Quartz-enhanced photoacoustic spectroscopy for gas sensing applications

This chapter aims to introduce quartz-enhanced photoacoustic spectroscopy (QEPAS) for trace gas sensing. Starting from the basic physical principles, the QEPAS technique is described. This is followed by a detailed theoretical analysis and experimental study regarding the influence of quartz tuning forks (QTFs), which is the core of a QEPAS sensor, on sensing performance. This study suggests guidelines for the realization of custom QTFs optimized for QEPAS operation. Results obtained by exploiting QEPAS configurations with custom QTFs, capable of increasing the QEPAS signal-to-noise ratio by more than two orders of magnitude, are reviewed. The minimum detection limits reached for the most performant QEPAS sensors described in literature are reported, for all gas species detected so far, along with a comparison to other optical detection techniques for gas sensing. Finally, novel QEPAS approaches, such as the use of high-finesse optical cavities, fiber-coupled sources, and novel acoustic modules for simultaneous dual-gas detection, are reviewed.