Comparison of ion trajectories in vacuum and viscous environments using SIMION: Insights for instrument design

The effective use of electrostatic and magnetic fields to control ion motion serves as the foundation for many scientific instruments. The advent of powerful personal computers and simulation programs, such as SIMION, have served to broaden the understanding of ion motion in vacuum throughout the scientific instrument community. The relatively recent development of a statistical dynamics simulation user program for SIMION 7.0 has provided a more accessible view into ion motions at elevated pressures and the opportunity to more fully illuminate ion motion differences between vacuum and viscous environments (e.g., electrostatic refraction, motion through grids, and magnetic fields). The loss of kinetic energy limits options for controlling ion motion in viscous conditions. For refraction, only net accelerating field methods (converging or diverging) are possible in viscous regimes. Motion of ions around wires in grids also has more severe consequences in viscous conditions than in vacuum. For magnetic fields, a good “rule-of-thumb” is that an ion must be able to complete a significant fraction of its cyclotron radius between collisions for the magnetic field to affect ion diffusion. At slightly reduced pressures, combined magnetic and electrostatic fields can be used to control ion trajectories. The difference in ion behavior is governed by the fact that kinetic energy of ion motion is retained in vacuum, but lost to collisions with the bath gas in viscous atmospheres. Because of the loss of kinetic energy, ion behavior in electrostatic and magnetic fields under viscous conditions is dramatically different than in vacuum.