Fig. 1. Commercially available standard quartz tuning forks with a resonant frequency of 32 kHz

Fig. 2. QEPAS spectrophones with (a) on-beam and (b) off-beam configurations[17]

Fig. 3. Quartz enhanced photoacoustic detection module for mid-infrared region. (a) CAD model; (b) prototype

Fig. 4. Typical quartz-enhanced photoacoustic spectroscopic sensor system

Fig. 5. Principle of modulation cancellation method and experimental setup[45]

Fig. 6. (a) BF-QEPAS principle; (b) waveforms of scanning, piezoelectric and beat frequency signals[49]

Fig. 7. QEPAS spectrophone based on two QTFs[13]

Fig. 8. (a) Configuration #1 of QEPAS spectrophone based on two QTFs; (b) configuration #2 of QEPAS spectrophone based on two QTFs[13]

Fig. 9. Schematic of double acoustic micro-resonator spectrophone[25]

Fig. 10. Schematic of the I-QEPAS optical path configuration[53]

Fig. 11. Schematic of the evanescent wave QEPAS configuration[56]

Fig. 12. (a) Definition of length (Lp), width (W), and thickness (T) of tuning fork arm; (b) size comparison of the standard and the 6 customized quartz tuning forks[59]

Fig. 13. Pressure distributions in acoustic resonator with (a) a large and (b) a small gaps in the middle[61]

Fig. 14. Schematic of single-tube on-beam QEPAS spectrophone[62]

Fig. 15. Single-tube on-beam QEPAS spectrophone operating in overtone resonant mode[64]