Towards Direct Molecular Speciation of Atmospheric Gaseous Oxidized Mercury

Mercury is a persistent environmental pollutant entering the atmosphere mostly in its elemental form. The mechanism of mercury oxidation is poorly constrained because of the nearly non-existent knowledge of molecular identities of atmospheric Gaseous Oxidized Mercury (GOM), severely hindering our ability to evaluate the fate of mercury in the environment and its magnification in food chains. We will present our recent work focused on the development of a direct approach for molecular speciation of GOM. The method is based on the ion drift - chemical ionization mass spectrometry (ID-CIMS), where GOM molecules react in a drift tube at a reduced pressure with an appropriate reagent ion to form well-defined product ions, which are detected by a quadrupole mass spectrometer equipped with a low-noise counting electron multiplier. In our study, HgCl₂ was used as proxy for GOM. The reagent ions were chosen based on quantum chemical investigation of several possible reaction mechanisms, including charge transfer, ion transfer, and ion-molecule clustering. HgCl₂ was found to react quantitatively with SF₆^-, CO₃^-, CO₄^-, and HNO₃ NO₃^- reagent ions through one or more of the aforementioned mechanisms. We expect HNO₃ NO₃^- to be most useful for real-world applications because this ion is least affected by ubiquitous atmospheric chemicals, such as water. The current detection limit of low-pressure CIMS towards HgCl₂ is about 1 part per trillion with a 1 minute integration time. With the use of the atmospheric chemical ionization, this limit can be reduced to below 10 parts per quadrillion, allowing direct analysis of the atmospheric GOM.