Speciation of volatile forms of selenium and inorganic selenium in sediments by gas chromatography–mass spectrometry

Abstract

Methods are presented for the speciation of both inorganic and organic compounds of selenium, including selenite, selenate, dimethylselenide (DMSe), dimethyldiselenide (DMDSe), diethyselenide (DESe) and diethyldiselenide (DEDSe). Parameters affecting the extraction efficiency from sediments are discussed and the performance of the methods is described. Inorganic selenium species are quantitatively leached from sediments using alkaline solutions. Separation of selenite and selenate is carried out using anion-exchange phases packed in cartridges. Selenite is analysed as piaselenol derivative using GC–MS. Selenate is determined 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 selenite and selenate using sample portions of 1 g. Volatile selenium species are desorbed from sediments using a dynamic headspace desorption method with active carbon as sorptive trap. The species are eluted with carbon disulphide. The four organic selenium species are directly analysed by GC–MS but sensitivity of the method is improved for dialkyldiselenides after derivatization with 1-fluoro-2,4-dinitrobenzene followed by extraction into ethyl acetate and evaporation to dryness under a N 2 stream. Extraction recoveries obtained by this method are higher than 75% for all the species considered in the present work. Estimated 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 and volatile organic selenium species in several sediments collected from different areas in the Southwest of Spain. Finally, incubation experiments were performed at two temperatures for 15 days on sediments and a raw sewage sludge collected from Ayamonte city. Only from this latter were both DMDSe and DESe generated.