Comparison of methods for particulate phase mercury analysis: sampling and analysis

Abstract.

Accurate and reliable sampling and analysis of mercury forms is an overriding aim of any atmospheric monitoring effort which seeks to understand the fate and transport of the metal in the environment. Although a fraction of the total mercury forms found in the atmosphere, particulate phase mercury, Hg_p, is believed to play a prominent role in both wet and dry deposition to the terrestrial and aquatic environments. Currently, microwave acid extraction and thermoreductive methodologies for analysis of Hg_p samples are widely used. We report on the potential for the use of a thermoreductive method for Hg_p analysis to evaluate and optimize it for use in routine monitoring networks. Pre-baked quartz filters can be placed in particulate samplers with well-characterized size cuts, such as dichotomous samplers and microoriface impactors. The thermoreductive methodology facilitates rapid analysis after sample collection. It requires no chemical extraction thereby eliminating the potential for contamination and generation of hazardous waste. Our results indicate that, on average, the thermoreductive method yields 30% lower values for fine fraction Hg_p when compared with microwave acid digestion. This may be due to matrix interferents that reduce the collection efficiency of mercury onto gold preconcentration traps. Results for total particulate mercury samples indicate that on average the thermoreductive method yields 56% lower values for the coarse fraction when compared with microwave acid digestion.

Experiments were also conducted in Detroit, MI, USA to investigate whether elevated reactive gaseous mercury (Hg²⁺ _g) in an urban environment can lead to an artifact during the collection of filters for Hg_p analysis. Our results indicate a significantly higher amount of Hg_p collected onto a filter using the conventional methodology as compared to a filter collected downstream of KCl-coated annular denuders in the absence of Hg²⁺ _g. These results point to the presence of Hg²⁺ _g as an artifact during Hg_p measurement. These results indicate that a denuder must be utilized upstream of a filter for Hg_p collection to prevent significant Hg²⁺ _g artifact formation.