Comparative study of high performance liquid chromatography species-specific and species-unspecific isotope dilution inductively coupled plasma mass spectrometry. A case study of selenomethionine and the origin of its oxidized form

The analytical procedures for accurate quantification of selenomethionine (SeMet) at various concentration levels and in different matrix of certified reference materials (CRMs): ERM DB151 skimmed powder milk, ERM DC210a wheat flour and SELM 1 enriched yeast were developed and compared. Two modes of application of isotope dilution, namely species-specific isotope dilution analysis (IDA) and species-unspecific isotope dilution analysis with the use of high performance liquid chromatography (HPLC) coupled to inductively coupled plasma mass spectrometry (ICP-MS) were evaluated. The excellent results for SeMet were obtained in this study, using of developed species-specific ID HPLC ICP-MS and addition of spike solution before extraction. With this approach, the stability of SeMet during sample preparation or sample storage can be track. It was also possible to state the presence or absence of SeMet in oxidized form, which was co-eluted with the other selenium species. Moreover, the origin of SeMetO in extracts can be confirmed by the calculation and comparison of isotope ratios from SeMet and SeMetO signals. Equal altered isotope ratios in both species indicate that SeMetO was formed during extraction or storage of sample and SeMet is only source of SeMetO. Although, different isotopic ratios can indicate that the signal also comes from SeMetO already present in the sample. All results obtained for SeMet by species-unspecific ID HPLC ICP-MS, were close to the certified reference values within their uncertainty, under the condition, when 2-mercaptoethanol was added before extraction as antioxidant agent. The species-specific and species-unspecific IDA HPLC ICP-MS approaches were validated and both can be used to establish references values and accurate quantification of selenomethionine in food related products at various concentration level. Moreover, in the case of species-specific IDA, which although is much more time-consuming than species-unspecific IDA, the origin of SeMeO in extracts can be clearly indicated.