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). © Copyright 2006 by the American Geophysical Union.