Application of liquid chromatography-atmospheric pressure chemical ionization mass spectrometry to the differentiation of stereoisomeric C19-norditerpenoid alkaloids.

Abstract

High-performance liquid chromatography-atmospheric pressure chemical ionization mass spectrometry (HPLC-APCI-MS) was successfully applied to stereoisomeric C19-norditerpenoid alkaloids at position 1. APCI-MS allowed the easy and precise control of the energy deposition by varying the drift voltage. Comparison of the breakdown curves, observed by changing the potential difference between the first electrode and the second electrode of the APCI ion source, revealed stereochemical dependence of different fragmentations. The APCI spectra of alkaloids were predominantly the [M+H]+ ion and major fragment ion, corresponding to the [M+H-H2O]+ ion or the [M+H-CH3COOH]+ ion, and comparison of the spectra showed that the abundance of fragment ions was significantly higher for C-1 beta-form alkaloids than for C-1 alpha-form alkaloids. The characteristic fragment ions were formed by the loss of a water, acetic acid or methanol molecule at position 8. The fragmentation mechanisms depending on the stereochemistry of the precursor ion could be discerned by recording the spectra in a deuterated solvent system of 0.05 M ammonium acetate in D2O-acetonitrile-tetrahydrofuran. Loss of D2O from the precursor ion gave the fragment ion. This result indicated that the proton of protonation was included in the leaving water molecule. The peak intensity ratio R=[M+H]+/[M+H-H2O]+ manifested the stereochemical differentiation of alkaloids at position 1.