A near-IR CO trace gas sensor based on quartz-enhanced photoacoustic spectroscopy (QEPAS) is evaluated using humidified nitrogen samples. Relaxation processes in the CO-N₂-H₂O system are investigated. A simple kinetic model is used to predict the sensor performance at different gas pressures. The results show that CO has a ~3 and ~5 times slower relaxation time constant than CH₄ and HCN, respectively, under dry conditions. However, with the presence of water, its relaxation time constant can be improved by three orders of magnitude. The experimentally determined normalized detection sensitivity for CO in humid gas is 1.556 × 10 − 8 W ⋅ cm − 1 / Hz 1 / 2 .

A selective and sensitive quartz enhanced photoacoustic spectroscopy (QEPAS) sensor, employing an erbium-doped fiber amplifier (EDFA), and a distributed feedback (DFB) laser operating at 1582 nm was demonstrated for simultaneous detection of ammonia (NH₃) and hydrogen sulfide (H₂S). Two interference-free absorption lines located at 6322.45 cm⁻¹ and 6328.88 cm⁻¹ for NH₃ and H₂S detection, respectively, were identified. The sensor was optimized in terms of current modulation depth for both of the two target gases. An electrical modulation cancellation unit was equipped to suppress the background noise caused by the stray light. An Allan-Werle variance analysis was performed to investigate the long-term performance of the fiber-amplifier-enhanced QEPAS sensor. Benefitting from the high power boosted by the EDFA, a detection sensitivity (1σ) of 52 parts per billion by volume (ppbv) and 17 ppbv for NH₃ and H₂S, respectively, were achieved with a 132 s data acquisition time at atmospheric pressure and room temperature.

A quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor for H₂S detection operating in near-infrared spectral range is reported. The optical source is an erbium-doped fiber amplified laser with watt-level optical power. The QEPAS spectrophone is composed of a quartz tuning fork with a resonance frequency of 7.2 kHz, a quality factor of 8500, and a distance between prongs of 800 µm, and two tubes with a radius of 1.3 mm and a length of 23 mm acting as an organ pipe resonator. With this spectrophone geometry, the photothermal noise contribution of the spectrophone was removed and the theoretical thermal noise level was achieved. The position of both tubes with respect to custom quartz tuning fork has been investigated as a function of signal amplitude, Q-factor, and noise of the QEPAS sensor when a high-power laser was used. Benefit from the linearity of the QEPAS signal to the excitation laser power, a detection sensitivity of 330 ppb for H₂S detection was achieved at atmospheric pressure and room temperature, when the laser power was 1.6 W and the signal integration time was set to 300 ms, corresponding to a normalized noise equivalent absorption of 3.15 × 10⁻⁹ W cm⁻¹/(Hz)^1/2. The QEPAS sensor was then validated by measuring H₂S in a biogas sample.