The Research And Application On Off-axis Quartz Enhanced Photoacoustic Spectroscopy Based On LED Source

Quartz enhanced photoacoustic spectroscopy(QEPAS)technology is a new approach of photoacoustic spectroscopy detection,in which high-quality quartz tuning forks are used instead of broadband microphones used in traditional ways,thus the QEPAS technology has the characteristics of large dynamic range and miniaturization structure.In general,the QEPAS sensor is equipped with the resonance tube to enhance photoacoustic signals,by which the detection sensitivity of the system is improved.Usually there are two kinds of resonance tube configuration:coaxial configuration and off-axis configuration.Because of the requirement for high quality of light beam,photoacoustic spectra analyzers of the coaxial configuration usually use lasers as light sources,therefore this kind of instruments generally have drawbacks such as high cost of and low flexibility.The off-axis quartz enhanced photoacoustic spectroscopy(OB-QEPAS)technology based on LED light sources we study on in this paper has the advantages of convenient adjustment of the optical path and low cost instrument,while the detection sensitivity achieved is basically the same with the coaxial devices,and the gas sensors based on this kind of configuration are easy to industrialized production.In this paper,the theoretical and experimental studies on the micro acoustic resonance tube are carried out on the basis of systematically research on off-axis quartz enhanced photoacoustic spectroscopy(OB-QEPAS)technology.A formula for numerically calculating the optimal micro-acoustic resonance(mR)'s parameters of OB-QEPAS-based sensor is obtained and the optimal design of the acoustic resonance tube is carried out.On the basis of theoretical and experimental studies,the measurement system is designed and developed based on the ARM+Window CE software and hardware platform.The entire system can be divided into four modules in terms of functionality:?Infrared light emission system ?Photoacoustic spectroscopy measurement module ?Arm control system ?data collecting system.A program of detecting the concentration of C02 is designed to test system sensitivity,in which a broadband spectrum light source of MIR-LED centered at 4.26?m is used.The linear fitting,data processing of the optical signal and the concentration of the experiment are carried out by the least square method.A detection limit of ppm order of magnitude is achieved,which is corresponding to a normalized noiseequivalent absorption coefficient(NNEA)of 10-8 order of magnitude.The results show that the OB-QEPAS technology based on LED light sources is a trace gases detection method of high precision and good reliability,which provides a new technical means for the development of low cost,convenient operation,and sensitive trace gas detection.