[en] The emission of greenhouse gases and the resulting global warming is one of the most important and challenging issues of the 21st century. Carbon dioxide is one of the major contributors to the greenhouse effect and its atmospheric abundance has growing constantly since the beginning of the industrialization. The isotope ratios n(13C)/n(12C) and n(18O)/n(16O) are important tools for studying the impact of anthropogenic CO2. Usually, isotopic compositions of CO2 are reported as δ-values, that express isotope ratios relative to an artifact based on a fossil calcite called VPDB. This relative VPDB scale was necessary, since absolute and SI-traceable isotope ratios of CO2 are currently not available, neither by isotope ratio mass spectrometry (IRMS) nor by optical isotope ratio spectroscopy (OIRS). In this study we present a potential way of deriving absolute carbon and oxygen isotope ratios of carbon dioxide via IRMS based on the gravimetric mixture approach. Besides practical improvements like an air buoyancy correction scheme for masses of gases, we show first results applying our method which demonstrate its feasibility, limitations, and achievable uncertainties. Also, we show the mathematics behind our approach and discuss further improvements and applications. Furthermore, we show how these absolute ratios can be used in field applications by OIRS methods including a new approach on OIRS uncertainty assessments according to the GUM. For this contribution we report on our recent results within in the European metrology research projects SIRS (16ENV06). and STELLAR (19ENV05).

[en] The pilot study CCQM-P204 was aimed at evaluating the level of compatibility of laboratories' measurement capabilities to value assign isotope ratios in samples of pure CO₂ gas, expressed as isotope delta values relative to the relevant international scale: δ¹³CVPDB and δ¹⁸OVPDB-CO₂. Pure CO₂ gas samples were prepared by the BIPM in batches of 10 samples of the same gas and circulated to participants for measurement. Each participant received four samples of CO₂ with a different nominal δ¹³CVPDB value: -1 per thousand; -9 per thousand; -35 per thousand; and -42 per thousand. The BIPM was also responsible for evaluating the homogeneity and stability of the samples. The co-coordinator IAEA received one sample per batch to confirm the batch-to-batch homogeneity. Within batch and between batch inhomogeneity was assessed and found to be negligible in comparison to the spread of results reported by participants. Participants used the analytical technique of their choice to measure the isotope delta values. They were requested to report the measurement results together with detailed information on their traceability, measurements methods and data treatment. Results of the comparison were to be compiled by the BIPM and evaluated jointly by the BIPM and the IAEA. The majority of participants reported results using DI-IRMS, and those that reported results based on laser spectroscopy techniques showed a very similar dispersion of results as for DI-IRMS, although generally with greater uncertainty. A total of nineteen participants reported their measurements, with two of them reporting results with different reference materials to provide more insight into the traceability of the measurements. The results were reported with traceability to three different VPDB scale realizations, notably VPDB, VPDB-LSVEC and VPDB2020, with 8, 7 and 6 results reported respectively for each of these. Participants agreed that results based on VPDB and VPDB2020 scale realizations should, in principle, lead to consistent results, whereas those based on VPDBLSVEC should show a bias that increased as samples became more depleted in ¹³C, with the bias approaching 0.2 per thousand for the most depleted sample. This bias was demonstrated by the participant reporting the most precise measurements based on the VPDB-LSVEC realizations, whereas for 2 participants using VPDB-LSVEC scale realizations other issues dominated the consistency of their results. The 3 laboratories using the NIST (8562,8563, 8564) reference materials (reported as on the VPDB-LSVEC scale), were highly consistent with each other, but the reported bias for the VPDB-LSVEC realization was not evident, with the historical method used for value assignment of the NIST RMs, and their relatively large uncertainty, being identified as possible causes for this. (author)