Laser absorption spectroscopy of rare and doubly-substituted carbon dioxide isotopologues.

Unambiguous detection of the clumped carbon dioxide isotopologue (COO)-C-13-O-16-O-18 with isotope ratio mass spectrometry is difficult due to isobaric interference on m/z = 47. We present an analytical technique based on direct absorption laser spectroscopy for precise, direct and simultaneous detection of all isotopologues involved in the isotope exchange reaction (CO2)-C-12-O-16 + (COO)-C-13-O-16-O-18 <-> (COO)-C-12-O-16-O-18 + (CO2)-C-13-O-16 and of (COO)-C-12-O-16-O-17. The achieved precision of 2 x 10(-5) for the (COO)-C-13-O-16-O-18/(CO2)-C-13-O-16 and (COO)-C-12-O-16-O-18/(CO2)-C-12-O-16 isotopologue ratios allows the equilibrium constant K of the isotope exchange reaction to be determined with an external reproducibility of better than 2 x 10(-5) (1 sigma) after 9 reference-sample comparisons. Each comparison requires 7 min. The isotopic composition of the pure gas can be simultaneously analyzed with a precision of 0.05 parts per thousand (1 sigma) for delta C-13 and delta O-18 and 0.15 parts per thousand (1 sigma) for delta O-17. The instrument deploys two interband cascade lasers (ICL) with center wavelengths of 4.3 and 4.4 mu m. A custom-built optical cell has been designed for single pass and multi pass optical paths (path length ratio 1:100); it allows simultaneous detection of rare and abundant isotopologues. The setup is capable to analyze pure CO2 samples of similar to 100 mu mol.