A comprehensive chromatographic method for sequential separation of Pb, Rb, Sr, Nd, Sm, Lu, U, and Hf for high-precision isotope analysis of micro-sized silicate samples

Abstract

Lunar basaltic materials, including samples returned by space missions and meteorites, represent the most precious and scarce materials for deciphering the origin and evolution of the Moon. To fully exploit these limited samples, this study presents a comprehensive protocol for the sequential separation of Pb, Rb, Sr, Nd, Sm, Lu, U, and Hf from micro-amounts of lunar basaltic and other compositionally similar precious samples, optimized for high-precision multi-isotopic analysis. Pb is first isolated from the sample matrix using well-developed HBr–HCl based anion-exchange chromatography, minimizing terrestrial contamination. A miniaturized cation-exchange column is then used to separate Rb and a fraction containing Ca, Sr, Ba, and REEs from the sample matrix. Subsequently, an Sr-specific resin column is employed to isolate Sr and Ba from Ca and REEs. Next, a home-made HEHEHP-based column is utilized for efficient isolation of Nd, Sm, and Lu from the Ca–REE mixture. A TODGA resin column is then used to further remove Pr from Nd. Finally, another TODGA column is applied to sequentially isolate U and Hf from matrix elements, with complete Lu and Yb removal from the Hf fraction. The procedure has been validated through isotopic analysis of the reference materials USGS BHVO-2 and BCR-2, using TIMS and MC-ICP-MS techniques. This method offers a robust protocol for acquiring U–Pb, Rb–Sr, Sm–Nd, and Lu–Hf isotopic data from micro-amounts (e.g., < 5 mg) of lunar basaltic samples. Moreover, it allows for the preservation of Ca, Ba, and a fraction containing major matrix elements such as Fe, Mg, Cr, Ti, and K, for potential further analysis of metal stable isotopes. While this method was initially developed to support comprehensive isotope analyses of lunar return samples, its design is broadly applicable to other small amounts of compositionally similar silicate materials.