Vapor generation of mercury and methylmercury in aqueous microdroplets produced by pneumatic nebulization

Pneumatic nebulization (PN) is commonly used for microdroplet (or aerosol) introduction from bulk solution in atomic spectrometry. Here, we found that the vapor generation process of mercury ion (Hg²⁺) and methylmercury ion (MeHg⁺) to volatile elemental mercury (Hg⁰) could spontaneously occur in aqueous microdroplets produced by PN without any added redox agent, electricity or radiation. The MeHg⁺ degradation might be caused by a hydroxyl radical (OH˙) and the Hg²⁺ reduction might be caused by the electron released from OH⁻, which were all formed spontaneously at the water-air interface of microdroplets produced by PN. With cold vapor atomic fluorescence spectrometry (CVAFS) as the detector for Hg⁰, the parameters in PN (including nebulizing gas pressure, microdroplet diameter, carrier solution flow rate, and solution pH) on the Hg²⁺ and MeHg⁺ vapor generation efficiencies were evaluated in detail. The vapor generation efficiency of Hg²⁺ by microdroplets was about 37.2%. The relative standard deviation (RSD) of Hg²⁺ and MeHg⁺ determined at 10 μg L⁻¹ were 2.5% and 3.4%, respectively. The detection limits (LODs) of Hg²⁺ and MeHg⁺ were 0.09 μg L⁻¹ and 0.43 μg L⁻¹, respectively. Compared with other vapor generation technologies, microdroplets induced vapor generation technology provides an innovative opportunity for green chemistry with low energy cost. Moreover, aqueous microdroplets may have provided a new route for Hg²⁺ reduction in clouds to participate in the global atmospheric mercury cycling.