A reference facility for the comparison of CO2 in Air isotope ratio standards

The commercial development of laser-based instruments over the last decade that can measure real-time isotopic ratio variations of greenhouse gases, and notably CO2, has allowed their application across a wide range of scientific and technical disciplines. Precise measurements can be achieved, and appropriate calibration strategies [1] and standards need to be applied to achieve accurate results and consistency with traditional mass spectrometric measurement methods. For CO2, calibration strategies can be based on using CO2 in air standards with the same isotopic ratios but containing different amount fractions, or the same amount fraction and different isotope ratios. This has resulted in the availability of calibration standards containing different isotope ratios at different amount fractions, which may or may not contain nitrous oxide. An international comparison programme at the BIPM (BIPM.QM-K4) is in development to demonstrate the equivalence of such standards, which would allow them to be used interchangeably by operators. The BIPM’s comparison facility is based on a dual inlet isotope ratio mass spectrometer with a custom built (BIPM) Air Trapping system (BAT) to extract CO2 from air mixtures using cryogenic separation for determination of δ13C and δ18O-CO2, with a correction for the N2O present in the sample. A procedure for regularly determining the relative ionization efficiency of N2O in relation to CO2 has been developed and is applied as a function of the amount fraction of N2O in the sample. Metrological traceability is achieved through a hierarchy of low-pressure CO2 standards with δ13C values nominally at -1 ‰, -35 ‰ and -43 ‰, calibrated on the VPDB scale via IAEA 603 carbonate standard material. Initial validation of the performance of the facility has been performed with the extraction of CO2 from gas mixtures within the range of 380 μmol mol−1 to 800 μmol mol−1 and δ13C and δ18O-CO2 values from 1 ‰ to -43 ‰ and -7 ‰  to -35 ‰, respectively. The method demonstrates excellent reproducibility, with standard deviations of 0.005% and 0.05%. for δ13C  and δ18O-CO2, respectively. In addition, the robustness of the N2O correction has been demonstrated by comparing δ13C  and δ18O-CO2 values from standards produced from the same CO2 source gas but at differing amount fractions. The performance and validation of the facility will be described.   [1] Flores, E., Viallon, J., Moussay, P., Griffith, D. W. T. & Wielgosz, R. I. Calibration strategies for FT-IR and other isotope ratio infrared spectrometer instruments for accurate δ13C and δ18O measurements of CO2 in air. Anal. Chem. 89, 3648–3655 (2017).