ION TRAJECTORIES IN EXACTLY DETERMINED QUADRUPOLE FIELDS

The use of multi-electrode structures as mass analysers is now very wi despread. These devices are employed as partial pressure analysers, ma ss filters and ion traps. The first of these devices to utilise an ele ctric quadrupole held for separating ions according to their mass-to-c harge ratio was introduced in the 1950s. The operation of these device s is based on the nature of particle trajectories between the electrod e rods. The lack of an exact theory up to now has precluded full explo itation of the device. In the quadrupole mass filter the relevant stab ility theory depends directly on the precise structure of the multipol e potential expansion. The exact coefficients in this expansion have b een determined. A number of trajectory simulations are performed using these coefficients. In addition a simple relationship is found for wh ich certain key coefficients vanish. A set of scaling rules has also b een discovered which enables immediate calculation of each higher orde r coefficient from a fixed set of values for the corresponding scaled coefficients.