Exploring δ C data compatibility in a broad δ C range: Reassembling δ C values obtained for NIST CO 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 δ C at ±0.01‰ and ±0.02‰, k = 1, respectively, is required for observations of greenhouse gases CO 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 CO in a broad δ C range and over years.

METHODS

We reassemble the δ C obtained in different laboratories for NIST CO RMs 8562-8564, which are pure CO gases. If the measurements are based on independent calibrations and reported on the same O correction framework, δ C of all three RMs can be used to estimate the long-term δ C data compatibility.

RESULTS

δ C values obtained in different laboratories for NIST CO RMs 8562-8564 (a range of δ C 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 δ C values from these laboratories agree within ±0.01‰, k = 1, over the entire δ C range and have uncertainties about two times lower than those obtained at NIST by combining data from several laboratories.

CONCLUSION

The δ C values for NIST CO 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 δ C of RMs 8562-8564. Second, this example of compatibility in δ C calibrations on pure CO , from -3.7‰ to -41.6‰, is relevant to stable isotope measurements of greenhouse gases CO and methane. Third, this confirms the high accuracy of the δ C(air-CO ) calibrations performed in 2003 and 2008 for the project Civil Aircraft for the Regular Investigation of the atmosphere Based on an Instrumented Container (CARIBIC).