Research On The Photoacoustic Spectroscopy For Trace Gas Detection And Applications

There is a wide range of gases not only playing important role in agriculture and industry, but also relating to public health and safety. The trace gases detection techniques have become an impotant branch of the modern analysis technologies. Trace gas measurements with photoacoustic spectroscopy (PAS) provide highly sensitive and selective detection in continuous-flow mode, suitable for online multicomponent trace gas analysis. In this dissertation, photoacoustic spectrometers for fire detection and for exhalation diagnosis are developed respectively, also a method for hydrogen detection is proposed. The main research works are outlined as follows:The guide for photoacoustic spectrometer design including IR sources, photoacoustic cells and microphones is discussed in detail by analyzing the photoacoustic signal generation process. Photoacoustic spectrometers based on thermal emitter, nonresonant PA cell and large microphone are compact and suitable for multicomponent gas detection, while photoacoustic spectrometers based on laser, resonant PA cell and small microphone are rather sensitve and suitable for trace gas detection. The phase and frequency information make it possible to detect gases without light absorption in IR band.In the development process of compact photoacoustic spectrometer, the integrated absorption coefficients of absorption gases at different absorption band are compared, thus the optimum parameters of the IR filters are determined:center wavelength of 4650nm,5350nm and 3700nm for CO, NO and H2S respectively. The light absorption efficiency and the cross interference are both considered.The operating frequency of 35Hz is determined based on the measured frequency curve, that ensures the PA signal excitation and detection most effective. Experimental results show that the detection limits of 1.6ppm,4.5ppm and 400ppm for CO, NO and H2S can be obtained respectively in less than 60s. The repeatability and linearity tested for CO in the concentration range of 0-987ppm are also fairly satisfactory.In the development process of highly sensitive photoacoustic spectrometer, the adsorption-desorption process caused by ammonia exchange between the cell surface and the gas phase is deeply analyzed, thus an H-type first longitudinal resonant PA cell made of Teflon is proposed. The sample gas is directed at tangent into the inlet located at center of the resonant cell and forced flowing in spiral along the surface. Experimental results show that the response time ofτPTFE 90%≈5 min and detection limit of 0.86ppb is achieved at 100SCCM sample flow rate.The method of cryogenic separation is used to separate CO2 in exhalation, which ensure the CO2 laser based photoacoustic spectrometer achieve high sensitivity for ammonia measurement. Experimental results of two volunteers before HD show that the exhaled ammonia concentrations are 4.73ppm and 3.54ppm respectively, which are in agreement with the results obtained in previous experiments by other scholars.A method for hydrogen detection is proposed, that makes it possible to measure all the seven types of gas in dissolved gas analysis by PAS. Experimental results show that the relation between resonant frequency change and hydrogen concentration in binary gas mixture is consistent with the theoretical expectation. The detection limit is about 200ppm, which can be further improved through temperature compensation.The hardware and software of the photoacoustic spectrometers are fully optimized based on virtual instrument technique combined with multitasking program provided by LabVIEW platform. The control and measurement ability of hardware is closely combined with the computer data processing, and complex tasks are organized efficiently.In summary, the capability to monitor multicomponent trace gas is provided by the IR rhermal emitter based compact photoacoustic spectrometerfor and the CO2 laser based sensitive photoacoustic spectrometer developed in this dissertation. The potential applications as fire gas detection, condition measurement of power transformers and breath gas analysis in medicine are also discussed.