High Precision Methane Isotopologue Ratio Measurement at Ambient Mixing Ratios Using Integrated Cavity Output Spectroscopy

M.F. Witinski, D.S. Sayres, and J.G. Anderson, High Precision Methane Isotopologue Ratio Measurement at Ambient Mixing Ratios Using Integrated Cavity Output Spectroscopy, App. Phys. B: Lasers and Optics, doi: 10.1077/s00340-010-3957-2, March 2010.

The demonstration of an ultra-sensitive infrared spectro- scopic technique for analysis of carbon isotopologues of methane is presented. Using off-axis integrated cavity output spectroscopy (ICOS) near 1294 cm−1, we have achieved a δ13C measurement precision of 1.5‰ in 1 s and 0.21‰ in 200 s while sampling dry air with a 12CH4 mixing ratio of 1.846 ppmv. This method will allow for high time response, high-sensitivity stable isotope analysis in the troposphere without the need for sample preconcentration, and could have major implications for better understanding of carbon-cycle dynamics. This work also addresses the relationship between the scanning frequency of the laser, the characteristic cavity frequency, and the line shapes of spectral features in ICOS.