A fragmentation study on four C<sub>19</sub>-diterpenoid alkaloids by electrospray ionization ion-trap time-of-flight tandem mass spectrometry

<div><p>High-resolution electrospray ionization ion-trap time-of-flight tandem mass spectrometry (HR-ESI-IT-TOF-MS<sup><i>n</i></sup>) in positive-ion mode was used to determine the accurate masses and fragmentation pathways of four C<sub>19</sub>-diterpenoid alkaloids, aconitine (<b>1</b>), yunnaconitine (<b>2</b>), crassicauline A (<b>3</b>), and benzoylmesaconine (<b>4</b>). The [M+H]<sup>+</sup> ions of compounds <b>1</b>–<b>4</b> were readily observed in conventional single-stage mass spectrometry. Based on the MS<sup>1–6</sup> analyses, detailed fragmentation rules of the four compounds were proposed. The neutral losses of AcOH, MeOH, H<sub>2</sub>O, CO, C<sub>2</sub>H<sub>4</sub>, PhCOOH and <i>p</i>-OMePhCOOH segments were the characteristic eliminations from the precursor ions due to the presence of acetyl, methoxyl, hydroxyl, <i>N</i>-ethyl, benzoyl and <i>p</i>-methoxyl-benzoyl units in the structures. Benefited from the high resolution of the mass analyzer, the loss of 28 Da corresponding to CO or CH<sub>4</sub> 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-MS<sup><i>n</i></sup> technique to investigate the fragmentation of C<sub>19</sub>-diterpenoid alkaloids provided useful information to understand their fragmentation behaviors.</p></div>