Generation of O2 from BaSO4 Using a CO2−Laser Fluorination System for Simultaneous Analysis of δ18O and δ17O

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Abstract
With the observation of mass-independent isotopic anomalies in numerous atmospheric molecules, the ability to measure both δ17O and δ18O in a range of samples is needed. Sulfate oxygen isotopic studies conventionally report only δ18O values. Recent findings indicate that sulfate δ17O and δ18O values, particularly the Δ17O value (= δ17O − (0.52)(δ18O)), can provide independent information on the origin, mixing, and transformation of sulfate in the atmospheric and surface environments, which is not resolvable by only δ18O measurements. Existing methods for analyzing sulfate δ17O and δ18O are extremely laborious and demand high-purity BrF5. Here we report a novel method of generating O2 directly from Barite (BaSO4) for simultaneous analysis of δ18O and δ17O by isotope ratio mass spectrometry (IRMS). The method utilizes a CO2−laser fluorination system that can also be used to quantitatively generate O2 from silicates and oxides. Partial but consistent oxygen yields from BaSO4 are obtained for samples >4 mg. Correction factors of +9.4‰ for δ18O and 4.89‰ for δ17O are obtained, and there is no deviation in the Δ17O value due to the nonquantitative O2 generation. The system may process more than a dozen samples per working day, with analytical error of ±0.05‰ and ±0.8‰ for Δ17O and δ18O, respectively. This new method is ideal for studies emphasizing an accurate sulfate Δ17O value.