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

Abstract

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 2116ppm) 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 100ppm). 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.