A model of non-congruent laser ablation as a source of fractionation effects in LA-ICP-MS

A model for laser ablation under conditions which are typical for LA-ICP-MS (He or Ar atmosphere, fluences below 100 J cm(-2)) is proposed that identifies the melted and subsequently re-solidified portion of the heated sample as a possible source for element-specific fractionation. Based on the full 3-dimensional description of the heat conduction problem, the heating, melting and evaporation of a sample is analytically modelled. Incorporation of radial heat losses provides a more realistic picture of the achievable surface temperature. Owing to the division of the ablation zone into a congruently vaporized part with stoichiometric mass removal and a zone of non-stoichiometric evaporation, the portion that laser ablation contributes to the total fractionation in LA-ICP-MS can be identified. The model describes the fluence dependence of fractionation, explains the size-dependent chemical composition of the particles, and the influence of the atmosphere on laser sampling. The theoretical predictions are compared with literature data and a fairly good agreement is found, especially for dilute alloys. Based on the model, optimal conditions for laser sampling in relation to a specific sample can be identified.