Near-infrared tunable diode laser absorption spectroscopy-based determination of carbon dioxide in human exhaled...

A spectroscopic detection system for the accurate monitoring of carbon dioxide (CO₂) in exhaled breath was realized by tunable diode laser absorption spectroscopy (TDLAS) in conjunction with a vertical-cavity surface-emitting laser (VCSEL) and a multipass cell with an effective optical path-length of 20 m. The VCSEL diode emitting light with an output power of 0.8 mW, covered the strong absorption line of CO₂ at 6330.82 cm^-1 by drive-current tuning. The minimum detectable concentration of 0.769‰ for CO₂ detection was obtained, and a measurement precision of approximately 100 ppm was achieved with an integration time of 168 s. Real-time online measurements were carried out for the detection of CO₂ expirograms from healthy subjects, different concentrations were obtained in dead space and alveolar gas. The exhaled CO₂ increased significantly with the increasing physical activity, reaches its maximal value at the beginning of respiratory compensation and then decreased slightly until maximal exercise. The developed measurement system has a great potential to be applied in practice for the detection of pulmonary diseases associated with CO₂ retention.