Calibration for CO2 stable isotopic composition measurements onto the VPDB scale via the primary carbonate RMs, NBS-19 or IAEA-603, is well known and accepted, and a consistent ion correction framework has been recommended and is in wide use. However, when measurements from different laboratories are compared, either by the analysis of common samples, such as occurs in round robin or other coordinated programs, or by the comparison of atmospheric records, inter-laboratory differences are reported that are frequently larger than the uncertainty reported by the laboratories for the combination of their measurement uncertainty and their link to the primary calibration. Why is this and is it significant? Is it because there are components of the "in use" procedures and corrections that need further refinement? For example, in the case of atmospheric CO2 measurements, is it due to other processes involved in data correction, such as the correction for N2O that is cryogenically processed with the CO2? Is it because we fail to properly consider the impact of measurement uncertainties? By looking into the details of methods for producing records of the stable isotopic composition of atmospheric CO2 it may be possible to establish recommendations to improve comparability and traceability for these measurements.
Topic : Theme 2: Accuracy requirements for atmospheric composition measurements across economic sectors, and temporal and spatial scales.
Reference : T2-A19
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