| With the accelerating process of urbanization and industrialization,the emission of nitrogen oxides seriously threatens human health.Among them,nitric oxide has a complex set of effects on tropospheric atmospheric chemical reactions,which lead to photochemical smog,increased tropospheric ozone,acid deposition,nitrate aerosols,and sulfate formation.As the culprit of various environmental problems,nitric oxide has been a research hotspot for a long time.Not only that,but nitric oxide also plays an important role in medical diagnosis.With the continuous development of gas detection technology,the biological functions of nitric oxide have been gradually excavated,including vascular tone regulation,inflammatory response regulation,neurotransmission,immune regulation,stimulation of pro-inflammatory cytokine expression and antibacterial effect.It has important value in the diagnosis of ocular surface diseases,lung cancer,asthma,chronic obstructive pulmonary disease and other diseases.However,the current nitric oxide detection technology has problems such as low sensitivity,complex structure,high price,and difficult calibration,which seriously affect its development.Therefore,it is urgent to develop a nitric oxide detection device with high sensitivity,simple structure,low cost and no calibration.Tunable diode laser absorption spectroscopy（TDLAS）technology has the characteristics of simple experimental setup,high detection sensitivity,fast response speed,high gas selectivity,high system stability,and low operating cost.After decades of development,gas detection devices based on TDLAS technology continue to emerge,which has always been a research hotspot in various gas concentration detection fields.Accordingly,this thesis uses TDLAS technology to build a nitric oxide detection system,and conducts a detailed study of the system.The overall arrangement of this thesis is as follows:firstly,the basic principle of TDLAS gas concentration detection is introduced.Secondly,the distribution of line intensity was analyzed based on HITRAN2008 database,and the maximum absorption line intensity was found at the base band of nitric oxide（5.2μm）.The absorption spectrum of nitric oxide in the fundamental frequency band was simulated.Taking the detection environment of human exhaled gas as an example,water（6%）and carbon dioxide（5%）will affect the absorption spectrum of nitric oxide.Therefore,the absorption line which is not interfered by water gas and carbon dioxide is selected,and the dual R（6.5）absorption line with the fundamental frequency band at 1900.07 cm-1is selected to detect the concentration of nitric oxide.Thirdly,a nitric oxide detection system was built based on Quantum Cascade Laser Absorption Spectroscopy（QCLAS）technology.Considering the factors such as laser cost,power and integration,the distributed feedback continuous-wave single-mode quantum cascade laser is adopted,with the wavelength tunable range of1899.33～1906.21 cm-1,and the maximum output power of 16.54 m W.The semiconductor cooled Hg Cd Te mid-infrared detector with peak response of 5～6μm is used.In order to improve the detection sensitivity,the self-developed Herriott optical multi-pass cell is used with compact structure,small volume,and optical path of 26.88 m.At the same time,the pressure sensor and temperature and humidity sensor are integrated to facilitate the monitoring of experimental conditions.Fourthly,the optimal pressure of the system was0.3 atm and the optimal modulation amplitude was 0.029 cm-1through the nitric oxide standard gas detection experiment.Different concentrations of nitric oxide gas were prepared by mass flowmeter,and the concentration of nitric oxide was calibrated.The linearity is 99.99%,and the detection sensitivity was 5.71 ppm/V;then the Allan variance analysis was carried out after the continuous operation of the system for 8 h,and the detection limit of the system was 12.05 ppb at 9.20 s.Fifthly,one healthy volunteer was tested for exhaled nitric oxide 5 times within 1 h,and the average peak value was 0.43 V,indicating the feasibility of this system for detecting exhaled nitric oxide in human body.This thesis successfully developed a high sensitivity and high precision nitric oxide detection system based on QCLAS,which can be widely used in the detection of trace nitric oxide in various environments,and lay a foundation for the application of exhaled nitric oxide detection in clinical diagnosis in the future. |
