Rapid and Precise Analysis of Carbon Dioxide Clumped Isotopic Composition by Tunable Infrared Laser Differential Spectroscopy

Abstract

The high precision measurement of doubly substituted ("clumped") isotopologues in CO₂ is a topic of significant interest in isotope geochemistry. Here we describe the performance of a new isotope ratio laser spectrometer using tunable infrared laser differential absorption spectroscopy (TILDAS). The TILDAS instrument has two continuous-wave lasers to simultaneously measure the four isotopologues involved in the ¹²C¹⁶O₂ + ¹³C¹⁶O¹⁸O ⇆ ¹³C¹⁶O₂ + ¹²C¹⁶O¹⁸O exchange reaction. CO₂ samples are trapped in a low volume (∼250 mL) optical multipass cell with a path length of 36 m. Each sample is compared to a reference gas, and clumped isotopologue precision of 0.01‰ (SE) is achieved within 20 min for 15 μmol samples. Similar precision is also achieved for bulk isotopic composition. The degree of rare isotope clumping in excess of strictly random distribution (Δ_16O13C18O) measured by this TILDAS instrument varies linearly with theoretically calculated values and shows a very weak dependence on bulk isotopic composition.