HCl ppb-level detection based on QEPAS sensor using a low resonance frequency quartz tuning fork

In this report, a sensitive quartz-enhanced photoacoustic spectroscopy (QEPAS) based hydrogen chloride (HCl) sensor using a quartz tuning fork (QTF) with a resonance frequency of 30.72 kHz was demonstrated for the first time. A fiber-coupled, continuous wave (CW), distributed feedback (DFB) diode laser emitting at 1.74 μm was employed as the excitation laser source. Wavelength modulation spectroscopy and a 2nd harmonic detection technique were used to reduce the sensor background noise. An acoustic micro-resonator (mR) was added to the QTF sensor architecture to improve the signal amplitude. For the reported HCl sensor system operating at atmospheric pressure, a 550 ppbv (parts per billion by volume) minimum detection limit at 5739.27 cm^-1 was achieved when the modulation depth and the data acquisition time were set to 0.23 cm^-1 and 1 s, respectively. The ppb-level detection sensitivity and robust design of the QEPAS technique makes it suitable for use in environmental monitoring and other applications.