NIST's Chemical Sciences Division Gas Sensing Metrology Group has participated in over 40 Consultative Committee for Amount of Substance-Metrology in Chemistry (CCQM) Gas Analysis Working Group (GAWG) Key Comparison and Pilot studies since 1993. The CCQM conducts international Key comparisons to establish equivalence among measurements made by National Metrology Institutes (NMIs). GAWG Key Comparisons during the past five years included measurement of BTEX, ammonia, carbon dioxide, monoterpenes, propane, hydrogen chloride, nitrous oxide, and automotive emissions gases. In addition to key comparison activities, the Gas Sensing Metrology Group routinely contributes to GAWG task group activities.
The results from these Key comparisons will be used to benchmark the appropriate Calibration and Measurement Claims (CMCs) of the participating NMIs. CMCs are basically the declared capabilities that an NMI offers to customers, and they are included in a searchable database at the BIPM web site. CMCs that are common among the NMIs are identified appropriate comparisons proposed. NIST CMCs are benchmarked based on carefully designed and executed comparisons using the methods normally used for those analyses for NIST's customers.
In addition to Key comparisons, NIST often participates in task group activities of the Gas Analysis Working Group (GAWG). Task groups are often formed to address technical issues and develop guidance for the NMI community. Examples of task groups NIST members of the Gas Sensing Metrology Group have participated in over the past five years include: Task Group on Designing Key Comparisons, Broad Scope Claims Harmonization Task Group, Task Group on Purity Analysis, GAWG Strategy Task Group, and Task Group on Developing Guidelines to Support New CMCs. Recently, GAWG has included expert institutions and external stakeholders in task group activities. Listed below are two additional large task groups which include external stakeholders:
The technical approaches used for the GAWG Key Comparisons are the same as those used for the CMCs. In this way, the CMCs can be benchmarked through the CCQM studies. The GAWG has decided to conduct two types of Key Comparisons: 1) the Core Key Comparisons and 2) Analytical Challenge Key Comparisons. The Core Key Comparisons will test an NMI's core capabilities to produce gas standards, and each NMI will be required to participate in these exercises periodically. An example of a current active Core Key Comparison is 1000 µmol/mol propane of which NIST is a participant. These comparisons will be very broad in scope and cover a large number of non-reactive gas CMCs. The Analytical Challenge Key Comparisons will cover a very narrow scope, usually only the compound being compared. An example of these comparisons is the recent ammonia key comparison which covers only ammonia in the concentration range of 10 µmol/mol to percent levels.