Exploring δ13C data compatibility in a broad δ13C range: Reassembling δ13C values obtained for NIST CO2 RMs 8562–8564 at different laboratories

Rationale Stable isotope datasets obtained in different laboratories over years must be compared and combined. The strict compatibility in δ13C at ±0.01‰ and ±0.02‰, k = 1, respectively, is required for observations of greenhouse gases CO2 and methane. That implies the long‐term compatibility of calibrations realized at different laboratories. Thus far, this level of compatibility has not been demonstrated for pure CO2 in a broad δ13C range and over years. Methods We reassemble the δ13C obtained in different laboratories for NIST CO2 RMs 8562–8564, which are pure CO2 gases. If the measurements are based on independent calibrations and reported on the same 17O correction framework, δ13C of all three RMs can be used to estimate the long‐term δ13C data compatibility. Results δ13C values obtained in different laboratories for NIST CO2 RMs 8562–8564 (a range of δ13C from ~ −3.7‰ to ~ −41.6‰) from 1998 to 2020 agree within their uncertainties. Two NIST studies (1999 and 2004) have combined data from several laboratories using different Isotope ratio mass spectrometry (IRMS) methods and somewhat different approaches. Three single‐laboratory studies (2003, 2008, and 2020) have used the same type of IRMS and similar approaches. The δ13C values from these laboratories agree within ±0.01‰, k = 1, over the entire δ13C range and have uncertainties about two times lower than those obtained at NIST by combining data from several laboratories. Conclusion The δ13C values for NIST CO2 RMs 8562–8564 obtained in three laboratories using the same IRMS (2003, 2008, and 2020) agree within ±0.01‰, k = 1. First, this confirms the long‐term stability in δ13C of RMs 8562–8564. Second, this example of compatibility in δ13C calibrations on pure CO2, from −3.7‰ to −41.6‰, is relevant to stable isotope measurements of greenhouse gases CO2 and methane. Third, this confirms the high accuracy of the δ13C(air–CO2) calibrations performed in 2003 and 2008 for the project Civil Aircraft for the Regular Investigation of the atmosphere Based on an Instrumented Container (CARIBIC).