A method for precise analysis of trace element/calcium ratios in carbonate samples using quadrupole inductively coupled plasma mass spectrometry

Abstract

An improved method has been developed using a quadrupole inductively coupled plasma mass spectrometer (Q‐ICP‐MS) to analyze carbonate samples for multiple trace element to calcium ratios. Problems alleviating shorter‐term drift have been overcome by manipulating the instrument to bracket each trace metal measurement with a calcium measurement, while longer‐term drift is corrected by bracketing each sample analysis with external matrix‐matched standards, of typical foram composition. Our method is optimized for measuring the ratios, Sr/Ca, Mg/Ca, Zn/Ca, Cd/Ca, Ba/Ca, Mn/Ca, and U/Ca, from a single carbonate sample but could be adapted to include a range of alternative ratios. Evaluation has shown the method to be extremely precise. At less than 1 RSD (1σ) for 3 trace metal to calcium ratios, we have achieved comparable precision to a sector field ICP‐MS instrument and an advance over previous methods using the Q‐ICP‐MS instrument. Precision (RSD) for each ratio, based on 96 sample analyses in 5 batches made over a period of 9 months using a 100 mg/L Ca standard, is 2.2% for U/Ca, 1.7% for Cd/Ca, 1.9% for Zn/Ca, 1.1% for Ba/Ca, 0.7% for Mn/Ca, and 0.7% and 0.6% for Sr/Ca and Mg/Ca, respectively. Accuracy of the method is confirmed by comparing standard ratios with those obtained from an ICP‐AES instrument and by comparing sample analyses of NEAP4B material from the North Atlantic with data measured using thermal ionization mass spectrometry (TIMS) and flame atomic absorption spectroscopy (FAAS).