Structural characterization and thermally induced isomerization investigation of cis- and trans-vitamin K1 using ion mobility mass spectrometry

Abstract

Since some organic isomers have different activity, choosing of an appropriate quantitative strategy for their reference material (RM) development is critical. Value assignment is the core process during RM production and, recently, mass spectrometry (MS) based quantitative methods have been the primary approaches commonly adopted. However, due to the different chemical stability, the results obtained from MS and other non-ionized analysis methods might be inconsistent. From this argument, in this work, cis- and trans-vitamin K₁ were selected as they have a different spatial structure and bio-activity. Initially, liquid chromatography with the detection of fluorescence and MS were respectively employed for the quantitative comparison, and a 9.73% isomer quantity difference was observed. Next, ion mobility coupled MS (IM-MS) was adopted for the structural investigation of isomer ions. After optimization, the isomer mixture was separated in a drift tube within several milliseconds. Collision cross-section values calculated using an IM-MS matched well with the theoretical values (2.43% of minimum deviation), suggesting that the IM-MS method was reliable. Afterwards, by raising the desolvation temperature in the ion source, the relative content of the trans-isomer had increased. Tandem mass spectra of the isomers resolved by chromatographic and IM were not identical. All the results indicated that in-source isomerization occurred for the vitamin K₁ isomers. The different thermodynamic stabilities of the isomers might bring uncertainty to the quantification results when using MS methods, thus, understanding such in-source isomerization is conducive to choosing an appropriate analytical approach and then the isomer RM can be developed with higher accuracy.