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Anal. Chem. 1984, 56, 1806-1808
LITERATURE CITED (1) Sloneker, J. H. Methods Carbohydr. Chem. 1072, 6 , 20-24. (2) Nelson, C. R.; Gratzl, J. S. Carbohydr. Res. 1078, 60, 267-273. (3) SweeleY, c. c.; Bentley. R.; Maklta, M.; Wells, w. w. J . Am. Chem. SOC. 1063, 85, 2497-2507. (4) Morrlson, I. M.; Perry, M. B. Can. J. Blochem. 1066,44, 1115-1126. (5) Stecher, P. G. "The Merck Index", 7th ed.; Merck: Rahway, NJ, 1960;pp 530-531. (6) Newth, F. H. Adv. Carbohydr. Chem. 1051, 6 , 83-106. (7) Hcdge, J. E.; Rlst, C. E. J. Am. Chem. SOC. 1053. 75, 316-322.
(8) Stanek, J.; Cerny, M.; Kocourek, J.; Pacak. J. "The Monosaccharldes"; Academlc Press: New York, 1963;p 660.
R E C E ~ for J J review February 6, 1984. Accepted May 1,1984. The mention of firm names or trade products does not imply that they are endorsed or recommended by the U.S. Department of Agriculture over other firms or similar products not mentioned.
Determination of Selenium(IV) in Seawater by Gas Chromatography after Coprecipitation with Hydrous Iron(I I I) Oxide K. W. Michael Siu* and Shier S. Berman Division of Chemistry, National Research Council of Canada,' Montreal Road, Ottawa, Ontario, Canada K I A OR9
Selenium( I V ) concentratlon in seawater was determined by gas chromatography with electron capture detection after coprecipltatlon with hydrous iron( I II)oxide. Quantitatlve precipitation of Se(1V) was obtained at pH 5.0. Following a brlef, 15 min, stlrrlng and settilng period, the copreclpltate was filtered and dissdved in hydrochloric acid. Selenium was derlvatlred to 5-nitroplarselenoi, extracted into toluene and Introduced into the chromatograph. The detection limit was 1 pg injected or 5 ng of Se/L of seawater using a 200-mL sample. The precislon was 8 % at the 0.025 pg of Se/L level.
The concentration and fate of selenium in the marine environment have received considerable interest recently. This is due partially to the dual nature of selenium, whose essential and toxic levels are not that far apart. Gas chromatography (GC) has been used to determine selenium in a large array of materials after derivatization to form volatile piazselenols. Because of its high sensitivity, the electron capture detector (ECD) is used almost exclusively. The application of this technique has been reviewed ( I ) . Recently, we reported the analyses of marine sediments (2). This paper deals with analysis of open seawater. Selenium exists in seawater principally as Se(1V) and -(VI). It is debatable as to which oxidation state is thermodynamically more stable in seawater; however, both species usually coexist. The ratio of Se(1V) to Se(V1) varies from region to region and also along depths within a particular region (3). The various analytical techniques that are suitable for the determination of selenium in environmental waters have been discussed recently (3). Most methods require some form of preconcentration due to the very low quantities of selenium in natural waters. Gas chromatography has been used to determine various inorganic selenium species after derivatization to form 4,6-dibromopiazselenol(4)or 5-nitropiazselenol (5). For the former derivative, the reagent 1,2-diamino-3,5dibromobenzene is not commercially available. While the reagent for the latter, 1,2-diamino-4-nitrobenzene7 is available, its use in direct open ocean water analysis is not without drawbacks. The extraction efficiency of 5-nitropiazselenol is only about 70% at a toluenelaqueous ratio of 100, a ratio often 'NRCC 23343. 0003-2700/84/0356-1606$01.50/0
necessary for unpolluted waters. This means that experimental conditions have to be strictly controlled to obtain reproducible selenium recovery. Further, the large sample size (100-500 mL) requires addition of about 20 mg or more of derivatization reagent, whose reaction side-products with seawater are coextracted with piazselenol. This results in a complex chromatogram, and in most cases, requires capillary GC to resolve the piazselenol peak from the others. We find a 15-m SE-30 column performs satisfactorily. However, precision of the analysis is low, and each chromatographic run takes about half an hour (6). Some form of preconcentration seems desirable. It is well-known that a host of elements coprecipitates with hydrous iron(II1) oxide. Depending on the pH of the medium, which influences the surface charge of the precipitates, either cations or anions may be coprecipitated (7). Selenite @eo:-) is one of the anions that have been studied. Most methods call for addition of a surfactant, typically sodium lauryl sulfate, to facilitate collection of hydrous iron(II1) oxide (7-10).While it is true that precipitates from low ionic strength solutions are often colloidal and difficult to filter, those from solutions of high salt content, such as seawater, are often coarse and filter well, thus making addition of the collector surfactant unnecessary. It is interesting to note that selenium(V1) is not coprecipitated by hydrous iron oxide. Both previous work (11) and our results in a preliminary study show that practically no Se(V1) is brought down. Thus the coprecipitation process is species selective. Quite a few analytical techniques have used hydrous iron(111) oxide for selenium(1V) preconcentration including spectrophotometry (8, II), atomic absorption (7,9 ) , and neutron activation (IO). Spectrophotometry and atomic absorption necessitate dissolution of the collected hydrous oxide. Further, many procedures require the removal of iron (usually by ion exchange) prior to determination (7,8, 11) This paper discusses the rapid coprecipitation of selenium(1V) from seawater with hydrous iron(II1) oxide, the derivatization of selenium to 5-nitropiazselenol, and its subsequent determination by GC-ECD.
EXPERIMENTAL SECTION Instrumentation. A Varian VISTA 6000 GC equipped with a constant-currentECD was used. The column, held isothermally at 200 "C, was a 1-m borosilicate tube packed with 5% OV-225
Published 1984 by the Amerlcan Chemical Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 11, SEPTEMBER 1984
coated on acid-washed Chromosorb W, SO/lOO mesh. Nitrogen (
