Zinc isotope fractionation analyses by thermal ionization mass spectrometry and a double spiking technique

Abstract

The aim of this work was to develop thermal ionization mass spectrometry (TIMS) isotopic procedures to measure Zn isotope fractionation (δZn) in natural materials. This work represents the most recent development of Zn isotope measurements and the first δZn identification in terrestrial materials using TIMS and a double spike technique. The developed procedures evaluate and solve several critical analytical issues involved in TIMS Zn isotope analysis. For example, no more than 1 μg Zn was used for isotopic analyses which, considering the high ionization potential and low thermal ionization of Zn, represents a useful breakthrough in Zn isotope TIMS analysis. The effect of the ion exchange process on δZn was assessed and found equal to +0.07 ± 0.02‰ amu −1 per column. The ionization efficiency of Zn was enhanced to 0.22 ± 0.07%, which is four times more than what was achieved previously. The magnitude of δZn accompanied by the 95% confidence associated uncertainties were calculated relative to the IRMM 3702, using a Monte Carlo approach for each individual analysis, while the calculated average of δZn for number of analysis was accompanied by a 95% confidence calculated using GUM Workbench software. δZn values where always calculated using two different sets of isotopes which always agreed within uncertainty. These developments enabled sub-per mil δZn to be revealed relative to δZn zero for natural materials. Most of the samples measured are Standard Reference Materials SRMs, where, except for BCR-1 and BIR-1, this is the first time Zn isotopic fractionation has been measured in these samples. No previously published results for Zn isotopic fractionation have been published on these samples using double spiking. Consistent δZn of ∼+ 0.3‰ amu −1 was found in 5 sediments from a range of localities. δZn in two metamorphic samples is similar to that found in igneous rocks but different to that found in sedimentary rocks, which is consistent with our understanding that high temperature and pressure processes do not fractionate the composition of chalcophile elements. The isotope fractionation of Zn in a clay sample is within uncertainties the same as the sediments. The isotope fractionation of Zn of −0.088 ± 0.070‰ amu −1 was also measured in a standard rice sample. δZn in Antarctic Krill of +0.21 ± 0.11‰ amu −1 was found to be similar to the average δZn of +0.281 ± 0.083‰ amu −1 for marine sediments. River water was fractionated by −1.09 ± 0.70 ‰ amu −1, while restrained tap water yielded the maximum isotope fractionation of −6.39 ± 0.62‰ amu −1. δZn in high pure Zn standard materials ranged from −5.11 ± 0.36‰ amu −1 for AE 10760 to +0.12 ± 0.16‰ amu −1 for Zn IRMM 10440 with some evidence for a relationship between Zn isotope fractionation and its purity. All of the measured isotope fractionation yields an atomic weight within the IUPAC atomic weight of Zn.