High-Precision Measurements of ⁴⁴Ca/⁴⁰Ca Using Multiple-Collector Inductively Coupled Plasma Mass Spectrometry without Collision-Cell Technology

Abstract

Herein, we present a new method for determining the Ca isotopic composition of geological samples. To eliminate matrix elements from Ca, a column chromatography method was developed using a N,N,N′N′ tetraoctyl-1,5-diglycolamide (TODGA) resin. The Ca isotopic compositions were measured by a multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS) without collision cell equipment, especially that direct measurement to 44 Ca/ 40 Ca can be achieved. To mitigate the interference from 40 Ar during 40 Ca measurement, the cold plasma technique was used to suppress the Ar + generation, resulting in a background Ar + intensity of <300 mV, in contrast to the conventional hot plasma conditions, which typically yield thousands of volts for Ar + intensities. Given the potential for a concentration mismatch between the sample and bracketed standard solutions to cause an intensive shift in measured Ca isotopic compositions, a correction for the [Ca] match is needed. To avoid matrix effects arising from residue matrix elements, it is crucial to limit the concentrations below 1% of Ca for most matrix elements (including Al, Mg, K, Na, and Sr) and below 1‰ for Fe. Notably, the tolerance of residue Sr is effectively improved compared to measurements with CC-MC-ICP-MS and traditional Hot-plasma-SSB-MC-ICP-MS methods with the conventional hot plasma technique, thereby lowering the complexity of column chemistry. The measured δ 44/40 Ca, δ 44/42 Ca, and ε 40 Ca values for eight reference materials agree well with previously reported values within analytical uncertainties. This method demonstrates long-term precision is better than 0.10‰ (two standard deviations) for both δ values (i.e., δ 44/40 Ca and δ 44/42 Ca). We anticipate that the proposed method will benefit the growth of the Ca isotope data set and foster an increase in the application of Ca isotope in Earth science studies.