Determination of Barium Isotopic Ratios in River Water on the Multiple Collector Inductively Coupled Plasma Mass Spectrometer

抄録

Barium (Ba) isotopes have been booming while providing ubiquitous traceability to various geochemical/oceanographical processes in recent years. Accurate and precise determinations of Ba isotopes is the main precondition to apply them to tracing various processes. However, a particular determination method is lacking for Ba isotopic ratios in river water. Compared to the double-spike method which needs expensive spikes, intensive laboratory operation, and complex calibration, sample-standard bracketing (SSB) is an easier and more convenient method to obtain isotopic data on the multiple collector inductively coupled plasma mass spectrometer. In this study, to obtain the Ba isotopic ratio of river-water samples with a rather lower Ba concentration than igneous rock, we experimented with K, Ca, Na, Mg, and Ce doping tests. The acidity and concentration matches to explore suitable SSB method conditions for river water Ba isotopic determinations on an MC-ICP-MS. The results showed no obvious matrix effects to Ba isotopes when K/Ba, Ca/Ba, Na/Ba, and Mg/Ba <5, and Ce/Ba <0.2, indicating that the matrix effects are negligible after column purification of Ba. HNO₃ concentration match tests showed that within a 25% acidity difference, there would be also no systematic effects to a Ba isotopic determination. However, Ba isotopic determinations are highly sensitive to the Ba concentration difference between the standard and the sample. We observed a mismatch between the Ba concentration and its isotopic ratios, even when the concentration was strictly within a 5% difference. Nevertheless, the on-peak zero strategy (i.e. subtraction of HNO₃, gas, and instrumental blanks) worked well to solve the mismatch situation. With this easy strategy, the Ba isotopic ratios of batch natural rive-water samples with various Ba concentrations will become easily accessible.