Separation of Mg, Ca and Fe from geological reference materials for stable isotope ratio analyses by MC-ICP-MS and double-spike TIMS

Abstract

Near quantitative separation of analyte elements from the sample matrix is commonly required to obtain precise and accurate stable isotope data, especially if MC-ICP-MS is used in conjunction with the standard-sample bracketing (SSB) technique for mass bias drift correction. Here, we report a robust procedure that allows for the combined chemical separation of Mg, Ca and Fe, using ion-exchange columns that contain 1 ml AG50W-X8 (200–400 mesh) cation exchange resin. Magnesium was separated for isotope ratio analyses from many geological sample types by a single pass through this column. For Mg purification, Be, Ti, Mn, Fe and Al are selectively eluted using dilute HF and an acetone–HCl mixture. The separation of Mg from Ca-dominated carbonate samples and/or a combination of Mg, Ca and Fe separation from the same sample aliquot, is achieved by the adsorption of Ca and Fe onto the ion-exchange resin from 10 M HCl. Following purification, geological reference materials, including water, bone, carbonate and sediment samples, igneous and sedimentary rocks and a chondritic meteorite were analysed by MC-ICP-MS (Mg and Fe isotopes) and double-spike TIMS (Ca isotopes). Average external repeatabilities were ±0.16‰ for ²⁶Mg/²⁴Mg, ±0.26‰ for ⁴⁴Ca/⁴⁰Ca and ±0.05‰ for ⁵⁶Fe/⁵⁴Fe (2sd; n ≥ 5). Comparison with published data documents the accuracy of the results. For Mg isotope analyses using SSB-MC-ICP-MS, matrix-induced mass bias effects were studied using element additions. The artificial matrices left a memory in subsequent standard analyses, likely due to depositions on the cones. This observation allowed for the detection of matrix effects in unknown samples. Finally, the current status of Mg and Ca zero-delta reference materials is briefly discussed.