Detection of Atmospherically Relevant Mixed Mercuric Compounds by Chemical Ionization Mass Spectrometry

A significant fraction of atmospheric mercury enters the aquatic and terrestrial environments via the deposition of gaseous oxidized mercury (GOM). The photochemically produced GOM comprises many molecules, but most experimental work employs HgCl₂ and HgBr₂ as surrogates. This calls for a more realistic representation of GOM for the development of detection and sampling methods. Here, several mixed mercuric compounds XHgY (X and Y are Cl^-, Br^-, I^-, NO₂ ^-, and NO₃ ^-) were synthesized by exchange reactions involving mercuric and nonmercuric compounds, isolated in crystalline form, and characterized by Raman spectroscopy. Vapor above these solids was analyzed by ion drift-chemical ionization mass spectrometry (ID-CIMS), using the SF₆ ^-, HNO₃·NO₃ ^-, and I^- reagent ions. When X and Y are both halogens, XHgY readily volatilizes and can be detected by ID-CIMS, based on unique ion products XHgY·F^-, XHgY·NO₃ ^-, and XHgY·I^-. On the contrary, mixed mercuric salts containing at least one nonhalide ligand cannot be volatilized and detected. Furthermore, solid ClHgNO₂ and BrHgNO₂ rapidly oxidize upon exposure to ambient air, and the oxidation is accompanied by hydrolysis to form red HgO. We conclude that ClHgNO₂ and BrHgNO₂ are insufficiently redox-stable to survive multiday preconcentration from ambient air on sorbents and insufficiently volatile to be thermally desorbed for analysis in their original form.