Quadrupolar ion excitation for radiofrequency-only mass filter operation.

Trajectory calculations are used to model a mass filter based on the radiofrequency (rf)-only operation of a linear quadrupole with resonant quadrupole excitation of ions (resonant excitation applied with the same spatial electric field as the main quadrupole rf field). Ions are not trapped, but pass continuously through the quadrupole. Excited ions gain axial kinetic energy in the fringe field at the quadrupole exit, overcome a stopping potential and are transmitted to an external detector.

Ion cloud model for a linear quadrupole ion trap.

If large numbers of ions are stored in a linear quadrupole ion trap, space charge causes the oscillation frequencies of ions to decrease. Ions then appear at higher apparent masses when resonantly ejected for mass analysis. In principle, to calculate mass shifts requires calculating the positions of all ions, interacting with each other, at all times, with a self-consistent space charge field.