A Dual-Gas Sensing System Based on QEPAS and LITES

A dual-gas sensing system based on quartz-enhanced photoacoustic spectroscopy (QEPAS) and laser-induced thermoelastic spectroscopy (LITES) has been proposed. This system utilized two quartz tuning forks (QTFs) with distinct resonance frequencies, corresponding to the demodulation frequencies of the two-component gas for effective detection of the gas components. The QEPAS signal of each gas component was simultaneously detected by two QTFs to increase the signal strength, and through each QTF to achieve an integrated detection of QEPAS and LITES. The configuration featured a dual-off-beam acoustic micro-resonator (AMR), which integrated both QTFs to facilitate dual-gas QEPAS detection and enhancement. In this setup, each QTF and AMR detected and amplified the QEPAS signals from both gas components, while the output signals from the two QTFs were superimposed for calculating gas concentrations based on their respective frequency signals. Compared to conventional QEPAS systems employing a single QTF and off-beam AMR, this novel structure benefited from enhanced signal response due to the involvement of a second QTF. The two laser beams passed through two AMRs were then directed into a dual-channel absorption gas cell, generating second harmonics resulting from gas absorption. Then these second harmonics signals are incident on the side arm of each respective QTF for LITES signal excitation. The combined outputs of both QEPAS and LITES signals were used to accurate calculation of dual-gas concentrations using frequency division multiplexing (FDM) technique, thereby achieving significant signal enhancement. The minimum detection limits (MDLs) for C<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>H<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and CH<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> were determined to be 2.68 ppmv and 5.49 ppmv respectively, demonstrating that this dual-gas sensing system exhibited high sensitivity.