Correction for the 17O interference in δ(13C) measurements when analyzing CO2 with stable isotope mass spectrometry

Abstract

Measurements of δ( 13 C) determined on CO 2 with an isotope-ratio mass spectrometer (IRMS) must be corrected for the amount of 17 O in the CO 2 . For data consistency, this must be done using identical methods by different laboratories. This report aims at unifying data treatment for CO 2 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 CO 2 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 17 O and 18 O abundances. With the currently accepted N( 13 C)/N( 12 C) of 0.011 180(28) in VPDB (Vienna Peedee belemnite) reevaluation of data yields a value of 0.000 393(1) for the oxygen isotope ratio N( 17 O)/N( 16 O) of the evolved CO 2 . The ratio of these quantities, a ratio of isotope ratios, is essential for the 17 O abundance correction: [N( 17 O)/N( 16 O)]/[N( 13 C)/N( 12 C)] = 0.035 16(8). The equation [δ( 13 C) ≈ 45 δ VPDB-CO2 + 2 17 R/ 13 R ( 45 δ VPDB-CO2 – λ 46 δ VPDB-CO2 )] closely approximates δ( 13 C) values with less than 0.010 ‰ deviation for normal oxygen-bearing 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( 18 O). The linear approximations are easy to implement in a data spreadsheet, and also help in generating a simplified uncertainty budget.