ION EJECTION FROM FOURIER-TRANSFORM MASS-SPECTROMETRY TRAPPED ION CELLS DUE TO NONADIABATIC CHANGES IN TRAPPING POTENTIALS

The z-axis ejection of ions due to non-adiabatic changes in trapping potentials in Fourier transform mass spectrometry (FTMS) trapped ion cells is evaluated. This ejection mechanism is a kinetic energy rather than a mass dependent phenomenon which has important consequences for quantitative measurements that involve ion flight to or from the trapped ion cell. Ion trajectory simulations indicate that a large percentage of ions is ejected when gated trapping is used in external source spectrometers and that at least 20% of the initial ion population is lost in typical dual cell FTMS experiments. A z-axis equation of motion is developed to approximate the anharmonic effects of the grounded conductance limit in dual cell experiments and to allow an investigation of macroscopic properties of the resulting ion cloud including spatial and kinetic energy distributions. The model predicts that an initial z-axis coherence to the ion cloud exists after gated trapping events in dual cell instruments which results in periodic fluctuations in FTMS signal amplitude in non-uniform excitation fields. This effect was verified experimentally with a pluse sequence that yields the predicted z-axis trapping frequencies in cubic cells. © 1990.