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[1] Changes in the Earth's climate caused by global warming are a looming problem that poses serious challenges not only for our generation but for future generations. An accurate determination of CO 2 gas plays a critical role in this field of research. The measurement of greenhouse gases is pivotal to understanding the changes in Earth's climate and needs to be carried out with a high degree of accuracy. Precision measurements on a 0.1 mmol/mol scale may provide research data for precisely monitoring the continuing changes that the planet is undergoing. The World Meteorological Organization (WMO) has recommended that carbon dioxide concentrations in air can be measured by comparing these with national reference gases using a nondispersive infrared (NDIR) analyzer to standardize international data. The CO 2 molecules absorb the distinctive resonant frequencies in IR spectrometers. The NDIR analyzers usually use narrow band path filter to determine 12 CO 2 in all carbon dioxide molecules, which can possibly ignore the measurement of 13 CO 2 partially or totally. However, if the carbon isotopic abundances of CO 2 samples deviate from those in standard CO 2 gas, the NDIR measurement will not be exact. For accurate measurements, producers of reference gas mixtures either must use gas with natural isotopic abundances, or report the isotopic abundances of CO 2 . In order to document shifts based on isotopic variability, we prepared artificial air as CO 2 reference gas mixtures gravimetrically with CO 2 having different carbon isotopic signatures to study the resulting isotopic variations. We used different d13 C values of two CO 2 source gases, A and B, corresponding to À41.97% and À14.88%, respectively, which were measured using an isotope ratio mass spectrometer. One set of reference gas mixtures (A1 to A5) was prepared from the CO 2 source of d 13 C = À41.97%, and the other set of reference gas mixtures (B1, B2) was prepared from that of d 13 C = À14.88%. The CO 2 abundances of the two sets of mixtures were compared by using NDIR. The reproducibility test for the set A showed that the data are consistent within uncertainty (calibration line was obtained by the best secondary polynomial least squares fit). The uncertainty of CO 2 concentration in the reference gas mixtures are 0.06 mmol/mol with a 95% confidence level. The reproducibility of the NDIR measurement is 0.012 mmol/mol (standard deviation). The difference between the set A (A1 to A5) and set B (B1, B2) was found to be 0.17 ± 0.01 mmol/mol, which is in excellent agreement with the theoretically predicted value of 0.17 mmol/mol.

We used NDIR for this measurement (Siemens, Ultramat 6E). Configuration of analysis system: gas cylinder-> regulator-> MFC-> NDIR-> response comparison-> results Sample cell flow: 800 mL/min, Reference cell flow: 800 mL/min Cell pressure: 1.94 Kg/cm 3 Calibration Standards: The calibration standards for CCQM-K52 were prepared by gravimetric method including 0.93 %mol/mol of Ar in KRISS. Therefore, the matrix is different from that of coordinating Lab., which does not contain Ar. All source gases were analyzed impurities for purity analysis. The primary standards with 0.014% overall uncertainty (k=2) are used.

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 µmol/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 µmol/mol and consistent with the uncertainties given to their standard mixtures. The standard deviation of the residuals of all participating laboratory is 0.182 µmol/mol.With respect to impurity analysis, overall argon amounts of the cylinders are in the region of about 3 µmol/mol; however; four cylinders showed an argon amount fraction over 10 µmol/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).

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