Research on High Sensitive Detection of HCl Trace Gas Based on QEPAS Technology

Hydrogen chloride (HCl) is a toxic and harmful gas. It is of great significance to detecte it with high sensitivity. But up to now there has been a few research reports about the HCl detection using laser spectroscopy. Quartz-enhanced photoacoustic spectroscopy (QEPAS) technology was invented not long ago. This technique uses a commercially available mm sized piezoelectric quartz tuning fork (QTF) as an acoustic wave transducer. A high Q-factor and a ~32.7 kHz resonance frequency of the QTF improve QEPAS selectivity and immunity to environmental acoustic noise. QEPAS sensor has the advantages of high sensitivity, selectivity and compactness. In this paper, 5 000 ppm HCl∶N₂ mixture was selected as the target analyte. A continuous-wave distributed feedback (DFB) single mode diode laser emitting at 1 742.38 μm was used as the exciting source and QEPAS technology was adopted as the detection method. In order to reduce the sensor background noise and simplify the data process, wavelength modulation spectroscopy and a 2nd harmonic detection technique were employed. In this research, acoustic micro-resonator (mR) in the acoustic detection module and the effect of acoustic wave improvement was discussed firstly. The “on beam” architecture of mR was selected and the length and inner diameter of the mR tubes were selected to be 4 and 0.5 mm, respectively. In the experiments, the influence of modulation depth of laser wavelength on QEPAS signal amplitude was investigated. When the acquisition time was 1 sec and modulation depth was 0.23 cm^-1, a 1σ minimum detectable concentration limit of 815 ppb was obtained. The corresponding normalized noise equivalent absorption (NNEA) coefficient was 7.41×10^-9 cm^-1·W·Hz^-1/2. In the future, the detection sensitivity can be further improved by adding water vapor into the target HCl gas to enhance the V-T (vibrational-translational) relaxation rate of HCl molecular.