Abstract
A crystal quartz tuning fork (QTF) was used as a detector to collect and amplify laser-induced photoacoustic and photothermal waves simultaneously for trace chemical analysis. A wavelength modulation technique was applied to the proposed quartz-enhanced photothermal-acoustic spectroscopy (QEPTAS) to improve the detection signal-to-noise ratio. The QTF detector was exposed to the illumination of a near-infrared distributed feedback laser at distances of 1 m and 2 m to evaluate the QEPTAS sensor performance. The QEPTAS sensor performance was determined by detecting water vapor in ambient air using a near-infrared distributed feedback laser with a power of ~10 mW and a wavelength of 1.39 μm. With an optimized modulation depth of 0.47 cm−1, the normalized noise equivalent absorption (NNEA) coefficients of 8.4 × 10−7 W·cm−1·Hz−1/2 and 3.7 × 10−6 W·cm−1·Hz−1/2 were achieved for a distance of 1 m and 2 m, respectively. The developed QEPTAS technique reduces the requirements for laser beam quality, resulting in a simple but robust sensor structure and demonstrates the ability of remote sensing of gas concentrations.
Highlights
Photothermal spectroscopy (PTS) and photoacoustic spectroscopy (PAS) are well-established methods for experimental physics, chemistry, and biology
quartz-enhanced photothermal-acoustic spectroscopy (QEPTAS) obtained the maximum signal amplitude when the laser spot was positioned at the center of the quartz tuning fork (QTF) prong spacing
The proposed QEPTAS is a useful tool for the investigation of photoacoustic and photothermal effects simultaneously
Summary
Photothermal spectroscopy (PTS) and photoacoustic spectroscopy (PAS) are well-established methods for experimental physics, chemistry, and biology. Photoacoustic spectroscopy is an optical detection method widely used for trace gas analysis Transducers such as microphones [4,5,6], cantilevers [7,8,9,10], or piezoelectric elements [11,12,13] were used to detect acoustic waves and convert them into mechanical or electrical signals. A highly sensitive PTS technology called quartz-tuning-fork enhanced photothermal spectroscopy (QEPTS) was developed for trace gas analysis by use of a QTF [31]. In QEPTS, the laser beam was focused on a spot with the diameter of ~36 μm, located in a particular area of the QTF prong where no silver layer was coated In this manuscript, quartz-enhanced photothermal-acoustic spectroscopy (QEPTAS) was developed for trace gas analysis. QEPATS removes the requirement for laser beam quality and provides the possibility of the remote sensing of gas concentrations
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