Double resonant quartz-enhanced photoacoustic spectroscopy with a spherical resonator

In this Letter, double resonant quartz-enhanced photoacoustic spectroscopy (QEPAS) with a spherical resonator is proposed for the first time to our knowledge. This spherical resonator, which functions both as an acoustic resonator and a gas chamber, facilitates efficient sound wave coupling between the resonator and the quartz tuning fork (QTF), enabling double resonance. The frequency response and acoustic field distribution of the radial spherical resonator were calculated using the finite element method. The radius of the spherical resonator was optimized to 27.34 mm to realize perfect resonance coupling with the QTF. A novel T-tip QTF with a low resonance frequency of 8.97 kHz was positioned at the center of the spherical resonator to detect the photoacoustic signal though its piezoelectric effect and resonance characteristics. Acetylene (C 2 H 2 ) and methane (CH 4 ) were selected as target gases to evaluate the performance of the QEPAS sensor. Experimental results showed that incorporating the spherical resonator enhanced the 2 f signal amplitudes by factors of 13.03 and 13.29 for the C 2 H 2 -QEPAS and CH 4 -QEPAS systems, respectively. The long-term stability and optimal detection performance of the designed sensor were evaluated using Allan deviation analysis. The designed QEPAS sensor achieved minimum detection limits (MDLs) of 337.2 ppb for C 2 H 2 and 422 ppb for CH 4 at average times of 900 s and 1000 s, respectively.