  Significant systematic errors in high-precision Penning trap mass
spectrometry can result from electric and magnetic field imperfections. An
experimental procedure to minimize these uncertainties is presented for the
on-line Penning trap mass spectrometer ISOLTRAP, located at ISOLDE/CERN. The
deviations from the ideal magnetic and electric fields are probed by measuring
the cyclotron frequency and the reduced cyclotron frequency, respectively, of
stored ions as a function of the time between the ejection of ions from the
preparation trap and their capture in the precision trap, which influences the
energy of their axial motion. The correction parameters are adjusted to
minimize the frequency shifts.
