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.