Assessment of the accuracy of the 17 O correction algorithm used in δ13 C determinations by CO2 mass spectrometry.

Abstract

High-accuracy δ13 C 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 δ13 C values. δ13 C determinations by mass spectrometry use an 17 O isobaric correction on m/z 45, where 17 O abundance is calculated from the measured 18 O with the 17 O-18 O 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 δ13 C biases have not been verified using 17 O measurements. To verify the 17 O correction and to estimate potential δ13 C biases, we compile measured 17 O 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. δ13 C biases are calculated based on 17 O deviation from the 17 O-18 O relationship assumed in the 17 O correction. To estimate δ13 C biases accurately, the VPDB-CO2 framework for expressing 17 O excess is defined and linked to the δ13 C scale definition. δ13 C 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). For the first time, high accuracy of δ13 C data obtained by CO2 mass spectrometry using the 17 O correction with fixed λ = 0.528 has been demonstrated using measured 17 O data. δ13 C biases estimated are within the best IRMS precision (±0.014‰, k = 2) and can be neglected in most practical applications. To obtain high-quality δ13 C data, it is strictly necessary that all data are treated on the VPDB-CO2 scale.