Photoacoustic measurements of ammonia by a telecommunication diode laser

A distributed-feedback (DFB) telecommunication diode laser radiating at 1.53 µm was used for photoacoustic (PA) detection of ammonia molecules in the gas phase. The frequency of the diode laser was adjusted to an absorption line of ammonia and modulated at the first longitudinal resonant frequency (3.8 kHz) of the cylindrical resonator of the photoacoustic cell. For noise reduction a differential method with two acoustic resonators and a lock-in amplifier was used. To increase the Q factor of the cell electro-polished stainless steel resonators were applied. For reducing the adsorption problem of the polar ammonia molecule, the remaining surface of the cell was covered with a 30 µm Teflon coating. The adsorption behavior of ammonia with different gas flows was examined in detail. With this small system, it is possible to perform fast trace gas measurement of ammonia in the parts per million (ppmV) concentration range. The sensitivity of the photoacoustic cell could be determined to 5.0 ×10-9 Wcm-¹ (S/N = 1). The photoacoustic ammonia sensor could be applied for measurement and control of ammonia concentrations in exhaust gases of vehicles, of industrial plants, and for monitoring ammonia emissions by livestock farming.