Differences in Fragmentation between [M + H]⁺ and [M + Li]⁺ Ions in Fast Atom Bombardment Mass Spectrometry

Abstract

In order to understand differences in fragmentation between [M+H]⁺ and [M+A]⁺ ions (A=Li, Na, K, Rb, and Cs) in more detail, dissociation reactions of [M+H]⁺ and [M+Li]⁺ ions of N-t-butoxycarbonyl (Boc)-α-methylalanine (BMA) and N-t-Boc-α-methylalanine methyl ester (BMAM) in fast atom bombardment mass spectrometry (FAB-MS) were examined. Fragmentation and structural features of the [M+H]⁺ and [M+Li]⁺ ions are interpreted by using tandem mass spectrometry (MS/MS). The order of the activity of dissociation reaction [M+H]⁺[M+Li]⁺ is suggested from the relative peak intensities in the FAB mass spectra, I[M+H-fragment]⁺/I[M+H]⁺ and I[M+Li-fragment]⁺/I[M+Li]⁺. Furthermore, MNDO optimized structures of the [M+H]⁺ and [M+Li]⁺ ions are compared with the structures of the [M+H]⁺ and [M+Li]⁺ ions proposed by FAB-MS.