Anal. Chem. 1903, 55,2047-2050 (6) Il’icheva, I. A,; Degtereva, I. F.; Dolmanova, I. F.; Petrukhina, L. A. Metody Anal. Kontrolya Kach. Prod. Khim. Promsti. 1078. 4 , 65-66; Chem. Abstr. 1078, 8 9 , 190385n. (7) Lisetskaya, G. S.; Bakal, G. F. Ukr. Khim. Z h . (Russ. Ed.) 1970, 36, 709-71 2. (8) Motoharu, T.; Norio, A. Anal. Chim. Acta 1087, 3 9 , 485-490. (9) “Standard Methods for ihe Examination of Water and Wastewater”,
2047
14th ed.; APHA-AWWA-WPCF: Washington, DC, 1976; Sec. 3228,p
260.
RECEIVED for review February 28, 1983. Accepted July 15, 1983.
Simultaneous Determination of Arsenic, Antimony, and Selenium in Marine Samples by Inductively Coupled Plasma Atomic Emission Spectrometry Elisabeth de Oliveira,’ J. W. McLaren,* a n d S. 8. B e r m a n Analytical Chemistry Section, Chemistry Division, National Research Council of Canada, Ottawa, Ontario, Canada KIA OR9
A method is described for the determination of arsenic, antimony, and selenium In marine samples by continuous hydride generation inductively coupled plasma atomic emission spectrometry. A variety of sample dissolution procedures and hydride generation reaction conditions were evaluated in an attempt to eslabllsh optimal conditions for the simultaneous determination of all three elements. Detection limits of 1 fig L-’ for As and Sb and O.!; Fg L-’ for Se have been achieved. Results of analyses of NRCC and NBS reference materials demonstrate the applicability of the technique to biological and geological marine samples.
A series of reports from this laboratory (1,344 have shown that inductively coupled plasma atomic emission spectrometry (ICP-AES) is a suitable technique for the simultaneous determination of major, minor, and trace elements in a variety of marine samples. In (addition, a number of publications (6-1 7) have demonstrated that hydride generation, followed by introduction of the gaseous hydrides into an ICP, is a suitable method for the determination of several trace elements, including arsenic, antimony, and selenium, for which detection limits with conventional pneumatic nebulization are inadequate for inarine samples. A variety of sample digestion procedures have been shown to be suitable for the determination of arsenic (8, 11, 12), selenium (lo), arsenic and antimony (9, 14), or arsenic and selenium (15) in various materials; however, rather few procedures permitting the simultaneous determination of arsenic, antimony, and selenium have been described (13,16,1;3. The optimal reaction conditions for the generation of the hydrides can be quite different for the various elements. The type of acid and its concentratialn in the sample solution often have a marked effect on sensitivity. Additional complications arise because many of the hydride-forming elements exist in two oxidation states which (are not equally amenable to borohydride reduction. For example, potassium iodide is often used to prereduce As(V) and Sb(V) to the 3+ oxidation state for maximum sensitivity, but this can also cause reduction of Se(1V) to elemental Eielenium from which no hydride is formed. For this and other reasons Thompson and co-workers found it necessary to develop a separate procedure for the Present address: Instituto de Quimica, Universidade de SBo Paulo, Caixa Postal 20780, SHo Paulo, Brasil.
determination of selenium in soils and sediments (10) although arsenic, antimony, aind bismuth could be determined simultaneously (9). Recently, a method for simultaneous determination of As(III), SSb(III),and Se(1V) in water samples was reported in which the problem of reduction of Se(1V) to Se(0) by potassium iodide was circumvented by adding the potassium iodide after the addition of sodium borohydride (16). Goulden et al. (15) have reported the simultaneous determination of As, Sb, Se, Sn, and Bi in water samples, but it appears that in this case the generation of AsH3 and SbH3 occurs from the 5+ oxidation state. This report describes the application of a simple continuow hydride generation system coupled to a low-power (1.4 kVV) ICP to the determination of trace concentrations of arseniic, antimony and selenium in marine samples. A variety of sample dissolution procedures and hydride generation reaction conditions were evaluated in an attempt to establish optimal conditions for the simultaneous determination of all three elements. In addition, the effect of the oxidation state of the elements on hydride formation in dilute hydrochloric acid solution was studied. EXPERIMENTAL SECTION Apparatus. The custom ICP-echelle spectrometer used in this work has been described in previous publications (1-5). Hydride generation was accomplished in a continuous mode by using two channels of a four-channel peristaltic pump (Gilson Instrument Co., Minipuls 11) to deliver sample and borohydride reagent to a phase separator modified from that of Thompson et al. (6): a schematic diagram of the assembly employed is shown in Figure 1. An air bubble maintained by the surface tension at the junction of the two horizontal arms of the “T” prevents mixing of tlhe reagent and sample until the two solutions begin to flow down the vertical arm into the phase separator. This results in a smooth and continuous generation of hydrogen which is not significantly disturbed by changeover from one sample to the next, or to tlhe blank. The gaseous hydrides and hydrogen are swept from tlhe phase separator and into the ICP by a continuous flow of argon. Reagents. Acids used in this work were purified by subboiliing distillation (18). High-purity water was produced by passing distilled water through a deionizing system (Cole Parmer Instrument Co., Chicago IL). Other reagents were analytical grade. A solution of 170sodium borohydride in 0.1 M sodium hydroxide was prepared every day from NaBH, powder (Alfa Inorganics, Danvers, MA, 99%). A stock solution (1000 mg L-l) of As(V) was prepared by dissolution of As203in aqua regia and dilution with 3 M hydrochloric acid. Stock solutions of Sb(V) and Se(V1)were prepared by dissolution of antimony and selenium metals in aqua regia followed by dilution with 3 M hydrochloric acid. Solutions
0O03-27Q0/83/0355-2047$01.5Q/O 0 1983 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 55, NO.
13,
NOVEMBER 1983
4.5 mm dia
SAMPLE
I )
I
19 mm dio.
4
Table I. Standard Operating Conditions Plasma 1.4 kW (reflected, < 10 W) 1 4 L min-' 0.5 L min-' 1 2 mm above the load coil entrance, 100 pm; exit 100 pm 0.5 mm As, 193.696 nm; Sb, 206.833 nm; and Se, 196.026 nm
rf forward power plasma gas flow auxiliary gas flow observation height slit widths slit height wavelengths
Hydride-Generation Apparatus sample acidity sample flow NaBH, concentration NaBH, flow carrier gas flow
1 M HCl 6.2 mL min-' 1%(in 0.1 M NaOH) 3.0 mL min-' 0.8 L min-'
6.41
I
J -L
Figure 1. Gas-liquid phase separator.
of arsenic(III), antimony(III), and selenium(1V) were prepared by the reduction methods described in the next section. Dissolution Procedures. Biological tissues were dissolved by a procedure similar to that described by Agemian and Thomson (19) for determination of arsenic and selenium in fish tissue. This method involves an initial open vessel digestion at room temperature with concentrated nitric acid, followed by digestion with a mixture of nitric, perchloric, and sulfuric acids on a hot plate. After digestion is complete (i.e., a colorless solution is obtained), the volume of acid is reduced to 5 mL by evaporation but without charring. Dilutions to volume were made with 1 M hydrochloric acid. Four methods of dissolution were evaluated for marine sediments: (1)The acid digestion procedure described above for biological tissues. Crock and Lichte (20) recently described a similar procedure, involving hydrofluoric as well as nitric, perchloric, and sulfuric acids, for dissolution of geological materib prior to arsenic and antimony determination by atomic absorption spectrometry. (2) Fusion with sodium hydroxide, as described by Goulden et al. (15) but using porcelain or nickel crucibles. (3) Acid digestion with a mixture of nitric, perchloric, and hydrofluoric acids in sealed Teflon vessels, as described by McLaren et al. (5). (4) Fusion with potassium hydroxide. Four grams of KOH and 0.5 g of the sample are placed in a nickel crucible. The crucible is placed in a furnace which is then heated to 500 "C for ' 1 2 h. The crucible is then cooled and the contents are dissolved in 50 mL of 1 M hydrochloric acid. The precipitated silicic acid is allowed to settle or is filtered before analysis. RESULTS AND DISCUSSION Optimization of Operating Parameters. By use of the previous literature on hydride generation ICP-AES (6-1 7) as a guide, a brief investigation of the effects of various operating parameters on sensitivity and detection limits was conducted. The optimal operating conditions are shown in Table I. Under these conditions very few problems of accidental extinction of the plasma were encountered, despite the large amount of hydrogen introduced along with the hydrides. Smooth operation of the drain from the phase separator was found to be quite sensitive to the carrier gas flow rate. At flow rates less than 0.8 L min-l, the formation of bubbles (of hydrogen) in the U-tube hindered smooth flow, regardless of
0
1
2
3 4 [HCI], Molar
5
6
7
Flgure 2. Influence of sample acidity on sensitivity. All solution concentrations 0.1 rng L-'; As(III), Sb(III), and Se(1V)solutions were prepared as described in the text.
sample and reagent flow rates. On the other hand, the back pressure created by a carrier gas flow higher than 0.8 L m i d tended to force all the waste liquid out of the U-tube. In either case, the sudden change in back pressure usually extinguished the plasma. Fortunately, a carrier gas flow rate of 0.8 L min-I was found to be optimal with respect to detection limits in addition to promoting smooth operation of the drain. At this flow rate, the atomic emission signal stabilized approximately 1 min after introduction of the sample to the hydride generation apparatus. The effect of hydrochloric acid concentration on sensitivity was studied over a range of 1 M to 5 M for arsenic and antimony, and from 1 M to 6 M for selenium. As shown in Figure 2, the response is invariant within this concentration range for As(II1) and Sb(II1) (as reported by Thompson et al. (6)),as well as for Se(1V) and As(V). For Sb(V),an increase in acid concentration causes a decrease in sensitivity. High concentrations of hydrochloric acid cause a slow reduction of Se(VI) to Se(1V) at room temperature, as shown by the dashed line on Figure 2. As a result of this investigation, an acid matrix of 1M hydrochloric acid was chosen for further work. Prereduction Studies. The oxidative attack used in many of the digestion procedures leaves arsenic and antimony in the 5+ oxidation state and selenium in the 6+ oxidation state. As shown in Figure 2, higher sensitivity is achieved for As(II1) and Sb(II1) than for As(V) and Sb(V), although for arsenic the improvement is only about 2-fold. Several previous workers (9,12,16) have reported the reduction of As(V) and Sb(V) to the 3+ oxidation state, prior to the hydride generation, by the addition of a potassium iodide solution. In the
ANALYTICAL CHEMISTRY, VOL. 55, NO. 13, NOVEMBER 1983
permitting simultaneous determination of As(II1) and Sb(III), but not selenium. Linearity and Detection Limits. Calibration linearity and detection limits were investigated under the conditions detailed in Table I. Typical calibrations were linear over about 3 I l 2 orders of magnitude of concentration, Le., up to about 5 mg L-l. The relative sensitivities, as well as the detection limits found for As(III), As(V), Sb(III), Sb(V), and Se(IV), are shown in Table I[. The detection limit is defined as that concentration of analyte which gives a response equal to three times the standard deviation of the blank or background value. This blank is the ordinate intercept calculated from the linear regression analysis ad the calibration data (5). Despite considerable differences in the relative sensitivities, equivalent detection limits of 1pg L-l were determined for As(II1) and As(V);an even greater sensitivity improvement for Sb(II1) over Sb(V) results in only about a %fold improvement in the detection limit. Analysis of Biological Materials. Results obtained for three NBS standard reference materials, based on at least two determinations, are presented in Table 111. Precisions are expressed as 95% confidence intervals. Since the three wavelengths indicated in Table I were not available on either of our multielement cassettes, it was necessary to perform the analyses sequentially. Sb(V) was reduced to Sb(II1) (with KI) and Se(V1) was reduced to Se(1V) (with KBr) prior to hydride generation. All results were within the ranges certified by the National Bureau of Standards, with the exception of arsenic in NBS Oyster Tissue. Analysis of Marine Sediments. Results obtained for NBS and NRCC refiarence marine sediments, using the four dissolution procedures described in the Experimental Section, are presented in Table IV. Each result is the mean of at least
Table 11. Sensitivities and Limits of Detection for As(V), As(III), Sb(V), Sb(III), and Se(1V) As(II1) As(V) Sb(II1) Sb(V) Se(1V) slope of calibration graph, nA/ (mg L - ' ) limit of detection, wg L"
32.1
12.7
64.5
12.2
19.0
1
1
0.4
1
0.5
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present work, this technique was evaluated by adding 1mL of a solution 1 M in potassium iodide and 10% (w/v) in ascorbic acid to 100 mL of a 0.1 mg L-l solution of arsenic and antimony. As shown in Figure 3, the reduction is quantitative after 24 h. Unfortunately, this procedure also reduces selenium to the zero oxidation state, from which no hydride is formed. Selenium(V1) must be reduced to Se(1V) prior to hydride generation. In previous work (6, 7,10,13,15,16,17,21) this prereduction has been accomplished with either potassium bromide or hydrochloric acid. In the present work, two methods were found to be suitable. In the first, 1 mL of a 1 M KBr solution and 2.5 mL of concentrated hydrochloric acid were added to the sample solution (100 mL) which was then heated at 50 "C for !SO min. The second method involved addition of 10 mL of concentrated hydrochloric acid1 to the sample followed by heating a t 90 "C for 15 minutes. Neither of the two methods used for reduction of Se(V1) to Se(1V) resulttr in any reduction of As(V) and Sb(V) to the 3+ oxidation state. Thus, with the current apparatus, a choice must be made between (conditionspermitting simultaneous determination of As(V), Sb(V), and Se(1V) and conditions Table 111. Results for Eiiological Samples sample NBS 1566 Oyster Tissue NBS 1571 Orchard Leaves NBS 1577 Bovine Liver a
As, found,
As, certified,
Pg/g
Pg/g
Sb, certified, Pglg
13.4 i 1.9
Sb, found, Pglg 0.4 i 0.3
11.1i 1.1
11.9 i 0.6
10.2 f 2.0
2.8 t 0.2
2.9 i. 0.3
a
(0.055)
0.3 F 0.2
Se, found, Pg/g 1.7 i 0.2
Se, certified,
0.06
0.08 i 0.01
i
0.02
1.0 * 0.1
Pglg
2.1 i 0.5
1.1 t 0.1
Below of the limit of detection.
-
Table KV. Results for Near-Shore Marine Sediments for Four Different Dissolution Proceduresa sample
As, found,
As, certified,
Pglg
Pg/g
Sb, found,
Sb, certified,
Se, found, P g/g
Pg/g
Se, certified, Pg/g
WBS 1646 procedure 1 procedure 2 procedure 3 procedure 4
10.7 i 0.9 12.2 i 1.0 11.0 f 0.9 11.0 i. 0.9
11.6 i 1.3
0.18 i 0.09 c c 0.36 i. 0.09
(0.4)
0.3 i 0.1 0.61 * 0.02 0.59 i. 0.06
(0.6) (0.58 i 0.05)b
C
NRCC MESS-1 procedure I procedure 2 procedure 3 procedure 4
10.8 i 0.8
11.6 i 0.3 12.2 i 1.1 11.4 i 0.8
0.33 i 0.09 ll.l i 1.4
0.73 i 0.08
c c
0.71 i 0.03
0.29 i 0.07 0.33 i 0.05 0.35 ?r 0.02
0.34
i
0.06
C
NRCC BCSS-1 procedure 1 10.8 i. 0.9 0.22 i 0.06 procedure 2 11.6 i 1.8 10.6 i 1.2 c 0.59 procedure 3 11.4 i 1.1 c procedure 4 11.2 i 0.5 0.61 i 0.06 a The dissolution procedures are described in the Experimental Section, capture detection, as described in the text. Inconsistent. results.
f
0.06
0.3 i 0.2 0.42i 0.09 0.44 i 0.03
0.43 i 0.06
C
Results by gas chromatography with electron -
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 13, NOVEMBER 1983
gas-liquid separator, the main virtue of which is its simplicity 7 0 0 7 and low cost. On the other hand, the detection limits indicated
z W
5
/
I
/
I
I
I 1:
5.0c
1.5) AS
-0.1
A-A-A
T
4
ASPd log t , min
Figure 3. Effect of the addition of 1 M potasslum iodide to 0.1 mg L-I solutions of As and Sb. three determinations, with the precision expressed as a 95% confidence interval. Arsenic and antimony were determined as As(V) and Sb(V), while selenium(V1) was prereduced to Se(1V) with hydrochloric acid. For arsenic, all values agreed well with the values reported by the U.S. National Bureau of Standards or the National Research Council of Canada. For antimony, only the potassium hydroxide fusion procedure yielded accurate results. Two dissolution procedures provided consistent results for selenium: sodium hydroxide fusion and acid digestion in sealed Teflon vessels. Selenium determinations in sediments were also performed in this laboratory by using the same acid digestion procedure, followed by reaction of the selenium with 4-nitro-o-phenylenediamine and gas chromatographic determination with electron capture detection (22). The values obtained by the two methods agree quite well. The inconsistent results obtained for antimony, using dissolution by sodium hydroxide fusion (procedure 2) or by mixed acid digestion (procedure 3), are probably due to incomplete dissolution of the samples. The inconsistent selenium results, using the potassium hydroxide fusion (procedure 4), are perhaps due to loss of selenium at the temperature used for the fusion (500 "C). The low but constant results obtained for antimony and selenium by using the nitric/perchloric/sulfuric acid digestion (procedure 1)are probably due to incomplete dissolution of the samples. CONCLUSION Although the determinations of arsenic, antimony, and selenium were performed sequentially in this work, the method was developed with simultaneous determination of all three elements in mind. With the present apparatus, this can be achieved only if arsenic and antimony are determined as h ( V ) and Sb(V). Recently, Nygaard and Lowry (16)described a unique order of reagent mixing that allowed simultaneous generation of the hydrides of As(III), Sb(III), and Se(1V). It was reported that reduction of Se(1V) to Se(0) by potassium iodide could be avoided by adding this reductant after the addition of the borohydride reagent. Unfortunately, it was not possible for us to evaluate this technique with our present
in Table I11 suggest that the improvement in detection limits for arsenic and antimony would not be large. All four dissolution procedures studied were found to be suitable for arsenic determinations in biological and geological marine samples, but only one (potassium hydroxide fusion) yielded accurate results for antimony in marine sediments and only two (sodium hydroxide fusion or a nitric/perchloric/ hydrofluoric acid digestion in sealed Teflon vessels) were appropriate for determination of selenium in marine sediments. Thus, the development of a single procedure for the simultaneous determination of arsenic, antimony, and selenium (and perhaps other hydride-forming elements) in marine materials by hydride generation ICP-AES requires careful consideration not only of the oxidation-reduction chemistry of these elements and its influence on the hydride generation process but also of the chemistry of dissolution of these elements, particularly in geological materials. ACKNOWLEDGMENT The gas chromatographic determinations of selenium were performed by K. W. Michael Siu of this laboratory. We also thank H. B. MacPherson for many useful discussions regarding sample dissolution and hydride generation chemistry. Registry No. Arsenic, 7440-38-2;antimony, 7440-36-0;selenium, 7782-49-2. LITERATURE CITED (1) Berman, S.S.;McLaren, J. W.; Willle, S. N. Anal. Chem. 1980, 52, 488-492. (2) Berman, S.S.;McLaren, J. W. Appl. Spectrosc. 1978, 32, 372-377. (3) Berman, S. S.;McLaren, J. W.; Russell, D. S. I n "Developments in Atomic Plasma Spectrochemlcal Analysis"; Barnes, R. M., Ed.; Heyden: Philadelphia, PA, 1981; pp 586-600. (4) McLaren, J. W.; Berman, S. S. Appl. Spectrosc. 1981, 35, 403-408. (5) McLaren, J. W.; Berman, S. S.;Boyko, V. J.; Russell, D. S. Anal. Chem. 1981, 53, 1802-1808. (6) Thompson, M.; Pahlavanpour, 6.; Walton, S. J. Analyst (London) 1978, 703,566-579. (7) Thompson, M.; Pahlavanpour, 8.; Walton, S. J.; Kirkbright, G.F. Analyst (London) 1978, 103, 705-713. (8) Fry, R. C . ; Denton, M. 6.; Wlndsor, D. L.; Northway, S. J. Appl. Spectrosc. 1979, 3 3 , 399-403. (9) Pahlavanpour, 6.; Thompson, M.; Thorne, L. Analyst (London) 1980, 105,756-761. (10) Pahlavanpour, B.; Pullen, J. H.; Thompson, M. Analyst (London) 1980, 105, 274-278. (11) Broekaert, J. A. C . ; Lek, F. Fresenlus 2. Anal. Chem. 1980, 300, 22-27. (12) Nakahara, T. Anal. Chim. Acta 1981, 131, 73-82. (13) Wolnik, K. A.; Fricke, F. L.; Hahn, M. H.; Caruso, J. A. Anal. Chem. 1981, 53, 1030-1035. (14) Pahlavanpour, 6.; Thompson, M.; Thorne, L. Analyst (London) 1981, 106 467-47 1. (15) . . Goulden. P. D.: Anthonv. D. H. J.: Austen, K . D. Anal. Chem. 1981, 53,2027-2029. (16) Nygaard, D. D.; Lowry, J. H. Anal. Chem. 1982, 5 4 , 803-807. (17) Hahn, M. H.: Wolnlk, K. A.; Fricke, F. L.;Caruso, J. A. Anal. Chem. 1982, 54,1048-1052. (18) Dabeka, R. W.; Mykytiuk, A,: Berman, S . 6.;Russell, D. S. Anal. Chem. 1978. 48. 1203-1207. (19) Agemian, H.; Thomson, R. Analyst (London) 1980, 105, 902-907. (20) Crock, J. G.; Llchte, F. E. Anal. Chim. Acta 1982, 144, 223-233. (21) Robberecht, H.; VanGrieken, R. Talanta 1982, 29, 823-844. (22) Slu, K. W. M.; Berman, S. S . Anal. Chem. 1983, 55, 1603-1605 I
RECEIVED for review March 9, 1983. Accepted July 11, 1983. E.O. thanks the FundacBo de Amparo a Pesquisa do Estado de SBo Paulo for financial support.
