Surface coatings for analysis of semivolatile mercury compounds

Mercury pollution impacts human and environmental health. Most mercury pollution is emitted to the atmosphere, and atmospheric transport and chemistry determine when and where mercury deposits from the atmosphere into aquatic systems. Oxidized mercury compounds are particularly important because they deposit quickly to ecosystems. They exist in the low ppq range and are semivolatile and chemically labile, creating unique challenges for sampling and analysis. They are retained by virtually any surface, and when they sorb to a surface, they tend to reduce to elemental mercury, which re-volatilizes. Commercial instrumentation has been found to have a strong low bias because of this phenomenon, leaving no established techniques for oxidized mercury measurement. Our team is developing methods to identify and quantify atmospheric mercury compounds, as well as calibration methods, in an attempt to fill this gap. One of the most critical challenges of this work is finding materials and conditions that minimize sorption and reduction of oxidized mercury compounds while they travel through measurement and calibration instrumentation. We have tested several fluoropolymers and silicon-based coatings at a range of temperatures. All the materials and coatings retained some oxidized mercury and resulted in some reduction to the elemental form, but Sulfinert-coated stainless steel appears to perform best, at least after an equilibration time of 1-2 hours. Also, system temperatures of 120-180 degrees C appear to best balance the competing needs to keep semivolatile mercury compounds in the gas phase and avoid thermal decomposition. We have successfully used Sulfinert coatings in this temperature range in several novel analytical methods for oxidized mercury, including cryogenic preconcentration followed by analysis by gas chromatography-mass spectrometry (GC-MS), and inn novel NIST-traceable calibration systems.