Effect of carbon isotopic variations on measured CO 2 abundances in reference gas mixtures

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 μmol/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 δ 13 C values of two CO 2 source gases, A and B, corresponding to −41.97‰ and −14.88‰, 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 δ 13 C = −41.97‰, and the other set of reference gas mixtures (B1, B2) was prepared from that of δ 13 C = −14.88‰. 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 μmol/mol with a 95% confidence level. The reproducibility of the NDIR measurement is 0.012 μmol/mol (standard deviation). The difference between the set A (A1 to A5) and set B (B1, B2