Simultaneous Detection of 12CH4, 13CH4, and Related Isotope Ratio Exploiting a Frequency-Multiplexed Mid-Infrared Quartz-Enhanced Photoacoustic Sensor

We report the development of a dual-gas Quartz-Enhanced Photoacoustic Spectroscopy (QEPAS) sensor operating in the mid-infrared range for the simultaneous detection of 12CH4 and 13CH4. The sensor employs a frequency-modulated multiplexing scheme using two distributed-feedback quantum cascade lasers to independently excite the fundamental (fo) and overtone (f1) vibrational modes of a quartz tuning fork coupled with resonator tubes. The f0-demodulated signal is dedicated to monitoring 12CH4, while the f1-demodulated signal selectively quantifies 13CH4, enabling the analysis of the isotopic composition of methane samples. Calibration measurements demonstrated a linear response of the QEPAS signal to varying 13CH4 concentrations in CH4-based samples diluted in N2, with a precision of 1‰ in detecting isotopic delta ratio variations for 1% CH4 mixtures at 0.8 s integration time. The proposed system is suitable for real-time, high-precision isotopic methane sensing aimed at applications such as environmental monitoring, geochemical tracing, and industrial process control.