Breath Detection And Analysis Based On Laser Absorption Spectroscop

Endogenous gases in exhaled breath are closely related to physiological status and many of them can be used as biomarkers for non-invasive diagnosis of diseases.For instance,the detection and analysis of CO₂ and NO gases during respiration enables initial diagnosis of lung diseases.Tunable diode laser absorption spectroscopy（TDLAS）,as a common trace gas detection technique,has the advantages of high sensitivity,good selectivity,and fast response time.In this dissertation,CO₂ and NO gases in exhaled breath are detected and analyzed based on TDLAS technique.The main work and results are summarized as follows.Firstly,a versatile platform for breath detection is designed and built.According to the demand of high sensitivity and fast time response in the dynamic process of exhaled gas,the spectral line selection method for TDLAS breath gas detection is sorted out and summarized,and then the target bands of 2.004μm and 5.26μm are selected for CO₂ and NO to be measured,respectively.The versatile platform is integrated by a vital part of this platform which is a circular gas cell with a volume of 38m L and an optical range of 3.49m.The platform can realize the functions of water filtering and real-time sampling of exhaled gas,and furthermore achieve a measurement time resolution of80ms.Additionally,the highly sensitive dynamic process measurement of different biomarkers in breath can be realized by changing the laser light sources.Secondly,monitoring and analysis of human breath capnography are achieved.Based on direct absorption spectroscopy,an online processing program for spectral data is designed by using the platform combined with a 2004nm semiconductor laser,including dynamic calibration of wavenumber,wave peak deduction,baseline correction and Voigt profile fitting.The system is evaluated by a 4.5%CO₂ standard gas detection experiment,and the results show that the detection accuracy is±0.3%and the detection limit is 175.45ppm.The capnography of a pneumothorax patient and a healthy subject is monitored and analyzed,which shows that a significantly different trends exist in the saturation phase.In addition,the capnography of healthy subjects is measured and analyzed during normal breathing,after exercise and after meal to obtain some physiological indicators such as expiratory frequency,end tidal partial pressure of CO₂ and CO₂ elimination per breath that can be used for lung diagnosis,which demonstrates the potential of this system for the diagnosis of lung diseases and the detection of metabolic functions.Thirdly,the detection and analysis of NO in human breath are achieved.By means of a 5.26μm mid-infrared quantum cascade laser as the light source,the optimal modulation voltage of 100m V is obtained by simulation analysis based on the wavelength modulation spectroscopy technique with comprehensive consideration of the second harmonic signal amplitude and spectral line overlap.The performance of the system is then evaluated by different concentrations of NO standard gas detection experiments.The results suggest that the linearity of the system is 0.99663,the measurement error is±3.2%and the detection limit is 12.8ppb,respectively.The concentration curves of exhaled NO in long-term smoker and non-smoker are measured,which show that the peak exhaled NO concentration of the former is～25%lower than that of the later.The inhalation of excessive exogenous NO may inhibit the production of intracellular nitric oxide synthase which in turn leads to a decrease in exhaled gas NO concentration and increase the risk of respiratory infections.