Thermal degradation of O ‐acyl oximes under electron impact ionization conditions

Thermal degradation of O ‐acyl oximes under electron impact ionization (EI) conditions has been examined. The EI mass spectra gave fragment ions originating from thermal degradation, as well as molecular ions M +· and normal fragment ions due to unimolecular decomposition of the M +· ions. The fragment ions due to thermal degradation of O ‐acyl oximes were identified by using fast‐atom bombardment (FAB) mass spectrometry with a liquid matrix together with the collision‐induced dissociation (CID) technique. While the FAB mass spectra of O ‐acyl oximes gave, preferentially, M +· ions together with some characteristic fragment ions, as well as the molecular related ions [M+H] + and [M−H] + , the spectra showed little evidence of the fragment ions which may originate from thermal degradation, under EI conditions. The CID spectra of selected M +· ions formed by electron impact also showed little of such fragment ions. The thermal degradation was directly confirmed by measuring the FAB mass spectra of O ‐acyl oximes that had been heated in a sealed sample tube.