22 - Development of a Traceable Calibration for Gaseous Oxidized Mercury Species Based on Non-Thermal Plasma Approach

"Calibration for atmospheric HgII species at ambient concentration levels, along with sampling, is one of the major problems in atmospheric Hg speciation. Properties of HgII species such as high adsorption and reactivity complicate the development of a reliable calibration method.

To solve the problem of HgII calibration, we have developed a calibration approach based on non-thermal plasma oxidation of Hg0 to HgII. The calibration procedure began with the production of Hg0 by quantitative reduction of HgII in aqueous solution with SnCl2 and aeration. Hg0 was then oxidized to different Hg species in a stream of He and traces of reaction gas (O2, Cl2, or Br2) by non-thermal plasma. By changing the reaction gas, three different species of HgII were produced: HgO, HgCl2 and HgBr2. Since validation and oxidation efficiency estimation for the developed calibration at ambient HgII concentration levels is not possible by conventional methods (i.e. isotope dilution inductively-coupled plasma mass spectrometry), a highly sensitive 197Hg radiotracer (""hot"" Hg) was used for validation. While the obtained oxidation efficiencies were quantitative for HgO and HgCl2 it was not the case for HgBr2 due to difficulties in the production and handling of Br2 gas. The presence of each produced species was confirmed by qualitative measurements using temperature programmed quadrupole mass spectrometry (TPD-QMS).

Although validation and oxidation efficiency evaluation were performed with 197Hg radiotracer, the calibration method had to be further tested with non-radioactive (""cold"") mercury to simulate the intended use and demonstrate metrological traceability. Traceability to System of Units (SI) was achieved through traceability to SRM NIST 3133 and evaluation of measurement uncertainty.

Combined with validated sampling procedures, the developed calibration could provide comparable HgII measurements and thus the foundation for the future of atmospheric mercury speciation."

Authors: Jan Gačnik,I gor Živković, Sergio Ribeiro Guevara, Jože Kotnik, Sreekanth Vijayakumaran Nair, Andrea Jurov, Uroš Cvelbar, Teodor Andron, Milena Horvat