Mass-dependent z-excitation of ions in cubic traps used in FTMS

Fourier transform mass spectrometric ion excitation methods such as rapidly scanned radio frequencies (chirps) are intended to put momentum into the ion cyclotron mode only. However, momentum will also be transferred to the trapping oscillation mode of the ions, as is shown from both ion trajectory simulations and experimental data. Momentum transfer occurs because of a synchronization between ion motion and temporal changes of the z-component of the excitation field to produce a net average force toward the nearest trap plate. Because a stronger excitation field is associated with chirp excitation relative to rf bursts, the phenomenon is particularly apparent with chirps. To a rough approximation, the synchronization effect increases with decreasing mass, decreasing trap voltage, and increasing excitation amplitude. When the excitation amplitude is increased to accommodate higher magnetic fields, the effect is expected to increase also. The most troublesome manifestation of this effect is mass-dependent ejection of ions from the cell. Methods for reducing z-excitation are proposed.