  Doppler imaging, a technique which inverts spectral line profile variations
of an Ap star into a two-dimensional abundance maps, provides new observational
constraints on diffusion mechanism in the presence of a global magnetic field.
A programme is presented here with the aim to obtain abundance distributions of
at least five elements on each star, in order to study how different diffusion
processes act under influence of a stellar magnetic field. The importance of
this multi-element approach is demonstrated, by presenting the abundance maps
of helium, magnesium, silicon, chromium and iron for the magnetic B9pSi star CU
Virginis.
