We present a quartz enhanced photoacoustic spectroscopy (QEPAS) gas sensor designed for precise monitoring of ammonia (NH) 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 NH. 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 NH QEPAS sensors, corresponding to a normalized noise equivalent absorption coefficient of 1.4 × 10 W cm Hz. A five-day continuous monitoring for atmospheric NH is performed, verifying the stability and robustness of the presented QEPAS-based NH sensor.

中文翻译：

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结合锤形音叉和 9.55 µm 量子级联激光器的 ppb 级 NH3 光声传感器

我们推出了一款石英增强光声光谱 (QEPAS) 气体传感器，专为精确监测 ppb 级浓度的氨 (NH) 而设计。该传感器基于新型定制石英音叉 (QTF)，具有发射波长为 9.55 µm 的中红外量子级联激光器。具有锤形爪几何形状的定制 QTF 也经过表面凹槽修改，被设计为声换能器，在大气压下提供 9.5 kHz 的低谐振频率和 10263 的高品质因数。此外，采用50℃的温度和260标准立方厘米/分钟（sccm）的大气体流量来减轻NH的极化分子产生的吸附和解吸效应。凭借 80 mW 光功率和 300 ms 锁定积分时间，检测限达到 2.2 ppb，这是迄今为止文献中报道的 NH QEPAS 传感器的最佳值，对应于归一化噪声等效吸收系数1.4 × 10 瓦·厘米·赫兹。对大气 NH 进行了五天的连续监测，验证了所提出的基于 QEPAS 的 NH 传感器的稳定性和鲁棒性。