Rapid quadruple sulfur isotope analysis at the sub-micromole level by a flash heating with CoF3

Stable sulfur isotope ratio has been useful for tracing a wide range of biogeochemical processes. The conventional SF6 method allows us to make high-precision analyses of all four of the stable isotopes (32S/33S/34S/36S), though the fluorination step is time consuming and requires the hazardous elemental fluorine (F2). Here we show a novel flash-fluorination technique for instantaneous preparation of SF6 from sulfide samples by reaction with cobalt fluoride (CoF3) instead of F2. Flash heating by using a Curie-point pyrolyzer dramatically simplifies the fluorination procedure. A series of experiments demonstrated that silver sulfide (Ag2S) can be converted into SF6 at 590 °C within 3 s with sufficient reproducibilities for δ34S (± 0.4‰), Δ33S (± 0.01‰) and Δ36S (± 0.1‰). Instrumental fractionation is negligible for δ34S and Δ33S, but should be corrected for Δ36S, when fluorinating small samples less than 150 nmol. Further, application of the method to various other sulfur compounds shows that pyrite and several organic sulfur species can be directly converted into SF6, though more work is necessary for improving the accuracy of isotopic analysis comparable to that of Ag2S. Nevertheless, the new CoF3-flash-fluorination technique provides fast, convenient and safe procedure for high-precision analysis of quadruple sulfur isotopes.