Ppb-level detection of ammonia based on QEPAS using a power amplified laser and a low resonance frequency quartz tuning fork

In this report, an ultra-high sensitive quartz-enhanced photoacoustic spectroscopy (QEPAS) based ammonia (NH₃) sensor using a power amplified diode laser and a low resonance frequency quartz tuning fork (QTF) was demonstrated for the first time. A fiber-coupled, continuous wave (CW), distributed feedback (DFB) diode laser with a watt level output power boosted by an erbium-doped fiber amplifier (EDFA) was used as the QEPAS excitation source. A QTF with a resonance frequency of 30.72 kHz was employed as an acoustic wave transducer. The modulation depth in the wavelength modulation spectroscopy (WMS) based QEPAS system was optimized theoretically and validated by experimental measurements. For the reported NH₃ sensor system, a 418.4 ppbv (parts per billion by volume) minimum detection limit at a NH₃ absorption line of 6533.4 cm⁻¹ was achieved when the modulation depth was set to the optimum value of 0.188 cm⁻¹. The ppb-level detection sensitivity verified the design of the reported QEPAS method and makes it suitable for use in environmental monitoring and other applications.