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/08026