Vaporization and ionization of laser ablation generated aerosols in an inductively coupled plasma mass spectrometer - implications from ion distribution maps

The distribution of elemental ions in the plasma of an inductively coupled plasma mass spectrometer ( ICPMS) was investigated with laser ablation for sampling of silicate materials. The distributions were determined by scanning the ICP vertically across the sampler orifice of the mass spectrometer at four different sampling depths. Different laser wavelengths were used for ablation with argon and helium as carrier gases. The distributions show a wider radial spread for all ions when helium is used as carrier gas and when the fraction of large particles in the aerosol is small, which is a result of faster vaporization of the aerosol and higher mobility of vapor and ions when helium is present in the central channel of the ICP. This causes the axial distribution to become smaller because the ions distribute over the ICP more rapidly. Mass dependencies are pronounced in the radial profiles, especially at short sampling depth and for incompletely vaporized aerosols. Lighter ions show generally a wider radial spread, in accordance to the greater mobility of atoms and ions of low atomic mass. This dependence is reduced with increasing sampling depth. The axial profiles on the other hand show no pronounced dependency on mass. The relative width of the axial profiles slightly increases with sampling depth for heavier ions, which is attributed to a change in their kinetic energy when the plasma is sampled at higher temperature and reduces the fraction of ions transmitted by the ion optics. Elements with high ionization energies are characterized by shorter axial and wider radial distributions at large sampling depth, which is a result of the change in electron temperature that decreases with sampling depth and increases with distance from the axis of the ICP.