Abstract. The pilot study evaluated the level of comparability of laboratories' preparative capabilities for gravimetric nitrogen monoxide/nitrogen primary reference mixtures in the range (30–70) µmol/mol. The comparison was designed so that measurements would be performed at a central laboratory (the BIPM) and measurement results compared to values assigned by each national metrology institute (NMI) based on gravimetry using regression analysis.

The advantages and complications in organizing a comparison with measurements performed at a central laboratory have been clearly demonstrated, notably:

• analytical measurement uncertainties can be reduced;
• a degree of equivalence parameter and its uncertainty can be calculated;
• regression analysis and therefore reference values for the comparison are highly dependent on the data set chosen for regression analysis and the uncertainty ascribed to the analytical measurement system within the central laboratory.

Participating laboratories reported standard uncertainties for gravimetric preparation which ranged from 0.01% to 0.22% relative to the nitrogen monoxide mole fraction value.

Following FTIR analysis, the standards of three laboratories (six gas standards in total) were omitted from the regression analysis data set, as a significant difference between reported and measured values of impurity contents was observed. An additional standard was removed from the regression analysis set as its deviation from the regression line was of the same order of magnitude as the standards already omitted. A regression line consistent with the remaining calibration data (15 gas standards) could be obtained by increasing estimates of the coordinating laboratory's measurement uncertainty by approximately a factor of three, resulting in analytical standard uncertainties of 0.12% (at 70 µmol/mol) and 0.27% (at 30 µmol/mol), and predicted standard uncertainties of the nitrogen monoxide mole fractions of 0.09 µmol/mol. Reported standard uncertainties related to gravimetric preparation ranged from 0.004 µmol/mol to 0.11 µmol/mol with a median of 0.03 µmol/mol. Nevertheless, comparison of the results of CCQM-P73 to previous key comparisons for nitrogen monoxide in nitrogen at 100 µmol/mol (CCQM-K1.c and EUROMET.QM-K1.c) illustrates the reductions in uncertainties that can be achieved through a comparison with measurements performed at a central facility.

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The final report has been peer-reviewed and approved for publication by the CCQM-GAWG.