Exhaled Breath Detection Using Hollow Waveguide Based Tunable Absorption Laser Spectroscopy

Exhaled breath analysis offers noninvasive disease diagnostics and metabolic status monitoring.Monitoring carbon dioxide concentration during the process of breathing has a great importance in the field of medical diagnostics.Methane is related to intestinal problems,including lactose intolerance,intestinal canal and obesity.In this study,the application of tunable diode laser in the measurement of exhaled CO2 and CH4 was demonstrated.Firstly,the analysis of hollow waveguide was presented.The Trace Pro was used for the simulation of the relationship between physical path length and the radius of bent hollow waveguide,the inner diameter was presented.In addition,the relationship between physical path length and the radius of bent hollow waveguide was verified by the measurement of physical path length in the bent hollow waveguide.And the effective optical path length(EOPL)is an important parameter according to the laws of Beer-Lambert.The measurement of EOPL for a 284.0 cm hollow waveguide,based on the absorption of CO2 was demonstrated.The repeatability error of effective optical path length was calculated as 0.08 cm and the detection limit was 0.057 cm.Secondly,the measurement of exhaled CO2 was conducted.Hollow waveguide served as the gas cell in the sensor.Meanwhile,an optimal CO2 line at 2003 nm was selected according to disturbance from main exhaled components.A sensor for exhaled CO2 was presented and a CO2 detection experiment was performed using real breath cycles.The optical fringe in the sensor was analyzed and suppressed.The detection limit was 60 ppm and linearity was 0.99916.Thirdly,a hollow waveguide(HWG)-enhanced tunable laser absorption spectroscopy sensor was demonstrated for the detection of exhaled methane in the mid-infrared range.Based on our experience of exhaled CO2 sensor and HWG,an evolved HWG sensor to detect exhaled CH4 was established.In addition,real-time experiments involving human exhaled methane were performed in the laboratory and the concentration of exhaled CH4 after drink was compared with the concentration before drink.The detection limit and linearity for the system was presented.The influence of temperature and pressure was studied.And the detection limit reaches 48 ppb at 0.1 s for the sensor,which was with high sensitivity and high time resolution.In conclusion,the sensor using HWG based on TLAS was suitable for exhaled breath detection.The shortcoming of the traditional gas cell,the volume of which is large,was overcome and real-time measurement of exhaled CO2 and CH4 was derived.It is easy to apply in other exhaled breath detection by changing the hollow waveguide.