Meeting the demand for δ13C-CO2, δ18O-CO2, δ13C-CH4 and δ2H-CH4 Reference Materials for Climate Monitoring

The introduction of widely available optical isotopic ratio spectroscopy (OIRS) has enabled in-field measurements of isotopologues of carbon dioxide and methane (CO₂ and CH₄), allowing the source apportionment of greenhouse gases through distinguishing between natural and anthropogenic emissions of CO₂ and CH₄. At present, there are no existing internationally accepted reference materials which provide traceability for the calibration of δ¹³C-CO₂, δ¹³C-CH₄, δ²H-CH₄ and δ¹⁸O-CO₂ OIRS measurements which meet the World Metrological Organisation’s data compatibility goals to underpin global measurements.

We present the latest progress achieved during the EMPIR project Stable Isotope Metrology to Enable Climate Action and Regulation (STELLAR), on the development of primary reference materials (PRMs) which are traceable to the SI for amount fraction and existing isotope ratio scales. The production and certification of pure and ambient amount fraction reference materials is discussed alongside the sensitivities of the production and sampling process to isotopic fractionation. Methodologies for mitigating fractionation during PRM production and sampling will be outlined.

The production of the air matrix and the effects of trace gases and water vapour in the air matrix on the isotope ratio of the ambient PRMs is presented. The effects of trace gases in the matrix on the measurement of the isotope ratio of the ambient amount fraction CO₂ and CH₄ PRMs is also discussed for OIRS techniques.

We demonstrate achievement of the targeted uncertainties in δ¹³C_VPDB-CO₂ of ± 0.1 ‰ and ± 0.5 ‰ for δ¹⁸O_VPDB-CO₂. Uncertainty budgets are presented and the main contributions to the uncertainty budget are highlighted. The stability of the isotope ratio of the PRMs with pressure and time (2 years) is also discussed alongside comparisons with existing scales for amount fraction.