Electron ionization mass spectrometry of citrus limonoids

Electron ionization (EI) mass spectrometry was used to differentiate four structurally closely related citrus limonoid aglycones, including limonin, nomilin, obacunone, and deacetylnomilin. The limonoids were isolated and purified from citrus seeds. Structures of major fragment ions were elucidated by high-resolution mass spectrometry (HRMS) and fragmentation pathways were proposed. The fragmentation patterns observed in the EI spectra can be used as important references for the positive characterization of limonoid aglycones.