Multi-pass quartz-enhanced photoacoustic spectroscopy-based trace gas sensing.

A multi-pass quartz-enhanced photoacoustic spectroscopy (MP-QEPAS)-based trace gas sensor is reported. In MP-QEPAS, a multi-pass laser beam pattern through the prong spacing of a quartz tuning fork (QTF) is obtained by means of two right-angle prisms. A large QTF with the prong length of 17 mm and prong spacing of 0.8 mm was employed to increase the passage of multi-pass time and ease the alignment of the beam reflection pattern through the QTF. This multi-pass configuration allows the laser beam to pass through the QTF prong spacing six times. Water vapor (H2O) was chosen as target gas to investigate the performance of the MP-QEPAS sensor. Compared to a conventional QEPAS measurement, the MP-QEPAS technique provided an enhancement of signal level of ∼3.2 times.