Role of laser-induced melting and vaporization of metals during ICP-MS and LIBS analysis, investigated with computer simulations and experiments

The role of surface melting and vaporization during laser ablation of several metals is investigated, presenting results from a numerical model and experimental determinations. The model takes into account the laser-solid interaction, melt-pool development and evaporation of the target material, followed by vaporization and plume expansion in 1 atm He background gas, plasma formation, and laser-plasma interaction. Surface melting and vaporization are shown to be responsible for the formation of a non-stoichiometric particle fraction. A comparison is made between several metals (Cu, Zn, Al, Fe, Mn and Mo) to evaluate whether they are potentially prone to fractionate in a laser-induced aerosol. It is found that the material properties, such as target surface reflectivity, optical absorption coefficient, thermal diffusivity, and melting/boiling temperature, have a pronounced influence on the target surface temperature and the amount of laser-induced vaporization. Consequently, the plume expansion, plasma formation and plasma shielding will also be different for different target metals. Our model predicts that the Al vapor yield is much lower than for the other metals, indicating that the ablation of Al should be mostly attributed to melt mobilization. The actual crater volume produced during laser ablation experiments using some target metals was directly measured, as a means to assess the role of surface vaporization on the overall mass removal extent. This showed a dependency on material and irradiance, so that for certain operating conditions surface vaporization cannot be neglected, potentially leading to fractionation. At low irradiance the micro-sampling process becomes less reproducible, both from the physical and chemical perspective. At high irradiance, phase explosion and droplet expulsion greatly enhance the ablation rate, leading to more regular sampling conditions.