THEORY OF HIGH-RESOLUTION MASS-SPECTROMETRY ACHIEVED VIA RESONANCE EJECTION IN THE QUADRUPOLE ION TRAP

Beginning with the pseudopotential well approximation for ion motion in the quadrupole ion trap, analytical expressions are derived for the ion resonance ejection line shape. Incorporation of terms for ion-molecule collisions and linear variation with time of the resonance excitation frequency enables line shapes to be calculated and displayed as a function of scan rate and relaxation time. Derivation of the relationship between mass and frequency line widths allows the mass resolution to be evaluated as a function of fundamental ion trap operating parameters. The results account for the synergistic effect on resolution when resonance ejection is combined with retarded scan rates and indicate that for any non-zero scan rate there is an optimum collisional damping value corresponding to a resolution maximum. The effects of the experimental variables mass/charge, neutral pressure, and scan rates on mass resolution and accuracy are also evaluated in terms of the model and illustrated In convenient plots.