One-step chromatographic purification of K, Ca, and Sr from geological samples for high precision stable and radiogenic isotope analysis by MC-ICP-MS

Abstract

Multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) is widely used in stable and radiogenic isotope analysis of various elements because of the extremely high precision and accuracy. At present, stable K, Ca, and radiogenic Sr isotopes are increasingly used as a combined proxy of geological processes. However, time consuming and cumbersome multi-step chemical procedures hinder their development and application. This study presents a method for one-step chromatographic separation of K–Ca–Sr from matrix elements, and the purification procedure is based on AG50W-X12 cation exchange resin without any intervening evaporation step. The K fraction was first collected using 10 mL 2 M (mol L⁻¹) HCl, the Ca fraction was then collected using 18 mL 2 M HCl, and the Sr fraction was collected at last using 10 mL 3 M HCl. K–Ca–Sr isotopic ratios were measured by using MC-ICP-MS (Nu Plasma 3) with a sample–standard bracketing (SSB) method. The stability of our chemical procedure was demonstrated by replicate measurements of ten international reference materials of rocks and seawater. The analytical results obtained for these reference materials are consistent with previously reported values within analytical errors. The external reproducibility of a BCR-2 standard sample was ±0.05‰ (2SD, N = 9) for δ^41/39K, ±0.04‰ (2SD, N = 9) for δ^44/42Ca, and 0.000012 (2SD, N = 4) for ⁸⁷Sr/⁸⁶Sr. Thus, this optimal separation method improves the experimental efficiency and reduces cross-contamination.