Selenium Speciation in Aqueous Solutions Using a Hydride Generation Atomic Absorption Spectrophotometry Technique

A sensitive analytical technique for the determination of soluble selenium species [Se(IV), Se(VI), Se(0,-II), dimethyl selenide (DMSe), and oxidized methylated organo-Se compounds], using hydride generation atomic absorption spectrophotometry is described. Dimethyl selenide is purged out of a solution and trapped on a U-tube immersed in liquid N2. After controlled heating of the sample trap, DMSe is quantified in a quartz flame-in-tube atomizer aligned in the optical path of an atomic absorption spectrophotometer. Selenite [Se(IV)] and oxidized methylated Se-compounds are reduced by NaBH4 in 4 M HCl to H2Se and DMSe, repectively. Entrained by a He carrier gas the H2Se and DMSe are trapped in a liquid N2 cooled U-tube. Their separation is accomplished by controlled heating of the sample trap. Hydrogen selenide and DMSe carried out of the trap into the atomization cell are detected as two distinct and sharp absorption signals. Oxidative and/or reductive chemical digestions are used to quantitatively convert total soluble Se or Se(VI) to Se(IV), while the original Se(IV) stays unchanged. The digestions are analyzed for Se(IV). Selenate [Se(VI)] is calculated from the difference between the Se(IV) and [Se(VI)+Se(IV)] analysis. The difference between the total Se content and the [Se(VI)+Se(IV)] analysis represents the Se(0,-II) fraction. The technique has been applied for Se speciation in sediment-water extracts.