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

Measurements of delta(C-13) determined on CO2 with an isotope-ratio mass spectrometer (IRMS) must be corrected for the amount of O-17 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) 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 lambda, which relates differences in O-17 and O-18 abundances. With the currently accepted N(C-13)/N(C-12) 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(O-17)/N(O-16) of the evolved CO2. The ratio of these quantities, a ratio of isotope ratios, is essential for the O-17 abundance correction: [N(O-17)/N(O-16)/[N(C-13)/N(C-12)] = 0.035 16(8). The equation [delta(C-13) approximate to (45)delta(VPDB-CO2) + 2 R-17/R-13 ((45)delta(VPDB-Ca2) - lambda(46)delta(VPDB-CO2))]closely approximates delta(C-13) values with less than 0.010 % deviation for normal oxygenbearing materials and no more than 0.026 `700 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 delta(O-18). The linear approximations are easy to implement in a data spreadsheet, and also help in generating a simplified uncertainty budget.