Ion trajectory simulation of linear multipole ion traps for analysis of spatial ion distribution

We explore the spatial distribution of ions in a linear multipole radio-frequency ion trap by ion-trajectory simulation using the SIMION software. As a typical example, a two-dimensional column density is characterised for Ag2+ ions stored in several types of the trap. Taking into account the effects of the space charge and ion–buffer He collisions, we are able to reproduce a ring profile of an ion distribution experimentally observed for a linear octopole trap at a high density of ions close to the space-charge limit. This simulation demonstrates also that the profile of the ion distribution is dependent on the number of ions stored and the temperature of a buffer He gas. As for a quadrupole trap, an ion distribution is shown to be located around the central axis of the trap as observed by experiment. An influence of the electrode structure is also discussed for the quadrupole trap.