Mass Spectrometric Behavior of Four Typical Stemona Alkaloids

The mass spectrometric behavior of four Stemona alkaloids was studied. Electron impact (EI) mass spectra showed the molecular ion with moderate intensity [M−99]+ ion as base peak. The other ions were relatively weak. EI spectra provided limited information for determination of structure of the alkaloids. However, electrospray ionization (ESI) and atmosphere pressure chemical ionization (APCI) mass spectra showed the pseudomolecular ions [M+H]+ of the four compounds as base peak. The characteristic ions [M−100]+ and [M−74]+ corresponding to the α-methyl-γ-lactone ring were also observed . The results showed that MS2 spectra associated with soft ionization could provide more structural information. The fragmentation pathways of the four compounds were deduced. The mass spectrometric behavior reflected their structural similarities and differences. Among all the methods, soft ionization technique is the best choice for the structural identification of Stemona alkaloid. It will serve as an important tool in the structural identification of this kind of compounds.