Relation of signal sensitivity and ion z-motion in cubic cells. Theory and implication for ion kinetic studies

The temporal effects of ion evaporation and ion relaxation along the z-axis (parallel to the magnetic field) on ion signal intensity are addressed in this paper. An understanding of these ion dynamical phenomena is important for quantifying ion abundance and measuring rates of ion/molecule reactions with Fourier transform mass spectrometry (FTMS) and trapped cell ion cyclotron resonance (ICR) mass spectrometry. A theory is developed to account for the effects of the z-axis oscillation amplitude of trapped ions on the ion signal. As the z-motions are damped and the ions relax to the center of the cell, ion signal increases. The qualitative features of the theory are tested for three broad classes of ions: unreactive, modestly reactive, and highly reactive in bimolecular, ion/molecule reactions. Ion compression, accomplished by steadily increasing the trap voltage, is introduced as a means of expediting z-axis relaxation so that temporal variations of signal due to relaxation phenomena can be minimized. Ion evaporation, on the other hand, can be avoided by using higher trapping fields in the cell. The implications of these phenomena for ion kinetic studies are discussed.