An off-beam (OB) detection approach is suggested and experimentally investigated and optimized for quartz-enhanced photoacoustic spectroscopy (QEPAS). This OB-QEPAS configuration, very simple in assembly, not only allows for use of larger excitation optical beams and facilitating optical alignment but also provides higher enhancement of photoacoustic signals than previously published results based on the common on-beam QEPAS under the same experimental conditions. A normalized noise equivalent absorption coefficient $\left(1\sigma \right)$ of $5.9\times {10}^{-9}{\mathrm{cm}}^{-1}\mathrm{W}∕{\mathrm{Hz}}^{1∕2}$ was obtained for water vapor detection at normal atmospheric pressure.

We applied for the first time, to our knowledge, broadband off-beam quartz-enhanced photoacoustic spectroscopy (BB-OB-QEPAS) to trace ${\mathrm{NO}}_2$ detection using a broadband blue laser diode centered at $450\mathrm{nm}$. A detection limit of $18\mathrm{ppbv}$ (parts in ${10}^9$ by volume) for ${\mathrm{NO}}_2$ in ${\mathrm{N}}_2$ at atmospheric pressure was achieved with an average laser power of $7\mathrm{mW}$ at a $1\mathrm{s}$ integration time, which corresponds to a $1\mathrm{\sigma }$ normalized noise equivalent absorption coefficient of $4.1\times {10}^{-9}{\mathrm{cm}}^{-1}\mathrm{W}/{\mathrm{Hz}}^{1/2}$. An Allan variance analysis was performed to investigate the long-term stability of the BB-OB-QEPAS-based ${\mathrm{NO}}_2$ sensor.

A novel spectrophone sensor prototype consisting of a T-shaped acoustic microresonator (T-mR) in off-beam quartz-enhanced photoacoustic spectroscopy (T-mR QEPAS) is introduced for the first time. Its performance was evaluated and optimized through an acoustic model and experimental investigation via detection of water vapor in the atmosphere. The present work shows that the use of T-mR in QEPAS based sensor can improve the detection sensitivity by a factor of up to ~30, compared with that using only a bare QTF. This value is as high as that obtained in a conventional “on-beam” QEPAS, while keeping the advantages of “off-beam” QEPAS configuration: it is no longer necessary to couple excitation light beam through the narrow gap between the QTF prongs. In addition, the T-mR is really suitable for mass production with high precision.