Quantitative Conversion of Sulfate Oxygen for High-Precision Triple Oxygen Isotope Analysis

Triple oxygen isotope composition (Δ'¹⁷O) of sulfate carries information on the sulfur cycle as well as signatures of atmospheric O₂, O₃, and surface water of the geological past. However, existing analytical techniques can only achieve a partial oxygen yield during the conversion from sulfate to the analyte O₂, which results in isotope fractionation and impedes the Δ'¹⁷O comparison with other oxygen-bearing compounds (e.g., H₂O) on the VSMOW-SLAP scale. Here, we present an analytical method involving high-temperature graphite reduction, CO discharge, and Pt-catalyzed CO₂-O₂ isotope exchange, the R-D-E method, that achieves ∼100% conversion of sulfate oxygen to CO₂. Compared with the widely used CO₂-laser-fluorination technique, our R-D-E method has a better Δ'¹⁷O precision (9 per meg) and requires a smaller sample amount, as low as 4 μmol of sulfate (equivalent to 1 mg BaSO₄), with both Δ'¹⁷O and δ¹⁸O being determined simultaneously. Using this method, we report the triple oxygen isotope compositions of three international reference materials (NBS-127, IAEA-SO-5, and IAEA-SO-6) and three in-house references on the VSMOW-SLAP scale. We found that the sulfate Δ'¹⁷O values measured on the partially yielded O₂ from the fluorination technique are 10-200 per meg higher than those obtained by our R-D-E method, depending on the oxidant used (e.g., F₂ or BrF₅). Incomplete conversion of sulfate oxygen during the high-temperature graphite reduction would result in lower δ¹⁸O and higher Δ'¹⁷O values. This work highlights the needs for recalibration and reinterpretation of the published sulfate Δ'¹⁷O data when sulfate Δ'¹⁷O values of interest are small.