Measuring 13 C‐enriched CO 2 in air with a cavity ring‐down spectroscopy gas analyser: Evaluation and calibration

Rationale Cavity ring‐down spectroscopy (CRDS) is becoming increasingly popular for δ 13 C‐CO 2 analysis of air. However, little is known about the effect of high 13 C abundances on the performance of CRDS. Overlap between 12 CO 2 and 13 CO 2 spectral lines may adversely affect isotopic‐CO 2 CRDS measurements of 13 C‐enriched samples. Resolving this issue is important so that CRDS analysers can be used in CO 2 flux studies involving 13 C‐labelled tracers. Methods We tested a Picarro G2131‐i CRDS isotopic‐CO 2 gas analyser with specialty gravimetric standards of widely varying 13 C abundance (from natural to 20.1 atom%) and CO 2 mole fraction ( x CO 2 : <0.1 to 2116 ppm) in synthetic air. The presence of spectroscopic interference between 12 CO 2 and 13 CO 2 bands was assessed by analysing errors in measurements of the standards. A multi‐component calibration strategy was adopted, incorporating isotope ratio and mole fraction data to ensure accuracy and consistency in corrected values of δ 13 C‐CO 2 , x 12 CO 2 , and x 13 CO 2 . Results CRDS measurements of x 13 CO 2 were found to be accurate throughout the tested range (<0.005 to 100 ppm). On the other hand, spectral cross‐talk in x 12 CO 2 measurements of standards containing elevated levels of 13 CO 2 led to inaccuracy in x 12 CO 2 , total‐ x CO 2 ( x 12 CO 2  +  x 13 CO 2 ), and δ 13 C‐CO 2 data. An empirical relationship for x 12 CO 2 measurements that incorporated the 13 C/ 12 C isotope ratio (i.e. 13 CO 2 / 12 CO 2 , R C O 2) as a secondary (non‐linear) variable was found to compensate for the perturbations, and enabled accurate instrument calibration for all CO 2 compositions covered by our standard gases. Conclusions 13 C‐enrichement in CO 2 leads to minor errors in CRDS measurements of x 12 CO 2 . We propose an empirical correction for measurements of 13 C‐enriched CO 2 in air by CRDS instruments such as the Picarro G2131‐i.