Speciation of selenium-containing small molecules in urine and cell lysate by CE-ICPMS with in-capillary enrichment

The quantitative speciation of selenium in biological systems is highly important for evaluating health status and elucidating transformations of Se species in physiological and pathological processes. Hyphenation of capillary electrophoresis with inductively coupled plasma mass spectrometry (CE-ICPMS) is promising for this purpose. However, the unfavorable or insufficient sensitivity for selenium analysis with CE-ICPMS seriously limits its practical applications in biological analysis, e.g., cell analysis. Therefore, it is crucial to improve the detection sensitivity for Se species. In this study, CE-ICPMS sensitivities for five selenium species (selenocystamine (SeA), methyl-2-acetamido-2-deoxy-1-seleno-β-d-galactopyranoside (SeSug 1), selenomethionine (SeMet), Se-Methylselenocysteine (MeSeCys) and selenocystine (SeCys)) were improved by in-capillary stacking via pH gradient between the zones of sample-leading buffer and the incorporation of isopropanol. The improvement on sensitivity of up to 9.9 folds was achieved in different biological samples, with LODs of 0.29-0.52 μg L^-1. This approach was further applied for Se speciation in cell lysate, urine and culture medium. It showed that SeMet was more readily reduced in the medium and favorably accumulated by HepG2, HuH-7 and HCCLM3 cells with respect to SeSug 1 and MeSeCys. In cells, all the three Se species were largely transformed into other Se species. Furthermore, more than 70 % of SeMet reduced in medium was transformed into unknown Se species after 48-h interaction with cells.