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 ion‐trap mass spectrometers corresponds to the adduct ion [I(H 2 O)] − . 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‐ 2 H 4 ]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. 79 Bromobenzoate anions produce similar results showing an ion at m/z 97 for [ 79 Br(H 2 O)] − . 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. Copyright © 2006 John Wiley & Sons, Ltd.