Optical gas sensors are excellent candidate as powerful tool for trace gas detection and monitoring in a large number of applications, such as environmental monitoring, industrial process control and petrochemical industry. Among most sensitive optical techniques, quartz-enhanced photoacoustic spectroscopy (QEPAS) is capable of extremely high detection sensitivities with a compact detection module. It exploits a quartz tuning fork (QTF) as a sharply resonant acoustic transducer to detect weak photoacoustic excitation and allowing the use of extremely small volumes . QEPAS technique does not require an optical detector, it is wavelength independent, it is immune to environmental noise. These factors, together with and its proven reliability and ruggedness, represent the main distinct advantages with respect to other laser-based techniques for environmental monitoring and in situ detection.
Starting from the basic physical principles governing the QEPAS technique, this presentation will review the main results achieved by exploiting custom QTFs for QEPAS sensing, like the extension of operation in the THz range and intracavity-QEPAS. Novel approaches, such as the use of single tube microresonator systems or QTFs overtone modes operation will be also discussed. Finally, examples of QEPAS real-word applications will be reviewed.