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Comparison of sample preparation methods for stable isotope analysis of dissolved sulphate in forested watersheds

Kang, Phil-Goo, Mayer, Bernhard, Mitchell, Myron J.
Isotopes in environmental and health studies 2012 v.48 no.3 pp. 410-420
anion exchange resins, barium sulfate, forested watersheds, oxygen, stable isotopes, sulfur
Pretreatment methods for measuring stable sulphur (δ³⁴S) and oxygen (δ¹⁸O) isotope ratios of dissolved sulphate from watersheds have evolved throughout the last few decades. The current study evaluated if there are differences in the measured stable S and O isotope values of dissolved sulphate from forested watersheds when pretreated using three different methods: Method 1 (M1): adsorb sulphate on anion exchange resins and send directly to isotope facility; Method 2 (M2): adsorb sulphate on anion exchange resins, extract sulphate from anion exchange resins, and send the produced BaSO₄ to the isotope facility; and Method 3 (M3): directly precipitate BaSO₄ without anion exchange resins with the precipitates being sent to the isotope facility. We found an excellent agreement of the δ³⁴Sₛᵤₗₚₕₐₜₑ values among all the three methods. However, some differences were observed in the δ¹⁸Oₛᵤₗₚₕₐₜₑ values (M1 versus M2:−1.5 ‰; M1 versus M3:−1.2 ‰) associated with possible O contamination before isotope measurement. Several approaches are recommended to improve the pretreatment procedures for δ¹⁸Oₛᵤₗₚₕₐₜₑ analysis.