Exploring δ¹³ C data compatibility in a broad δ¹³ C range: Reassembling δ¹³ C values obtained for NIST CO₂ RMs...

Stable isotope datasets obtained in different laboratories over years must be compared and combined. The strict compatibility in δ¹³ C 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 δ¹³ C range and over years.

We reassemble the δ¹³ C 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 ¹⁷ O correction framework, δ¹³ C of all three RMs can be used to estimate the long-term δ¹³ C data compatibility.

δ¹³ C values obtained in different laboratories for NIST CO₂ RMs 8562-8564 (a range of δ¹³ 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 δ¹³ C values from these laboratories agree within ±0.01‰, k = 1, over the entire δ¹³ C range and have uncertainties about two times lower than those obtained at NIST by combining data from several laboratories.

The δ¹³ C 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 δ¹³ C of RMs 8562-8564. Second, this example of compatibility in δ¹³ C 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 δ¹³ C(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).