Quartz‐Enhanced Photoacoustic Spectroscopy for Trace Gas Sensing

Photoacoustic spectroscopy (PAS) was established itself over the last decades as a powerful detection technique, capable of measuring trace gas concentrations at part‐per‐billion level. The basic principles of laser PAS are provided with a discussion on photoinduced acoustic wave generation and its detection. Quartz‐enhanced photoacoustic spectroscopy (QEPAS), where a quartz tuning fork (QTF) is used to detect acoustic waves, is presented in detail as an improvement of the conventional microphone‐based PAS. QEPAS does not require an optical detector, requires extremely small volumes, laser wavelength independent, and immune to environmental noise. The discussion covers several aspects of QTF resonance properties, regarding flexural modes, dissipation processes, and quality factor (Q‐factor). The main advantages achievable by exploiting custom QTFs for QEPAS sensing and novel approaches such as the excitation of overtone modes are highlighted and discussed. Finally, the most performant QEPAS sensors for real‐world applications are reviewed. Environmental monitoring of CH4, CO, and N2O and a QEPAS sensor for leak detection in mechatronic systems as well as hydrocarbon detection in petrochemical industry are few examples of QEPAS sensors already tested in out‐of‐laboratory environments. The obtained results clearly demonstrate the potential of QEPAS‐based sensors for in situ operations.