Resonance excitation of ions stored in a quadrupole ion trap. Part 1. A simulation study

Abstract

Preliminary findings are presented of a simulation study of resonance excitation of gaseous ions stored within a quadrupole ion trap. The ion trap was operated in the common mode wherein a radio-frequency drive potential was applied to the ring electrode while the end-cap electrodes were grounded or held at a small potential. The arrangement chosen for computer simulation of resonance excitation discussed here is that in which an auxiliary potential is applied to both end-cap electrodes such that these electrode potentials are in phase with each other. The effects of resonance excitation are manifested in the following ways: temporal variation of ion kinetic energy; temporal variations of axial and radial displacements from the trap centre; direction of ion ejection; and emergence of new frequency components of ion motion. The extent of resonance excitation has been examined as a function of ion mass, trapping parameters a u and q u , ion initial position and velocity, drive and auxiliary potential phase angles, delayed imposition of auxiliary potential, auxiliary potential amplitude and frequency, and mass of an inert collision gas. Four principal findings are reported. Irradiation of trapped ions at the two fundamental secular frequencies, axial and radial, does not lead exclusively to ejection of ions in the axial and radial directions, respectively: there is clearly a degree of perturbation of ion radial motion upon irradiation at the axial fundamental secular frequency, and vice versa, which can become dominant under certain conditions and lead to a change in direction of ion ejection. Excitation at theoretically predicted secular frequencies other than the fundamental frequencies is virtually negligible, as expected from the magnitudes of the C 2 n coefficients. Strong resonance absorption occurs at a series of frequencies which are not present in the motion of the unexcited ions; these frequencies are designated as new frequencies. The motion of ions subjected to resonance excitation often becomes characterized by additional frequency components which we have designated as induced frequencies.