A Method for Rapid and Precise Triple Oxygen Isotope Measurements via High-Temperature Conversion to CO Followed by Nickel-Catalyzed CO to CO ₂ Conversion and Laser Spectroscopy

Triple oxygen isotopic compositions (¹⁶O, ¹⁷O, ¹⁸O) have conventionally been measured via isotope ratio mass spectrometry using O₂ as an analyte. Conversion of sample oxygen to O₂ typically utilizes fluorination chemistry or catalytic equilibration between CO₂ and O₂. Recently, laser spectroscopy has become a viable alternative for triple oxygen isotope (Δ'¹⁷O) measurements due to its ease and rapid throughput. Laser spectrometers are currently available for Δ'¹⁷O analysis of either H₂O or CO₂ as the analyte gas. So far, these instruments have been used to measure Δ'¹⁷O of water, carbonate (CO₂ liberated by acid digestion), and atmospheric CO₂ samples. We present a new method for high-precision Δ'¹⁷O analysis of CO₂ via tunable infrared laser direct absorption spectroscopy that is compatible with a wider range of geochemically important materials. This approach involves converting sample oxygen to CO₂ in two steps. First, the sample oxygen is liberated and reduced to CO by high-temperature conversion at 1450 °C in the presence of excess elemental carbon. Then, CO is catalytically converted to CO₂ over hot nickel at 350 °C. The conversion process is rapid (10 to 30 min) and quantitative. Spectroscopic Δ'¹⁷O analysis of the resulting CO₂ takes approximately 45 min. By measuring several oxygen isotope standards, we demonstrate that the method is precise (1σ = 12 per meg for procedural replicates) and accurate (within 11 per meg of previously reported values). The method can be applied to most pyrolytic materials where quantitative oxygen conversion is attainable, such as sulfate, phosphate, nitrate, and oxide minerals, water, and organic molecules.