Elemental fractionation in femtosecond laser ablation is studied by ICP-MS by applying successive single laser shots to binary metallic and semiconductor samples as well as to multi-component glasses. Fractionation can be observed in the first laser shots in particular if the laser fluence is near the ablation threshold of the sample. However, the element ratio in the laser-sampled masses changes from shot to shot until it reaches an asymptotic fluence-independent value representing stoichiometric sampling. The asymptotic stoichiometric ratios can be obtained with fewer shots if higher laser fluences are applied. It is shown by electron probe X-ray analysis that different elemental ablation probabilities modify the element compositions in the surface layers of the laser craters until equilibrium conditions are obtained. These conditions can be reached by applying many shots of low laser fluence or with one high-fluence laser shot only. The experimental data reveal that in most cases the elemental ablation probability can be correlated with the respective ionization energies of the elements, i.e., the elements with lower first ionization energy have higher ablation probability. No or only very weak fractionation was observed when elements with nearly the same ionization energies were sampled.