Hydrodynamic sputtering as a possible source for fractionation in LA-ICP-MS

A model is proposed that identifies Kelvin–Helmholtz instabilities as one of the major sources of large particle production in LA-ICP-MS measurements under optimized conditions. The model describes the impact transfer from the expanding plasma to the liquid melt layer and the subsequent generation of droplets of variable sizes. The generated droplet spectrum is found to be dependent on melt layer thickness, surrounding atmosphere and laser parameters. The calculation of the melt layer thickness is based on a solution of the three dimensional heat conduction equation with moving interface boundaries that was previously introduced. Beside the calculation of the particle distribution function, the fluence dependency of the distribution and the particle size dependent chemical composition is explained by the model. Preliminary experiments with brass samples confirm the model calculations. The proposed model allows identifying optimal conditions for laser sampling in the regime which is typically used in LA-ICP-MS.