Initiation of coherent magnetron motion following ion injection into a Fourier transform ion cyclotron resonance trapped ion cell

Abstract

The initiation and growth of coherent magnetron motion for ions injected into a Fourier transform ion cyclotron resonance trapped ion cell is investigated. The phenomenon is demonstrated for laser desorption/ionization (LDI) of metals and salts and for electrospray ionization (ESI) of multiply charged proteins. In all cases, the rate of growth of the magnetron orbit increases proportionally with trap potential, ion density, and external detection circuit resistance. The data support resistive damping of the magnetron motion as the primary relaxation mechanism leading to ion ejection from the cell within time periods ranging from a few hundred milliseconds to tens of seconds. Factors associated with ion injection that contribute to initiation and growth of the destabilizing motion are length of the injection period relative to the magnetron period and extent of off-axis injection. In general, if the injection period is smaller than one period of the magnetron motion, as is the case for LDI, and the injection point is severely off the centerline of the cell, coherent growth is observed instantaneously and ions are expelled from the cell within a few hundred milliseconds. If the LDI ions are introduced along the centerline, coherent motion leading to expulsion is still observed but initiation times increase ten-fold. For the case in which ESI ions focused along the centerline are injected into the trap for a time much longer than the period of a magnetron oscillation, initiation and growth of coherent magnetron is only observed at much longer times and with a larger trap potential.