An evaluation on the extraction capability of anion exchange membranes for high-precision sulfur isotope measurement by multiple-collector inductively coupled plasma mass spectrometry

Abstract

Anion exchange membranes (AEMs) are adept at extracting sulfate for sulfur isotope analyses by multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) from natural samples typically with low sulfate concentrations. But up to now, their capability for sulfate extraction is still lacking adequate research. In this study, a series of detailed tests on AEMs for sulfate extraction were developed, which included the tolerance of pH, the effect of competitive anions, and the lowest concentration limit of sulfate uploading. The optimal scope of pH for sulfate exchange is from 3 to 11. Approximately over 90% of sulfate recoveries and reliable sulfur isotope analyses can be achieved when the concentrations of nitrate, chloride, phosphate, carbonate, and bicarbonate are limited in 0.5 mmol per L per cm² of a piece of AEM. In practice, we suggest that the applicable concentrations are able to increase to 10 times, except for phosphate. The lowest uploading concentration of sulfate that can be adsorbed by the AEM without sulfur isotope fractionation is further detected as 0.5 μmol L⁻¹ though the recovery of sulfate decreases when its concentration is lower than 0.01 mmol L⁻¹. This research offers insight into realizing accurate and precise sulfur isotope analyses for natural freshwater and marine pore water by MC-ICP-MS.