Rapid, simultaneous separation of Sr, Pb, and Nd by extraction chromatography prior to isotope ratios determination by TIMS and MC-ICP-MS

Abstract

A straightforward separation scheme is described for the separation of Sr, Pb, and Nd from silicate rocks. It allows the concomitant isolation, without any intervening evaporation, of these three elements of great interest in radiogenic isotope geology and cosmochemistry. Following digestion with HF–HNO₃, the sample residue is dissolved in 1 M HNO₃. After addition of ascorbic acid to reduce Fe(III) to Fe(II), this solution is passed through two tandem columns containing 250 μL of Sr Spec and TRU Spec extraction chromatography resins, respectively. The upper Sr Spec column extracts Sr and Pb, while the lower TRU Spec column extracts the LREE. Sr and Pb are back-extracted with 0.05 M HNO₃ and 6 M HCl, respectively. The LREE are eluted directly onto a longer column containing 300 mg of the HDEHP-based EXC material Ln Spec, to obtain, through sequential elution with 0.25 M HCl, a Nd fraction free of any Sm contribution. The whole procedure is achieved within a single working day. The Sr and Nd fractions separated in this way are ready for isotope ratio measurements by TIMS (Sr, Nd) or MC-ICP-MS (Nd). The Pb fraction is converted to the nitrate form before isotopic analysis by MC-ICP-MS. The potential of this method is exemplified by analysis of different powder aliquots of several iron-rich, international standard rocks of basaltic composition.