Final report of CCQM-P204, comparison on CO2 isotope ratios in pure CO2
Authors: Viallon J , Choteau T , Flores E , Idrees F , Moussay P , Wielgosz R I , Loh Z , Allison C , Huang L , Chivelscu A , Camin F , Krajnc B , Ogrinc N , Fioravante A de Lima , Fasciotti M , Monteiro T V C , Garrido B C , Rego E C P , Wollinger W , Augusto C R , Michel S , Lee J S , Lim J K , Daëron M , Kassi S , Moossen H , Hai L , Zhou Z , Srivastava A , Shimosaka T , Webber E Mussel , Hill−Pearce R , Brewer P , Chartrand M , Rienitz O , Ebert V , Flierl L , Braden-Behrens J , Nwaboh J , Emad A , Simsek A , Chubchenko I 
Published: November 2023

Main text
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 ‰; −9 ‰; −35 ‰; and −42 ‰. 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, measurement 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 VPDB-LSVEC should show a bias that increased as samples became more depleted in 13C, with the bias approaching 0.2 ‰ 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.
In general, for all results the dispersion was greater than expected based on the measurement uncertainties reported by participants. This dispersion increased as the samples became more depleted in 13C, so that results that were traceable to realizations of the VPDB scale that could be considered equivalent (VPDB and VPDB2020) did not lead to ensembles that were fully consistent within their stated uncertainties. Either the reduced chi-squared or Birge Ratio provide easily calculated quantities to characterise lack of consistency in a data set, where consistent data would lead to values of unity for either of these, and discrepant data leading to increased values. This is most readily demonstrated by considering results based on DI-IRMS with traceability to the VPDB scale through either VPDB and VPDB 2020 realizations, where the standard deviation of 16 results was 0.043 ‰ and a Birge Ratio of 2.7 calculated for nominally −1‰ for δ13C, and the standard deviation was 0.12 ‰ and a Birge Ratio of 2.9 calculated at nominally −9 ‰ for δ18O. For the samples where the nominal δ13C value was −42 ‰, the standard deviation of 17 results was 0.085 ‰ and a Birge Ratio of 4.5 calculated for δ13C, and the standard deviation was 0.24 ‰ and a Birge Ratio of 3.4 calculated for δ18O at nominally −36 ‰. The observed magnitude of the standard deviation of results can also be compared to the standard uncertainty of the IAEA-603 materials certified values (0.01 ‰ for δ13C and 0.04 ‰ δ18O) and the smallest standard uncertainties reported by a participant (0.005 ‰ for δ13C, and 0.01 ‰ for δ18O). These results indicate an underestimation of uncertainty for reported results, especially for those with the smallest uncertainties. Components of uncertainty that should be reviewed before future comparisons include: the uncertainty contribution from reference materials; the uncertainty associated with the phosphoric acid reaction with carbonate reference materials; corrections and uncertainties related to cross-contamination effects in the IRMS; appropriate methods for combining uncertainties of multiple reference materials and accounting for their correlations. A retreatment of results, which normalizes results to the −1 ‰ and −42 ‰ δ13C samples, leads to improvement in the consistency of results as demonstrated for measurements on the nominally −35 ‰ δ13C, −30 ‰ δ18O samples for which the standard deviation is reduced to 0.034 ‰ and 0.057 ‰ for δ13C and δ18O respectively (from 0.072 ‰ and 0.198 ‰ without normalization). The results of the comparison indicate that once issues of non-ideal methods and use of LSVEC are removed, discrepancies in results arise from the challenges in accurately transferring delta values from carbonate reference materials to CO2 gaseous samples, and that these issues can be reduced by having appropriate gaseous reference standards for calibration when measuring CO2 gaseous samples. This is consistent with the identical treatment principle that is preferred in the isotope ratio measurement community.
An analysis of results is presented in this report, with further consideration of the impact of the measurement method, the scale, and the reference materials. Uncertainties reported by participants are detailed and analysed, highlighting important differences in the uncertainty sources considered. Although CCQM-P204 was a comparison organised within the CCQM/GAWG and IRWG, no reference value was calculated, mainly because not all results appeared to be strictly on the same scale. Instead, a list of recommendations is provided to encourage more harmonised measurement practices and reach better consistency in future comparisons on similar materials.
To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database https://www.bipm.org/kcdb/.
The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).