Laser Absorption Spectroscopy of Rare and Doubly Substituted Carbon Dioxide Isotopologues

Unambiguous detection of the clumped carbon dioxide isotopologue 13C16O18O 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 12C16O2 + 13C16O18O ↔ 12C16O18O + 13C16O2 and of 12C16O17O. The achieved precision of 2 × 10-5 for the 13C16O18O/13C16O2 and 12C16O18O/12C16O2 isotopologue ratios allows the equilibrium constant K of the isotope exchange reaction to be determined with an external reproducibility of better than 2 × 10-5 (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 δ13C and δ18O and 0.15‰ (1σ) for δ17O. 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 CO2 samples of ∼100 μmol.