Ppb-Level Quartz-Enhanced Photoacoustic Detection of Carbon Monoxide Exploiting a Surface Grooved Tuning Fork

A compact and sensitive carbon monoxide (CO) sensor was demonstrated by using quartz enhanced photoacoustic spectroscopy (QEPAS) exploiting a novel 15.2 kHz quartz tuning fork (QTF) with grooved surfaces. The custom QTF was designed to provide a quality factor as high as 15 000 at atmospheric pressure, which offers a high detection sensitivity. A large QTF prong spacing of 800 μm was selected, allowing one to avoid the use of any spatial filters when employing a quantum cascade laser as the excitation source. Four rectangular grooves were carved on two prong surfaces of the QTF to decrease the electrical resistance and hence enhance the signal amplitude. With water vapor as the catalyst for vibrational energy transfer, the sensor system using the novel surface grooved QTF achieved a CO minimum detection limit of 7 ppb for a 300 ms averaging time, which corresponds to a normalized noise equivalent absorption coefficient of 8.74 × 10^–9 cm^–1W /√Hz. Continuous measurements covering a seven-day period for atmospheric CO were implemented to verify the reliability and validity of the developed CO sensor system.