JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, MARCH 1994, VOL. 9 28 5

Determination of Selenium and Arsenic in Mineral Waters With Hydride Generation Atomic Absorption Spectrometry* Marjan Veber, Ksenija Cujes and Sergej GomiSCek Department of Chemistry and Chemical Technology, University of Ljubljana, P. 0.6. 537, Ljubljana SLO-67007, Slovenia

A continuous flow hydride generation (HG) technique was applied for the determination of Se and As in highly mineralized mineral waters. The atomization was performed either conventionally in a heated silica tube or in the graphite furnace after collection and thermal decomposition of the hydrides. In most cases the determination of As in real samples was not subject to serious problems. Significant matrix effects were found only for mineral waters with higher Mg, Na and sulfate concentrations. With the conventional flow through HG technique reasonably good linearity (r = 0.998) and satisfactory precision [relative standard deviation (RSD)<5%] were obtained in the concentration range from 1 to 50 pg I-'. For the determination of lower concentrations of As and Se, in situ preconcentration in the graphite furnace was applied. Within 120 s of hydride introduction for As and 150 s for Se at a sample flow rate 0.5 ml min-', favourable preconcentration factors were obtained that yielded a limit of detection (30,) of 0.02 pg I-'. The precision of this procedure was satisfactory in the concentration range 0.5-10 pg I-', the RSD being 2-16% for both elements under investigation. Keywords: Selenium; arsenic; electrothermal atomic absorption spectrometry; hydride generation; precon- centration; mineral water

The determination of As and Se in mineral waters is important because of their toxicity, additionally Se is also considered to be an important essential element.' The maximum concen- tration levels for drinking water recommended by the World Health Organization (WHO) are 10 pg 1- ' for Se and 50 pg 1- ' for A s 2 The concentrations in natural waters are usually much lower, and therefore, in order to obtain reliable quantitative determinations, as well as in speciation studies, sensitive analyt- ical techniques should be used.

Electrothermal atomic absorption spectrometry (ETAAS) is commonly applied for the determination of both As and Se in water sample^.^-^ To eliminate interferences caused by the matrix and to improve the parameters of analytical procedures, different preconcentration techniques are usually These techniques are typically time consuming and not very convenient for routine analysis, therefore, direct approaches are desirable. The hydride generation (HG) techniques have become well established for the routine determination of traces of Se and As in a variety of samples. The continuous HG generation systems have some advantages compared with batch procedures. Such systems, especially when they are performed using the flow injection approach, have produced very efficient procedures giving excellent precision, with low sample and reagent consumption, high speed and often fewer

The technique also offers good possibilities for automation." In order to improve the sensitivity of the hydride technique to ng 1- ' levels different preconcentration procedures have been introduced. Whereas Van Cleuvenbergen et aL2* used a cold trapping technique, which was applied for speciation studies of As in natural waters, Tsalev and co-workers21,22 proposed a two-step analytical procedure for the determination of hydride forming elements. The first step was trapping the corresponding hydrides in a cerium(1v)-iodide absorbing solution, and the second step direct determination by ETAAS. With this approach, which was proposed also for the analysis of mineral waters, limits of detection (LODs) in the sub-ng ml-' range were obtained.

Recently, in situ preconcentration of hydride forming elements in a graphite furnace has also been introduced. The technique, which was originally proposed by Drasch et

* Presented at the XXVIII Colloquium Spectroscopicurn Internationale (CSI), York, UK, June 29-July 4, 1993.

has been successfully applied for the determination of hydride forming elements in various matrices at the ng ml-' level. Uthus et ~ 1 . ~ ~ used a similar approach for the determination of As in biological samples, Lee25 applied the furnace hydride trapping technique for the determination of picogram amounts of Bi in environmental samples and Andreae26 determined inorganic Te species in natural waters. Sturgeon et al. further improved this technique, moreover, they have also studied the mechanisms for the sorption and atomization of metallic hydrides in the graphite furnace.27 They have applied a HG graphite furnace technique for the determination of As, Sb and Se in sea-water as well as in marine tissues and sediment^.^^-^' Enhancement of the trapping efficiency was achieved with suitable modification of the graphite surface. It was proved that Pd serves as a good absorber for arsenic and selenium hydrides in the graphite furnace, therefore, the addition of Pd to the graphite tube enables high trapping efficiency to be

In the present investigation the continuous flow HG tech- nique was applied for the determination of Se and As in highly mineralized mineral waters. The atomization was performed either conventionally in a heated silica tube or in the graphite furnace after collection and thermal decomposition of the hydrides.

Experimental Instrumentation The atomic absorption measurements were performed by using a Perkin-Elmer Model 2280 spectrometer with a Model HGA- 400 graphite furnace, and a Model 561 chart recorder was used to follow the analyte absorbances. Arsenic and Se elec- trodeless discharge lamps (Perkin-Elmer), operating at 5 W, were used as sources, Arsenic and Se absorbances were meas- ured at 193.7 and 196.0nm, respectively, with the spectral bandpass set at 0.7 nm.

The HG system, which is shown schematically in Fig. 1 was built in the laboratory. A four channel peristaltic pump, Ismatec SA, Model MS-reglo with Tygon tubing was employed. Flow rates were controlled either with the pump controls or by the selection of tubings with different diameters. Reactor coil (length = 100 or 75 cm, 1 mm i.d.) and gas-liquid separ- ators were made of Pyrex glass (Fig. 2). Two types of gas-liquid separators were used for different sample and carrier gas flow rates.

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286 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, MARCH 1994, VOL. 9

To the atomizer (graphite furnace)

NaBH, Gas-liquid separator

Mixing coil rn To waste

Fig. 1 Schematic diagram of the hydride generation system

(a) To the quartz cell

t 30

I

Drain I "r

b I inlet To the furnace

t

u Fig. 2 Gas-liquid separators used for: (a) the continuous flow hydride generation system; and (b) in situ preconcentration of hydrides in the graphite furnace. All dimensions in mm

Reagents

Standard solutions

Selenium standard solutions (1.000 g 1-I) were prepared by dissolving of 0.2190 g of Na,SeO, ( Se") or 0.1320 g of Na,SeO, (Se'') in 20 ml of de-ionized water (Millipore, Milli-Q) to which 1.0ml of concentrated HCl was added and diluted to 100 ml with de-ionized water. Arsenic standard solutions (1.000 g 1-I) were prepared by dissolving 0.1320 g of As,03 (As"') or 0.4816 g of As205.5H20 (As') in 2 ml of 20% NaOH solution. The solutions were acidified with HCl to a final pH of 2 and diluted to 100ml with de-ionized water. Working solutions for both elements at lower concentrations were prepared daily by appropriate dilution of stock solutions.

Other reagents, 1 YO m/v solutions of NaBH, (Merck) in 1 YO NaOH, a 20% KI solution and a 10% solution of ascorbic acid were prepared by dissolving pro analysi grade reagents (Merck) in de-ionized water. Palladium solution was prepared by dissolving PdCl, in HN03, and diluted with de-ionized water.

Procedures

Sampling SampIes of mineral waters were stored in polyethylene vessels after filtration through 0.45 pm membrane filters, and acidified with HCl (pH 2) at 4 "C.

Continuous flow determination of As and Se

For the determination of As, an appropriate volume of sample 115-25 ml) was measured into a 50 ml calibrated flask, As was reduced to the As"' state with the addition of 4 ml of concen- trated HC1, 1 ml of 1% ascorbic acid and 1 ml of 20% KI solution and the solution was diluted to 50ml. Selenium was reduced to the Se" form by heating the solution for 12 min at 90°C after the addition of 10 ml of HC1 into 15 ml of sample solution. Afterwards, the solution was cooled to room tempera- ture, diluted to 50 ml and analysed.

The measurements were performed with the Ar carrier gas flow rate of 40 1 h-' and reagent and sample flow rates of 10 ml min-l. The reducing reagent used in most of the experi- ments was 1% NaBH,.

Determination of As and Se by ETAAS after in situ precon- centration in the graphite furnace

Before each measurement, the graphite surface was modified with Pd; 50 pl of Pd solution (100 pg 1-I) were injected into the graphite tube and dried. The hydrides were introduced into the graphite tube through the sample introduction opening with a 20 cm long quartz tube (2 mm i.d., 3 mm 0.d.). The tube was narrowed at the end to give a 1 mm 0.d. tip and was connected to the outlet of the gas-liquid separator. During the hydride trapping the graphite tube was heated to 300°C and the internal shield gas flow was stopped or reduced (gas stop and/or mini-flow set-up). Hydride generation was per- formed continuously with a carefully controlled collection time and sample/reagents flow rates. The most commonly applied reagent and sample flow rates were 0.5 mlmin-I, and the carrier Ar gas flow rate was set to approximately 5 1 h-'. After the selected time of HG the sample and reagent flows were stopped and after 15-20s purging of the system the quartz tube was removed from the furnace and ETAAS measurements were carried out under the experimental conditions shown in Table 1.

Results and Discussion The aim of this investigation was to elaborate a simple and reliable procedure for the determination of trace amounts of Se and As in highly mineralized mineral waters. For these types of water the varying content of major components as well as a great variety of minor components, with regard to the occurrence of elements and their concentration levels, is characteristic. The content of some major components in the samples of waters investigated is shown in Table 2. The concen- trations of the minor components, such as heavy metals, are at the low pgl-' level (e.g., Co, Cr, Ni and Pb 1-10, Cd 0.01 and Cu 1-35 pg 1-l). It is to be expected that such matrices interfere seriously with direct determination by ETAAS of both elements under in~estigation.~ Although the chemical modifier (Pd-Mg nitrate) enables the application of charring tempera- tures up to 1000 "C, significant matrix interferences were

Table 1 Furnace temperature programmes

Operation Step Temperature/"C Time/s'

AS- Trapping 1 300 30-300 Analysis 2 450 15

3 2200 4 4 2650 2

Se- Trapping 1 300 30-300 Analysis 2 450 15

3 2400 4 4 2650 2

Internal gas flow rate/ ml min -

0 200

0 200

0 200

0 200

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JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, MARCH 1994, VOL. 9

G 60

50

2 .= 40 2

30

4- .- c L

0

l5 20 3 n a

1 0 -

287

- - - -

-

-

Table2 Composition of main components in the mineral waters investigated (in mg I - ' )

Component

Na+ K + CaZ +

Mg2+ Fe

F- a- HCO; so; -

A

90-115 20-28

110-180 30-65 7-1 1

0.2-0.4 1-11 1-25

800-1 10

Mineral water

B C 1300-1350 1550-1 620 200-220 10-30 250-300 41 0-450 100-110 1050-1150

5-13 4-8

0.7 0.1 140-160 50-90 270-350 2050-2250

4500-4800 7800-8700

D

10-15 2-5 5-60 35 -

0.1 5.4

40 3 30

observed for both elements, especially in more highly min- eralized mineral waters samples. The interferences could be reduced only by dilution of samples (e.g., 1 +9), however, this approach is not suitable for the determination at trace levels of both elements under consideration.

In an effort to avoid these problems and to achieve better sensitivities HG techniques were chosen and investigated. The continuous generation system used in this work was optimized and the appropriate reagent concentrations and sample, reagent and carrier gas flow rates were determined and some relevant interferences for reliable analysis were studied. Since both elements are usually present in natural samples at higher oxidation states the reduction to the As"' and Se" state is necessary prior to HG. While successful reduction of As was achieved within a short time after the addition of ascorbic acid and KI, the reduction of Se species required only heating with HCl for a longer period. It was established that the same reduction time can be used for different types of samples (Fig. 3). On the basis of the experimental work performed, a reduction time of 12min was selected and used in all further experiments.

Although the interferences influencing hydride formation have been studied in detail in the past, the influence of some cations is investigated here because of the complexity of the composition of mineral waters (mentioned above) and the unfavourable concentration ratios. In Figs. 4 and 5 the ratio of absorption signals ( A : A,,), measured from solutions contain- ing different concentrations of metal ions given in Table 3, and those obtained from aqueous standard solutions, are presented. A slight decrease in signals (10-15%) for As was observed in the presence of the interferents. For Se a decrease in the signals was observed only for solutions containing higher concen- trations of K+. Among anions, the influence of the sulfate anion was noticed (Table4). Since some types of mineral waters can contain higher concentrations of sulfate ions, these interferences must be considered.

The continuous flow HG system is suitable for the analysis

70

0 2 4 6 8 10 14 Reduction ti me/m i n

Fig. 3 Reduction efficiency for mineral waters A, B and C

0.40

0.20

0

Solution 1 0 Solution 2

Na K Ca Mg Zn Fe Al Mn

Fig.4 Influence of some cations on formation of ASH,. A,= absorbance measured in aqueous solutions, A = absorbance measured in the presence of an interferent

1.20

1 .oo

0.80

3 0.60

0.40

0.20

0

Solution 1 OSolution 2

Na K Ca Mg Zn Fe Al Mn

SeH,. A,= FL.5 Influence of some cations on formation of - absorbance measured in aqueous solutions, A = absorbance measured in the presence of an interferent

Table3 Composition of solutions (1 and 2) used for interference studies

c/mg 1-'

Element

Na K Ca Mg Zn Fe Mn A1

1

500 100 100 500

5 0.4 0.1 0.1

2

1500 500 500

1500 50 10 1 1

of samples in the pg1-I concentration range. Their determi- nation is usually not subject to serious problems. To some extent, matrix effects were observed for the Mg-Na rich mineral waters and they were reflected in the reduced slope (up to 30%) of the calibration curves. To avoid errors, the evaluation of the experimental data should be accomplished with the standard additions technique. The estimated LODs (3a blank signals for a solution with the same matrix components typical of the majority of samples) for this technique were found to be 0.15 pg 1-' under the proposed conditions. Reasonably good linearity (r = 0.998) and satisfactory precision (RSD < 5 YO)

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1.00

% 0.80

$ 0.60 9

2

B 3 0.40 0 c, -

0.20

Table 4 Influence of SO4'- on the absorption signals of As and Se; the maximum amount of sulfate in the samples is 0.035 mol 1-'

-

-

-

-

-

[~O,~-]/rnol I - ' As Se

0.001 0.97 1 .o 0.005 0.96 0.97 0.010 0.95 0.98 0.050 0.80 1 .oo 0.100 0.80 0.98 0.500 0.65 1 .oo

* A =absorbance measured in the solution with interferring ion; A, = absorbance measured in the aqueous standard solution.

were obtained in the concentration range from 1 to 50 pg 1-'. These parameters were not significantly subject to day-to-day variations.

For the determination of lower concentrations of As and Se, direct preconcentration of As and/or Se in the graphite furnace was examined. Preliminary experiments have shown that the precision of measurements was not satisfactory (RSD being 20-30%) when the same sample and reagent flow rates as in previous experiments were used. Therefore, measurements at lower sample and reagent flow rates required modification of the HG system. To reduce dead volumes, a different gas-liquid separator [Fig. 2(b)] was constructed and the reactor coil was shortened to 75 cm.

The selection of the correct carrier gas (Ar) flow rate is vital, because it contributes to the transport of the hydrides into the graphite furnace and also influences the mixing of reagents in the reactor coil and therefore it leads to a more efficient HG. As can be seen from the data shown in Fig. 6 the carrier gas flow rate should be maintained between 2 and 5 1 h-l. In this range there is no significant influence of small changes in Ar flow rate on absorption signals, while at lower flow rates not only lower sensitivity but also poorer precision is noticeable.

A plot of absorbance as a function of sample flow rate is shown in Fig. 7. Under the experimental conditions used this relationship is linear for flow rates up to 1.0ml min-'. In Fig. 7 RSD (%) levels for absorbance measurements are also shown, which indicate that beyond flow rates of 0.5 ml min-' signal reproducibility deteriorates, probably owing to exper- imental difficulties related to the precise time control of the HG. This problem could be overcome with suitable automation of the system. The linear relationship between absorbance and collection, which is presented in Fig. 8, was observed for both elements, and indicates good trapping efficiency. When all the parameters mentioned above were carefully controlled, rectilin- ear dependence of measured absorbance on concentrations of analytes in the solutions was obtained. The correlation

O.'O 7 35

W g 0.50

8 0.40 I]

3 0.30 5 $ 0.20

2

Cn

C - 0.10 '- z * c-

25

20 - s n -

15 V, K

10

5

0 1 2 3 4 5 6 7 8 Ar flow rate/l h- '

Fig. 6 uersus carrier gas flow rate for 5 pg I-' of Se; collection time, 60 s

Absorbance signals and relative standard deviation of signals

-0 0 0.20 0.40 0.60 0.80 1.00

Flow r a t e h l min-'

Fig.7 Dependence of sample flow rate on the absorbance of the analyte for 5 pg I- ' of Se; collection time 60 s

O'*O I 0.64

3 0.48

n 8

h: 5 0.32 Cn c -

0.16

0 60 120 180 240 300 360 420 Time/s

Fig.8 Absorbance signals for As and Se versus collection time, (concentrations of both elements 1.0 pg I-', sample flow rate 0.5 ml min-l)

coefficients were better than 0.995 in the concentration range 0.5-5 pg 1-I.

From the slopes of calibration curves the characteristic masses were calculated and they were found to be 22.5 f 1 and 17.5 _+ 1 pg for Se and As, respectively. These values are higher than the values reported in the l i t e r a t ~ r e , ~ ~ , ~ ~ which is probably the result of using less reliable equipment. The values are, however, comparable to direct ETAAS measurements which were obtained with the same apparatus using platform atomiz- ation with Pd as modifier (22 pg for Se and 16 pg for As).

The precision of this procedure under optimum experimental conditions is good, the RSD being 2 4 % for both elements under investigation for concentrations 50-fold above the LODs.

Even at relatively low sample flow rates (0.5 ml min-') and with a short time of hydride introduction, favourable precon- centration factors were obtained. The preconcentration efficiency as well as the reproducibility of parallel runs is illustrated by recorder tracings shown in Fig. 9. However, it has to be emphasized that although a high temperature cleaning step was performed after each analysis, a memory effect was observed after several analyses (Fig. 9, C), therefore additional cleaning of the graphite tube at the maximum temperature could usefully be performed after 10- 15 trapping procedures. The estimated LOD under the experimental con- ditions described (150 s collection time, sample flow rate 0.5 ml min-') calculated on the variability of the blank signals ( 3ob) was 0.02 pg I-' for Se, while the same value was obtained for As in 120 s. The different collection time which is required to obtain the same values of LOD for both elements is related to the characteristic masses for ETAAS measurements. The

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JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, MARCH 1994, VOL. 9 289

1 I - Time

Fig. 9 Recorder tracings of the ETAAS measurements; absorption signals after in situ preconcentration of Se ( 5 pg 1-') standard solution: A, direct determination by ETAAS (20 p1 of sample); B, in situ preconcentration (60 s collection of SeH, from the same analyte solution, sample flow rate 0.5 mlmin-'); and C, blank signals (no sample introduction in the furnace) after six collection procedures

experimental data also indicate that LODs could be improved with longer collection times and/or with different flow rates of the sample solution and reagents. More attention should, nevertheless, be paid in this case to the laboratory conditions and the use of purer chemicals, since, under the proposed conditions the blank values, especially in the case of As, prevented better figures of merit from being achieved. The magnitude of the absolute blank at a sample flow rate of 0.5 ml min-' and 2 min collection time varied from day-to- day and corresponded to 130-160 pg of As and 75-100 pg of Se in the graphite tube.

Some samples of mineral and thermal waters from different regions of Slovenia were analysed with both hydride procedures (Table 5 ) . The evaluation of results was achieved with the standard additions technique (at least three additions to the aliquots of the sample). The slopes of calibration curves obtained for real samples were, with few exceptions, compar- able to those of the standard solutions for most of the samples investigated. Owing to the lack of certified standards for mineral waters the accuracy of the method was estimated with the analysis of spiked samples in which the concentration levels of As and Se were below the LOD. In most samples examined, the recovery test showed good agreement of the results with the calculated values (Table 6). The disagreement in results shown by the sample of thermal water Al(D) suggests that the presence of some other components is important in the generation of selenium hydride. As this sample of thermal water differed from the others in that it contained a higher concentration of sulfide it can be assumed that the presence of sulfide influences the generation of selenium hydride. The analysis of waters with higher sulfide concentrations requires additional investigation.

Table 5 Determination of As and Se in some mineral water samples. Results (in pg 1-') are mean of five analyses, confidence interval was calculated with t-test, P = 0.09

~~

As

6.0 & 0.6* 6.3 -1 0.6* 3.1 0.4* 2.2 f 0.3"

270 & 8 180-15 20-12

Se

< 0.02* 0.10* 0.20*

< 0.02*

< 0.02* < 0.02 < 0.02"

*ETAAS, in situ preconcentration.

Table6 Recovery test for As and Se in samples by ETAAS, in situ preconcentration. Results are averages of three replicates

As/pg 1 - ' Se/pg 1 - ' Sample Added Found Added Found

V-L (A) 10 9.5f 1 1.0 1.1 f0 .2 V-C (A) 10 9.6_+1 1.0 0.95 f 0.2 V3/66 (C) - - 0.5 0.4k0.1

- - 0.5 0.45 kO.1 RgSP (C) A/1 (D) 10 9.6-t 1 20 1 3 k 2 B/1 (D) 10 1Of l 0.5 0.5 f 0.1

Conclusions Continuous HG techniques are suitable for the determination of Se and As in highly mineralized mineral waters. Compared with other techniques, e.g., ETAAS, HG methods are less subject to interferences. The interferences which do occur can be overcome by applying the standard additions technique. The concentrations of both elements at concentrations greater than 1 pg 1 - l can be determined with satisfactory precision by using the conventional approach with atomization in a heated silica tube; the determination of lower levels of As and Se requires more sensitive techniques. In situ preconcentration was found to offer much lower LODs and give reliable results, and therefore can be recommended for routine analysis.

The financial support of the Ministry of Sciences and Technology of Slovenia is gratefully acknowledged.

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References Maxwell, M. H., Kleema, C . R., and Narins, R. G., Clinical Disorders of Fluid and Electrolyte Metabolism, McGraw-Hill, New York, 1987. Codex Alimentaruis, World Health Organization, Rome, 1988, vol. 12. Stein, V. B., Canelli, E., and Richards, A. H., At. Spectrosc., 1980, 1, 61. Welz, B., Schlemmer, G., and Mudakavi, J. R., J. Anal. At. Spectrom., 1988, 3, 695. Bozsai, G., Schlemmer, G., and Grobenski, Z., Talanta, 1990, 37, 545. Mentasti, E., Nicolotti, A., Porta, V., and Sarzanini, C., Analyst, 1989, 114, 1113. Van Elteren, J. T., Gruter, G. J. M., Das, H. A., Brinkman, U. A. Th., Int. J . Enuiron. Anal. Chem., 1990, 43, 41. Kamada, T., and Yamamoto, Y., Talanta, 1980, 27, 473. Hudnik, V., and GomiSEek, S., Anal. Chim. Acta, 1984, 157, 135. Thompson, K. C., and Thomerson, D. R., Analyst, 1974, 99, 595. Robbins, W. B., and Caruso, J. A., Anal. Chem., 1979, 51, 889. Cutter, G. A., Anal. Chim. Acta, 1980, 98, 59. Ebdon, L., and Wilkinson, J. R., Anal. Chim. Acta, 1982, 136, 191. Welz, B., and Schubert-Jacobs, M., At. Spectvosc., 1991, 12, 91. McLaughlin, K., Dadgar, D., Smyth, M. R., and McMaster, D., Analyst, 1990, 115, 275. DEdina, J., Frech, W., Lundberg, E., and Cedergren, A., J . Anal. At. Spectrom., 1989, 4, 143. Brindle, I. D., Alarabi, H., Karshman, S., Le, X.-c., Zheng, S., and Chen, H., Analyst, 1992, 117, 407. Flow Injection Atomic Spectroscopy, ed. Burguera, J. L., Marcel Dekker, New York, 1989. Narasaki, H., J. Anal. At. Spectrom., 1988, 3, 517. Van Cleuvenbergen, R. A., Van Mol, W. E., and Adams, F. C., J. Anal. At. Spectrom., 1988, 3, 169. Tsalev, D. L., and Mandjukov, P. B., Microchem. J., 1987, 35, 83. Tsalev, D. L., Mandjukov, P. B., and Stratis, J. A., J. Anal. At. Spectrom., 1987, 2, 135. Drasch, G., Meyer, L., and Kanert, G., Fresenius' 2. Anal. Chem., 1980, 304, 141. Uthus, E. O., Collings, M. E., Cornatzer, W. E., and Nielson, F. H., Anal. Chem., 1981, 53, 2221. Lee, D. S., Anal. Chem., 1982, 546, 1682. Andreae, M. O., Anal. Chem., 1984,56,2064.

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27 Sturgeon, R. E., Willie, S. N., and Berman, S. S., Anal. Chem., 1985,57, 2311. 44, 339.

28 Sturgeon, R. E., Willie, S. N., and Berman, S. S., J. Anal. At. Spectrom., 1986, 1, 115.

29 Sturgeon, R. E., Willie, S. N., and Berman, S. S., Fresenius’ 2. Anal. Chem., 1986, 323, 788.

30 Sturgeon, R. E., Willie, S. N., Sproule, G. J., and Berman, S. S., J. Anal. At . Spectrom., 1987, 2, 719.

31 Doidge, P. S., Sturman, B. T., and Rettberg, T. M., J. Anal. At. Spectrom., 1989, 4, 251.

32 Li, Z., Ni, Z.-m., and Shan, X.-q., Spectrochim, Acta, Part B, 1989,

33 An, Y., Willie, S. N., and Sturgeon, R. E., Spectrochirn. Acta, Part B, 1992, 47, 1403.

Paper 3/048171 Received August 10, 1993

Accepted October 10, 1993

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