Anal. Chem. 1985, 57,6-9
6
Preconcentration of Selenium and Antimony from Seawater for Determination by Graphite Furnace Atomic Absorption Spectrometry R. E. Sturgeon,* S. N. Willie, and S. S. Berman Division of Chemistry, National Research Council of Canada, Ottawa, Ontario, Canada K l A OR9
Concentration of Se( IV), Sb( I I I), and Sb(V) from samples of coastal and open-ocean seawater is described uslng a comblnatlon of multielemenl chelation wlth ammonium pyrrolldine dlthlocarbamate and subsequent adsorption on C18bonded sllica gel. Eluted anaiytes are taken up In a malrlxfree acidic solution suitable for graphlte furnace AAS. Detectlon llmlts for Se( IV), Sb(I I I ) , and Sb(V) are 7, 50, and 50 ng/L, respectively, based on a 300-mL sample volume. Precislon of the determination of Se( IV), Sb( I I I), and Sb(V) was 1 4 % , l o % , and 10% RSD at concentratlons of 0.02, 0.2, and 0.2 pg/L, respectively. Accuracy of results was assessed by comparison wlth those obtalned by other, independent, analytical technlques.
Direct determination of Se and Sb in seawater by electrothermal AAS is not feasible due t~ insufficient instrumental sensitivity. Concentrations of the total dissolved forms of these elements in open ocean waters are approximately 0.2 wg/L Sb (1, 2) and 0.1 pg/L Se (3, 4 ) . With the exception of a few electrochemical methods for direct analyses of Sb (5),concentration and/or matrix separation of these analytes prior to their determination is necessary. Such techniques have included coprecipitation (6), chelation-solvent extraction (7, 8),and hydride generation (1, 4). The latter method has gained the most widespread acceptance, and optimum results are achieved when the hydride is generated from a sample concentrate (1)or following cold trapping concentration of the hydride (4, 9). Each method provides advantages and disadvantages that must be considered with regard to the scope of the study and the available laboratory facilities. An alternative procedure, complexation of Se(IV), Sb(III), and Sb(V) with ammonium pyrrolidine dithiocarbamate followed by adsorption of these complexes onto a column of CI8-bonded silica gel, described here, allows for a simple, rapid, and quantitative concentration of these elements into a clean matrix suitable for subsequent analysis by GFAAS. Concentration factors of 200-fold are readily obtained, and determination of both Se(1V) and Se(VI) is feasible.
EXPERIMENTAL SECTION Apparatus. A Perkin-Elmer Model 5000 atomic absorption
spectrometer equipped with an HGA-500 graphite furnace and Zeeman background correction was used. A Perkin-Elmer electrodeless discharge lamp (Se) and Westinghouse hollow cathode lamp (Sb) served as line sources. These were run at the manufacturers' recommended wattage or current. Measurements were made at the 196.0-nm line for Se and 217.6-nm line for Sb. A spectral band-pass of 0.2 nm was used. Sample aliquots of 10 or 20 WLwere delivered to the furnace with a P.E. AS-40 autosampler, and integrated or peak-height absorbance was recorded on a Versagraph 800 series strip-chart recorder (Linear Instruments Corp., Irvine, CA). Pyrolytic graphite-coated tubes were used throughout. Reagents. Analytes were concentrated by adsorption of their pyrrolidine dithiocarbamate (APDC) complexes onto a bed of
CIB-bonded silica gel (Bondapak, Porasil B, 37-75 Wm; Waters Assoc., Milford, MA) The polypropylene reservoir and C18-gel column support have been described elsewhere (10). The C18-bondedsilica gel was used without further purification. All acids, solvents, and other reagents were purified as previously reported (11). Stock solutions (1000 mg/L) of Se(1V) and Se(V1) were prepared by dissolution of NazSe03and Na2Se04(BDH Chemicals Ltd., Poole, England) in 1 M HC1 and 1 M HNO,, respectively. A stock solution of Sb(II1) (1000 mg/L) was prepared by dissolution of Sb203 (Baker) in 1 M HC1. A soiution of Sb(V) was obtained by oxidation of the Sb(II1) with excess KBr03 in 5 N HC1 (12). AU stock solutions were stored in cleaned polypropylenebottles, and working standards of lower concentrationswere prepared fresh daily by serial dilution of the stock solution with 1% HN03 for Sb(V) and Se(V1) and with 1% HCl for Sb(II1) and Se(1V). Solutions (5% (w/v)) of APDC were purified after filtration by repeated extraction with CHCl, and dissolved CHCl, removed by sparging the solution with purified N, for several minutes. Subsurface (50 m deep) samples of near-shore North Atlantic seawater were obtained as part of the ICES fifth round intercalibration for trace metals in seawater,Nantes, France,1982 (13). The samples (salinity 35.4%) were filtered and acidified to pH 1.6 with "OB. Open-ocean North Atlantic seawater was obtained southeast of Bermuda from a depth of 1300 m (14). These samples were unfiltered, had a salinity of 35.07%, and were acidified t o pH 1.6 with "OB. All seawater was stored in precleaned polypropylene or polyethylene bottles. Procedure. A clean laboratory equipped with laminar flow benches and fume cupboards providing a class 100 working environment was used for sample preparation. Polyethylene gloves (unpowdered) were worn during all sample manipulation steps. A borosilicate glass column, slurry loaded with 600 mg of C18-bondedsilica gel suspended in subboiling distilled methanol, was converted to an aqueous mobile phase, as described in ref 10. For the determination of Se(IV), Sb(III), and Sb(V), 300400-mL aliquots of seawater were adjusted to pH 1.2 by addition of high-purity HC1,1.6 mL of purified 5% APDC was added, and the sample was passed through the column at a flow rate of 10 mL/min. Following passage of the sample, the column was washed free of residual seawater by using 10 mL of pH 1 deionized water (acidified with HCl) containing 20 mg/mL APDC, The adsorbed metal chelates were eluted from the column with 10 mL of methanol. This eluate was collected in a 30-mL quartz crucible, evaporated to near dryness in the presence of 400 fiL of concentrated HNOB,and then diluted to 4.0 mL with 1%HN03. Several precautions were necessary with antimony during evaporation of solutions. Substantial losses occurred (50%)with simple evaporation of the methanol following initial addition of 0.4 mL of "0,. Losses were avoided by subsequent addition of a further 0.4 mL of HNO, when nearly all the methanol had evaporated. Furthermore, Sb could also be lost if the final solution was reduced much below 0.3 mL in volume. Total selenium (hence, Se(V1) by difference) was determined on a separate 400-mL aliquot of sample because reduction of Se(V1)to Se(1V)prior to complexationwith APDC was somewhat involved, requiring heating and a higher concentration of acid. A reaction based on the HBr/Br2 redox buffer proposed by Uchida e t al. (8) was adopted for this study. Samples were heated to near boiling for 15 min in their 1-L polypropylene reservoirs using a vented microwave oven (15). After the solution was cooled to room
Published 1984 by the American Chemical Society 0003-2700/85/0~57-0006$01.50/0
ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985
temperature, excess Brz was reduced with NHzOH-HC1and the sample was processed as outlined above for Se(1V) determinations. Blanks were determined by applying these procedures to 25-mL aliquots of deionized water and also to 25-mL volumes of seawater which had been previously stripped free of qnalyte by prior passage through the CIScolumn. The column was reslurried between runs to ensure proper packing of the bed, thoroughly flushed with methanol, and converted to an aqueous mobile phase. All concentrates were stored in 30-mL screw-capped polypropylene bottles until analyzed by GFAAS. Calibration was achieved by means of appropriate metal standards in 1% P N 0 3 as well as by the addition of metal spikes to aliquots of the sample concentrate in order to obtain a matrix match. The method of additions was used during initial methods development. A matrix modifier, prepared by dissolution of high purity Ni in HN03, was used with all samples, which were made 0.01% in Ni, sufficient for stabilization of the analytes during an 800 "C ashing temperature. Optimum furnace parameters for the atomization of Se and Sb were determined to be plafform sampling of 2O-pL volumes in combination with an 800 "C ash and maximum power heating to 2700 "C. Peak height absorbance was recorded.
Table I. Analysis of Open-Ocean Seawatern
RESULTS AND DISCUSSION Selective formation and extraction of the piazselenole complex of Se(1V) prior to determination by fluorimetry (16) or gas chromatography (8)have become common analytical practice. Attempts were therefore made to sequester Se(1V) from seawater by using a column of silica-iwmobilized 5nitro-l,2-~henylenediamine, similar to the successful procedures utilized for several other trace metals onto a column of silica-immobilized 8-hydroxyquinoline (17). Quantitative removal of Se(1V) from deionized water at pH 2 occurred but required 10 mL of 50% (v/v) HNO, to elute it from the bed. Recovety from seawater proved no better than 50%. This method of concentration was therefore abandoned in favor of one based on complexation by APDC and adsorption onto a column of C18-bonded silica gel. This approach is not selective for Se (although it permits differentiation of Se(1V) and Se(Y1)) but allows the simultaneous recovery of Sb and Bi, as well as elements such as Pb, Zn, As(III), Sn(II), and V(V) (18). Recovery from acidic media (7) is advantageous when determinations are to be made on acid-preserved samples. Recovery Efficiency. A 10-mL volume of methanol was sufficient to completely elute metal-APDC complexes from the CI8bed. Direct analysis of the methanolic concentrates is not recommended due to the volatility of the solvent and its affinity for the graphite tube wall which resulted in excessive sample spreading and soaking. Separate recovery experiments indicated no losses of Se or Sb during evaporation of the methanol and takeup in HN03. Recovery of spikes (500 ng added to 300 mL of seawater) into aqueous concentrates averaged 94 f 7% ( n = 21) and 97 f 12% ( n = 19) for Se and Sb, respectively. With the experimental conditions used here (pH 1.2 seawater), only Se(1V) was recovered, whereas both Sb(II1) and Sb(V) were recovered. Speciation is discussed more fully in a subsequent section. Recovery of spikes was independent of flow rate from approximately 2 mL/min (gravity feed) to 32 mL/min (aspirated). At flow rates above 20 mL/min, aspiration created voids in the lower portion of the C18 bed. Although this was not deleterious to performance, the elution rate was reduced due to air trapped in the voids. Flow rates of 10-20 mL/min were therefore used. Recovery of analytes was quantitative from 500 mL volumes of seawater. Aliquots of 300-400 mL were adequate for these determinations. Analytical Blanks. Analytical blanks were prepared from both 25-mL volumes of distilled, deionized water (DDW) and
Table 11. Analysis of Coastal Seawatern
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concentration, rg/L Se(1V) Sb(II1) and Sb(V) 0.019 0.024 0.021 0.021 0.022
0.19 0.23 0.21 0.19 0.17
0.018
0.022 0.027 0.024 mean
0.022 f 0.003
0.20 f 0.02
independent values
0.025 f O.OOlb 0.026 f 0.003c
0.21 f 0.01d 0.24 f 0.02c
NASS-1 reference seawater (14). Piazselenol extraction, GC Activated charcoal adsorption, NAA (20, 21). Electrochemical analysis, ASV-HMDE ( 5 ) .
(19).
concentration, pg/L Se(1V) Sb(II1) and Sb(V) 0.020
0.023 0.017 0.025 0.022 0.016 0.018
0.020 0.017 mean independent values
0.020 f 0.003
0.21 0.17 0.19 0.19
0.15 0.26 0.26 0.24 0.21 f 0.04 0.19 f 0.03b
ICES fifth round intercalibration for trace metals in seawater, Nantes, France, 1982 (13). Electrochemical analysis, ASVHMDE (5). seawater which had been previously stripped of analyte. Several blanks were also prepared from 300-mL volumes of stripped seawater. Average blanks were for Se 1.7 f 0.7 ng (n = 11)and for Sb 9 f 5 ng (n = 21). There was no significant difference between the DDW and seawater blanks. Sources of contamination include reagents, labware, and sample manipulation. Use of polyethylene gloves (unpowdered) was imperative for Se determinations, otherwise blanks were invariably higher and significantly more variable. Seawater Analyses. Results for the analysis of the coastal (ICES) and open-oceqp seawater (NASS-1) are given in Tables I and 11. In both cases, analytes were concentrated 75-fold from 300-mL volumes of seawater. Calibration was achieved by spiking an aliquot of the concentrate with the element of interest. Direct calibration against acid-matched standards was not successful. The matrix in the concentrate generally enhanced signals relative to those of a standard. The accuracy of the method was assessed by comparison of results with those obtained using other, independent analytical techniques. Good agreement with these other values is evident from the data in Tables I and 11. Whereas total S b was determined in these samples, only Se(1V) could be reliably determined (see comments on speciation). Additional determinations of Se(1V) concentrations in these samples were obtained by coprecipitation of Se(1V) with iron hydroxide, selective extraction of the piazselenole from this concentrate, and subsequent quantitation by gas chromatography (19). Results are also reported for the
ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985
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Table 111. Analytical Figures of Merit
element Se(1V) Sb(II1) and Sb(V)
abs
concn, detection
2 15
7 50
peak-height detection, limitD(300 sensitivity, precision,* limit, ng mL), ng/L A/pg % RSD 0.00028 0.00028
14 at 3 d.LC 10 at 4 d.1
Defined as the concentration of analyte which gives a response equivalent to 3 times the standard deviation of the blank, based on a 300-mL sample volume. *Expressedas a coefficient of variation of the analytical determination (cf. Tables I and 11). cd.l. = detection limit. determination of Se(1V) and S b by neutron activation analysis (20)following concentration of their chelates by sorption onto activated charcoal (21). Additional values for S b were obtained by using anodic stripping voltammetry at a hanging mercury drop electrode
(5)* According to recent, reliable data, the concentrations of Se(1V) in uncontaminated seawaters are quite low (
