A highly sensitive MC-ICPMS method for Cd/Ca analyses of foraminiferal tests

Abstract

A new, highly sensitive technique for the precise and accurate determination of Cd/Ca ratios in foraminiferal shells is presented and validated. The excellent accuracy and precision of the method reflects the application of an isotope dilution (ID) protocol for both Cd and Ca, and isotopic analysis by multiple collector inductively coupled plasma mass spectrometry. The determination of the Cd abundances involves simultaneous measurements of ¹¹⁰Cd and ¹¹¹Cd with a dual ion counting system, whereas the ID analyses for the major element Ca were carried out by multiple collection with Faraday cups. The performance of the method was verified by analyzing multiple samples of the planktonic foraminifer Orbulina universa that were handpicked from a sediment core taken in the North Atlantic. Analyses of unspiked samples that consumed as little as ∼200 ng Ca, indicate that the Ca ID measurements are accurate to about ±0.2% (all errors are 1 RSD). For the Cd ID analyses, the accuracy and precision is about ±1.3% and ±3% for measurements that consumed 3–12 pg and 0.5–1.5 pg of Cd, respectively. Repeated analyses of spiked O. universa tests yielded a reproducibility of ±0.7% for the Cd/Ca ratio, based on measurements that each consumed about 5 pg and 700 ng of natural Cd and Ca, respectively. The method is characterized by a total procedural Cd blank of 112 ± 44 fg (1 s.d.), which results in a detection limit of 131 fg (3 s.d.). This demonstrates that the new technique is superior to published methods for the determination of foraminiferal Cd/Ca ratios, particularly with regard to the acquisition of precise data for samples of limited size.