High-precision Ca isotopic measurement using a large geometry high resolution MC-ICP-MS with a dummy bucket

Abstract

Wide application of stable Ca isotopes requires reliable production of high-precision data and in recent years multiple collector inductively coupled plasma mass spectrometers (MC-ICP-MS) have been used as an alternative to traditional thermal ionization mass spectrometers. The main challenges in using MC-ICP-MS to measure stable Ca isotopes are scattered ions and peak tailing of high level ⁴⁰Ar⁺ and ⁴⁰Ca⁺ ion beams, along with variable interferences. In this study we optimize Ca isotopic measurements on a large geometry high resolution MC-ICP-MS (Nu Plasma 1700) operating in conventional hot Ar-plasma mode. ⁴⁰Ar⁺ and ⁴⁰Ca⁺ were trapped by a specially designed dummy bucket, to prevent scattering of ions that might affect the Ca isotopic measurement. Strontium and matrix elements in samples were eliminated to blank levels by chemical separation. Instrumental mass bias of Ca isotopes was corrected by a sample-standard bracketing method using a purified sea water Ca isotope standard. Potential effects of Ar-related interferences, Ca-hydride, acid molarity, concentration mismatch and matrix elements were evaluated. The results from both measurements and modeling suggest that ⁴⁰Ar⁺ peak tailing and Ca-hydride cause negligible Ca isotopic offset under normal MC-ICP-MS operating conditions. The precision and accuracy were validated using the measurement of twelve well-characterized international reference materials and a pure Ca standard solution. The long-term external precisions are better than ±0.07‰ (2SD) for δ^44/42Ca and ±0.10‰ (2SD) for δ^43/42Ca. Calcium isotopic compositions of all reference materials measured in this study agree with previously published data within uncertainties.