Laser Absorption Spectroscopy of Rare and Doubly Substituted Carbon Dioxide Isotopologues

Abstract

Unambiguous detection of the clumped carbon dioxide isotopologue 13 C 16 O 18 O 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 12 C 16 O 2 + 13 C 16 O 18 O ↔ 12 C 16 O 18 O + 13 C 16 O 2 and of 12 C 16 O 17 O. The achieved precision of 2 × 10 -5 for the 13 C 16 O 18 O/ 13 C 16 O 2 and 12 C 16 O 18 O/ 12 C 16 O 2 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 δ 13 C and δ 18 O and 0.15‰ (1σ) for δ 17 O. 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 2 samples of ∼100 μmol.