New standards for isotope measurements of CO2 for atmospheric and biogeoscience applications

Progress in the development of pure CO2 gas standards for δ13C, δ18O and Δ47 measurements as well as CO2 in air gas standards (with mole fractions in the range 350 µmol/mol to 800 µmol/mol) for δ13C, δ18O measurements are described. Initial results indicate the potential to produce standards with internal consistencies at the 0.005 ‰ level for δ13C and standard uncertainties of 0.015 ‰ in relation to the VPDB scale, with the magnitude of the latter principally limited by the homogeneity of primary carbonate reference materials.An initial driver for standards development was the requirement for appropriate calibration strategies and standards [1]  to support commercially developed laser-based instruments that have grown in number over the last decade. These analysers can measure real-time isotopic ratio variations of greenhouse gases, and notably CO2, allowing their application across a wide range of scientific and technical disciplines. The development of appropriate standards and calibration methods has required the links and traceability to primary carbonate materials via the IRMS dual inlet reference method to be re-examined.Outputs of the project so far include:Establishment of a facility to produce stable pure CO2 gas standards in 6L cylinders at 2 bar with δ13C values from -1 ‰ to +45 ‰ vs VPDB, with internal consistency approaching the 0.005 ‰ level, and an effective calibration option for dual inlet IRMS systems as demonstrated in the international comparison CCQM-P204 completed in 2021 [2];Studies of Δ47 values of mixtures of different pure CO2 gas, and the reproducibility and stability of these and their potential to act as reference standards for clumped isotope ratio measurements with IRMS systems;The development and validation of a cryogenic Air Trapping system to extract CO2 from air for determination of δ13C and δ18O-CO2 with IRMS, including a correction for the N2O present in samples. The facility is currently being used for another international comparison (CCQM-P239) of CO2 in in air standards from 15 institutes containing CO2 over 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, and will demonstrate the level of equivalence of new CO2 in  air isotope  ratio standards currently being produced.[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).[2]  J Viallon et a, Final report of CCQM-P204, comparison on CO2 isotope ratios in pure CO2,  2023 Metrologia 60 08026 DOI 10.1088/0026-1394/60/1A/0802