 2 0.20 1 I I I I I 0 20 40 60 80 100 Oxygen flow rate/ml min-' Fig.2 Measurements for optimization of the oxygen flow rate. (a) Blank value at A 53 and B 50 m/z. (b) Analyte signals from Cr"' and CrV1 100 ng ml-' each A 53Cr"1; B 53CrV1; C s°Cr"'; D 50CrV1 .- -2 .g 8 - C al C Y - 6 4 2 0 100 200 Time/s 300 Fig. 3 Measurements for optimization of TBAA content. (a) A in the absence of TBAA; B 5 x lop3; and C 5 x mol 1-1 TBAA in the samples and a constant concentration of 3 x lop3 moll-' in the eluent. (b) A in the absence of TBAA; B 5 x lop3; and C 5 x moll-' TBAA in the eluent and a constant concentration of 3 x lov5 moll-' in the sample196 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 Increasing the concentration of TBAA in the sample results in a loss of sensitivity with almost no change in the retention time For the measurements represented in Fig. 3(b) the TBAA concentration in the eluent was kept fixed at the low value of 3 x moll-' while the concentration of TBAA in the sample was varied.A minimum concentration of about 5 x mol 1-1 is obviously necessary to achieve satisfactory separation with highest sensitivity for both species; this was therefore included in the final procedure. The preceding investigation was performed using a mixture of methanol and water as the eluent which was known to result in good chromatographic resolution. However variation of the concentration of methanol in the eluent was of course also taken into account for optimization of the operating conditions.From the results represented in Fig. 4 it can be seen that the methanol content influences both the retention time and the signal intensity. A concentration of 25% methanol was chosen as an acceptable value in order to achieve high sensitivity for both species and well separated signals. It is one of the peculiarities of HHPN that the sample uptake rate can be varied over a broader range than is usually accessible with pneumatic nebulization. For the HHPN nozzle chosen a range of between 0.5 and 3 ml min-' can be covered. The results of the corresponding measurements are shown in Fig. 5. The maximum intensity of both species remains constant for sample uptake rates of from 0.8 up to 1.6mlmin-'. A value of 1 ml min-' was chosen as being suitable. It should be mentioned that complete elution of CrV1 after injection will take more than 2 min.This is much longer than expected from the sample uptake rate. The reason is a pro- nounced dispersion effect caused by the high dead volume of the nebulizer and the desolvation system. Nevertheless chrom- atographic working conditions could be chosen here so as to resolve both species. In general however it might be possible 9 1 1 I I 0 100 200 300 400 Time/s Fig. 4 Optimization of methanol concentration in the eluent A 10; B 20; C 30; D 40; and E 50% Time/s Fig.5 C 1.2; and D 1.6 ml min-' Optimization of the eluent liquid flow rate A 0.8; B 1.0 to reduce the analysis time significantly by application of a dispersionless sample introduction system. Analytical Measurements The first measurement using the chosen parameters was a reproducibility test with seven repeat injections of a mixture of both Cr species at a concentration of 100 ng ml-' each.The reproducibility of the retention time is very good. Fluctuations in the signals as can be seen from Figs. 3 and 4 are mainly caused by the greater instability of the plasma owing to the high carbon and oxygen load and can only partially be ascribed to pulsations of the pump. Nevertheless the relative standard deviation (RSD) of the integrated signals amounts to 2.9% for both Cr species which is nearly the same as is obtained for the analysis of aqueous solution. Finally a calibration procedure was performed using four solutions with concentrations of up to 100 ng ml-' of both Cr species.The recorded single ion monitoring profiles for 50Cr + are shown in Fig. 6. With blank value correction the cali- bration procedure provides satisfactory linearity with detection limits (3s) of 0.6 ng ml-l for Cr"' and 1.8 ng ml-' for CrV1. The CrV1 shows a slightly lower sensitivity which is attributed to a certain depression of the signal by the organic environ- ment. The calibration curve is linear over more than two orders of magnitude down to the ng ml- ' region. On evaluating 53Cr the detection limits are worse by a factor of about four. Recovery and stability of the Cr"' were checked by injection of this species alone. A signal was seen to appear without being retained for the Cr"' but this did not exceed 6% of the peak area of the final CrV' signal.This signal must be attributed to a partial reduction of CrV' to Cr"' for the chosen working conditions of pH 3. Therefore all investigations were performed with CrV1 solutions which were prepared immediately before use. As a preliminary application the whole procedure was applied to measurements of the local tap water and of a mineral water with quantification by standard additions for both species. For the mineral water the single ion monitoring profiles of 'OCr+ that were recorded are presented in Fig. 7. Again doubly distilled water was used for blank measurement. A depression of the CrV' signal by 50-70% is observed for both samples which is stronger for the mineral water which has the higher salt content. However the determination of Cr"' is not affected.Actual investigations indicate that compen- sation for the signal depression could be possible due to the presence of a higher amount of acetic acid and TBAA in the sample. Nevertheless detection limits for the determination of CrV1 in real samples are in the low ng ml-' region as can be 7.50 D C . . . . . . . . . . . . . . .. . L . .- . . . . - . . .~ . . . . . . . . 0 100 200 300 Tirne/s Fig. 6 Calibration with varying concentrations of both Cr"' and Cr" ion-monitoring profiles at 50 m/z A blank; B 10; C 50; and D 100 ng ml-'JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL. 9 197 1.50 I I 0 100 200 300 Time/s Fig.7 Test analysis of a mineral water sample showing recorded single ion monitoring profiles of 'OCr+ with standard additions of both Cr"' and Cr" A blank; B sample; C sample + 5 ng m1-I; and D sample + 10 ng ml- ' estimated from these measurements. No signal for Cr"' is observed for the tap-water sample whereas for the mineral water a Cr"' signal corresponding to almost 4 ng ml-' appears.In both samples a third species occurs in the chromatogram after the CrV' signal the identity of which cannot be confirmed so far. It should be mentioned that owing to the higher detection limit this species was not observed when applying ICP-AES instead of ICP-MS. In conclusion the results demon- strate promising possibilities for the on-line determination of Cr"' and CrV1 in drinking water. Comparative Assessment From the work published so far no procedure is known for the speciation analysis of Cr by ICP-MS which enables determination of Cr"' and CrV' in only one step.For the determination of Cr"' alone Roehl and Alforq~e'~ have reported detection limits of about 1 ng ml-I for an application with ICP-MS but nearly identical detection limits were obtained by colorimetry. Procedures that were primarily developed for TCP-AES can often be adapted to ICP-MS with only minor modification so that ICP-AES procedures for the determination of CrV' will also be considered here for comparative assessment of the results obtained. With any procedure developed for the deter- mination of Cr"' Cr"' can always be determined by measure- ment of the total Cr content and subtraction of the CrV' value. The relevant data available from the literature are compiled in Table 2. In comparison with AES the detection limits obtained in the present work offer a certain improvement by a factor of about four with the additional advantage of reduced analysis time but for evaluation of these results one should be aware that the determination of Cr is in some way a worst case scenario because the interferences introduced by carbon have the strongest disturbing influence for this particular element.From this point of view the work of Cox et (see Table2) could be a promising alternative also for ICP-MS. However for future work the advantage of the described procedure could be for pre-concentration of Cr" as has been shown by Posta et a1.,16 to be additionally applied. Preliminary results show that detection limits for CrV' can be improved by more than one order of magnitude by an increase in the volume of the sample loop.Further improvement of the analysis time by automation using a commercial HPLC auto- sampler is also possible. Conclusion Hydraulic high pressure nebulization as a functional part of an HPLC system is a powerful means for providing an interface-free coupling of HPLC to ICP-MS with the additional advantage of improved sensitivity. This has been demonstrated in the present work by development of a pro- cedure for the speciation analysis of Cr by application of ion- pair chromatography in the determination of Cr"' and CrV' by ICP-MS in one step. Addition of oxygen to the aerosol gas and effective desolvation were necessary prerequisites in order to apply ICP-MS as a highly selective and sensitive detection technique.Ion-pair chromatography has been applied to separ- ation of the species so that the procedure described here can be used for speciation analysis of other elements with only minor modification. Detection limits down to 1 ng ml-' were achieved with the main limitation being interferences. An even better analytical advantage can be attained from this promising technique by addition of a pre-concentration step which could be useful for improving the detection limits significantly and to extend the applicability to species of other elements with environmental significance. For a full evaluation of the capabili- ties of the technique the work must also be verified by high mass resolution which is the subject of future studies. This work was supported financially by the Bundesministerium fur Forschung und Technologie and by the Ministerium fur Wissenschaft und Forschung des Landes Nordrhein-Westfalen. Table 2 Detection limits (DL) for different ICP applications of ICP for speciation analysis of Cr Technique* HPLC-DIN-ICP-AES DIN-ICP-AES HPLC-USN-ICP-AES HHPN-ICP-AES TS-HPLC-ICP- AES FI-ICP- AES IC-ICP-MS HPLC-HHPN-ICP-MS Crvl Cr"' DL/ng ml-' 20 20 12 12 8 4 4 2 2 1.4 0.2 1 0.6 1.8 Time/min Reference 8 LaFreniere et al.ref. 7 8 LaFreniere et al. ref. 7 4 1-2 Wang and Jiang ref. 9 Berndt and Luo ref. 17 3 Roychowdhury and Koropchak ref. 6 1 Cox et al. ref. 21 8 Roehl and Alforque ref. 13 1-2 This work CrV1 *IC = ion chromatography; FI =flow injection. t Not specified.198 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1994 VOL.9 1 2 3 4 5 6 7 8 9 10 11 12 References Broekaert J. A. C. in Metal Speciation in the Environment ed. Broekaert J. A. C. Gucer S. and Adams F. Nato AS1 Series G Ecological Sciences Springer Verlag Berlin 1990 vol. 23 p. 213. Heumann K. G. in Metal Speciation in the Environment ed. Broekaert J. A. C. Gucer S. and Adams F. Nato AS1 Series G Ecological Sciences Springer Verlag Berlin 1990 vol. 23 p. 153. Okamoto K. Spectrochim. Acta Part B 1991 46 1615. Sperling M. Xu S. and Welz B. Anal. Chem. 1992 64 3101. Syty A. Christensen R. G. and Rains T. C. At. Spectrosc. 1986 7 89. Roychowdhury S. B. and Koropchak J. A. Anal. Chem. 1990 62 484. LaFreniere K. E. Fassel V. A. and Eckels D. E. Anal. Chem. 1987 59 879. Shum S. C. K. Neddersen R. and Houk R. S. Analyst 1992 117 577. Wang S.-R. and Jiang S.-J. J. Chin. Chem. SOC. (Taipei) 1991 38 327. Berndt H. Fresenius’ Z . Anal. Chem. 1988 331 321. Houk R. S. and Jiang S . J. J. Chromatogr. Libr. 1991 47 101. Vela N. P. Olson L. K. and Caruso J. A. Anal. Chern. 1993 65 585A. 13 14 15 16 17 18 19 20 21 Roehl R. and Alforque M. M. At. Spectrosc. 1990 11 210. Jakubowski N. Feldmann I. Stuewer D. and Berndt H. Spectrochim. Acta Part B 1992 47 119. Syty A. Christensen R. G. and Rains T. C. J. Anal. At. Spectrom. 1988 3 193. Posta J. Berndt H. Luo S . K. and Schaldach G. Anal. Chem. 1993 65 2590. Berndt H. and Luo S . K. J. Anal. At. Spectrom. submitted for publication. Griepink B. in Metal Speciation in the Environment ed. Broekaert J. A. C. Giicer S. and Adams F. Nato AS1 Series G Ecological Sciences Springer Verlag Berlin 1990 vol. 23 p. 361. Jakubowski N. Feldmann I. and Stuewer D. Spectrochim. Acta Part B 1992 47 107. Luo S. K. and Berndt H. Spectrochim. Acta Part B submitted for publication. Cox A. G. Cook I. G. and McLeod C . W. Analyst 1985 110 332. Paper 3/04551 J Received July 27 1993 Accepted October 5 1993