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.