Efficient separation and high-precision analyses of tin and cadmium isotopes in geological materials

Abstract

This paper presents a new method for the separation of Sn and Cd from geological matrices followed by high-precision isotope analyses that include low abundance isotopes (<1.25%). The new technique is of specific interest for the detection of small mass-independent nucleosynthetic or cosmogenic isotope variations in meteorites and other planetary materials. We also report a new precise estimate for Sn isotope abundances. The method employs a combination of ion exchange and extraction chromatography together with multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). Tin is separated from the sample matrix using an anion-exchange resin, followed by removal of remaining matrix elements and organics using the TRU and Pre-filter resins, respectively. The matrix fraction from the TRU resin step is further purified to isolate Cd using a two-stage anion exchange procedure. Analyses of Sn and Cd standard solutions doped with interfering elements were employed to define thresholds for tolerable amounts of interference producing elements. Our data demonstrate that our new procedure produces purified Sn and Cd solutions with sufficiently low levels of contaminants for high precision Sn and Cd isotope analyses. Removal of U is important for Sn isotope data because of doubly charged U ions. The internally normalised Sn isotope data of the two standard solution (NIST SRM 3161a and SPEX CLSN2-2Y) are in excellent agreement with previous data. Based on repeated analysis of independently processed lake sediment aliquots, an external reproducibility (intermediate precision) (2SD) is achieved of ±110 ppm for ¹¹²Sn/¹²⁰Sn, ±170 ppm for ¹¹⁴Sn/¹²⁰Sn, ±160 ppm for ¹¹⁵Sn/¹²⁰Sn, ±21 ppm for ¹¹⁷Sn/¹²⁰Sn, ±13 ppm for ¹¹⁸Sn/¹²⁰Sn, ±20 ppm for ¹¹⁹Sn/¹²⁰Sn, ±22 ppm for ¹²²Sn/¹²⁰Sn and ±24 ppm for ¹²⁴Sn/¹²⁰Sn. Replicate Sn analyses of the carbonaceous chondrite Allende are in excellent agreement with those of the lake sediments. For Cd isotope analyses, the lake sediment yields an external reproducibility (2SD) of ±170 ppm for ¹⁰⁶Cd/¹¹¹Cd, ±200 ppm for ¹⁰⁸Cd/¹¹¹Cd, ±34 ppm for ¹¹⁰Cd/¹¹¹Cd, ±18 ppm for ¹¹²Cd/¹¹¹Cd, ±24 ppm for ¹¹³Cd/¹¹¹Cd and ±15 ppm for ¹¹⁴Cd/¹¹¹Cd.