Harmful influences of confinement field non-linearities in mass identifying for a Fourier transform quadrupole ion trap mass spectrometer

The Fourier transform operating mode requires an external ion source and a large amplitude of the ion motion during confinement. Consequently, even far from a resonance line, ions experience non-linearity effects during confinement due to the maximum amplitude dispersion of the ion cloud during confinement connected to that at the beginning of the confinement. For a same mass-to-charge ratio ion cloud, spectrum is constituted by the superimposition of close-frequency peaks. As a result, mass resolution is limited by peak splitting. In addition to the case of a single confined ion, the operating mode is simulated based on an ion cloud's with initial positions and velocities drawn from Gaussian laws. The mass resolution limit is reported as a function of the initial-condition broadening of the ion cloud. For example, simulation results give a mass resolution of 3250 at the mass 130 u for an operating point βz=0.74 and with an ion cloud having, at the beginning of the confinement for the axial direction, 4 mm for the mean position and 2 mm for the standard deviation.