The influence of the laser ablation process on isotopic fractionation of copper in LA-MC-ICP-MS

Abstract

Isotopic fractionation during in-situ analysis of copper using laser ablation (LA) MC-ICP-MS has been investigated. Results show that particles <200 nm transported in the aerosol are isotopically lighter by up to 0.7‰ compared to the larger particles. The dependence of this isotopic fractionation on the ablation time, together with isotopic effects caused in the ICP during particle vaporisation, explains the variations of isotope ratios observed during LA-MC-ICP-MS. LA-MC-ICP-MS measurements of copper isotope ratios in metallic copper typically show large systematic within-run variations of up to several permil (‰) depending on the ablation time and, correspondingly, large offsets from solution data for the same sample when compared directly. Analysis of small particles generated by vapour condensation after the laser impact, and larger particles formed by ejection of liquid droplets from the crater, show that the laser impact results in isotopic fractionation of ⁶⁵Cu⁺/⁶³Cu⁺ between different aerosol particle size fractions of up to 0.7‰. This fractionation cannot be overcome by removal of large particles from the aerosol that are not completely vaporised and ionised in the ICP source. However, separation of the large particles is shown to significantly reduce within-run fractionation, as well as improving the accuracy relative to solution data, and is therefore recommended for the analysis of copper metal. Temporal variations in the isotopic composition of particulate vapour condensate are explained by a crater effect that influences the residence time of large particles in the laser-generated plasma plume, leading to preferential vaporisation of the lighter isotopes.