Isotope Analysis as a tool for climate metrology at PTB: a novel approach to oxygen-17 correction

The isotopic composition of carbon dioxide is a powerful tool in many scientific areas and normally reported as isotope δ’s, viz. δ¹³C_VPDB in case of carbon and δ¹⁸O_VPDB-CO2 in case of oxygen. These two isotopic quantities must be calculated from the measured molecular quantities. This includes an ¹⁷O correction, which is an important step in data evaluation. Due to the measurement conditions, typically available experimental information is insufficient on ¹⁷Oand the calculative correction must be done iteratively. The fact that there is no analytical solution complicates the calculation of δ¹³C_VPDB and δ¹⁸O_VPDB-CO2 as well as the calcuation of the associated uncertainties. Therefore, Brand et al. [1] suggested a linear approximation which performs quite well. Moreover, Brand et al. presented a simplified scheme for uncertainty estimation. Here, we present an alternative approximation [2] which outperforms the established one leading to much smaller deviations from the exact solutions and to uncertainty calculations according to the Guide to the Expression of Uncertainty in Measurement (GUM) [3]. These approximations are implemented in an EXCEL Add-in, which allows potential users to gain full control over their data evaluation and to check the data received from commercial IRMS software in a spreadsheet.

This work is embedded in PTB’s commitment to the metrology for environment and climate which is overseen by the Innovation Cluster Environment & Climate [4].

References

[1]	W. A. Brand, S. S. Assonov und T. B. Coplen, „Correction for the 17O interference in δ(¹³C) measurements when analyzing CO₂ with stable isotope mass spectrometry (IUPAC Technical Report),“ Pure Appl. Chem., Bd. 82, p. 1719–1733, January 2010.
[2]	L. Flierl und O. Rienitz, „OCEAN – an EXCEL Add-in for 17O Correction using a novel Approximation,“ MethodsX, p. 102529, 2023.
[3]	Joint Committee for Guides in Metrology, „JCGM 100: Evaluation of Measurement Data - Guide to the Expression of Uncertainty in Measurement,“ 2008.
[4]	Physikalisch-Technische Bundesanstalt, „Innovation Cluster Environment & Climate,“ [Online]. Available: https://www.ptb.de/cms/en/research-development/ptbs-innovation-clusters/innovationscluster-umwelt-klima.html.