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Real-time measuring of CO 2 isotopes ( 13 CO 2 , 12 CO 2 , and 18 OC 16 O) in exhaled breath using a mid-infrared hollow waveguide gas sensor incorporating a 2.73 μm distributed feedback laser was proposed and demonstrated for the first time based on calibration-free wavelength modulation spectroscopy. The measurement precisions for δ 13 C and δ 18 O were, respectively, 0.26 and 0.57‰ for an integration time of 131 s by Allan variance analysis. These measurement precisions achieved in the present work were at least 3.5 times better than those reported using direct absorption spectroscopy and 1.3 times better than those obtained by calibration-needed wavelength modulation absorption spectroscopy. Continuous measurement of three isotopes in the breathing cycle was performed. Alveolar gas from the expirogram was identified, and the 13 C/ 12 C and 18 O/ 16 O ratios were found to be almost constant during the alveolar plateau, which enables optimization of breath sampling and provides accurate information on metabolic processes. The 13 C/ 12 C and 18 O/ 16 O isotope ratios at the alveolar plateau of five breath cycles were averaged, yielding δ 13 C and δ 18 O values of (-24.3 ± 3.4) and (-30.7 ± 2.6) ‰, respectively. This study demonstrates the feasibility of real-time analysis of 13 C- and 18 O-isotopes of human breath CO 2 in clinical applications and shows its potential for diagnosing respiratory-related diseases with high sensitivity, selectivity, and specificity.

Real-Time Monitoring of 13C- and 18O-Isotopes of Human Breath CO2 Using a Mid-Infrared Hollow Waveguide Gas Sensor

Real-Time Monitoring of ¹³C- and ¹⁸O-Isotopes of Human Breath CO₂ Using a Mid-Infrared Hollow Waveguide Gas Sensor