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