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

Rationale

High-accuracy δ
¹³C values are required for observations of greenhouse gases CO₂ and methane, and, consequently, for international reference materials (RMs). Recently, another application, clumped isotope geothermometry of natural carbonates, has demonstrated the requirement for high-accuracy δ
¹³C values. δ
¹³C 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 δ
¹³C biases have not been verified using ¹⁷O measurements.

Methods

To verify the ¹⁷O correction and to estimate potential δ
¹³C biases, we compile measured ¹⁷O data for carbonate RMs, for a range of natural carbonates that are typically analyzed in clumped isotope geothermometry and for CO₂ in isotope equilibrium with natural waters including plants and biota. δ
¹³C biases are calculated based on ¹⁷O deviation from the ¹⁷O–¹⁸O relationship assumed in the ¹⁷O correction.

Results

To estimate δ
¹³C biases accurately, the VPDB-CO₂ framework for expressing ¹⁷O excess is defined and linked to the δ
¹³C scale definition. δ
¹³C 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 isotope ratio mass spectrometry (IRMS) (around ±0.014‰, k = 2).

Conclusions

For the first time, high accuracy of δ
¹³C data obtained by CO₂ mass spectrometry using the ¹⁷O correction with fixed λ = 0.528 has been demonstrated using measured ¹⁷O data. δ
¹³C biases estimated are within the best IRMS precision (±0.014‰, k = 2) and can be neglected in most practical applications. To obtain high-quality δ
¹³C data, it is strictly necessary that all data are treated on the VPDB-CO₂ scale.