Advancing Primary Direct Method for Absolute 13C/12C Isotope Amount Ratio Measurement of Carbon Dioxide in Air Using Infrared Absorption Spectroscopy.

We present a comprehensive study aimed at advancing the NIST accurate isotope ratio infrared spectroscopy (AIR-IS) system (Fleisher et al., Nat. Phys. 17, 889-893, 2021), as a primary direct method for determining the absolute 13C/12C isotope amount ratio of CO2 in air. The AIR-IS system employs a dual-wavelength cavity ring-down spectrometer to measure the abundance of the two most prevalent isotopologues of carbon dioxide: 12C16O2 and 13C16O2. This study introduces key methodological improvements, including quantification of surface-associated effects, enhancement of laser-to-cavity coupling, and refinement of the underlying spectroscopic model. To illustrate its application, we analyze three significant CO2-in-air samples: (i) the natural Northern Continental air NIST Standard Reference Material, SRM® 1720, and (ii) two synthetic mixtures derived from pure CO2 gases sourced from the international P204 comparison study, establishing traceability of their 13C/12C isotope ratios to the International System of Units (SI). Our results highlight AIR-IS's analytical performance, achieving 0.08‰ long-term precision for the absolute 13C/12C isotope ratio of the Northern Continental air standard monitored over five months, demonstrating agreement in isotope delta values within 0.3(0.2)‰ across a broad range (-1.515 to - 43.119‰ on the Vienna Peedee Belemnite, VPDB scale) for international samples, and constraining surface-associated uncertainty contributions to 0.2‰. AIR-IS measurements further provide a critical SI-traceable reference for CO2-in-air synthetic mixtures. Finally, we determine the 13C/12C isotope amount ratio for VPDB, contributing to independent efforts to refine its accepted value. Taken together, these results reinforce AIR-IS's potential as a primary direct method for stable carbon isotope ratio measurements with implications for atmospheric research and isotope metrology.