Yubing Yang, X.D Sheng, Guangzhen Gao, Hui Yang, Tingdong Cai
High-sensitivity measurements of both molecular concentrations and isotope ratios ( 13 C/ 12 C) of CO 2 and CH 4 are crucial for identifying their sources and tracking their dynamics within the carbon cycle. To achieve high-precision and simultaneous 13 C/ 12 C ratio analysis of CO 2 and CH 4, we developed, to the best of our knowledge for the first time, a sensor system based on frequency-division multiplexed off-axis cavity-enhanced absorption spectroscopy (FDM-OA-CEAS). The system enables concurrent detection of four isotopic species ( 12 CO 2, 13 CO 2, 12 CH 4, and 13 CH 4 ), allowing direct determination of δ 13 C for both CO 2 and CH 4 . Benefiting from a kilometer-scale effective absorption path length, two pairs of CO 2 and CH 4 isotope absorption lines with relatively weak near-infrared line strengths were selected as sensing targets. Each pair of absorption lines can be acquired in a single laser scan, improving detection efficiency and reducing measurement cost. For the four isotopic species, the detection limits are 0.109 ppm for 12 CO 2, 0.025 ppb for 13 CO 2, 0.030 ppb for 12 CH 4, and 0.008 ppb for 13 CH 4 . Furthermore, the detection sensitivity of δ 13 C reaches 0.0108 ‰ for CO 2 and 0.086 ‰ for CH 4 with an integration time of 720 s and 1200 s. These results demonstrate the system’s capability for simultaneous, high-precision isotope analysis, highlighting its potential as a practical analytical chemistry tool for environmental and isotopic studies.