Measuring ¹³ C-enriched CO₂ in air with a cavity ring-down spectroscopy gas analyser: Evaluation and calibration

Cavity ring-down spectroscopy (CRDS) is becoming increasingly popular for δ¹³ C-CO₂ analysis of air. However, little is known about the effect of high ¹³ C abundances on the performance of CRDS. Overlap between ¹² CO₂ and ¹³ CO₂ spectral lines may adversely affect isotopic-CO₂ CRDS measurements of ¹³ C-enriched samples. Resolving this issue is important so that CRDS analysers can be used in CO₂ flux studies involving ¹³ C-labelled tracers.

We tested a Picarro G2131-i CRDS isotopic-CO₂ gas analyser with specialty gravimetric standards of widely varying ¹³ C abundance (from natural to 20.1 atom%) and CO₂ mole fraction (xCO₂ : <0.1 to 2116 ppm) in synthetic air. The presence of spectroscopic interference between ¹² CO₂ and ¹³ CO₂ 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 δ¹³ C-CO₂ , x¹² CO₂ , and x¹³ CO₂ .

CRDS measurements of x¹³ CO₂ were found to be accurate throughout the tested range (<0.005 to 100 ppm). On the other hand, spectral cross-talk in x¹² CO₂ measurements of standards containing elevated levels of ¹³ CO₂ led to inaccuracy in x¹² CO₂ , total-xCO₂ (x¹² CO₂  + x¹³ CO₂ ), and δ¹³ C-CO₂ data. An empirical relationship for x¹² CO₂ measurements that incorporated the ¹³ C/¹² C isotope ratio (i.e. ¹³ CO₂ /¹² CO₂ , RCO2) as a secondary (non-linear) variable was found to compensate for the perturbations, and enabled accurate instrument calibration for all CO₂ compositions covered by our standard gases.

¹³ C-enrichement in CO₂ leads to minor errors in CRDS measurements of x¹² CO₂ . We propose an empirical correction for measurements of ¹³ C-enriched CO₂ in air by CRDS instruments such as the Picarro G2131-i.