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??g/L to greater than 50mg/L with sulfur species concentrations reaching greater than 1000mg/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 ?? 65 Cu values. Here we report the extent of the mass interference caused by SO 4 2- breakthrough when measuring ?? 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??g/L Cu SRM 976 samples spiked with 500mg/L SO 4 2- resulted in an average ?? 65 Cu of -3.50?????5.42??? following single anion column separation with variable SO 4 2- breakthrough but an average concentration of 770??g/L. Following double anion column separation, the average SO 4 2- concentration of 13??g/L resulted in better precision and accuracy for the measured ?? 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 ?? 65 Cu and may require the use of a double ion exchange column procedure. ?? 2010.