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Abstract
Double spiking is conventionally used to make accurate determinations of natural massdependent isotopic fractionations for elements with four or more stable isotopes. Here we document a methodology which extends the effective application of double spiking to three isotope systems. This approach requires making a mixture with isotope ratios that lie on a ‘critical curve’ where the sample − doublespike mixing line and the tangent to the instrumental massbias curve are coincident. Inversion of the mixing equations for such a mixture leads to a solution for the sample fractionation which is independent (to first order) of the uncertainty in the instrumental massbias and, hence, independent of any massdependent artefacts in the measurement such as those produced by residual matrix not completely removed by prior chemical purification. In practice, mixtures can be made which yield an accuracy conservatively estimated to be ~0.005‰/amu. The precision of the method is explored as a function of doublespike composition for Mg, Si and K isotope systems. We show that for Mg and Si measurement precision is not compromised by the compositions of viable critical mixtures nor by uncertainty magnification during inversion of the equations. Thus, double spiking provides a valuable means to obtain robust, high precision isotopic measurements of Mg and Si. For K, however, the low abundance of ^{40}K in the optimal critical mixture places a significant practical limitation on the application of double spiking to analyses of this element.
Original language  English 

Pages (fromto)  7889 
Number of pages  12 
Journal  Chemical Geology 
Volume  451 
Early online date  21 Dec 2016 
DOIs  
Publication status  Published  Feb 2017 
Keywords
 Doublespike
 isotopic fractionation
 threeisotope systems
 magnesium isotopes
 silicon isotopes
 potassium isotopes
 mass spectrometry
 MCICPMS
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Projects
 2 Finished


Constraints on terrestrial differentiation from the isotopic fractionation of major elements
1/04/14 → 31/03/18
Project: Research
Profiles

Dr Christopher D Coath
 School of Earth Sciences  Senior Research Fellow
 Geochemistry
Person: Academic , Member