  Phase-imaging ion-cyclotron-resonance technique has been implemented at the
Penning-trap mass spectrometer JYFLTRAP and is routinely employed for mass
measurements of stable and short-lived nuclides produced at IGISOL facility.
Systematic uncertainties that impose limitations on the accuracy of
measurements are discussed. It was found out that the phase evolution of the
radial motion of ions in a Penning trap during the application of
radio-frequency fields leads to a systematic cyclotron frequency shift when
more than one ion species is present in the trap during the cyclotron frequency
measurement. An analytic expression was derived to correctly account for the
shift. Cross-reference mass measurements with carbon-cluster ions have been
performed providing the mass-dependent and residual uncertainties.
