Cavity ring-down spectroscopy using an EC-QCL operating at 7.5 µm for direct monitoring of methane isotopes in air

Abstract

A number of atmospheric pollutants and greenhouse gases have strong fundamental vibrational transitions within the spectral range of 7.5–8 µm, which marks the region as particularly important for trace gas sensing. Here, we report the development of a mid-infrared continuous-wave (cw) cavity ring-down spectroscopy (CRDS) technique coupled with an external-cavity (EC) mode-hop-free quantum cascade laser (QCL) operating at 7.5 µm. We validated the EC-QCL based high-resolution cw-CRDS system by measuring 12CH4 and 13CH4 isotopes of methane (CH4) which served as a benchmark molecule. The direct, quantitative and selective measurements of 12C and 13C isotopes of CH4 in ambient air as well as in human breath samples in the levels of parts per billion by volume were made by probing one of the strongest fundamental vibrational transitions of CH4 arising from the asymmetric bending (ν4 band) vibrations of the bonds centred at ~1327.244 cm−1 and ~1332.946 cm−1, respectively. We achieved a noise-equivalent absorption coefficient of 1.86 × 10−9 cm−1 Hz−1/2 with 100 Hz data acquisition rate for the current cw-CRDS spectrometer. The current high-resolution cw-CRDS system could be further exploited to harness the full advantage of the spectral region covering 7.5–8 µm to monitor several other trace molecular species along with their isotopic compositions.