Total determination and quantitative speciation analysis of selenium in yeast and wheat flour by isotope dilution analysis ICP-MS

Isotope dilution analysis (IDA) has been used to carry out the determination of total Se as well as Se speciation analysis in samples of yeast and wheat flour using an ICP-(Q)MS with an octapole reaction system (ORS). Total selenium was determined after microwave digestion of the samples, whereas enzymatic hydrolysis with protease and lipase was used for the speciation studies. Speciation analysis was performed by using an anion exchange column in order to separate the Se compounds. The separated Se species, eluting from the column, were quantified on-line by post-column IDA using an enriched Se-77 spike solution, after adequate conversion of intensity chromatograms into mass flow chromatograms. To do this, the Se-78/Se-77 ratio was monitored, avoiding the (ArAr)-Ar-38-Ar-40 polyatomic interference on the Se-78 isotope by pressurizing the octapole chamber with hydrogen. Under such conditions significant formation of SeH+ ions occurred which had to be mathematically corrected. Results showed that the main seleno-compound found in both matrices was selenomethionine, with concentration levels of approximately 59% of total Se for wheat flour and of 68% for yeast. Se(IV) and Se(VI) were detected in yeast samples, but only Se(IV) was present at a level to be quantified. Conversely, those two inorganic species of Se were not detected in wheat flour. The obtained results were included in an inter-laboratory exercise, organised by the University of Plymouth Enterprise Ltd. (UK), with excellent agreement between our results and the average obtained from the rest of the participants.