Laurie Besley

Final report on international key comparison CCQM-K53: Oxygen in nitrogen

Gravimetry is used as the primary method for the preparation of primary standard gas mixtures in most national metrology institutes, and it requires the combined abilities of purity assessment, weighing technique and analytical skills. At the CCQM GAWG meeting in October 2005, it was agreed that KRISS should coordinate a key comparison, CCQM-K53, on the gravimetric preparation of gas, at a level of 100 ?mol/mol of oxygen in nitrogen. KRISS compared the gravimetric value of each cylinder with an analytical instrument. A preparation for oxygen gas standard mixture requires particular care to be accurate, because oxygen is a major component of the atmosphere. Key issues for this comparison are related to (1) the gravimetric technique which needs at least two steps for dilution, (2) oxygen impurity in nitrogen, and (3) argon impurity in nitrogen. The key comparison reference value is obtained from the linear regression line (with origin) of a selected set of participants. The KCRV subset, except one, agree with each other. The standard deviation of the x-residuals of this group (which consists of NMIJ, VSL, NIST, NPL, BAM, KRISS and CENAM) is 0.056 ?mol/mol and consistent with the uncertainties given to their standard mixtures. The standard deviation of the residuals of all participating laboratory is 0.182 ?mol/mol. With respect to impurity analysis, overall argon amounts of the cylinders are in the region of about 3 ?mol/mol; however; four cylinders showed an argon amount fraction over 10 ?mol/mol. Two of these are inconsistent with the KCRV subset. The explicit separation between two peaks of oxygen and argon in the GC chromatogram is essential to maintain analytical capability. Additionally oxygen impurity analysis in nitrogen is indispensable to ensure the preparative capability. Main text. 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 kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

International comparison CCQM-P41 Greenhouse gases. 2. Direct comparison of primary standard gas mixtures

In this second part of this study, primary standard gas mixtures (PSMs) as used for calibrations for the greenhouse gases carbon dioxide and methane have been compared. The nominal amount-of-substance fraction levels were 365 µmol/mol for carbon dioxide and 1.8 µmol/mol for methane. The matrix was synthetic air, simulated by a mixture of nitrogen, oxygen (209 mmol/mol) and argon (9.3 mmol/mol). The measurements took place in the spring of 2003. In the protocol, it was foreseen to have the PSMs produced for this study in a small range, enabling regression analysis on the data to assess the consistency of the gas mixtures. The direct comparison of the PSMs shows agreement for carbon dioxide at 365 µmol/mol nominal within 0.52 µmol/mol (0.14% relative) standard uncertainty. One cylinder lies between the 95% and 99% boundaries of the regression line, and after removing this cylinder from the dataset the standard deviation of the x-residuals reduces to 0.18 µmol/mol (0.05% relative). This value for the standard deviation is substantially greater than the stated standard uncertainties for several cylinders. The standard deviation of the x-residuals for methane at 1.8 µmol/mol nominal is 0.011 µmol/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.

International comparison CCQM-K54: Primary standard gas mixtures of hexane in methane

The aim of this comparison is to evaluate the gravimetry and purity verification of gas mixtures prepared by introducing a liquid into a gas cylinder. This type of preparation is well known from, e.g., ethanol in nitrogen/air (CCQM-K4), and n-hexane in synthetic natural gas (CCQM-K16). The choice for n-hexane in methane is based on the relevance of natural gas primary standard mixtures and higher hydrocarbons in synthetic natural gas (for, e.g., dew point calibrations). Furthermore, n-hexane is relatively straightforward to analyse using a GC/FID (gas chromatograph equipped with a flame ionisation detector) with good repeatability. Eight laboratories participated. After the data evaluation, the result of one laboratory was eliminated to arrive at a consistent subset. The quality of the fitted line was not overly good, leading to small discrepancies in the degrees-of-equivalence calculated for three other laboratories. Only four laboratories are consistent with the KCRV. Main text. 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 kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

International comparison CCQM-P41 Greenhouse gases. 1. Measurement capability

The measurement of greenhouse gases in the atmosphere is one of the activities to monitor changes in the global environment, as some of these gases are presumed to contribute to climate change. The CCQM Gas Analysis Working Group has organised a pilot comparison for carbon dioxide and methane at ambient levels to compare primarily the capabilities of national metrology institutes (NMIs) for measuring these species in air. The participation in this comparison was not limited to signatories of the MRA, as is usually the case. Laboratories from the World Meteorological Organisation (WMO) were invited as well, and two participated. The project has been split in two parts, a comparison of the (analytical) measurement capability, and a high-accuracy comparison of primary standard gas mixtures for greenhouse gases. In this first part, the measurement capability was compared between NMIs and WMO laboratories. The nominal amount-of-substance fraction levels are 365 µmol/mol for carbon dioxide and 1.8 µmol/mol for methane. The matrix was synthetic air, simulated by a mixture of nitrogen, oxygen (209 mmol/mol) and argon (9.3 mmol/mol). The measurements took place in the spring of 2003. The protocol used for this comparison was the same as used so far in the gas analysis area for key comparisons and studies of this type. The reference values were assigned to each gas mixture on the basis of the gravimetric preparation, taking into account the results from the purity verification. In particular for the value assignment of methane, traces of methane in the matrix gases (nitrogen, oxygen and argon) can influence the value assigned to the gas mixture appreciably. The results for methane agree within 4% relative, and for most participants even within 2% relative at an amount of substance fraction level of 1.8 µmol/mol. For carbon dioxide all results agree within 1% relative, and for most the agreement is even better: within 0.5% relative at an amount of substance fraction level of 365 µmol/mol. The methane data show a discrepancy of approximately 0.025 µmol/mol between the averages of the WMO laboratories and the NMIs, which confirms results of earlier comparisons. Measurement traceability is quite differently established in NMIs and WMO laboratories, and may account for this difference. Further work is needed to find out the causes of this difference. 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.

Final Report on International comparison CCQM K23ac: Natural gas types I and III

At the highest metrological level, natural gas standards are commonly prepared gravimetrically as PSMs (primary standard mixtures). This international key comparison is a repeat of CCQM-K1e-g. The mixtures concerned contain nitrogen, carbon dioxide and the alkanes up to butane. The only difference with CCQM-K1e-g is the addition of iso-butane to the list. The results usually agree within 1% (or better) with the key comparison reference value. For ethane, nitrogen and carbon dioxide, the agreement is within 0.5% (or better), and for methane within 0.1% (or better) of the KCRV. Main text. 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 kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

International Comparison CCQM K23b-Natural gas type II

At the highest metrological level, natural gas standards are commonly prepared gravimetrically as PSMs (Primary Standard Mixtures). This international key comparison is a repeat of CCQM-K1e-g. The mixtures concerned contain nitrogen, carbon dioxide and the alkanes up to butane. The only difference from CCQM-K1e-g is the addition of iso-butane to the list. The agreement of the results in this key comparison is very good. For all parameters, with a few exceptions, the results agree within 0.5% (or better) with the key comparison reference value. For methane, the results are generally within 0.1% (or better) of the KCRV. Main text. 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 kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).