Application of isotope dilution analysis for the evaluation of extraction conditions in the determination of total selenium and selenomethionine in yeast-based nutritional supplements

Isotope dilution analysis (IDA) has been used to quantify total selenium, total solubilized selenium, and the selenomethionine (SeMet) amount in yeast and yeast-based nutritional supplements after acid microwave digestion and different enzymatic extraction procedures. For this purpose, both a Se-77-enriched SeMet spike, previously synthesized and characterized in our laboratory, and a Se-77(VI) spike were used. In the analysis of the nutritional supplements, the SeMet spike was added to the sample and extracted under different conditions, and the Se-78/Se-77 and Se-80/Se-77 isotope ratios were measured as peak area ratios after high-performance liquid chromatography (HPLC) separation and inductively coupled plasma mass spectrometry (ICP-MS) detection. The formation of SeH+ and mass discrimination were corrected using a natural SeMet standard injected every three samples. Similarly, total solubilized selenium was measured in the extracts after enzymatic hydrolysis using the Se-77-enriched SeMet as a spike by direct nebulization without a chromatographic separation. To establish a mass balance, total selenium was also determined by IDA-ICP-MS on the yeast tablets after microwave digestion using Se-77(VI) as a spike. Results showed that all enzymatic procedures tested were able to solubilize total selenium quantitatively from the solid. However, the recovery for the species SeMet, the major selenium compound detected, was seriously affected by the enzymatic procedure employed and also by the matrix composition of the supplement evaluated. For the yeast sample, SeMet recovery increased from 68 to 76% by the combined use of driselase and protease. For the nutritional supplements, the two most effective procedures appeared to be protease and driselase/protease, with a SeMet recovery ranging from 49 to 63%, depending upon the supplement evaluated. In the case of in vitro gastrointestinal enzymolysis, the results obtained showed 26-37% SeMet recovery, while the rest of selenium was solubilized as other unknown compounds (probably Se-containing peptides).