Sensitive Detection of Ammonia Based on Quartz-Enhanced Photoacoustic Spectroscopy

This letter describes a sensitive quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor for measuring ammonia (NH 3 ) concentrations based on a custom quartz tuning fork (QTF) with a small gap of 200 μ m. The signal was enhanced by adding a microresonator (mR) with an optimum length of 5 mm to the bare QTF. Wavelength modulation spectroscopy with second-harmonic detection was adopted to reduce the noise level and simplify the data processing. After optimization of the laser modulation depth, a minimum detection limit of 22.6 ppm was obtained.