COLLISION-MEDIATED AXIAL EJECTION OF RADIALLY EXCITED IONS AS A MASS-DEPENDENT ION LOSS MECHANISM IN FOURIER-TRANSFORM ION-CYCLOTRON RESONANCE MASS-SPECTROMETRY

In the Fourier transform ion cyclotron resonance (FT-ICR) experiment performed at pressures above 10(-7)Torr, the primary signal damping mechanism for low mass ions excited to large cyclotron radii is shown to be axial ejection following high energy collisions with neutrals. Experimental evidence to support this ion loss process is presented. At increased pressure, "citation profiles acquired as a function of increasing radial kinetic energy exhibit a reduction both in signal magnitude and excite maxima which is attributed to FT-ICR signal loss for higher energy ions. This signal loss increases with decreasing mass of the kinetically excited ion. FT-ICR mass resolution of odd harmonics is found to increase by a factor that is proportional to harmonic order which suggests that the signal reduction is due to ion ejection from the cell rather than phase randomization or a reduction in ion cyclotron energy. Arguing in favor of axial rather than radial ejection, the erosion of FT-ICR excitation profiles at high energies is eliminated as trap potentials are increased. These results suggest that sensitivity gains achieved with high energy excitation should be sacrificed in favor of low energy excitation if quantitative broadband FT-ICR data are required, for example in ion/molecule and collision-induced dissociation experiments. High energy axial ejection will be of increasing concern with the use of larger superconducting magnets since cyclotron kinetic energy scales with the square of magnetic field, without a compensating increase in the trapping electric field.