Research On Breath Gas Detection Based On Wavelength Modulation Spectroscopy

Breath gas analysis is a medical field with broad application prospects,which can provide a nondestructive,real-time,high-sensitivity detection as well as the realization of fixed-point disease diagnosis and metabolic status monitoring.Laser spectroscopy not only has high sensitivity and high selectivity,but also can achieve the ability of real-time monitoring,which has a very important role in the field of medical diagnosis.This paper mainly introduces the theory of wavelength modulation spectroscopy?WMS?and its application in respiratory gas detection.A tunable distributed feedback?DFB?laser with a center wavelength of 2.73?m was used as a light source to build a breathing gas detector based on tunable diode laser absorption spectroscopy?TDLAS?.The single-pass cell with a length of about 88 cm was used as the gas chamber,and the spectral lines at 3659.402 cm^-1 and 3659.934 cm^-1 were selected for simultaneous measurement of CO₂ and water vapor in the respiratory gas.Through concentration calibration,the linear correlations of CO₂ and water vapor were 0.99945 and 0.99679,respectively,when the CO₂ and water vapor concentrations were less than 35%and2.3%,respectively.By optimizing the experimental parameters,the detection sensitivity of exhaled CO₂ and water vapor reached 0.433%and 137 ppmv(10^-6),respectively.In the real-time monitoring of respiratory cycle process,the changes of CO₂ and water vapor content were stable at 5.3%and 2.2%,respectively,and the response time of concentration was about 4.5 s.In the integral time of 17 min,the detection limit of CO₂ can reach 149 ppmv.The results show that the measurement system can measure the respiratory gas stably and accurately.However,the excessive volume of the single-pass cell is not conducive to respiratory gas analysis.In addition to consuming excessive respiratory gas,it also increases the concentration response time.In order to solve these problems,a hollow fiber with a length of 1 m is used to replace the traditional single-pass cell as the gas chamber.The inner diameter of hollow fiber is only 1 mm and the volume is 0.78 cm³,which can greatly save gas sample consumption and experimental space.By measuring different CO₂ concentration,it was found that the concentration response time was shorter in the process of the rise and fall,which were 1.4 s and 1 s respectively.In addition,the detection limit of exhaled CO₂ is 61 ppmv for a 16 s optimum integration time and the corresponding equivalent noise sensitivity is 7.7×10^-6 cm^-1Hz^-1/2.Compared with the direct absorption method,the detection sensitivity of WMS method is improved by about 22 times.The results show that the setup based on hollow fiber can shorten the concentration response time and increase the detection limit compared to the traditional single-pass cell.