Metrological calibration strategies and uncertainty assessments for spectroscopic mid-IR isotope ratio measurements in carbon dioxide

Optical isotope ratio spectroscopy (OIRS) has recently gained popularity and maturity in isotope research because it simplifies the measurement process and makes isotope ratio measurements in atmospheric greenhouse molecule gases, e.g. CO₂, N₂O, CH₄, more accessible to field research and monitoring networks. OIRS is advantageous in terms of high time resolution, possibility of in-situ measurements and simplified sampling process. However, compared to traditional, high accurate isotope ratio mass spectrometry (IRMS), spectroscopic methods are not yet well established for the highest precision quantification of delta values and agreed recommendations for best practices are still missing. From a metrological point of view, the concept of uncertainty assessment in OIRS field measurements needs to be reported in accordance to the terms of the “Guide to the expression of uncertainty in measurement” (GUM, ISO/IEC Guide 98-3:2008). GUM compliant uncertainty estimation is necessary to assess the metrological comparability of OIRS measurement results. 

In this study we discuss calibration strategies for δ¹³C-CO₂ and δ¹⁸O-CO₂ measurements with commercial mid-IR OIRS instrumentation (Thermo Scientific Delta Ray), an uncertainty budget for OIRS measurements estimated according to the GUM, and compare the calibration strategies recommended by the manufacturer with our new metrological calibration procedure, which considers aspects so far limiting the accuracy as, e.g., matrix gas effects, CO₂ concentration dependence, instrumental drift corrections, and isotope calibration gas uncertainties.

Acknowledgments. This study has received funding from the European Metrology Programme for Innovation and Research (EMPIR) co-financed by the EURAMET Participating States and from the European Union's Horizon 2020 research and innovation programme as of the SIRS project (16ENV06).  PTB is a member of the European Metrology Network for Climate and Ocean Observation.