Recent advances in quartz-enhanced gas-phase photoacoustic spectroscopy

A new approach to detecting photoacoustic signals in gas media is reported. Instead of using a gas-filled resonant acoustic cavity and a microphone as in traditional photoacoustic spectroscopy (PAS) the sound energy can also be detected using a high Q piezoelectric crystal element. Quartz crystal tuning forks can be used as resonant transducer elements for photoacoustic gas sensing. Recently, we introduced a novel approach to PAS called quartz-enhanced photoacoustic spectroscopy (QEPAS). QEPAS takes advantage of the extremely high quality factor Q of quartz crystals. Feasibility experiments of a gas sensor utilizing a quartz tuning fork (TF) resonating at 32.768 kHz and a fiber coupled distributed-feedback (DFB) diode laser operating at 1.53 μm have demonstrated a normalized detection limit of 7.2×10^-9 W/Hz¹2 / using NH₃ as the target gas. This sensitivity is in the same range as the best reported result for conventional PAS. The sensitivity limiting factor of QEPAS is the fundamental thermal noise of the TF.