Simultaneous detection of C2H6, CH4, and δ13C-CH4 using optical feedback cavity-enhanced absorption spectroscopy in the mid-infrared region: towards application for dissolved gas measurements

Simultaneous measurement of C2H6 and CH4 concentrations, and of the delta C-13-CH4 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 mu m radiation from an inter-band cascade laser (ICL) into a V-shaped high-finesse optical cavity. A minimum absorption coefficient of 2.8 x 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 C2H6, CH4, and delta C-13-CH4, 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.