A comprehensive atmospheric pressure photoionization (APPI) mass spectrometry investigation of hexamethonium bromide is reported. This bisquaternary ammonium salt is a model system for the investigation of multiply charged species and elucidation of ion formation processes. It has been used to elucidate the physico–chemical phenomenon occurring when photoionization is carried out at atmospheric pressure. First, the in-source fragmentations were studied for aqueous solutions of the salt with the photoionization lamp switched off, i.e. under thermospray conditions. It is shown that, in this mode of operation, fragmentations are minor and may be classified into two classes, namely dequaternization and charge separation, arising from the two precursors, M²⁺ and [M+Br]⁺. Second, the fragmentation patterns have been monitored in dopant-assisted APPI for different dopants (toluene, toluene-d₈, anisole and hexafluorobenzene) at various amounts. At low dopant flow rates, the [M+Br]⁺ and M²⁺ ions are still observed. As the flow rate is increased, these precursor ions lose intensity and are finally suppressed for all three dopants. Comparison of toluene and toluene-d₈ reveals that H atoms may be transferred from the dopant to the molecular ions, very likely mediated by the solvent. The role of the solvent (water) was also investigated by using heavy water. Apart from the thermospray fragmentations, which are also observed in APPI, several fragmentation pathways appear to be specific to the photoionization process. Photoionization efficiencies are measured by determination of the relative photoionization cross sections with respect to toluene. It is found that, when the ionization efficiencies are taken into account, the depletion of the precursors as a function of the dopant flow rates is the same for all three dopant molecules. This result shows that the precursor ions are depleted by reactions with the photoelectrons released from the dopant. Three additional mechanisms are proposed to account for this effect: electron transfer or H atom transfer from negatively charged water nanodroplets and H atom transfer from the dopant.

Keywords: atmospheric pressure photoionization (APPI); bisquaternary ammonium ions; fragmentation mechanisms; dissociative recombination; photoelectron; radical; H atom transfer