Effects of crater development on fractionation and signal intensity duringlaser ablation inductively coupled plasma mass spectrometry

The effects of crater development on ICP-MS signal intensities and elementa l fractionation have been presented in this work. Craters formed after repe titive 266-nm Nd/YAG laser ablation with 1.0-mJ pulses had a cone-like shap e. The laser ablation rate (ng/s) depended on the laser irradiance (laser p ulse energy per unit time and unit area), decreasing as irradiance increase d. In contrast, the particle entrainment/transport efficiency did not signi ficantly change with irradiance. As the crater-aspect ratio (depth/diameter ) increased above some threshold value of six, the Pb/U elemental ratio dep arted from the stoichiometric value. However, good stoichiometry of ablated mass could be achieved when experimental conditions were carefully selecte d. The exact mechanism of how crater development affects fractionation is n ot well understood. In this work, actual irradiance was introduced instead of a nominal value. Actual irradiance decreased as the crater deepened due to changes of the effective area, sampled by the laser beam. (C) 2000 Elsev ier Science B.V. All rights reserved.