Laser Ablation ICP-MS: particle size-dependent isotopic fractionation of copper in laser-generated aerosols

Laser induced isotopic fractionation Previous studies of laser ablation (LA) MC-ICP-MS analysis of Cu metal and sulphides showed reduced Cu/Cu ratios compared to solution nebulisation MC-ICP-MS of the same samples [1]. The fractionation of about 30ε (parts per 10,000) was reproduced repeatedly and observed, to a smaller degree, for Ag and Sb also. This deviation, which was attributed largely to preferential vaporisation of Cu in the ICP, seriously reduces the accuracy of the results. Recently published research has demonstrated that the origin of elemental fractionation in LA-ICP-MS is the result of a combination of laserand ICP-induced fractionation [2]. The laser impact produces two distinct phases in the ablation plume – relatively large ( m scale) melt droplets and much smaller (<10 nm) particles which are the product of condensation from a vapour phase. The more volatile elements were shown to be enriched in the condensate which is much more efficiently vaporized in the ICP than the larger melt particles. A similar effect can be expected for isotopic fractionation, where the lighter isotopes could be enriched in the smaller particles leading subsequently to lower measured Cu/Cu ratios.