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Licensed Unlicensed Requires Authentication Published by De Gruyter 2016 (Print 2010)

Correction for the 17O Interference in Δ(13C) Measurements When Analyzing Co2 With Stable Isotope Mass Spectrometry[*]

  • Willi A. Brand , Sergey S. Assonov and Tyler B. Coplen

Abstract

Measurements of δ(13C) determined on CO2 with an isotope-ratio mass spectrometer (IRMS) must be corrected for the amount of 17O in the CO2. For data consistency, this must be done using identical methods by different laboratories. This report aims at unifying data treatment for CO2 IRMS by proposing (i) a unified set of numerical values, and (ii) a unified correction algorithm, based on a simple, linear approximation formula. Because the oxygen of natural CO2 is derived mostly from the global water pool, it is recommended that a value of 0.528 be employed for the factor λ, which relates differences in 17O and 18O abundances. With the currently accepted N(13C)/N(12C) of 0.011180(28) in VPDB (Vienna Peedee belemnite) reevaluation of data yields a value of 0.000393(1) for the oxygen isotope ratio N(17O)/N(16O) of the evolved CO2. The ratio of these quantities, a ratio of isotope ratios, is essential for the 17O abundance correction: [N(17O)/N(16O)]/[N(13C)/N(12C)] = 0.03516(8).

The equation [δ(13C)45δVPDBCO2+217R/13R (45δVPDBCO2λ46δVPDBCO2)] closely approximates δ(13C) values with less than 0.010 ‰ deviation for normal oxygenbearing materials and no more than 0.026 ‰ in extreme cases. Other materials containing oxygen of non-mass-dependent isotope composition require a more specific data treatment. A similar linear approximation is also suggested for δ(18O). The linear approximations are easy to implement in a data spreadsheet, and also help in generating a simplified uncertainty budget.

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