Metrology for mercury measurements
Expert speaking: Iris de Krom – Scientist at VSL, the Netherlands & Coordinator 19NRM03 SI-Hg project
Measurements play an important role in many aspects of daily life. For technological developments and innovations too, such as green fuels, nanotechnology and radiotherapy, extremely accurate measurement is of great importance. In international trade it is necessary that we measure with the same dimensions. A bolt manufactured in one country would otherwise never fit on a nut from a different country. Thanks to the SI system (SI stand for Système Internationale) measurement is done nationally and internationally with the same dimensions. The unit system defines seven basic SI units, namely: the second (time), the metre (length), the kilogram (mass), the kelvin (temperature), the candela (light intensity), the mol (quantity of substance) and the ampere (electrical current). These are fundamental units for physical quantities, that were accepted and recommended by the general conference on weights and measures (Conférence Générale des Poids et Mesures, CGPM). All other SI units can be derived from these basic units. Via the Mutual Recognition Arrangement (MRA), signed by many countries in 1999, it was agreed that the countries would accept each other’s national standards and associated measurement results. In order to be able to compare together measurements that are conducted in different places or at different times, the measurements must be traceable. Running a new world marathon record can only be established if the measurements are mutually comparable. The same applies to the emission of mercury: if there are agreements on the emission, the measurements must be made in the same way to allow conclusions to be drawn. The traceability to primary standards is regulated via the National Metrology Institutes (NMIs). They manage the national measurement standards that are compared to each other internationally. Traceability is regulated by calibrating means of measurement. In a measurement there is always a comparison between an unknown quantity with a quantity whose properties are known (standard or reference material) and taking into account the measurement uncertainty.
The SI-Hg project
Requirements for SI-traceability of mercury measurement results exist in European Directives: the Industrial Emissions Directive 2010/75/EU, the Air Quality Directive 2004/107/EC and the Waste Incineration Directive 2000/76/EC. Furthermore, worldwide the Minamata Convention calls for comparable measurement results and harmonised methodologies to analyse mercury and its compounds in emission sources. In former EMRP projects PartEmission and MeTra and EMPIR project MercOx primary standards and SI-traceable calibration methods for mercury in air have been developed at NMIs. Notwithstanding these efforts there are no standardised procedures that ensure the dissemination of the metrological traceability from primary to working standards among calibration and testing laboratories and in the field. Scientifically sound certification protocols, to assess the performance of elemental mercury gas generators in the form of formally accepted documentary standards, are of fundamental importance to guarantee the accuracy and comparability of the mercury measurement results in gas emission sources and in the atmosphere in Europe and globally. Within this project (19NMR03 SI-Hg) such metrological certification protocols are developed and validated. The certification protocols contribute to establishing a traceability chain from a primary mercury gas standard to measurement data obtained in gas emission sources and in the atmosphere. The protocols replace thereby other references currently used in mercury concentration measurements. The validation data contributes to the incorporation of the certification protocol into new and existing documentary standards in collaboration with standardisation committee CEN/TC264/WG8. Therefore, this project is of key importance to obtain traceable measurements of mercury concentrations which are essential to underpin global efforts to control and reduce the concentration of mercury in the environment, meet the obligations of legislation and to protect human health.
Elemental mercury gas generators
Work has been carried out compiling data of the performance characteristics and uncertainty sources of commercially available elemental mercury gas generators used in the field. Little information is available on the performance characteristics of the generators. All the more reason to perform the validation activities described throughout this project. Furthermore, uncertainty calculations have been set up for different types of elemental mercury gas generators.
Experimental approaches have been developed for the certification protocol and to assess the performance characteristics during the validation of the certification protocol. During the validation, the multipoint, bracketing and single point calibration methods will be tested to determine the most suitable approach for the certification of elemental mercury gas generators. Meanwhile the project is developing the first draft of the protocol for the SI-traceable certification of elemental mercury gas generators used in the field.
Oxidised mercury gas generators
The first step was to compile data about the performance characteristics and uncertainty sources of state-of-the-art dual analytical systems and commercially available oxidised mercury gas generators. Based on the findings a review was written by Lumex about state-of-the-art dual analytical systems, the review is published on the project website (www.si-hg.eu). Several systems have been developed able to determine oxidized and elemental mercury in parallel. In the course of these developments, major obstacles in trace analysis of highly reactive chemical compounds have been overcome. Yet, several problems remain i.e., the tendency of interconversion between mercury species during sampling and analysis, and the evaluation of the efficiency of the converter under different conditions. Another challenge is the qualification of a suitable SI-traceable calibration device for the instruments. Such a device would allow a more comprehensive metrological characterization of the systems, and hence the critical assessment and validation of analytical results produced by these instruments, and finally the comparison with data generated by other research groups with different equipment.
Furthermore, uncertainty calculations have been set up for different types of oxidised mercury gas generators.
Tests are planned to evaluate the thermal converter efficiency and undesirable species interconversion of state-of-the-art dual analytical systems and gas transfer efficiency of the gas generators, including the effect of oxidised mercury reversible adsorption, irreversible (or very slow reversible) absorption and chemisorption/reduction of gas generators.
Performance evaluation generators
Next year the performance evaluation of elemental mercury and oxidised mercury gas generators on the market will start. With the evaluation the project will gather data on the characteristics of mercury gas generators. In collaboration with the stakeholder committee possible collaborators will be contacted to participate in the performance evaluation of elemental and oxidised mercury gas generators. If you are interested to make equipment available for the evaluation please contact us.
The project website is online: www.si-hg.eu. The website contains detailed information about the project, project output such as project summaries, project reports and articles and news & events with information about workshops and conferences project members attend.
- Why Is Mercury Measurement Important for the Environment? Envirotech Online (envirotech-online.com)
- SI-traceable mercury measurement results with the VSL primary mercury gas standard - Jun 01 2021 - Iris de Krom - Environmental Science News Articles - Envirotech Online (envirotech-online.com)
- Download the Publishable Summary for 19NRM03 SI-Hg Metrology for traceable protocols for elemental and oxidised mercury concentrations