Abstract

We report a newly developed method for analyzing the sulfur isotope ratio (³⁴S/³⁴S) of trace amounts of sulfate. This method consists of two steps: 1) isolation of sulfate via anion chromatography with fraction collection (IC-FC), and 2) precipitation of nanomolar-scale sulfate as barite within a tin capsule (nano-BP) for isotope measurement. In the first step of anion chromatography with fraction collection, the sulfate ion is isolated from other components with baseline resolution on the chromatogram. In the second nanomolar barite precipitation step, microcrystalline barite precipitates through reaction with BaCl₂, followed by evaporation through heating. By conducting this reaction in a tin capsule for isotope ratio measurement, the collection or filtration of barite precipitates is not required. With our nano-EA/IRMS, sulfur content and isotope ratio can be determined in the 5–125 nmol sulfur content range. In this study, we further applied this method to carbonate-associated sulfate (CAS) extracted from natural samples, including foraminiferal shells from marine sediments and geological carbonates such as coral, limestone, and dolostone. Reliable δ³⁴S values were obtained from as little as 5–10 nmol of sulfur, with standard deviations typically within ±0.2–1.0‰, reflecting differences in material homogeneity. The method successfully resolved δ³⁴S variations among size fractions and stratigraphic layers in foraminifera and distinguished between carbonate types of different origins. These results demonstrate the method’s precision and robustness for analyzing CAS δ³⁴S in diverse natural materials.