The extension of the detection range is a crucial task for improving the effectiveness of gas sensors. In this work we propose a novel approach in quartz-enhanced photoacoustic detection in which the detection module is composed of a bare 15 kHz custom tuning fork (QTF), aligned with a 12 kHz QTF and mounted inside a vacuum-tight cell. This configuration allows the same molecule to be detected by the bare QTF at high concentration and by the 12 kHz QTF down to part-per-billion scale. The signal from the bare QTF is also useful to gather information on the gas matrix variation.
We report on a comparison between the piezoelectric and interferometric readouts of vibrations in quartz tuning forks (QTFs) when employed as sound wave transducers in quartz-enhanced photoacoustic trace gas sensors. We demonstrate the possibility to properly design the QTF geometry to enhance interferometric readout signal with respect to the piezoelectric one and vice versa. When resonator tubes are acoustically coupled with the QTFs, signal-to-noise ratio enhancements are observed for both readout approaches. These results open the way to the implementation of optical readout of QTF vibrations in applications where external electromagnetic field could distort the piezoelectric signal.
We report on the performance of new quartz tuning fork (QTF) designs optimized for quartz-enhanced photoacoustic spectroscopy (QEPAS). We investigated the impact on resonance properties of prong geometries differing from the standard rectangular one. We proposed a QTF with T-shaped prongs and a QTF with prongs having rectangular grooves carved on the surface. QTFs were implemented in a QEPAS sensor and performances were compared in terms of signalto-noise ratio (SNR). Then, QTFs were acoustically coupled with single- and dual-tube micro-resonator systems. A record x60 SNR enhancement factor with respect to the bare QTF was achieved with QTF having T-shaped prongs.
We report on a comparison between two quartz tuning forks (QTFs) employed for quartz-enhanced photoacoustic spectroscopy (QEPAS) having quadrupole and octupole electrode pattern configurations. With respect to the quadrupole, the implementation of the octupole pattern suppresses the fundamental mode and reduces by a factor of ~ 4.4 the electrical resistance for the first overtone mode with negligible variations of the related Q-factors. Both QTFs operating at the first overtone mode were implemented in a QEPAS sensor and the results showed that the octupole configuration provides a ~2.3 signal enhancement factor.
We report on the development of a gas sensor system based on quartz-enhanced photoacoustic spectroscopy (QEPAS) for the detection of trace levels of ethylene using a quantum cascade laser operating at ~ 10.3 μm. To realize a compact sensor architecture, a dedicated acoustic detection module was designed and implemented. The module includes a QEPAS spectrophone, composed of a quartz tuning fork, a micro-resonator tube and a low-noise pre-amplifier chip for the signal readout. The volume of the ADM is ~30 cm3. A minimum detection limit of 30 part-per-billion in concentration was obtained with a data acquisition time of 10 s.
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