Analysis of Characteristic Ion Reactions and Mechanism Exploration of Active Ingredients in Traditional Chinese Medicine by Direct Analysis in Real-Time Mass Spectrometry

Direct analysis in real-time (DART) ionization source coupled with mass spectrometer can directly and rapidly analyze raw samples in their native environment with high specificity and minimal or no sample preparation. In this study, DART combined with linear ion trap mass spectrometry (DART-LTQ MS) was used to analyze the active components of 33 traditional Chinese medicines and explore the generated gas-phase ion reaction mechanism. The parameters of MS and ion source were optimized, such as ionization gas and temperature. Under the optimized conditions, a series of regular characteristic ions were generated in DART-MS when using N₂ and He as the ion source working gas. It was found that when N₂-DART MS was used to analyze active compounds of low boiling points, regular characteristic ion reactions occurred in situ, mainly including oxidation reaction (M+nO+H⁺ or M+nO-H^- (n=1, 2, 3, 4, 5)) and dehydrogenation reaction (two hydrogens lost, M-2H±H^±). The methylation reaction (M+nCH₂+H⁺, M+nCH₂+(CH₃)₃NH⁺ or M+nCH₂-H^- (n=1, 2, 3, 4, 5)) occurred when detection of glycosides compound with highly polar and high-boiling by He-DART MS. Furthermore, the characteristic gas ion reaction mechanism of the active components in traditional Chinese medicine was intensively studied, and the exactly related reactions were discussed. The oxidation mechanisms of furanocoumarins and the dehydrogenation mechanisms of dihydroflavonoids were deduced according to the tandem mass spectrometry of the characteristic ion products and the proposed fragmentation pathways. The methylation reaction of glycosides and the action site were verified. And the reaction mechanisms were proposed and further confirmed by tandem mass spectrometry. The rapid analysis of the active ingredients in traditional Chinese medicine by N₂-DART under high temperature and high pressure provides unique and primary characteristics that is difficult to be formed by other methods, which also provides a theoretical reference for future research on the mechanism of DART-MS. This method is rapid, effective, which is suitable for qualitative determination of the active ingredients in traditional Chinese medicine.