Extraction and determination of elemental selenium in sediments--a comparative study

This paper proposes a new technique to extract elemental Se from soil and sediment samples. In this study, we have identified that the purchased red elemental selenium standard (PF-Se) was impure and rather consisted of a mixture of CS₂ soluble amorphous elemental Se (ca. 10%, w/w), water soluble oxidized Se (ca. 15-17%, w/w) and, CS₂ insoluble red monoclinic elemental Se. In more recent studies, a slow oxidation and a mineral phase transition of this sample was also observed. The solubility of the amorphous elemental Se in CS₂ was at least 0.64 mg L⁻¹. The black elemental Se purchased from Sigma-Aldrich had a much lower solubility in CS₂ (7.2 μg mL⁻¹) compared to that given in the literature. Any selenium compounds with electrical charge and polar nature is insoluble in CS₂. In a sodium sulphite solution, PF-Se was completely dissolved thus giving a clear indication of the lack of selectivity in that extraction system. Other comparative studies also demonstrated that over extraction did occur with the Na₂SO₃ method. Compared to Na₂SO₃, CS₂ extraction of elemental Se is not only much simpler, straightforward and with higher analytical precision, but also much more selective and accurate. With HG-AFS, the detection limit can reach as low as 1.0 ng g⁻¹ in sediment sample owing to a low reagent blank of CS₂ solvent.