Development and optimization of an OF-CEAS system for stable isotopic ratio analysis of methane ( δ ¹³ C-CH ₄ ) in the mid-infrared

High-precision analysis of the stable isotopic composition of atmospheric methane (CH4) is essential for attributing its sources and sinks and for a more precise understanding of the global methane cycle. Although conventional isotope ratio mass spectrometry (IRMS) provides high accuracy, it lacks in situ capabilities and provides only low data rates. Tunable laser spectrometers offer higher acquisition rates and high sensitivity. In accordance with the Beer–Lambert law, the absorption signal increases proportionally with the optical path length, requiring kilometer-scale paths for atmospheric CH4 detection and thus the use of optical cavities. Here, we present an optical feedback cavity-enhanced absorption spectroscopy (OF-CEAS) system for determining the isotopic ratio (δ13C) of the stable isotopologues 12CH4 and 13CH4 in the mid-infrared range at 3000 cm−1 (3333 nm wavelength). The optical setup comprises a high-finesse V-shaped cavity with highly reflective mirrors (r=0.999893), resulting in a theoretically effective path length of 22.53 km, which is integrated into an Invar cell with active temperature and pressure stabilization. Allan deviation analysis of the temperature regulation shows a minimum of σ=8.85 µK at an integration time of τ=1.6×104s, and for the gas temperature inside the cell, it gives a minimum of σ=600 µK at τ=6 s, which shows thermal stability that is compatible with an uncertainty in the isotopic ratio of ≤0.1 ‰ (Bergamaschi et al., 1994). Spectral measurements confirm active cavity locking, a symmetric mode structure and frequency calibration using a germanium etalon and a methane reference cell. The spectral resolution of the measurement is given by the free spectral range (FSR) of the cavity with FSR≈84 MHz. The system presented here demonstrates the stability and resolution required for high-precision isotopic methane analysis.