Isotope fractionation during ion beam formation in multi-collector inductively coupled plasma mass spectrometry

Instrumental mass discrimination in multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) was investigated with respect to influences after ion extraction from the plasma and during ion beam formation. The ions were implanted in targets which were subsequently subjected to spatially (radially) resolved isotopic analysis via laser ablation MC-ICP-MS or to bulk isotopic composition via conventional solution nebulization MC-ICP-MS. For the elements Cd, Pb and U, the isotope ratios show a strong variation as a function of distance from the center, whereas for lighter elements such as Li or Ni, such variation could not be detected by this approach, due to low signal-to-background ratios. The bulk isotope ratio of Li was found to be fractionated by 9–12% with respect to that in the sample at the earliest accessible location of the ion beam. Heavier elements show less isotope fractionation down to ≈ 0.4% for Pb and U. The combination of bulk and spatially resolved data suggests a combination of coulombic repulsion and scattering effects as main contributors to instrumental mass discrimination.