Isotopic ratio measurement of methane in ambient air using mid-infrared cavity leak-out spectroscopy

Abstract

We report on infrared laser spectroscopic measurements of the isotopic composition of methane (12CH4, 13CH4) in natural air samples with a cavity ring-down technique. A CO overtone sideband laser is utilized to excite a high-finesse cavity which provides an effective optical absorption path length of 3.6 km. We achieved a detection limit of 105 ppt methane in ambient air using an integration time of 20 s. This corresponds to a minimum detectable absorption of 1.9 × 10-9 /cm. Rapid determination of the 13C/12C isotopic ratio of methane in ambient air without sample preconcentration or gas processing is realized. The present system requires only few minutes for an isotopic ratio measurement with a precision of 11‰.