Sensitive Detection of CH4 and CO2 Using Frequency-Division-Multiplexing Based Quartz-Enhanced Photoacoustic Spectroscopy

A compact double-gas sensing system was reported for simultaneous sensitive CH4 and CO2 detection, this system combined quartz-enhanced photoacoustic spectroscopy (QEPAS) and frequency division multiplexing technique. Two continuous wave distributed feedback diode lasers operating at 1654 nm and 2004 nm were employed as the light sources. The photoacoustic signals were excited by simultaneously injecting sinusoidal modulation currents with independent frequencies near the resonant frequency of the QTF into the lasers，and were subsequently demodulated to obtain the corresponding second harmonic components of CH4 and CO2. The frequency-domain results showed no interference between the photoacoustic signals of the two gases. The relationships between CH4 and CO2 concentrations and the corresponding second harmonic signals have been calibrated, and linear correlation coefficients better than 0.994 were obtained. The system performance was further evaluated through Allan deviation analysis on CH4 and CO2 with given concentration. The minimum detection limits of CH4 and CO2 for the system were obtained as 0.58×10-6 and 1.32×10-6, corresponding to the normalized noise equivalent absorption coefficients (NNEA) of 7.2×10-9 cm-1W√Hz and 9×10-9 cm-1w√Hz, respectively.