Measurement of the ¹³C/¹²C of atmospheric CH ₄ using near-infrared (NIR) cavity ring-down spectroscopy

A near-infrared (NIR) continuous-wave-cavity ring-down spectrometry (CW-CRDS) device was developed with the goal of measuring seasonal changes in the isotopic composition of atmospheric CH₄ on Earth and eventually on Mars. The system consisted of three distributed feedback laser diodes (DFB-LDs), two of which were tuned to the absorption line peaks of ¹²CH₄ and ¹³CH₄ at 6046.954 cm ^-1 and 6049.121 cm^-1, respectively, and a third that measured the baseline at 6050.766 cm^-1. The multiple laser design improved the long-term stability of the system and increased the data acquisition rate. The acquisition frequency was further increased by utilizing a semiconductor optical amplifier (SOA) to initiate cavity ring-down events. The high repetition rate combined with the superhigh reflectivity mirrors yielded precise isotopic measurements in this NIR region, even though the line strengths of CH₄ in this region are 200 times weaker than those of the strongest mid-IR absorption bands. The current system has a detection limit of 1.9 × 10^-12 cm^-1, corresponding to 10 pptv of CH₄ at 100 Torr. For ambient air samples that contained 1.9 ppmv CH₄, the δ¹³C of the CH₄ was determined to be -48.7 ± 1.7‰ (1σ).