Negative ion electrospray ionization mass spectral study of dicarboxylic acids in the presence of halide ions

The negative ion electrospray ionization (ESI) mass spectra of a series of dicarboxylic acids, a pair of isomeric (cis/trans) dicarboxylic acids and two pairs of isomeric (positional) substituted benzoic acids, including a pair of hydroxybenzoic acids, were recorded in the presence of halide ions (F(-), Cl(-), Br(-) and I(-)). The ESI mass spectra contained M--H](-) and M+X](-) ions, and formation of these ions is found to be characteristic of both the analyte and the halide ion used. The analytes showed a greater tendency to form adduct ions with Cl(-) under ESI conditions compared with the other halide ions used. The isomeric compounds yielded distinct spectra by which the isomers could be easily distinguished. The collision-induced dissociation mass spectra of M+X](-) ions reflected the gas-phase basicities of both the halide ion and M--H](-) ion of the analyte. However, the relative ordering of gas-phase basicities of all analyte M--H](-) and halide ions could not account for the dominance of chloride ion adducts in ESI mass spectra of the analytes mixed with equimolar quantities of the four halides.