An isotope dilution approach for validating the output of mercury gas generators for mercury pollution monitoring†

Mercury (Hg) gas generators produce a continuous flow of Hg vapour and are used to calibrate detectors used in the field for Hg pollution monitoring. Typically, Hg gas generators are certified using empirical equations such as Dumarey or Huber which lead to results that can differ considerably and lack traceability. This work presents, for the first time, a novel online gas phase isotope dilution (IDMS) method for the accurate quantification of the output of Hg gas generators achieving SI traceability via NIST SRM 3133 certified reference material. To achieve this, a ¹⁹⁹Hg isotopically enriched standard was vapourised using a cold vapour generator and mixed with the gaseous output of a Hg gas generator. The ²⁰²Hg/¹⁹⁹Hg ratio of the gaseous blend was then measured by ICP-MS, and the generator output calculated using a single IDMS equation adapted for gas mixtures. The efficiency of the ¹⁹⁹Hg vapour generation is a key contributing parameter to the measurement uncertainty and was therefore quantified using a ¹⁹⁷Hg radiotracer and found to be greater than 99.5%. The feasibility of this method for validating the output of Hg gas generators was demonstrated by quantifying the output of one elemental Hg at an Hg flow of approximately 32 ng L⁻¹ and one oxidised Hg gas generator at an Hg flow of approximately 4 ng L⁻¹. In both cases, a relative expanded uncertainty (k = 2) of less than 9% was obtained. This method represents an important step towards improving the traceability, and therefore comparability of measurements for gaseous Hg which are essential for global environmental monitoring and reduction of atmospheric Hg pollution.