Speciation of selenium in diet supplements by HPLC-MS/MS methods

HPLC-MS/MS methods are developed to study the speciation of selenium in food supplements. The species most commonly present in supplements are considered: seleno-l-methionine (C5H11O2NSe), l-selenocystine (C6H12O4N2Se2), phenyl-l-selenocysteine (C9H11O2NSe), methyl-seleno-l-cysteine (C4H9O2NSe), methaneseleninic acid (CH3SeO2H), selenate (SeO42 -), selenocyanate (SeCN-) and selenite (SeO32 -). The chromatographic analysis is performed on a Luna C18 stationary phase: for the separation of the organic species the mobile phase is a mixture of methanol and water both 0.05% in trifluoroacetic acid, while for the separation of inorganic species the mobile phase is a mixture of methanol and tetrabutylammonium hydroxide 1.0 mM aqueous solution. Gradient elution conditions are used in both the separations. Two mass spectrometry analyzers are employed for detection, namely an ion trap for determination of the aminoacidic species and a triple quadrupole for the inorganic ones. The methods are applied in the analysis of six commercial food supplements bought in pharmacies and supermarkets and containing selenium under different speciation forms. Speciation results are compared with the data labelled and with the amount of total selenium, determined by ICP spectroscopy. It is worthwhile to underline that HPLC-MS analysis of supplements containing selenate and selenite, gives values respectively lower than those expected and non detectable values of selenite. On the basis of some experiments, these results are explained through the taking place of side reactions in which selenite and selenate are reduced by ascorbic acid that is present in the supplements at very higher amounts