Preparation of high-precision CO₂ with known triple oxygen isotope for oxygen isotope analysis

The objective of this work is to propose a more effective way to prepare an in-house CO₂ with known triple oxygen isotope compositions. The major experimental steps include: (1) the O₂ is combusted to CO₂ on a graphite rod at 750 °C with Pt-catalyst for 3-4 min; and (2) converted CO₂ is subsequently purified by two cryogenic traps. The results show high reproducibility of δ¹³C and δ¹⁸O values of the converted CO₂ within 0.010-0.020 ‰ and 0.006-0.010 ‰ (1σ, SD), and the identical δ¹⁸O value within error with that of the original O₂. Additionally, we have measured the triple oxygen isotope compositions of converted CO₂ using an O₂-CO₂ Pt-catalyzed oxygen-isotope equilibration method. The measured δ¹⁷O values of CO₂ show high reproducibility within 0.006 ‰ (1σ, SD), and are identical within error with the original O₂ as well. Notably, our experiments also found that the O₂ with heavier oxygen isotope ratios (δ¹⁸O > 40 ‰, VSMOW) might have a lesser conversion efficiency, and this effect, combined with the lighter isotope preferential fractionations during the reaction processes of O₂ to CO and CO to CO₂, may explain the observed lower ¹⁷O/¹⁶O and ¹⁸O/¹⁶O ratios of the converted CO₂ relative to the original O₂.