Twenty-Five Years of CIPM-CCQM Gas Analysis Key Comparisons.

International key comparisons under the International Committee for Weights and Measures' Mutal Recognition Arrangement (CIPM-MRA) provide the means to assess the equivalence of measurement standards and the comparability of calibration and measurement capabilities at National Metrology Institutes and Designated Institutes around the world. They also provide a snapshot of the state of the art in measurement science in terms of the realization of the definition of SI units and vital insights on where to focus future efforts to advance metrology. In the last 25 years, the Gas Analysis Working Group of the Consultative Committee for Amount of Substance: Chemistry and Biology coordinated 46 key comparisons, resulting in a wealth of data. From these data, we present a classification of the target components by difficulty, how the state of the art has changed since the working group was established, and the changes in performance of a selection of NMIs.