ABSTRACT
In this work, we developed off-plane quartz-enhanced photoacoustic spectroscopy (OP-QEPAS). In the OP-QEPAS the light beam went neither through the prong spacing of the quartz tuning fork (QTF) nor in the QTF plane. The light beam is in parallel with the QTF with an optimal distance, resulting in low background noise. A radial-cavity (RC) resonator was coupled with the QTF to enhance the photoacoustic signal by the radial resonance mode. By offsetting both the QTF and the laser position from the central axis, we enhance the effect of the acoustic radial resonance and prevent the noise generated by direct laser irradiation of the QTF. Compared to IP-QEPAS based on a bare QTF, the developed OP-QEPAS with a RC resonator showed a >10× signal-to-noise ratio (SNR) enhancement. The OP-QEPAS system has great advantages in the use of light emitting devices (LEDs), long-wavelength laser sources such as mid-infrared quantum cascade lasers, and terahertz sources. When employing a LED as the excitation source, the noise level was suppressed by â¼2 orders of magnitude. Furthermore, the radial and longitudinal resonance modes can be combined to further improve the sensor performance.
ABSTRACT
A ppbv-level mid-infrared photoacoustic spectroscopy sensor was developed for mouth alcohol tests. A compact CO2 laser with a sealed waveguide and integrated radio frequency (RF) power supply was used. The emission wavelength is â¼9.3 µm with a power of 10 W. A detection limit of â¼18 ppbv (1σ) for ethanol gas with an integration of 1 s was achieved. The sensor performed a linear dynamic range with an R square value of â¼0.999. A breath measurement experiment after consuming lychees was conducted. The photoacoustic signal amplitude decreased with the quality of lychee consumed, confirming the existence of residual alcohol in the mouth. During continuous measurement, the photoacoustic signal decreased in < 10 min when consuming 30 g lychee fruits, proving that the alcohol detected in exhaled breath originated from the oral cavity rather than the bloodstream. This work provided valuable information on the distinction of alcoholism and crime.
ABSTRACT
A novel spectroscopic method, named quartz-enhanced photoacoustic spectroscopy-conductance spectroscopy (QEPAS-CS), was first developed for gas mixture analysis. In QEPAS-CS, the advantage of photoacoustic detection and conductance analysis was realized by a quartz tuning fork (QTF). Two-component gas analysis was done by photoacoustic detection and conductance detection. For an explicit application, natural spider silk was used as a water vapor transducer to modify the QTF, making a conductance sensing channel. A 2004 nm laser diode was used as an excitation source for a photoacoustic sensing channel. Such a QEPAS-CS sensor was used for H2O/CO2 gas mixture analysis in a cell incubator. This provides a solution to calibrate an infrared photoacoustic spectroscopy gas sensor. This example effectively confirms the capacity of multigas analysis by the QEPAS-CS sensor.
