Non-contact quartz-enhanced photoacoustic spectroscopy

Abstract

Non-contact quartz-enhanced photoacoustic spectroscopy (NC-QEPAS) was proposed and developed for trace gas analysis. The NC-QEPAS aims at solving the problem that the quartz tuning fork (QTF) must be immersed in the gases for photoacoustic wave transducing, which limits its application for corrosive and dusty gas sensing. In this work, the QTF was isolated from the gas, realizing “non-contact” detection. An elastic parylene film was synthesized and then patched to the slit of a QEPAS gas cell. With an optimized coupling effect, the parylene film shows a resonance enhancement with the QTF and acoustic micro-resonator, realizing non-contact photoacoustic detection of gas. The NC-QEPAS not only increases the photoacoustic signal amplitude but also decreases the background noise. Compared to traditional contact QEPAS with QTF immersed in the gas, the NC-QEPAS shows a signal-to-noise enhancement factor of 13. A normalized noise equivalent absorption coefficient of 8.8 × 10−9 cm−1 W Hz−1/2 was achieved. Allan deviation shows good long-term stability of the NC-QEPAS sensor. With an integration time of 1000 s, the developed QEPAS sensor shows a detection limit of 0.4 ppm. The detection limit can be further improved with longer integration time.