An Analytical Method for Quantification of Sn Isotope Variations in Geological Materials Using MC-ICP-MS

Abstract

A robust analytical method is presented here, for measurements of Sn isotope variations with high precision using a multi-collector inductively coupled plasma-mass spectrometer (MC-ICP-MS). A silicate rock dissolution procedure was used, together with means to remove organic matter introduced by the resin during ion-exchange chromatography. The two-stage ion exchange chromatography efficiently isolates Sn from the matrix and isobarically interfering elements, from samples with variable Sn mass fractions (~ 0.03 to 7 μg g⁻¹). A double spike technique was also implemented to correct any secondary mass bias induced during sample processing and isotope ratio measurements. An intermediate precision (2s) of ± 0.47 was obtained for ε^122/118Sn (using internal normalisation; n = 70 over a period of 29 months), ± 0.021‰ for δ^122/118Sn (using double spike; n = 176 over a period of 29 months) and ± 0.059‰ for δ^122/118Sn (using Sb doping; n = 34 over a period of 4.5 months) for the Sn standard solution, NIST SRM 3161a (Lot# 140917). The δ^122/118Sn_{NIST SRM 3161a} of eleven geological reference materials are reported for the first time (BIR-1a, BRP-1, DTS-2b, ECRM 782-1, JP-1, OKUM, QLO-1a, RGM-2, SGR-1b, STM-2 and UB-N), with five others (AGV-2, BCR-2, BHVO-2, GSP-2 and W-2a) showing similar results to previous studies, generally with improved precision. The intermediate precision (2s) of replicate dissolutions of ten RMs (AGV-2, BCR-2, BHVO-2, BIR-1a, DTS-2b, GSP-2, OKUM, SGR-1b, STM-2 and W-2a) varied from 0.003 to 0.054‰. Typical repeatability precision (2SE) of individual measurements of these ten RMs over multiple measurement sessions varied from 0.031 to 0.297‰ (with ¹²⁰Sn⁺ beam intensity < 2.37 V) and 0.017 to 0.025‰ (with ¹²⁰Sn⁺ beam intensity > 2.37 V).