Mass defects and excitation energies are important properties which reveal the structure of atomic nuclei. Reaching out for the nuclear landscape's boundaries of existence of is a major challenge of modern experimental physics. The high-accuracy mass-spectrometry setups ISOLTRAP and SHIPTRAP, located at CERN/Geneva and GSI/Darmstadt, respectively, allow the determination of precise mass values of exotic nuclei with half-lives down to well below a second. The ions are delivered by the corresponding accelerator facilities and – due to their short lifetimes – have to be investigated "online". 

The experiments are confronted with further challenges. In particular, there are often isobaric contaminants in abundances many orders of magnitude higher than the ions of interest. For precision measurements, these contaminant species have to be removed. Furthermore, very exotic atoms come in only tiny amounts, and the event rates can be as low as just a few ion counts per hour. Nevertheless, ion-trap methods have been developed for dealing with these circumstances, such that either nuclei with lifetimes of only a few tens of milliseconds are accessible or that mass resolving powers exceeding 10 million and relative uncertainties in the 10^{-8} region can be reached.

The investigations are supported by the German Federal Ministry of Education and Research.