An unexpected ion-molecule adduct in negative-ion collision-induced decomposition ion-trap mass spectra of halogenated benzoic acids

The ion observed at m/z 145 when product ion spectra of iodobenzoate anions are recorded using iontrap mass spectrometers corresponds to the adduct ion [I(H₂O)^-. The elements of water required for the formation of this adduct do not originate from the precursor ion but from traces of moisture present in the helium buffer gas. A collision-induced decomposition (CID) spectrum recorded from the [M-H]^- ion (m/z 251) derived from 3-iodo[2,4,5,6-²H₄]benzoic acid also showed an ion at m/z 145. This observation confirmed that the m/z 145 is not a product ion resulting from a direct neutral loss from the carboxylate anion. ⁷⁹Bromobenzoate anions produce similar results showing an ion at m/z 97 for [⁷⁹Br(H₂O)]^-. The ion-molecule reaction observed here is unique to ion-trap mass spectrometers since a corresponding ion was not observed under our experimental conditions in spectra recorded with in-space tandem mass spectrometers such as triple quadrupole or quadrupole time-of-flight instruments.