Simultaneous High-Precision ¹³C Isotope Ratio Analysis of CO₂ and CH₄ Using an FDM-OA-CEAS Sensor

High-sensitivity measurements of both molecular concentrations and isotope ratios (¹³C/¹²C) of CO₂ and CH₄ are crucial for identifying their sources and tracking their dynamics within the carbon cycle. To achieve high-precision and simultaneous ¹³C/¹²C ratio analysis of CO₂ and CH₄, 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 (¹²CO₂, ¹³CO₂, ¹²CH₄, and ¹³CH₄), allowing direct determination of δ¹³C for both CO₂ and CH₄. Benefiting from a kilometer-scale effective absorption path length, two pairs of CO₂ and CH₄ 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 ¹²CO₂, 0.025 ppb for ¹³CO₂, 0.030 ppb for ¹²CH₄, and 0.008 ppb for ¹³CH₄. Furthermore, the detection sensitivity of δ¹³C reaches 0.0108 ‰ for CO₂ and 0.086 ‰ for CH₄ 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.