High-precision double-spike Sn isotope analysis of geological materials by MC-ICP-MS

Abstract

Stable Sn isotopes have the potential to constrain the behavior of Sn in various natural and anthropogenetic processes, however, studies on stable isotopes of Sn for terrestrial rock samples have been limited. Here, we present a high-precision Sn isotope analytical method of geological samples by the double spike technique. Samples were equilibrated with a ¹¹⁷Sn–¹²²Sn double spike during dissolution, then Sn was purified from the rock matrix using TRU spec resin. The purified Sn solutions were analyzed on a Nu 1700 Sapphire MC-ICP-MS, and data reduction was performed using Newton–Raphson iteration. Effects of acid mismatch, concentration mismatch, and matrix elements were systematically tested to evaluate their impact on the accuracy and repeatability of the measurements. We show that the mismatch of acidity exerts negligible influences on the Sn isotope analysis. By contrast, the accuracy of Sn isotopic measurements could be compromised if the Sn concentration of the sample is below 40% of that of the standard. To ensure the accuracy of Sn isotope analysis, the concentration match between the sample and standard should be better than ±20%. When the measured solution has varying mass ratios of [X]/Sn (ng g⁻¹/ng g⁻¹) lower than 0.5 for Mg, Ca, Fe, Ti, U, Ni, Ag, As, Ru, and Mo, the measured Sn isotope composition was not affected. However, Cd in Sn samples could affect the accuracy of Sn isotope analysis by the ¹¹⁷Sn–¹²²Sn double spike through the generation of ¹¹⁶CdH⁺ in plasma that interferes with ¹¹⁷Sn⁺. The Cd/Sn ratio needs to be below 0.01 to ensure analytical accuracy by the double spike method. The accuracy of this method was further verified by measuring the pure solution with both double spike and sample standard bracketing methods, as well as processing a synthetic solution through column chemistry. The Sn isotope compositions of geological samples measured using our method are reproducible and consistent with previous data from other laboratories. Additionally, Sn isotope data for geological reference materials BIR-1, RGM-1, and GSR-1 are reported for the first time. Based on repeated analyses of pure and geological reference materials, the long-term intermediate precision is better than ±0.069‰ on δ^122/118Sn for a spiked solution of 100 μg g⁻¹.