Simultaneous Detection of C 2 H 6 , CH 4 and d 13 C-CH 4 Using Optical Feedback Cavity Enhanced Absorption Spectroscopy in the Mid-Infrared Region: Towards Application for Dissolved Gas Measurements

Abstract. Simultaneous measurement of C 2 H 6 and CH 4 concentrations, and of the δ 13 C-CH 4 isotope ratio is demonstrated using a cavity enhanced absorption spectroscopy technique in the mid-IR region. The spectrometer is compact and has been designed for field operation. It relies on optical-feedback assisted injection of 3.3-μm radiation from an Interband Cascade Laser (ICL) into a V-shaped high-finesse optical cavity. A minimum absorption coefficient of 2.8 × 10 −9 cm −1 is obtained in a single scan (0.1 s) over 0.7 cm −1 . Precisions of 3 ppbv, 11 ppbv, and 0.08 ‰ for C 2 H 6 , CH 4 , and δ 13 C-CH 4 , respectively, are achieved after 400 s of integration time. Laboratory calibrations and tests of performance are reported here. They show the potential for the spectrometer to be embedded in a sensor probe for in situ measurements in ocean waters, which could have important applications for the understanding of the source and fate of hydrocarbons from the seabed and in the water column.