Anal. Chem. 1802, 5 4 , 307-309 (32) Korkes, S.; Campillo, A. D.; Ochoa, S. J. Blol. Chem. 1950, 187, 89 1-905. Werkman, C. H. Blochem. J. 1060, 74, 359-362. (33) Suzuki, I.; 134) . . Demetriou. J. A.: Drewes. P. A.: Gin, J. B. I n “Ciinlcal Chemlstrv”: Henry R., Cannon, D. C., Winkelman, J. W., Eds.; Harper and Row: Hagerstown, MD, 1974; Chapter 21. (35) Vennesland, B. I n ”Methods in Enzymology”; Colowick, S., Kapian, N., Eds.; Academic Press: New York, 1955; Voi. 11, pp 719-722.
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(36) Racker, E. I n “Methods in Enzymology”; Colowick, S., Kapian, N., Eds.; Academic Press: New York, 1955; Vol. 11, pp 722-725.
RECEIVED for review June 11, 1981. Accepted November 12, lg81*We are grateful to the Institutes Of (Grant GM-25308) for support of this research.
Determination of Inorganic Selenium Species in Groundwaters Containing Organic Interferences by Ion Chromatography and Hydride Generation/Atomic Absorption Spectrometry Dennis R. Roden and Dennls E. Tallman” Department of Chemistry, North Dakota State University, Fargo, North Dakota 58105
Natural waters containing appreclabie levels of dissolved organic campounds often present difficulties In thelr analysls for selenium. Oxldative dlgestlon of the sample usually removes organic interferences and permlts the determinatlon of total selenium. However, digestlon also destroys the natural distrlbutlon of selenium between Its common oxldatlon states, 44and 6+. I n this reporl we descrlbe a procedure which permlts speclatlon of lnorganlc selenium In groundwater samples obtained from a coal strlp mlnlng area. Many of these samples, even when spiked wlth appreclabie amounts of seienlum, totally suppress the release of selenium hydride. The method is based on separatlon of the selenlum species from the organlc interferent(s) by column chromatography on XAD-8 resin followed by hydride generatlon/graphite furnace atomic absorption determlnatlon.
The determination of selenium in ground and surface waters by hydride generation followed by atomic absorption (HGAA) is well established (1-7). During studies of groundwater geochemistry in the coal development region of western North Dakota, we have encountered several samples for which the selenium absorbance signal from HGAA is severely depressed by one or more interferents believed to be organic in origin. Oxidative digestion of the sample followed by reduction of the selenium to selenite does eliminate the signal suppressing interferent and permits the determination of total selenium. However, this approach precludes the ability to obtain information about the distribution of inorganic selenium between selenite and selenate in the original sample. Such species-specific information is necessary for understanding the geochemistry, potential toxicity, and mobility of selenium in the groundwater aquifer. Most of our problem groundwater samples were obtained from spoil banks and below spoil banks adjacent to lignite strip mines and had dissolved organic carbon values as high as 50 ppm. This observation, combined with a recent report (8) which suggests that humic acids interfere in the determination of selenium in soils, led us to suspect that the interferent was most likely organic, possibly humic in origin. Recent work by Curtis et al. (9) on humic acid fractionation prompted experiments in our laboratory which led to the observation
that the interfering substance(s) could be removed by passing the sample through a XAD-8 anion exchange column at pH 1.6-1.8. Selenite and selenate were found to pass through the column unchanged under these conditions, allowing speciation by standard hydride atomic absorption methods ( 7 , I O ) .
EXPERIMENTAL SECTION Instrumentation and Apparatus. The hydride generation and trapping apparatus was the same as that described by Tallman and Shaikh (IO) with the exception that their liquid nitrogen trap was replaced by a smaller trap 26 cm long made from 5 mm i.d. Pyrex tubing. The inside surface area of this trap was increased by making numerous indentations in the tube by intense localized heating and reduced internal pressure. The use of this smaller trap permitted substitution of a Styrofoam cup of warm tap water for the vari-ac and nichrome coil heater used by Tallman and Shaikh. The hydride generator was interfaced to a Perkin-Elmer Model 603/HGA-2100 graphite furnace atomic absorption spectrometer as described previously (10). Pyrolytically coated graphite furnace tubes were used, and the light source was a Perkin-Elmer selenium electrodeless discharge lamp. All data were collected with background correction. The instrument settings were as follows: slit width, 3 mm; wavelength, 196 nm; atomization temperature, 2650 “C; atomization time, 12 s; helium purge gas flow rates, 100-125 mL/min when sparging the hydride generator and 375425 mL/min when sweeping hydride from the cold trap to the furnace. The HGA-2100 furnace controller was set to go directly to atomization upon initiation of the temperature program and was interfaced to the Model 603 microprocessor so as to simultaneously initiate the read function of the spectrometer. The peak height mode of absorbance measurement was used throughout this study. The absorbance-time profiles were also recorded on a Heath Model EU-205-11 strip chart recorder. The ion exchange column was made of Pyrex glass tubing 28 cm in length with a 1.3 cm internal diameter. The column was slurry packed to a depth of 14 cm with XAD-8 resin (Amberlite). Column flow was controlled with a Teflon stopcock. Reagents. All chemicals were analytical reagent grade and were used without further purification. Solutions were prepared in distilled, deionized water (four cartridge Milli-Q system, Millipore, Inc.). Sample Preparation. Digestion of a sample was carried out by placing up to 50 mL of the water sample in a 250-mL flask containing five silica boiling beads and adding sufficient deionized water to bring the volume to 50 mL. Then 0.5 mL of 5% KMn04 was added and the appropriate standard addition was introduced at this point. The solution was boiled for 15 min followed by cooling in ice to room temperature. When cool, the excess per-
0003-2700/82/0354-0307$01.25/0@ 1982 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 2, FEBRUARY 1982
Table I. Determination of Total Selenium in Groundwaters, Effects of Sample Digestion on Analytical Data and Percent Recoveries Se concn in amt of Se(1V) sample % recovery sample identa sample,&ppb added, ng treatment peak absC of added Se deionized water
none
0.185
lOOd
digestion none
0.389 0.085
104 22
100
digestion none
0.454 0.007
92 2
looe
digestion
0.340
87
50
