SI traceable results of Hg mass concentration at saturation in air from a newly developed measurement procedure

Christophe R. Quétel1*#; Mariavittoria Zampella1#; Richard J. C. Brown2; Hugo Ent3; Milena Horvat4; Eduardo Paredes1, 5; Murat Tunc1, 6

1- European Commission - Joint Research Centre – Institute for Reference Materials and Measurements, Retieseweg 111-2440 Geel, Belgium.

2- Analytical Science Division, National Physical Laboratory, Teddington, Middlesex, TW11 0LW, UK.

3- VSL, Thijsseweg 11, 2629JA Delft, The Netherlands.

4- Jožef Stefan Institute, Jamova 39, Ljubljana, Slovenia.

5- University of Alicante, 03080 Alicante Spain.

6- TUBITAK-UME, P.K. 54 41470 Gebze, Kocaeli, Turkey.

* Corresponding author. Tel.: +32 14 57 1658

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# Co-first authors

Data most commonly used at present to calibrate measurements of mercury vapour concentrations in air come from a relationship known as the ‘Dumarey equation’. It uses a fitting relationship to experimental results obtained nearly 30 years ago. The way these results relate to the international system of units (SI) is not known. This has caused difficulties for the specification and enforcement of limit values for mercury concentrations in air and in emissions to air as part of national or international legislation. Furthermore there is a significant discrepancy (around 7% at room temperature) between the Dumarey data and data calculated from results of mercury vapour pressure measurements in presence of only liquid mercury. As an attempt to solve some of these problems a new measurement procedure is described for SI traceable results of gaseous Hg concentrations at saturation in mL samples of air. The aim was to propose a scheme as immune as possible to analytical biases. It was based on isotope dilution (ID) in the liquid phase with the 202Hg enriched certified reference material ERM-AE640 and measurements of the mercury isotope ratios in ID blends, subsequent to a cold vapour generation step, by inductively coupled plasma mass spectrometry. The process developed involved a combination of interconnected valves and syringes operated by computer controlled pumps, and ensured continuity under closed circuit conditions from the air sampling stage onwards.

Quantitative trapping of the gaseous mercury in the liquid phase was achieved with 11.5 µM KMnO4 in 2% HNO3. Mass concentrations at saturation found from five measurements under room temperature conditions were significantly higher (5.8% on average) than data calculated from the Dumarey equation, but in agreement (-1.2% lower on average) with data based on mercury vapour pressure measurement results. Relative expanded combined uncertainties were estimated following a model based approach. They ranged from 2.2% to 2.8% (k=2). The volume of air samples was traceable to the kilogram via weighing of water for the calibration of the sampling syringe. Procedural blanks represented on average less than 0.1% of the mass of Hg present in 7.4 cm3 of air, and correcting for these blanks was not an important source of uncertainty.

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