We report a pilot study organized within the Consultative Committee for Amount of Substance (CCQM), in which the ozone reference standards of 23 institutes have been compared to one common reference, the BIPM ozone reference standard, in a series of bilateral comparisons carried out between July 2003 and February 2005. The BIPM, which maintains as its reference standard a standard reference photometer (SRP) developed by the National Institute of Standards and Technology (NIST, United States), served as pilot laboratory. A total of 25 instruments were compared to the common reference standard, either directly (16 comparisons) or via a transfer standard (9 comparisons). The comparisons were made over the ozone mole fraction range 0 nmol/mol to 500 nmol/mol.Two reference methods for measuring ozone mole fractions in synthetic air were compared, thanks to the participation of two institutes maintaining a gas-phase titration system with traceability of measurements to primary gas standards of NO and NO2, while the 23 other instruments were based on UV absorption.In the first instance, each comparison was characterized by the two parameters of a linear equation, as well as their related uncertainties, computed with generalized least-squares regression software. Analysis of these results using the Birge ratio indicated an underestimation of the uncertainties associated with the measurement results of some of the ozone standards, particularly the NIST SRPs.As a final result of the pilot study, the difference from the reference value (BIPM-SRP27 measurement result) and its related uncertainty were calculated for each ozone standard at the two nominal ozone mole fractions of 80 nmol/mol and 420 nmol/mol.Main text. To reach the main text of this paper, click on Final Report.The final report has been peer-reviewed and approved for publication by the CCQM.
The evaluation results of the metrological performance of a dilution and a permeation standard for generating SI-traceable calibration gas mixtures of NO, SO2 and NO2 for ambient air measurements are presented. The composition of the in situ produced reference gas mixtures is calculated from the instantaneous values of the input quantities of the generating standards. In a measurement comparison, the calibration and measurement capabilities of five laboratories were evaluated for the three analytes at limiting amount of substance fractions in ambient air between 20 and 150 nmol mol−1. For the upper generated reference values the target relative uncertainties of ⩽2% (for NO and SO2) and ⩽3% (for NO2) for evaluating the laboratory results were fulfilled in 12 out of 13 cases. For the analytical results seven out of nine laboratories met the criteria for the upper values for NO and NO2, for SO2 it was one out of four. From the negative degrees of equivalence of all NO2 comparison results it was supposed that the permeation rate of NO2 through the FEP polymer membrane of the permeator was different in air and N2. Subsequent precision permeation measurements with various carrier gases revealed that the permeation rate of NO2 was ≈0.8% lower in synthetic air compared to N2. With the corrected NO2 reference values for air the degrees of equivalence of the laboratory results were improved and closer to be symmetrically distributed.
The mole fraction of argon in ambient dry air has been measured using gas phase chromatography calibrated with gravimetric gas standards prepared at the Laboratoire National de Métrologie et d'Essais. Different air samples, operators and gas standards were used to obtain a value of 0.9330 × 10 −2 mol mol −1 ± 0.0032 × 10 −2 mol mol −1 . This value is significantly different from the conventional value (0.917 × 10 −2 mol mol −1 ) used in the 81/91 formula of the Comité International des Poids et Mesures (CIPM) (Davies 1992 Metrologia 29 67-70) for the determination of air density during comparisons of mass standards.This value confirms the result obtained by the Korea Research Institute of Standards and Science, which found a concentration of 0.9332 × 10 −2 mol mol −1 ± 0.0006 × 10 −2 mol mol −1 by mass spectrometry in 2004.This new value explains the difference between the CIPM formula method and the artefact method used for the determination of air density.
Abstract. The European Directive (2008/50/EC) sets up, among other things, the limit values i.e. the maximum allowed concentrations at given time average in the air, for specified pollutants. Calibration of the measurement instruments needs to be performed at regular time intervals. In the framework of an European Joint Research Programme (JRP) named Metrology for Chemical Pollutants in Air (MACPoll) one task aims to provide harmonized dilution methods for air pollutant gases at low concentration for calibration and quality control purposes. The study focuses on the reactive gases nitrogen dioxide and sulphur dioxide at concentration levels corresponding to the limit values given in the European Directive (2008/50/EC). Nitrogen oxide (NO) is studied as well as NO2 because both of them are measured simultaneously for NOx. This work consists in improving the dilution methods for generating calibration standards for SO2, NO, NO2 at limit values and to validate them by an interlaboratory comparison.
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.Main text. To reach the main text of this paper, click on Final Report.The final report has been peer-reviewed and approved for publication by the CCQM-GAWG.
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