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

We report on infrared laser spectroscopic measurements of the isotopic comp osition of methane ((CH4)-C-12, (CH4)-C-13) in natural air samples with a c avity ring-down technique. A CO overtone sideband laser is utilized to exci te a high-finesse cavity which provides an effective optical absorption pat h length of 3.6 km. We achieved a detection limit of 105 ppt methane in amb ient air using an integration time of 20 s. This corresponds to a minimum d etectable absorption of 1.9 x 10(-9) /cm. Rapid determination of the C-13/C -12 isotopic ratio of methane in ambient air without sample preconcentratio n or gas processing is realized. The present system requires only few minut es for an isotopic ratio measurement with a precision of 11 parts per thous and.