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

摘要

The Fourier transform operating mode applied to a quadrupole ion trap results in an axial secular frequency spectrum of the simultaneously confined ions. During confinement, the ion trajectory must be as pure as possible, giving only one frequency peak for an ion population with the same mass-to-charge ratio. The simulation work presented here examines the harmful influences of the spatial defects (here, mainly electrode truncation) of the ion trap leading to a non-linear confinement field. The observed harmful influences on the ion’s axial trajectory and its corresponding frequency spectrum is studied based on a case of a single confined ion. Secular frequency of the motion is calculated as a function of the maximum amplitude of the axial and radial trajectories. In the operating mode used in this work, amplitude and frequency of the confinement potential remain constant. As a result, mass analysis is impaired on mass range sustaining the influence of a resonance line. Secular frequency peak amplitude is modified, coupling peaks gain amplitude and ions could be lost. Defective mass ranges have been estimated for a trap truncated at 2r0 and 3r0 and different maximum amplitudes of the ion motion. 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.