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

Precision measurements of the stable isotope ratios of oxygen (O/O and O/O) in CO are critical to atmospheric monitoring and terrestrial climate research. High-precision O 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 CO, 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 CO by TILDAS, requiring ∼8-9 μmol of CO (or 0.9 mg carbonate) in 50 min, plus ∼1.5 h for sample preparation and dilution of CO in N to a nominal 400 μmol mol. Overall reproducibility of Δ'O (CO) was 0.004‰ (4 per meg) for IAEA603 (SE, = 6) and 10 per meg for NBS18 (SE, = 4). Values corrected to the VSMOW2-SLAP2 scale are in good agreement with established techniques of high-precision IRMS, with the exception of Δ'O measured by platinum-catalyzed exchange of CO with O. Compared to high-precision IRMS, TILDAS offers the advantage of 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 O anomalies in a range of geological materials.