Laser Absorption Spectroscopy of Rare and Doubly Substituted Carbon Dioxide Isotopologues.

Unambiguous detection of the clumped carbon dioxide isotopologue COO with isotope ratio mass spectrometry is difficult due to isobaric interference on / = 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 CO + COO ↔ COO + CO and of COO. The achieved precision of 2 × 10 for the COO/CO and COO/CO isotopologue ratios allows the equilibrium constant of the isotope exchange reaction to be determined with an external reproducibility of better than 2 × 10 (1σ) 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‰ (1σ) for δC and δO and 0.15‰ (1σ) for δO. The instrument deploys two interband cascade lasers (ICL) with center wavelengths of 4.3 and 4.4 μm. 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 CO samples of ∼100 μmol.