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

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 (CO2)-C-12-O-16 + (COO)-C-13-O-16-O-18 reversible arrow (CO2)-C-13-O-16 + (COO)-C-12-O-16-O-18 exchange reaction. CO2 samples are trapped in a low volume (similar to 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 parts per thousand (SE) is achieved within 20 min for 15 mu mol samples. Similar precision is also achieved for bulk isotopic composition. The degree of rare isotope clumping in excess of strictly random distribution (Delta(16O13C18O)) measured by this TILDAS instrument varies linearly with theoretically calculated values and shows a very weak dependence on bulk isotopic composition.