Octupole electrode pattern for tuning forks vibrating at the first overtone mode in quartz-enhanced photoacoustic spectroscopy.

The design, realization, and performance analysis of an octupole electrode pattern configuration intended for the optimization of the charge collection efficiency in quartz tuning forks (QTFs) vibrating at the first overtone in-plane flexural mode is reported. Two QTFs having the same geometry, but differing in the electrode pattern deposited on the QTF prongs, have been realized in order to study the influence of the electrode pattern on the resonance quality factor and electrical resistance. A standard quadrupole pattern (optimized for the fundamental mode) and an octupole electrode layout have been implemented. Although both QTFs show the same resonance quality factor for the first overtone, the octupole pattern provides a reduction of the QTF electrical resistance by more than four times. The sensing performance of the two QTFs has been compared by employing them in a mid-IR quartz-enhanced photoacoustic sensor (QEPAS) system targeting a water absorption line. When operating at the first overtone mode, the QTF with an octupole electrode pattern provides a QEPAS signal more than two times higher with respect to the QTF employing the standard quadrupole configuration.