Simultaneous Detection of ¹² CH ₄ , ¹³ CH ₄ , and Related Isotope Ratio Exploiting a Frequency-Multiplexed Mid-Infrared Quartz-Enhanced Photoacoustic Sensor

High Resolution Image Download MS PowerPoint Slide 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 12 CH 4 and 13 CH 4 . The sensor employs a frequency-modulated multiplexing scheme using two distributed-feedback quantum cascade lasers to independently excite the fundamental (f o ) and overtone ( f 1 ) vibrational modes of a quartz tuning fork coupled with resonator tubes. The f 0 -demodulated signal is dedicated to monitoring 12 CH 4, while the f 1 -demodulated signal selectively quantifies 13 CH 4, enabling the analysis of the isotopic composition of methane samples. Calibration measurements demonstrated a linear response of the QEPAS signal to varying 13 CH 4 concentrations in CH 4 -based samples diluted in N 2, with a precision of 1‰ in detecting isotopic delta ratio variations for 1% CH 4 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.