SPECIATION OF VOLATILE FORMS OF SELENIUM AND INORGANIC SELENIUM IN SEDIMENTS BY GAS-CHROMATOGRAPHY MASS-SPECTROMETRY

Methods are presented for the speciation of both inorganic and organic compounds of selenium, including selenite, selenate, dimethylselenide (DMSe), dimethyldiselenide (DMDSe), diethyselenide (DESe) and diethyl diselenide (DEDSe). Parameters affecting the extraction efficiency fro m sediments are discussed and the performance of the methods is descri bed. Inorganic selenium species are quantitatively leached from sedime nts using alkaline solutions. Separation of selenite and selenate is c arried out using anion-exchange phases packed in cartridges. Selenite is analysed as piaselenol derivative using GC-MS. Selenate is determin ed after conversion into selenite. The detection limit achieved using the proposed method is 0.6 ng of Se g(-1) (wet mass basis) for both se lenite and selenate using sample portions of 1 g. Volatile selenium sp ecies are desorbed from sediments using a dynamic headspace desorption method with active carbon as sorptive trap. The species are eluled wi th carbon disulphide. The four organic selenium species are directly a nalysed by GC-MS but sensitivity of the method is improved for dialkyl diselenides after derivatization with 1-fluoro-?,4-dinitrobenzene foll owed by extraction into ethyl acetate and evaporation to dryness under a N-2 stream. Extraction recoveries obtained by this method are highe r than 75% for all the species considered in the present work. Estimat ed detection limits for DMSe, DMDSe, DESe and DEDSe using portions of 20 g of wet sediments are 33, 1.0, 22 and 2.3 ng of Se g(-1) (wet mass basis). The methods were applied to the determination of inorganic an d volatile organic selenium species in several sediments collected fro m different areas in the Southwest of Spain. Finally, incubation exper iments were performed at two temperatures for 15 days on sediments and a raw sewage sludge collected from Ayamonte city. Only from this latt er were both DMDSe and DESe generated. (C) 1998 Elsevier Science B.V. All rights reserved.