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

Triple oxygen isotope composition (Δ'O) of sulfate carries information on the sulfur cycle as well as signatures of atmospheric O, O, 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 O, which results in isotope fractionation and impedes the Δ'O comparison with other oxygen-bearing compounds (e.g., HO) on the VSMOW-SLAP scale. Here, we present an analytical method involving high-temperature graphite reduction, CO discharge, and Pt-catalyzed CO-O isotope exchange, the R-D-E method, that achieves ∼100% conversion of sulfate oxygen to CO. Compared with the widely used CO-laser-fluorination technique, our R-D-E method has a better Δ'O precision (9 per meg) and requires a smaller sample amount, as low as 4 μmol of sulfate (equivalent to 1 mg BaSO), with both Δ'O and δO 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'O values measured on the partially yielded O 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., F or BrF). Incomplete conversion of sulfate oxygen during the high-temperature graphite reduction would result in lower δO and higher Δ'O values. This work highlights the needs for recalibration and reinterpretation of the published sulfate Δ'O data when sulfate Δ'O values of interest are small.