ELECTRON-BEAM POTENTIAL DEPRESSION AS AN ION-TRAP IN FOURIER-TRANSFORM ION-CYCLOTRON RESONANCE MASS-SPECTROMETRY

Trapping of ions in the electron beam of a FTICR mass spectrometer is investigated and a simple model describing the confinement process is presented. Detection of resistive-wall destabilization of the magnetro n motion of ions in the trapped-ion cell is used to determine conditio ns for ion trapping within and escape from the electron beam. The mode l predicts a potential well that is dependent on electron beam current , energy, and dimension in defining its capacity for low energy ions. Plots of ion retention time versus ion number are consistent with a mo del in which ions are initially trapped in the electron beam but with increasing ion formation will eventually overcome the potential depres sion in the electron beam and escape into magnetron orbits. Based upon this model, expressions are derived for ion retention time which are then fit to the experimental data. The model is used to estimate ion n umber, initial magnetron radius and ion cloud shape and density. One e xample in which electron trapping is important in the FTICR experiment is in the efficient transfer of ions between dual trapped-ion cells. Ion transfer within the potential depression of the electron beam envi ronment is shown to be virtually 100% efficient over a 10 ms interval whereas all ions are lost to collisions with the conductance limit aft er 2 ms when transferring without the confining aid of the electron be am. Several analytical applications of electron traps in the ICR cell are now being investigated.