Evaluation of the carbon isotopic effects of NDIR and CRDS analyzers on atmospheric CO2 measurements

Non-dispersive infrared (NDIR) and cavity ring-down spectroscopy (CRDS) CO₂ analyzers use ¹²CO₂ isotopologue absorption lines and are insensitive to all or part of other CO₂-related isotopologues. This may produce biases in CO₂ mole fraction measurements of a sample if its carbon isotopic composition deviates from that of the standard gases being used. To evaluate and compare the effects of carbon isotopic composition on NDIR and CRDS CO₂ analyzers, we prepared three test sample air cylinders with varying carbon isotopic abundances and calibrated them against five standard cylinders with ambient carbon isotopic composition using CRDS and NDIR systems. We found that the CO₂ mole fractions of the sample cylinders measured by G1301 (CRDS) were in good agreement with those measured by LoFlo (NDIR). The CO₂ values measured by both instruments were higher than that of a CO₂ isotope measured by G2201i (CRDS) analyzer for a test cylinder with depleted carbon isotopic composition δ¹³C =-36.828‰, whereas no obvious difference was found for other two test cylinders with δ¹³C=-8.630‰ and δ¹³C=-15.380‰, respectively. According to the theoretical and experimental results, we concluded that the total CO₂ mole fractions of samples with depleted isotopic compositions can be corrected on the basis of their ¹²CO₂ values calibrated by standard gases using LoFlo and G1301 if the δ¹³C and δ¹⁸O values are known.