Preparation of high-precision CO 2 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 CO ₂ with known triple oxygen isotope compositions. The major experimental steps include: (1) the O ₂ is combusted to CO ₂ on a graphite rod at 750 °C with Pt-catalyst for 3-4 min; and (2) converted CO ₂ is subsequently purified by two cryogenic traps. The results show high reproducibility of δ ¹³ C and δ ¹⁸ O values of the converted CO ₂ within 0.010-0.020 ‰ and 0.006-0.010 ‰ (1 σ , SD), and the identical δ ¹⁸ O value within error with that of the original O ₂ . Additionally, we have measured the triple oxygen isotope compositions of converted CO ₂ using an O ₂ -CO ₂ Pt-catalyzed oxygen-isotope equilibration method. The measured δ ¹⁷ O values of CO ₂ show high reproducibility within 0.006 ‰ (1 σ , SD), and are identical within error with the original O ₂ as well. Notably, our experiments also found that the O ₂ with heavier oxygen isotope ratios ( δ ¹⁸ O > 40 ‰, VSMOW) might have a lesser conversion efficiency, and this effect, combined with the lighter isotope preferential fractionations during the reaction processes of O ₂ to CO and CO to CO ₂ , may explain the observed lower ¹⁷ O/ ¹⁶ O and ¹⁸ O/ ¹⁶ O ratios of the converted CO ₂ relative to the original O ₂ .