High-Precision Triple Oxygen Isotope Analysis of Carbon Dioxide by Tunable Infrared Laser Absorption Spectroscopy

Abstract

Precision measurements of the stable isotope ratios of oxygen (¹⁸O/¹⁶O and ¹⁷O/¹⁶O) in CO₂ are critical to atmospheric monitoring and terrestrial climate research. High-precision ¹⁷O measurements by isotope ratio mass spectrometry (IRMS) are challenging because they require complicated sample preparation procedures, long measurement times, and relatively large samples sizes. Recently, tunable infrared laser direct absorption spectroscopy (TILDAS) has shown significant potential as an alternative technique for triple oxygen isotope analysis of CO₂, although the ultimate level of reproducibility is unknown, partly because it is unclear how to relate TILDAS measurements to an internationally accepted isotope abundance scale (e.g., VSMOW2-SLAP2). Here, we present a method for high-precision triple oxygen isotope analysis of CO₂ by TILDAS, requiring ∼8-9 μmol of CO₂ (or 0.9 mg carbonate) in 50 min, plus ∼1.5 h for sample preparation and dilution of CO₂ in N₂ to a nominal 400 μmol mol^-1. Overall reproducibility of Δ'¹⁷O (CO₂) was 0.004‰ (4 per meg) for IAEA603 (SE, n = 6) and 10 per meg for NBS18 (SE, n = 4). Values corrected to the VSMOW2-SLAP2 scale are in good agreement with established techniques of high-precision IRMS, with the exception of Δ'¹⁷O measured by platinum-catalyzed exchange of CO₂ with O₂. Compared to high-precision IRMS, TILDAS offers the advantage of $\sim 10$ times less sample, and greater throughput, without loss of reproducibility. The flexibility of the technique should allow for many important applications to global biogeochemical monitoring and investigation of ¹⁷O anomalies in a range of geological materials.