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

Abstract

RationaleHigh‐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 of natural carbonates, has demonstrated the requirement for high‐accuracy δ13C values. 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.MethodsTo verify the 17O correction and to estimate potential δ13C 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. δ13C biases are calculated based on 17O deviation from the 17O–18O relationship assumed in the 17O correction.ResultsTo estimate δ13C biases accurately, the 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 isotope ratio mass spectrometry (IRMS) (around ±0.014‰, k = 2).ConclusionsFor 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.