Thermal fractionation of mercury in environmental matrices: a critical review of direct analyzer techniques

Understanding the biogeochemical transformations of mercury (Hg) and managing the associated ecological and human health risks require analytical approaches that go beyond total Hg and methylmercury (MeHg) measurements and resolve the forms that control the mobility and bioavailability of this metal. Yet no single speciation technique currently provides a routine, multi-matrix solution suitable for environmental monitoring. This review evaluates temperature-programmed thermal fractionation of Hg, implemented using direct Hg analyzers, as a pragmatic tool to bridge this gap. We summarize the theoretical basis of thermally defined Hg fractions and compare them with chromatographic, extraction-based, and spectroscopic approaches. Drawing on applications to aerosols, suspended particulate matter, and aquatic biota, we demonstrate how Hg fraction patterns reflect emission sources, catchment characteristics, atmospheric and hydrological forcing, and trophic transfer. In low-MeHg systems, thermal fractionation helps identify labile and refractory inorganic Hg pools that underpin MeHg production, ecological exposure, and ultimately human risk via seafood consumption. We also discuss methodological constraints, including the operational nature of fractions, matrix effects, and the need for harmonized protocols and reference materials. Finally, we outline how integrating thermal fractionation with targeted MeHg analysis, Hg stable isotopes, and reactive transport models can improve understanding of Hg cycling and support the prioritization of remediation and management strategies in Hg-impacted environments.