Assessment of the accuracy of the 17O correction algorithm used in δ13C determinations by CO2 mass‐spectrometry.

RATIONALE

High accuracy δ¹³C values are required for observations of greenhouse gases CO₂ and methane, and, consequently, for international reference materials (RMs). Recently, another application, clumped isotope geothermometry on natural carbonates, has demonstrated the requirement for high-accuracy of δ¹³C. δ¹³C determinations by mass-spectrometry use an ¹⁷O isobaric correction on m/z 45, where ¹⁷O abundance is calculated from the measured ¹⁸O with the ¹⁷O-¹⁸O relationship assumed with λ=0.528. This relationship is the key assumption of the algorithm proposed in 2003 and accepted by IUPAC in 2010. However, to date, this relationship and potential δ¹³C biases have not been verified using ¹⁷O measurements.

METHODS

To verify the ¹⁷O correction and estimate potential δ¹³C biases, we compile measured ¹⁷O data for carbonate RMs, a range of natural carbonates which are typically analyzed in clumped isotope geothermometry, and CO₂ in isotope equilibrium with natural waters including plants and biota. δ¹³C biases are calculated based on ¹⁷O deviation from ¹⁷O-¹⁸O relationship assumed in the ¹⁷O correction.

RESULTS

To estimate δ¹³C biases accurately, VPDB-CO₂ framework for expressing ¹⁷O-excess is defined and linked to the δ¹³C scale-definition. δ¹³C biases estimated for carbonate RMs are found within ±0.004 ‰, the biases estimated for natural carbonates and CO₂ in equilibrium with natural waters are mostly within ±0.010 ‰ (bidirectional distribution around zero). In all cases, the estimated biases are found within the best instrumental uncertainty of modern IRMS (around ±0.014 ‰, k=2).

CONCLUSIONS

In the first time, high accuracy of δ¹³C data obtained by CO₂ mass-spectrometry using the ¹⁷O correction with fixed λ=0.528 has been demonstrated using measured ¹⁷O data. δ¹³C biases estimated are within the best IRMS precision (±0.014 ‰, k=2) and can be neglected in most practical applications. To obtain high quality δ¹³C data, it is strictly necessary that all data are treated on the VPDB-CO₂ scale.