A Method for Rapid and Precise Triple Oxygen Isotope Measurements via High-Temperature Conversion to CO Followed by Nickel-Catalyzed CO to CO2 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.