The mass of a highly charged ion is the sum of the mass of the nucleus, the mass of the electrons and the electronic binding energies. High accuracy mass measurements on highly charged ions in a sequence of different charge states yield informations on atomic binding energies, i.e., the ionisation potentials. In our contribution we discuss the possibility of determining atomic binding energies of highly charged ions to better than 20 eV via cyclotron frequency measurements in a Penning trap. At this level of accuracy different contributions to the binding energies, like relativistic corrections, Breit corrections and QED corrections, can be measured.