Ppb-level NH₃ photoacoustic sensor combining a hammer-shaped tuning fork and a 9.55 µm quantum cascade laser

We present a quartz enhanced photoacoustic spectroscopy (QEPAS) gas sensor designed for precise monitoring of ammonia (NH₃) at ppb-level concentrations. The sensor is based on a novel custom quartz tuning fork (QTF) with a mid-infrared quantum cascade laser emitting at 9.55 µm. The custom QTF with a hammer-shaped prong geometry which is also modified by surface grooves is designed as the acoustic transducer, providing a low resonance frequency of 9.5 kHz and a high-quality factor of 10263 at atmospheric pressure. In addition, a temperature of 50 °C and a large gas flow rate of 260 standard cubic centimeters per minute (sccm) are applied to mitigate the adsorption and desorption effect arising from the polarized molecular of NH₃. With 80-mW optical power and 300-ms lock-in integration time, the detection limit is achieved to be 2.2 ppb which is the best value reported in the literature so far for NH₃ QEPAS sensors, corresponding to a normalized noise equivalent absorption coefficient of 1.4 × 10^-8 W cm^-1 Hz^-1/2. A five-day continuous monitoring for atmospheric NH₃ is performed, verifying the stability and robustness of the presented QEPAS-based NH₃ sensor.