Detection of Gaseous Mercuric Halides Using Acetate and Iodide Chemical Ionization Mass Spectrometry

Predicting the fate of gaseous oxidized mercury (GOM) in the atmosphere remains a challenge because no direct analytical methods exist to determine its molecular speciation. Conventional preconcentration techniques produce speciation losses and induce ligand exchange, obscuring the original composition of GOM. Direct analysis by chemical ionization mass spectrometry (CIMS) offers a path toward addressing this analytical challenge, but its success depends critically on the use of an appropriate ion-molecule chemistry. Previously, we showed the applicability of the negative-ion CIMS for detection of gaseous HgCl₂. Here, we demonstrate the first systematic evaluation of the detection of gaseous mercuric halides (HgCl₂, HgBr₂, and HgI₂) by acetate (CH₃COO^-·CH₃COOH) and iodide (I^-) reagent ions, which have been successfully used in the past for the detection of atmospheric trace chemicals. As soft Lewis bases, these reagent ions react promptly with mercuric halides through either complexation or ligand switching. Using a calibrated permeation source, we establish the sensitivity and limit of detection (LOD) for HgBr₂ with these and previously reported reagent ions. Measured sensitivities range from 3.8 to 21.0 cps/ppbv (for 1 Mcps reagent ion signal), and in the case of ligand switching, they agree with theoretical sensitivities. The current LOD of 53-268 pptv, obtained at a 2.5 Torr ion-molecule reaction pressure, is sufficient for laboratory studies of Hg chemistry and speciating GOM from combustion flue gases. Operating at higher ion-molecule reaction pressures will significantly enhance ionization efficiency, extending the applicability of CIMS toward direct detection of GOM in the atmosphere.