Single-step microwave digestion of food and biological samples for the quantitative conversion of Se into the +4 oxidation state

An improved single step microwave digestion procedure is described for providing the fast and easy exhaustive mineralisation of biological samples concomitantly with the quantitative conversion of any type of selenium compounds into Se(IV). In such a way, digested samples are directly suitable for the subsequent Se analysis at trace and ultratrace levels by both spectrometric methods such as HG-ICP-MS or HG-ICP-OES and differential pulse cathodic stripping voltammetry (DPCSV). It is based on the use, under suitably optimised microwave irradiation conditions, of a digestion mixture with a carefully tailored composition such that its redox potential is made lower than that allowing Se(IV) to be oxidized to Se(VI), but high enough to permit total destruction of biological or, in general, organic matrices. It consists of a nitric acid (65%, w/w) and hydrogen peroxide (30%, w/w) mixture in a volume ratio 5:1, frequently adopted for the mineralisation of organic and biological samples, but added simply with 0.25 g mL⁻¹ of NaCl. Successful application of the procedure, in terms of both repeatability and accuracy, to the quantification of selenium by the instrumental methods above in standard compounds and in a certified biological sample proved its good performance. The application to the Se determination in human blood plasma and in a wide variety of foods is also reported.