Two competing ionization processes in ESI-MS analysis of N-(1,3-diphenylallyl)benzenamines: formation of the unusual [M-H] ion versus the regular [M+H] ion.

A series of N-(1,3-diphenylallyl)benzenamine derivatives (M) were investigated by electrospray ionization mass spectrometry in the positive-ion mode. Both the anomalous [M-H] and the regular [M+H] were observed in the ESI mass spectra. The occurrence of [M-H] has been supported by accurate mass spectrometry, liquid chromatography mass spectrometry, and tandem mass spectrometry analysis. Calculation results indicated that formation of [M-H] is attributed to the ion-molecule reaction of M with the protonated ESI solvent molecule (e.g. CHOH) via hydride abstraction from a tertiary C-H. The competing ionization processes leading to [M-H] or [M+H] were significantly affected by the concentration of formic acid in the electrospray ionization solvent and the proton affinity of the N atom.