Innovative analytical strategy using ion-mobility shifting additive for isobaric selenium compound identification in selenomethionine standards by IMS

Selenium (Se) is a trace element which is both essential and toxic depending on its concentration and its chemical form. Selenomethionine (SeMet) is one of the widely used selenium standard during Selenium speciation studies. This work was focused on the elaboration of an analytical strategy for the detection and the structural elucidation of an isobaric Se interference, which is found in standard solutions of SeMet by high resolution mass spectrometry (Rm/Δm > 20.000). The structural elucidation of these compounds requires the isolation of the respective parent ion. Nevertheless, the mass difference between SeMet and its interference is less than 0.02Da which is well below the window selection of conventional techniques in mass spectrometry (Quadripole, ion trap). The empirical formula and double bound equivalent (DBE) of these ions suggest different tridimensional structures which lead to a discrimination depending on the ion mobility. This separation is observed, both in gaseous and liquid phase, by Ion Mobility Spectrometry (IMS), Capillary Electrophoresis (CE) and Liquid Chromatography (LC) which are hyphenated to mass spectrometry as detector. The separation efficiency of these ions by IMS and CE is improved by using specific shifting agents (18-Crown-6 Ether) selective to only one of these ions. This strategy has successfully separated the two isobaric ions present and leads to the structural elucidation of the isobar contaminant of SeMet.