Quartz-enhanced photoacoustic spectroscopy exploiting tuning fork overtone modes

We report on a quartz-enhanced photoacoustic sensor (QEPAS) based on a custom-made quartz tuning fork (QTF) to operate in both the fundamental and the first overtone vibrational mode resonances. The QTF fundamental mode resonance falls at similar to 3 kHz and the first overtone at similar to 18 kHz. Electrical tests showed that the first overtone provides a higher quality factor and increased piezoelectric current peak values, with respect to the fundamental flexural mode. To evaluate the QTF acousto-electric energy conversion efficiency, we operated the QEPAS in the near-IR and selected water vapor as the target gas. The first overtone resonance provides a QEPAS signal-to-noise ratio similar to 5 times greater with respect to that measured for the fundamental mode. These results open the way to employing QTF overtone vibrational modes for QEPAS based trace gas sensing. (C) 2015 AIP Publishing LLC.