An EC-QCL based dual-species (CH4/N2O) detection method at 7.8 µm in mid-IR region for simultaneous applications of atmospheric monitoring and breath diagnostics

A high-sensitive optical detection method was designed and developed at 7.8 μm mid-IR region by coupling a room-temperature operated continuous wave (cw) external-cavity quantum cascade laser (EC-QCL) with an astigmatic multipass cell. The detection strategy operates in the principle of wavelength modulation spectroscopy with second harmonic detection strategy (2f-WMS). We have shown that the methodology has the ability for simultaneous and real-time quantitative measurements of nitrous oxide (N2O) and methane (CH4) in ambient air and human exhaled breath in parts per billion (ppb) levels via an extremely narrow single QCL scan of ∼0.06 cm−1. The high-resolution rotational-vibrational interference-free 2f-WMS spectra of CH4 and N2O centred at 1297.8192 cm−1 and 1297.8314 cm−1, respectively were simultaneously acquired with 0.20 s data acquisition time in the optimized experimental conditions. Minimum detection limits of 6 ppb for N2O and 30 ppb CH4 were achieved by the cw-EC-QCL based 2f-WMS detection method, thus opening its broad applications in environmental monitoring and non-invasive biomedical diagnostics.