Mid-infrared quartz-enhanced photoacoustic sensor for ppb-level CO detection in SF6 gas matrix exploiting a T-grooved quartz tuning fork

An optical sensor for highly sensitive detection of carbon monoxide (CO) in sulfur hexafluoride (SF6) was demonstrated by using quartz-enhanced photoacoustic spectroscopy (QEPAS) technique. A spectrophone composed by a custom 8 kHz T-shaped quartz tuning fork (QTF) with grooved prongs and a pair of resonator tubes, to amplify the laser-induced acoustic waves, was designed aiming to maximize the CO photoacoustic response in SF6. A theoretical analysis and an experimental investigation of the influ-ence of SF6 gas matrix on spectrophone resonance properties for CO detection have been provided, and the performances were compared with the standard air matrix. A mid-infrared quantum cascade laser with central wavelength at 4.61 μm, resonant with the fundamental band of CO, and optical power of 20 mW was employed as light excitation source. A minimum detection limit of 10 ppb at 10 s of integration time was achieved and a sensor response time of ~ 3 minutes was measured.