Selenium speciation analysis at trace level in soils

This paper describes the development of an analytical methodology to determine speciation of selenium present in soils at trace level (μg kg−1). The methodology was based on parallel single extractions and high performance liquid chromatography hyphenated to inductively coupled plasma mass spectrometry (HPLC–ICPMS). Two complementary chromatographic separations were used to confirm Se species identity. Different extractants, selected on the basis of sequential extraction schemes, were compared. Ultrapure water, 0.1 mol L−1 phosphate buffer (KH2PO4/K2HPO4) at pH 7 and 0.1 mol L−1 sodium hydroxide extractants were finally chosen owing to their efficiency in extracting Se and compatibility with Se species stability. These extractants allow also assessing respectively water-soluble Se (i.e. the most mobile Se fraction), exchangeable Se (i.e. sorbed onto soil component surface) and Se bound to soil organic matter. This methodology gives thus information on Se mobility related to its distribution in soil with preservation of original Se speciation. Detection limits range from 3 to 29 ng(Se) L−1 and from 0.1 to 10 μg(Se) kg−1, allowing determination of Se species concentrations in extracts from soils containing native Se at trace level. The methodology was applied to three soils with total Se concentrations between 210 and 1560 μg(Se) kg−1.