High-precision Sm isotope analysis by thermal ionisation mass spectrometry for large meteorite samples (>1 g).

Abstract

This study presents a new procedure for high-precision Sm isotope analysis by thermal ionisation mass spectrometry (TIMS) for geological samples. A four-step chemical separation scheme results in sharp separation of Sm and Nd from the same sample aliquot. The first step utilises anion exchange resin to remove Fe from the sample solution. Two different liquid-liquid extraction resins are then used to isolate rare-earth elements (TRU-Spec) and purify Sm from Nd (DGA). Fractionation occurs on the DGA resin due to the nuclear field shift effect, but this is negligible if yields greater than 70% are achieved. Different analytical setups were tested to ascertain their ionisation efficiencies on TIMS. The effect of activators composed of Pt and Ta was tested on single Re filaments but the conventional double Re filament assembly provided efficient ionisation and more stable ion beams. The determination of nucleosynthetic isotope variations requires high precision for all Sm isotope ratios. We aimed to improve the precision on the scarce ¹⁴⁴Sm isotope (3% of all Sm). Static, multistatic and dynamic methods were tested. Isotope ratios were normalised to both ¹⁴⁷Sm/¹⁵²Sm and ¹⁵²Sm/¹⁴⁸Sm for comparison. The dynamic methods failed to provide better precision on ratios involving ¹⁴⁴Sm, whereas the multistatic method yielded improved precisions between 13 and 22 ppm (twice the standard deviation, 2 SD) on the ¹⁴⁴Sm/¹⁵²Sm ratio. Synthetic standards have variable Sm isotope compositions, thus requiring systematic and precise characterisation against terrestrial samples. Analyses conducted using this new procedure yielded high-precision values which were consistent with literature data for an array of terrestrial rock standards and the meteorite Allende.