Radial-cavity quartz-enhanced photoacoustic spectroscopy (RC-QEPAS) was proposed for trace gas analysis. A radial cavity with (0,0,1) resonance mode was coupled with the quartz tuning fork (QTF) to greatly enhance the QEPAS signal and facilitate the optical alignment. The coupled resonance enhancement effects of the radial cavity and QTF were analyzed theoretically and researched experimentally. With an optimized radial cavity, the detection sensitivity of QEPAS was enhanced by
>
1
order of magnitude. The RC-QEPAS makes the acoustic detection module more compact and optical alignment comparable with a bare QFT, benefiting the usage of light sources with poor beam quality.
Photoacoustic spectroscopy (PAS) is a boomingly developed spectroscopic method for trace gas analysis in the past decades. Unlike traditional laser absorption spectroscopy, an acoustic transducer is employed to detect the acoustic waves instead of optical waves. Microphone is a commonly used transducer to convert the acoustic signal into electrical signal, which is related to the gas concentration. In recent years, piezoelectric materials are playing important roles in the acoustic-to-electrical conversion process of photoacoustic system. In this article, recently used transducers in PAS such as quartz tuning fork, polyvinylidene fluoride, lead zirconate titanate, electromechanical film and LiNbO 3 , BiB 3 O 6 , NbCOB crystals are summarized and discussed in detail. The different spectrophone configuration, schematic diagram, and experimental system are reported. Side by side comparison is carried out to show the gas sensing performance of these piezoelectric transducers.
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.
In this Letter, clamp-type quartz tuning fork enhanced photoacoustic spectroscopy (Clamp-type QEPAS) is proposed and realized through the design, realization, and testing of clamp-type quartz tuning forks (QTFs) for photoacoustic gas sensing. The clamp-type QTF provides a wavefront-shaped aperture with a diameter up to 1 mm, while keeping Q factors > 104. This novel, to the best of our knowledge, design results in a more than ten times increase in the area available for laser beam focusing for the QEPAS technique with respect to a standard QTF. The wavefront-shaped clamp-type prongs effectively improve the acoustic wave coupling efficiency. The possibility to implement a micro-resonator system for clamp-type QTF is also investigated. A signal-to-noise enhancement of ∼30 times has been obtained with a single-tube acoustic micro resonator length of 8 mm, ∼20% shorter than the dual-tube micro-resonator employed in a conventional QEPAS system.
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