Near-infrared Laser Based Photoacoustic Spectroscopy For Multi-component Gas Detection And Medical Application

Trace gas detection plays an important role in the fields of atmospheric environment, industrial applications, medical diagnosis and life sciences. Among many detection techniques of trace gases, photoacoustic spectroscopy (PAS) has attracted a lot of interests as they enable high detection sensitivity, real time and continuous measurements. In this thesis, the near-IR laser photoacoustic spectroscopy for multi-component gas detection and its medical application of breath detection are deeply studied.Based on the theory of gas molecular absorption spectrum, the principle of wavelength modulation photoacoustic spectroscopy is discussed in detail. General harmonic expressions of wavelength modulation are given for the absorption line shapes of Lorentzian signals broadened by modulation. The linewidths and the signal intensities of second-harmonic have been calculated analytically as a function of the modulation amplitude. The general method of multi-component gas detection is given by analyzing the classic theory of photoacoustic spectroscopy. The method provides theories basis for detecting multi-component gas with photoacoustic spectroscopy.On the foundation of theoretical research, the photoacoustic multi-component gas detection system based on a near-IR tunable fiber laser and a resonant PA cell is designed and developed. The experimental investigation of multi-component gas mixture detection is conducted. Simultaneous and continuous measurement of trace water vapor, acetylene, carbon dioxide, and carbon monoxide (H2O, C2H2, CO and CO2) in gas mixtures are achieved. The detection limits (signal-to-noise ratio=l) of70ppm for H2O,2ppb for C2H2,4ppm for CO and4ppm for CO2were demonstrated with the detection system. The detection system has advantages of high level of linearity.The theoretical and clinical experimental studies of the breath ammonia monitoring in patients with end-stage renal disease (ESRD) undergoing hemodialysis are done on the basis of photoacoustic spectroscopy for multi-component gas detection. The breath ammonia pathological diagnosis and the specific requirement of sampling are analyzed. The dedicated mask of sampling and a teflon photoacoustic cell are designed. The gas path system are carried out constant temperature control. Aiming at the problem of high concentration of CO2and H2O interference with breath ammonia, the algorithm of multi-component gas measurement is proposed. The high sensitive ammonia measurements in high concentration of CO2and H2O is realized using the algorithm at atmospheric pressure. The minimum detection limit of16ppb (signal to noise ratio=1) in simulated breath samples (5.3%CO2and6.2%H2O (100%relative humidity at37℃)) is achieved.The breath ammonia levels of many healthy volunteers and six patients with ESRD were measured by means of photoacoustic spectroscopy breath analysis system. Experimental results indicated that, the breath ammonia levels of healthy volunteers are about300ppb. The initial concentration levels of breath ammonia are about2000ppb before dialysis treatment. These preliminary data indicate that breath ammonia levels decreased gradually as the treatment proceeded. Breath ammonia concentrations decreased to200～600ppb in the end stage of dialysis, which are close to the levels of healthy persons. The breath ammonia concentrations concentrations have decreased by more than65%. It is consistent with the current standard of the BUN.The breath analysis system of photoacoustic spectroscopy based on a near-IR tunable fiber laser has high sensitivity, multi-component gas simultaneous measurement, real time, continuous monitoring and suitable for clinical application. These works of the thesis will make some theoretical and experimental preparations for developing the medical breath analyser and the hemodialysis monitor.