Simultaneous identification of selenium-containing glutathione species in selenised yeast by on-line HPLC with ICP-MS and electrospray ionisation quadrupole time of flight (QTOF)-MS/MS

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Abstract
A method based on the on-line combination of ion-pair reversed-phase HPLC with ICP-MS and ESI-Q-TOFMS/MS was proposed for the identification of a number of Se-containing compounds in selenised yeast aqueous extracts without any pre-treatment or fractionation of the water extract. The on-line combination of reversed phase HPLC using a gradient of up to 40% methanol in 0.1% formic acid with ICP-MS was used for detection of multiple Se-containing species within a single chromatographic run of the water extract. The methanol load to the plasma was reduced two-fold by using post-column effluent dilution with ultra-pure water. This, along with a careful optimisation of the ICP parameters, allowed concentrations of up to 40% methanol in the mobile phase to be well tolerated by the ICP system with no significant changes in sensitivity or occurrence of plasma instability. Direct analysis of the water extracts by HPLC with on-line ESI-TOFMS allowed the accurate molecular mass determination of a range of Se compounds detected by HPLC-ICP-MS, including previously reported (m/z 198, 433 and 563) and unknown Se species. This was achieved thanks to the relatively high selectivity and compatibility of the chromatographic method with ESI-TOFMS. The accurate molecular mass data obtained by HPLC-ESI-Q-TOFMS/MS measurements of the [M + H]+80Se and [M + H]+78Se ions and their associated product ions allowed structural characterisation of the compounds. Two of these selenium compounds with m/z 402 and 661 for the [M + H]+80Se ions, identified as methyl Se-glutathione and a Se-glutathione species with a S–Se bridge between glutathione and Se-cysteinyl-gamma-glutamylglycine (GS-SeG) in selenised yeast, are reported here for the first time. Their empirical formulae were confirmed by the experimental data (measured accurate mass) with an error from calculated exact mass (of the derived empirical formulae) lower than 5 parts per million (ppm) for most of the ions detected. The presence of both of these Se compounds in two further Se-yeast supplements was confirmed by the same methodology.

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