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.