A fragmentation study on four C₁₉-diterpenoid alkaloids by electrospray ionization ion-trap time-of-flight tandem mass spectrometry

High-resolution electrospray ionization ion-trap time-of-flight tandem mass spectrometry (HR-ESI-IT-TOF-MSⁿ) in positive-ion mode was used to determine the accurate masses and fragmentation pathways of four C₁₉-diterpenoid alkaloids, aconitine (1), yunnaconitine (2), crassicauline A (3), and benzoylmesaconine (4). The [M+H]⁺ ions of compounds 1–4 were readily observed in conventional single-stage mass spectrometry. Based on the MS^1–6 analyses, detailed fragmentation rules of the four compounds were proposed. The neutral losses of AcOH, MeOH, H₂O, CO, C₂H₄, PhCOOH and p-OMePhCOOH segments were the characteristic eliminations from the precursor ions due to the presence of acetyl, methoxyl, hydroxyl, N-ethyl, benzoyl and p-methoxyl-benzoyl units in the structures. Benefited from the high resolution of the mass analyzer, the loss of 28 Da corresponding to CO or CH₄ segment in product ions was unambiguously distinguished. The losing sequence of the main substituent groups was summarized as: C(8)-acetyl>C(16)-methotyl>C(15)-hydroxyl>C(6)-methoxyl>C(1)-methoxyl/C(3)-hydroxyl>C(18)-methoxyl>>C(13)-hydroxyl. The sequential loss of (16)-methoxyl moiety and CO (generating from enol–ketone tautomerism) groups could be recognized as the characteristic eliminations for the compounds with C(16)-methoxyl and C(15)-hydroxyl groups simultaneously. The application of HR-ESI-IT-TOF-MSⁿ technique to investigate the fragmentation of C₁₉-diterpenoid alkaloids provided useful information to understand their fragmentation behaviors.