ION EXCHANGE CHROMATOGRAPHY AND MASS BIAS CORRECTION FOR CU AND ZN ISOTOPE RATIO MEASUREMENTS IN REFERENCE MATERIALS BY MC-ICP-MS AT CPGeo-USP.

The advance of multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) promoted the development of new research areas in isotope geochemistry, including the study of stable isotope metals, such as Cu and Zn (Araujo, et al, 2016). Recent studies have shown the great potential of stable metal isotopes to identify contaminant sources and pathways in the environment, comprehend biogeochemical processes, weathering and to reconstruct metal transfer in complex systems such as human body (Araujo, et al., 2016 and Oliveira et al., 2019). Systematic analyses of Cu and Zn isotopic compositions of USGS Reference Materials (BCR-2, BHVO-2, and AGV-2) were carried out at the Research Center of Isotope Geochemistry and Geochronology (CPGeo - USP). These measurements were performed on a Neptune MC-ICP-MS.

All chemical procedures were performed in class ISO 7 clean room equipped with class ISO 5 laminar flow hoods. About 50 mg of rock powder was dissolved with HF, HNO₃ and HCl in Savillex® beakers on hot plate at 100 ^oC. Cu and Zn from the same sample solution were purified using the ion exchange chromatographic resin AG MP1 (Biorad®). The first stage was the column packing with 0.6 mL anionic resin AG MP1, pre-cleaned with HNO₃ 0.5 M and H₂O. Then, the conditioning step was done using 4 mL HCl 7 M. The sample dissolved in 1 mL HCl 7M was loaded into the column and the resin was washed with 1.5 mL HCl 7 M. The eluted matrix was dried down and reconstituted for ion exchange separation of Pb. Cu was eluted with 10 mL HCl 7 M, Fe eluted with 7 mL HCl 2 M, and Zn eluted with 7 mL 0.5 M. Cu fraction required a second purification in the column to eliminate matrix interference.

MC-ICP-MS operated using hard extraction mode and low resolution for Cu and Zn analysis. The sample introduction system was a low-flow nebulizer (50 µL min^-1) connected to a Scot double-pass quartz cyclonic spray chamber (wet mode). Cu and Zn data collection was obtained through 40 cycles with 4 s integration for standard and sample measurements. HNO₃ 0.05 mol L^-1 solution (Cu and Zn) using a configuration of 10 cycles with 4 s integration, taken before and after samples and standard.

Stable isotopic variations (d-values) were reported relative to the certified international standards for Zn (JMC Lyon) and Cu (NIST-976), which were previously calibrated against our in-house standard references (named Zn_USP and  Cu_USP solutions). External normalization for Zn analysis was performed by doping samples and bracketing using Cu_USP solution. External normalization for Cu analysis was performed by doping samples and bracketing using Zn_USP solution.

The modified - Sample Standard Bracketing (m-SSB) was used to correct instrumental fractionation (mass bias) for Cu and Zn, considering the same fractionation of analyte and dopant.

Analysis of Zn JMC against IRMM-3702 standard (n=126) yielded d ⁶⁶Zn_JMC/IRMM values of -0.028 ± 0.05‰, d⁶⁶Zn_USP against JMC standard (n=39) yielded d ⁶⁶Zn_USP/JMC values of 0.04 ± 0.03‰, Cu_USP against NIST-976 Cu standard (n=25) yielded value of d⁶⁵Cu_USP/NIST976 0.17 ± 0.06‰. Analyses of BCR-2 at CPGeo yielded d⁶⁵Cu_NIST976 = 0.23 ± 0.06‰ (n=2), d⁶⁶Zn_JMC  = 0.29 ± 0.04‰ (n=6). Moeller et al. (2012) reported the following values for this RM: d⁶⁵Cu_NIST976 = 0.14 ± 0.03‰, d⁶⁶Zn_JMC  = 0.33 ± 0.07‰. Analyses of AGV-2 yielded d⁶⁵Cu_NIST976 = -0.02 ± 0.06‰ (n=2), d⁶⁶Zn_JMC  = 0.27 ± 0.03‰ (n=2). Freymuth et al. (2020) reported the following values for this reference material: d⁶⁵Cu_NIST976 = -0.01 ± 0.02 ‰ , d⁶⁶Zn_JMC  = 0.23 ± 0.04‰. Analyses of BHVO-2 yielded d⁶⁵Cu_NIST976 = 0.08 ± 0.04‰ (n=2), d⁶⁶Zn_JMC  = 0.33 ± 0.03‰ (n=5). Moeller et al. (2012) reported the following value for this RM: d⁶⁵Cu_NIST976 = 0.10± 0.04‰; and Araujo et al. (2016) published d⁶⁶Zn_JMC  = 0.25 ± 0.09 ‰. The isotopic compositions determined on these RM at CPGeo agree with the literature values.