Influence of sulfur-bearing polyatomic species on high precision measurements of Cu isotopic composition

An increased interest in high precision Cu isotope ratio measurements using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) has developed recently for various natural geologic systems and environmental applications, these typically contain high concentrations of sulfur, particularly in the form of sulfate (SO42−) and sulfide (S). For example, Cu, Fe, and Zn concentrations in acid mine drainage (AMD) can range from 100 µg/L to greater than 50 mg/L with sulfur species concentrations reaching greater than 1000 mg/L. Routine separation of Cu, Fe and Zn from AMD, Cu–sulfide minerals and other geological matrices usually incorporates single anion exchange resin column chromatography for metal separation. During chromatographic separation, variable breakthrough of SO42− during anion exchange resin column chromatography into the Cu fractions was observed as a function of the initial sulfur to Cu ratio, column properties, and the sample matrix. SO42− present in the Cu fraction can form a polyatomic 32S–14N–16O–1H species causing a direct mass interference with 63Cu and producing artificially light δ65Cu values. Here we report the extent of the mass interference caused by SO42− breakthrough when measuring δ65Cu on natural samples and NIST SRM 976 Cu isotope spiked with SO42− after both single anion column chromatography and double anion column chromatography. A set of five 100 µg/L Cu SRM 976 samples spiked with 500 mg/L SO42− resulted in an average δ65Cu of − 3.50‰ ± 5.42‰ following single anion column separation with variable SO42− breakthrough but an average concentration of 770 µg/L. Following double anion column separation, the average SO42−concentration of 13 µg/L resulted in better precision and accuracy for the measured δ65Cu value of 0.01‰ ± 0.02‰ relative to the expected 0‰ for SRM 976. We conclude that attention to SO42− breakthrough on sulfur-rich samples is necessary for accurate and precise measurements of δ65Cu and may require the use of a double ion exchange column procedure.