Evaluation of yeast-based selenium food supplements using high-performance liquid chromatography and inductively coupled plasma mass spectrometry

Six different brands of yeast-based selenium food supplements were obtained from local stores. These samples were analyzed for total selenium content and content uniformity by microwave nitric acid digestion. These supplements were also treated with milder extraction and hydrolysis conditions to analyze for the expected selenomethionine, a natural product of yeast when inoculated with selenium. A C8 high-performance liquid chromatographic (HPLC) column was used for the chromatographic determination of selenomethionine using a mobile phase of 89+10+1 (v/v/v) water-methanol-trifluoroacetic acid. Inductively coupled plasma mass spectrometry (ICP-MS) was used for the total selenium content and tablet content uniformity determination, and ICP-MS was used as the detector coupled with HPLC for the selenomethionine determination. A number of extraction and hydrolysis sample treatment procedures were tried. The two most effective appeared to be microwave heating with hydrochloric acid and with a simple overnight treatment with the enzyme Proteinase K. The six different brands of selenium supplements were found to have near label values based on total selenium, and they had reasonable tablet to tablet content uniformity values, but each brand had dramatically different profiles for the actual chemical form of selenium within the supplement. Only two brands had high levels of selenomethionine; one brand appeared to contain all inorganic selenium, and one brand appeared to contain greater than half inorganic selenium despite label claims of content being only selenomethionine.