International comparison CCQM-K74.2018: Nitrogen dioxide, 10 μmol mol-1

Abstract

Main textThe CCQM-K74.2018 comparison evaluated the level of compatibility of nitrogen dioxide (NO2) in nitrogen standards at nominal amount fractions of 10 μmol/mol provided by National Metrology Institutes. Fourteen laboratories participated in the comparison, submitting two standards each, with comparative measurements and comparison coordination undertaken by the BIPM. Measurements at the BIPM were performed from July 2018 to May 2019.The key comparison reference values (KCRV) for each submitted standard were established at three different times during the measurement period at the BIPM and based on the value and uncertainty provided by the BIPM's NO2 permeation system reference facility, operating with a standard measurement uncertainty of 0.04 μmol/mol, as demonstrated in the original CCQM-K74 comparison in 2009. Participants were required to report the values and measurement uncertainties assigned to their submitted standards before and after the measurement period at the BIPM, with all but one reported standard uncertainty ranging from 0.017 μmol/mol to 0.30 μmol/mol.The value of the participants' standards at the time of establishment of the KCRVs was determined from participants measured values. This was achieved by averaging all values when the measurements were consistent with each other in the two measurement periods. Alternatively, for standards that exhibited a decay in value, the values as a function of time were calculated using a model in which predicted the mid-point between the average value of participants' second series of values, and a linear decay from their first set of values. Relative standard uncertainties for participants' standards at the KCRV measurement times ranged from 0.03 μmol/mol to 0.25 μmol/mol.Values of standards from six of the fourteen participants agreed with the KCRV within their calculated uncertainties, with the remaining eight exhibiting biases ranging from 0.17 μmol/mol to 0.99 μmol/mol. The comparison has confirmed the challenge of controlling HNO3 levels in NO2 in N2 standards and accurately correcting for their presence as a function of time.The overall level of compatibility demonstrated in the comparison is not an improvement over that in the original comparison, the CCQM-K74 in 2009. The comparison protocol adopted for CCQM-K74.2018, was more challenging than the previous edition. In 2009, the travelling standards were prepared by one participant using the same cylinder type, surface treatment and preparation procedure. These standards were characterized for stability and with reference values provided by the BIPM. They exhibited a small decay rate that was calculated to have been no more than 0.1 nmol mol-1 per day loss of NO2, and accounted for by the addition of an uncertainty to the reference value. For CCQM-K74.2018, the standards were prepared by individual participants, with decay rates in different cylinders ranging from under 0.1 nmol mol-1 per day loss of NO2 to over 1 nmol mol-1 per day, and therefore on occasion an order of magnitude greater than in the original comparison. Whilst more challenging, the CCQM-K74.2018 study has provided an extended evaluation of the level of compatibility of NO2 in N2 standards.To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database https://www.bipm.org/kcdb/.The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).