Ion-molecule reactions relevant to the detection of atmospheric oxidized mercury by chemical ionization mass spectrometry

Mercury is a persistent environmental pollutant arising from anthropogenic and natural sources. Gaseous oxidized mercury (GOM) is significantly more water soluble than elemental Hg and should be the main form responsible for deposition unto environmental sinks, however, the speciation of GOM is poorly understood due to its relatively low concentrations and labile nature. We investigated ion-molecule chemistry of gas-phase HgCl₂ with SF₆ ^-, CO₃ ^-, and CO₂·O₂ ^- reagent ions by ion-drift chemical ionization mass spectrometry. Additionally, DFT and ab-initio computations coupled with rate constant calculations based on average dipole orientation (ADO) theory were used to evaluate the product stability, elucidate reaction pathways and corroborate experimental results. We found that the reaction of HgCl₂ with SF₆ ^-, CO₃ ^-, and CO₂·O₂ ^- forms HgCl2·F^- , HgCl₂·CO₃ ^-, HgCl₂O^- , and HgCl₂O₂ ^- product ions. These reactions are efficient in converting gaseous oxidized mercury to molecular ions, although competing reactions with O₃, NO_x, and H₂O may limit their application in atmospheric applications. The calculated stability of the complexes is in the 11 to 57 kcal-mol-1range.