Accurate determination of 87Rb/86Sr and 147Sm/144Nd ratios by inductively-coupled-plasma mass spectrometry in isotope geoscience: an alternative to isotope dilution analysis

The precise determination of 87Rb/86Sr and 147Sm/144Nd ratios in rocks and minerals is essential for isotope geology and geochronology. These determinations usually involve isotope dilution analysis, which requires the use of expensive isotopic spikes, consumes a significant fraction of precious thermal ionization mass spectrometer working time and, due to the repeated measurement of Rb isotopes, causes the performance of the Faraday cups to decay significantly after a certain period of time. The goal of this paper was to determine whether the direct determination by inductively coupled-plasma mass spectrometry (ICP-MS) of 85Rb/88Sr and 147Sm/145Nd ratios, easily transformed to 87Rb/86Sr and 147Sm/144Nd, could be an alternative to isotope dilution. A method was developed to dissolve samples under pressure with HNO3+HF in a microwave field, which completely dissolved Rb, Sr, Sm and Nd. Analyte vs. concentration functions were found to present some curvature effects, more intense in the case of Rb and Sr, which may cause poor accuracy in 85Rb/88Sr and 147Sm/145Nd determination. This effect, however, was satisfactorily corrected by (1) using an internal standard (103Rh) in both standard and sample solutions, and (2) fitting concentration vs. signal intensity to quadratic instead of linear functions. The accuracy and precision of the method calculated on ten replicates of the PM-S and WS-E standard rocks were found to be at least on the same order as those reported for isotope dilution analysis. It seems, therefore, that ICP-MS is capable of determining 87Rb/86Sr and 147Sm/144Nd ratios with nearly the same analytical quality as isotope dilution analysis but at a fraction of the time and cost.