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 (SO ₄ ^2- ) 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 SO ₄ ^2- 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. SO ₄ ^2- present in the Cu fraction can form a polyatomic ³² S- ¹⁴ N- ¹⁶ O- ¹ H species causing a direct mass interference with ⁶³ Cu and producing artificially light δ ⁶⁵ Cu values. Here we report the extent of the mass interference caused by SO ₄ ^2- breakthrough when measuring δ ⁶⁵ Cu on natural samples and NIST SRM 976 Cu isotope spiked with SO ₄ ^2- 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 SO ₄ ^2- resulted in an average δ ⁶⁵ Cu of - 3.50‰ ± 5.42‰ following single anion column separation with variable SO ₄ ^2- breakthrough but an average concentration of 770 μg/L. Following double anion column separation, the average SO ₄ ^2- concentration of 13 μg/L resulted in better precision and accuracy for the measured δ ⁶⁵ Cu value of 0.01‰ ± 0.02‰ relative to the expected 0‰ for SRM 976. We conclude that attention to SO ₄ ^2- breakthrough on sulfur-rich samples is necessary for accurate and precise measurements of δ ⁶⁵ Cu and may require the use of a double ion exchange column procedure.