High-Precision Synchronous Detection of Breath CO, CH4, and CO2 Using an NIR-WMS Sensor Based on a Dual-Channel Multipass Cell.

TDLAS-based breath sensors hold significant promise for clinical diagnostics because of their high selectivity, fast response, and operational simplicity. However, their application in high-precision, multicomponent breath analysis is often hindered by system complexity, high cost, and limited detection accuracy. To overcome these challenges, this study presents a near-infrared TDLAS system for the simultaneous monitoring of carbon monoxide (CO), carbon dioxide (CO2), and methane (CH4) in breath. The system integrates a spectral line optimization and selection strategy with a novel, miniaturized dual-path multipass cell and time-division multiplexing (TDM). To reduce the complexity of the system and improve the measurement accuracy, a closely spaced spectral line pair (CO: 4285.0 cm-1; CH4: 4284.5 cm-1) within the 2.3 μm band was selected for simultaneous detection using a single laser. The custom-designed multipass cell achieves an 88.52 m optical path within a 300 mL volume, ensuring both high sensitivity and compatibility with small breath samples. Furthermore, by leveraging the strong absorption lines of carbon dioxide within the 2 μm band, a dedicated 15 cm optical path was integrated into the multipass cell. Combined with the TDM technique, this approach enabled a compact dual-path, single-detector architecture, which significantly improved the integration level of the system. Laboratory evaluations demonstrated excellent linearity, with detection limits of 1.49 ppb for CO, 1.86 ppb for CH4, and 720.2 ppb for CO2, as determined by Allan deviation analysis. The system was successfully deployed in breath testing, effectively distinguishing end-tidal CO levels between smokers and nonsmokers, with concentration trends consistent with those in the established literature. This work validates a reliable and practical TDLAS sensor for real-time, multicomponent breath analysis, offering a viable pathway toward noninvasive, high-sensitivity diagnostic devices.