CCQM-K68.2019, nitrous oxide (N₂O) in air, ambient level, final report

Abstract

Main text The key comparison CCQM-K68.2019 was aimed at evaluating the level of comparability of laboratories' capabilities for preparing nitrous oxide in air primary reference mixtures at ambient amount fractions, in the range 320 nmol mol−1 to 350 nmol mol−1. The comparison was coordinated by the BIPM and the KRISS. It consisted in the simultaneous comparison of a suite of 2n primary gas standards, two standards to be prepared by each of the n participating laboratories. Two independent analytical methods were used by the BIPM to analyse the amount fraction of N2O in air, namely Gas Chromatography with an Electron Capture Detector (GC−ECD) and Quantum Cascade Laser Absorption Spectroscopy (QCLAS). Since the circulation of the Draft A report in April 2021, four meetings took place with participants to discuss the mathematical treatment of the comparisons results, and several models were proposed. The model chosen by participants is the Bayesian Errors−In−Variables regression with shades of dark uncertainty. In this final report, the Key Comparison Reference Values were obtained with this model, with calculations performed by B Toman and A Possolo. The key comparison CCQM-K68.2019 is considered to present an analytical challenge and therefore classified as Track C comparison in the CCQM nomenclature. 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).

Step Change in Improving the Accuracy of Nitrous Oxide Reference Materials for Underpinning Atmospheric Composition Measurements

“…Both key comparisons focused on the amount fraction range 320–350 nmol mol –1 . The results from CCQM-K68.2019, which required participants to send two reference materials of different amount fractions to the coordinating laboratory, indicate an improvement in comparability since the first study, with reported values closer to the reference value. Uncertainties were also generally smaller with the exception of two laboratories.…”

“…Figure 1 shows the results of two key comparisons (CCQM-K68.2019 17 and CCQM-K68 23 ) organized by the Consultative Committee for Amount of Substance: Chemistry and Biology�Gas Analysis Working Group (CCQM-GAWG) to assess the comparability of laboratory capabilities for underpinning atmospheric measurements of nitrous oxide. Both key comparisons focused on the amount fraction range 320−350 nmol mol −1 .…”

“…Two approaches were used to quantify the amount fraction of nitrous oxide in the matrix gas. The first involved a standard addition method. ,, A high-accuracy dilution device, developed at the NPL, referred to as the adjustable gas standard (AGS), was used to spike the matrix gas under test with known amounts of nitrous oxide. The AGS produced seven mixtures with dilution ratios between nominal values of 1:350 and 1:2000.…”

“…Recent developments at NMIs have led to improvements in the uncertainty of nitrous oxide reference materials. These have been demonstrated in the recent key comparison CCQM-K68.2019 . The National Physical Laboratory (NPL) has developed a synthetic primary reference material (PRM) of nitrous oxide by blending the target components with the main elements of ambient air (nitrogen, oxygen, and argon) in the appropriate ratios using gravimetry. , An expanded uncertainty of 1.1 nmol mol –1 was achieved for a mixture with a nominal composition of 330 nmol mol –1 .…”

“…An examination of the uncertainty budget reveals that the quantification of nitrous oxide in the synthetic air matrix gas is the largest contributor (accounting for just under half of the total uncertainty). 17 This work presents a new approach based on the NPL Boreas system 20 to quantify the amount fraction of the nitrous oxide impurity in the matrix gas with improved accuracy. It uses a novel cryogen-free sample preconcentration system, coupled with a dual-laser spectrometer.…”