Quartz-enhanced photoacoustic spectroscopic methane sensor system using a quartz tuning fork-embedded, double-pass and off-beam configuration.

Development of a methane (CH) sensor system was reported based on a novel quartz-tuning-fork (QTF)-embedded, double-pass, off-beam quartz-enhanced photoacoustic spectroscopy (DP-OB-QEPAS). A simplified and accurate numerical model was presented to optimize the DP-OB-QEPAS spectrophone and to enhance the detection sensitivity. A compact and fiber-coupled acoustic detection module (ADM) with a volume of 3 × 2×1 cm and a weight of 9.7 g was fabricated. A continuous-wave distributed feedback diode laser was used to target the CH absorption line at 6046.95 cm. With the combination of wavelength modulation spectroscopy (WMS) and second harmonic (2) detection technique, the CH sensor system reveals a 1σ detection limit of 8.62 parts-per-million in volume (ppmv) for a 0.3 s averaging time with an optimized modulation depth of 0.26 cm. The proposed CH sensor shows a similar or even lower level in the normalized noise equivalent absorption coefficient (NNEA) (1.8 × 10 cm∙W/√Hz), compared to previously reported QEPAS-based CH sensors.