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.
A simple and universal technique of the combination of uniaxial electrospinning with subsequent peculiar dual-crucible fluorination technology is innovatively designed and used to construct metallic fluorides yolk-shell nanofibers (YSNFs) and yolk-shell nanobelts (YSNBs) directly for the first time. As a case study, Yb 3+ /Er 3+ co-doped alkaline earth metal fluoride (BaF 2 , SrF 2 and CaF 2 ) as core layer and SiO 2 as shell layer are applied to construct the yolk-shell one-dimensional nanostructures . Under the optimum calcination temperature (600 °C) and time (5 h), the obtained MF 2 :Yb 3+ , Er 3+ /SiO 2 (M=Ba, Sr, Ca) samples are of pure cubic phase, and display the yolk-shell nanofibers and yolk-shell nanobelts with good size uniformity and green up-converting fluorescence. By HF acid treatment of the samples, it is not only proved that the shell layer is SiO 2 , but also independent core-layer MF 2 :Yb 3+ , Er 3+ (M=Ba, Sr, Ca) nanofibers and nanobelts with good size uniformity and excellent up-conversion luminescence are obtained. Especially, only fluorination process among the halogenation can form such exceptive YSNFs and YSNBs one-dimensional nanostructures , and the formation mechanisms of YSNFs and YSNBs are proposed and verified, and further the novel technique is established. The new technique possesses the merits of simple device and easy operation, large output, well-defined morphology and high preparative success rate of yolk-shell nanofiber and yolk-shell nanobelt, universality to metallic fluorides. The design philosophy and the construction technique for the exceptional yolk-shell one-dimensional nanostructures afford a facile approach for the fabrication of other functional metal fluorides yolk-shell one-dimensional nanostructures.
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