The Research Of Novel Resonant Quartz-enhanced Photoacoustic Spectroscopy

Quartz-enhanced photoacoustic spectroscopy?QEPAS?is one of the newly developed spectral absorption technologies in recent years.Compared to traditional microphone-based photoacoustic spectroscopy,QEPAS has the outstanding performance of high sensitivity,high selectivity,simple structure,small in-size and low cost.It has been successfully used in the area of environment monitoring,industrial process controlling and medical diagnosis.In QEPAS,the most common used methods for acoustic enhancement are on-beam enhancement and off-beam enhancement.The on-beam QEPAS has a high detection sensitivity,but it requires high quality of laser beam.And the off-beam QEPAS avoids the rigor request of laser beam quality,while performing a lower detection sensitivity.The drawbacks limit the application of QEPAS.In this article,the novel resonant QEPAS spectroscopy is proposed by realizing the multi-resonance based on the off-beam QEPAS.For its high detection sensitivity and low laser beam quality demand,the resonant QEPAS has a better development prospect in the real application.Based on the QEPAS technique,this dissertation focus on design of the spectrophone and model used for analyzing the acoustic wave enhancement;studying the system characteristics of resonant QEPAS and application of QEPAS system high sensitivity trace gas detection.The main content and innovations of the article are listed as follows:?1?Theoretical analysis of photoacoustic spectroscopy.Based on the theory of infrared spectral absorption,the process of photoacoustic effect and the detection of photoacoustic signal are studied.Taking the two level molecular system as an example,the physical processes of the photo-thermal conversion and thermal-acoustic excitation are analyzed.Based on the boundary conditions of the acoustic wave equation,the expression of sound pressure and amplitude is obtained.And the theory of wavelength modulation and demodulation is introduced.?2?Design and optimization of the novel resonant QEPAS spectrophone.Based on the principle of quartz enhanced photoacoustic spectroscopy,traditional QEPAS spectrophone is modeled and the energy leakage is analyzed.This article presents five kinds of photoacoustic acoustic configurations:the double tube same side resonance tube,the double tube different side resonance tube,the double tube side resonance tube,the elliptical cavity resonance cavity and the quartz overtone enhancement resonance tube.The theoretical calculation,finite element analysis and parameter optimization are carried out for each configuration.Finally,the correctness of the theory and simulation is verified by the experimental study,and the optimal parameters of five kinds of photoacoustic resonance acoustic sensors are obtained.?3?Establishment and characteristics study of novel resonant QEPAS system.For the above spectrophones,the QEPAS system is presented and the modules are introduced in detail.The modulation depth,laser power response and frequency characteristics are analyzed.?4?The environment influence on resonant QEPAS system.The influence of environment factors including temperature,pressure and humidity on QEPAS system is studied by theory and experiments.The background noise of the tuning fork is analyzed and two noise cancellation methods are introduced.The methods can improve the anti-interference performance of the system and enhance the reliability of applications.?5?Application of novel resonance QEPAS for trace gas detection.The application of resonant QEPAS system is investigated in the area of SF6 decomposition product detection and multi-gas detection.With the near-infrared laser power,high accuracy detection of H₂S is realized.Then the application of the novel resonant QEPAS in multi-gas detection is studied.By using the two-channel laser QEPAS,simultaneous detection of NH₃,H₂S and CO gases is also realized.The research provides a new technique for the realization of low cost and small in-size of trace gas detection.