Does deprotonated benzoic acid lose carbon monoxide in collision‐induced dissociation?

Abstract Decarboxylation is known to be the major fragmentation pathway for the deprotonated carboxylic acids in collision‐induced dissociation (CID). However, in the CID mass spectrum of deprotonated benzoic acid ( m/z 121) recorded on a Q‐orbitrap mass spectrometer, the dominant peak was found to be m/z 93 instead of the anticipated m/z 77. Based on theoretical calculations, 18 O‐isotope labeling and MS 3 experiments, we demonstrated that the fragmentation of benzoate anion begins with decarboxylation, but the initial phenide anion ( m/z 77) can react with trace O 2 in the mass analyzer to produce phenolate anion ( m/z 93) and other oxygen‐containing ions. Thus oxygen adducts should be considered when annotating the MS/MS spectra of benzoic acids.