Ma et al also reported a multi-quartz-enhanced photothermal spectroscopy (M-QEPTS) which using multiple QTFs for increasing signal amplitude. Ma et al designed a T-shaped QTF with a prong length of 9.4 mm and a resonance frequency of 9.38 kHz for H 2O detection based on in-plane quartz-enhanced photoacoustic spectroscopy (IP-QEPAS), in which, the laser beam acted on the plane of the QTF for increasing the PAS signal intensity. The QTF structure and the PAS excitation device were also able to be optimized for the performance improvement of QEPAS sensor. The above researches could improve the PAS signal intensity effectively by the application and optimization design of AMR structure. optimized the off-beam AMR to realize superior detection performance and a NNEA of 6.2 × 10 –9 cm −1 W Hz –1/2 was obtained for H 2O detection. proposed an on-beam structure that was based on three AMRs to improve the acoustic signal and obtained an MDL of 166 ppbv for water vapor (H 2O). A NNEA of 1.21 × 10 –8 cm −1 W Hz –1/2 was then obtained for carbon dioxide (CO 2) detection. used a single tube to construct an on-beam AMR to improve the acoustic signal. optimized the on-beam AMR structure and demonstrated a NNEA of 3.3 × 10 –9 cm −1 W Hz –1/2 for acetylene (C 2H 2) detection. first introduced the QTF and a resonant sound tube to detect and improve the acoustic signal and the normalized noise equivalent absorption (NNEA) coefficient obtained for methane (CH 4) detection was 1.2 × 10 −7 cm −1 W Hz –1/2. The off-beam AMR includes a relatively long tube with a small slit in its center that is placed on one side of the QTF. The on-beam AMR consists of a pair of short metal tubes that are placed on the two sides of a quartz tuning fork (QTF). Normally, there are two main types of AMR structure: the on-beam and the off-beam structures. To enable further enhancement of the PAS signal and the minimum detection limit (MDL), an acoustic microresonator (AMR) is generally used to amplify the acoustic signal. In the past years, the performance indexs have been greatly improved in these areas of photoacoustic spectroscopy (PAS) signal improvement, distributed gas sensing and multi-component gas detection. The developments and prospect of QEPAS gas sensing technique were summarized and discussed by Ma et al. Because of these outstanding performance characteristics, QEPAS was widely researched for use in trace gas detection. in 2002 and offers portability, small volume and an excellent photoacoustic response. The introduction Quartz-enhanced photoacoustic spectroscopy (QEPAS) was introduced by Kosterev et al. Minimum detection limits of 3.6 ppmv for C 2H 2, 34.71 ppmv for CH 4, 1.09 ppmv for H 2O, and 341.18 ppmv for CO 2 were obtained, and the linear responses reached 0.9982, 0.9969, 0.99843 and 0.99591 for C 2H 2, CH 4, H 2O and CO 2, respectively, at 1.5 s intervals. The four-component gas sensing scheme was used to detect acetylene (C 2H 2) at 1,532.83 nm, methane (CH 4) at 1,653.722 nm, water vapor (H 2O) at 1,368.597 nm and carbon dioxide (CO 2) at 1,577.787 nm for feasibility testing. Four-component gas sensing was achieved via time-division multiplexing of the distributed feedback laser driver currents. Four distributed feedback (DFB) lasers were connected to the four ends of the two off-beam AMRs using a fiber collimator for photoacoustic signal excitation. 2School of Physics Science and Information Technology and Shandong Key Laboratory of Optical Communication Science and Technology, Liaocheng University, Liaocheng, ChinaĪ compact and portable quartz-enhanced photoacoustic spectroscopy gas sensor was developed for four-component gas detection using two off-beam acoustic microresonators The two AMRs were placed in parallel on opposing sides of a quartz tuning fork for photoacoustic signal enhancement.1Department of Information Engineering, Shandong Jiaotong University, Jinan, China. Cheng Wang 1, Zongliang Wang 2 * and Xiyu Pang 1
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