A COLLISIONAL FOCUSING ION GUIDE FOR COUPLING AA ATMOSPHERIC-PRESSUREION-SOURCE TO A MASS-SPECTROMETER

The formation of a beam of ions produced by an ion source working at h igh background gas pressure is a very important process in any gas-dyn amic interface. While undergoing expansion into vacuum the ions acquir e large velocity and spatial dispersions. Recent experiments have show n that it is possible to obtain large improvements in the beam quality by subsequently passing the beam through an RF multipole at an interm ediate pressure. The multipole RF field constrains the ions close to t he axis, while the necessary dissipative forces are provided by collis ions with the gas. Presented here is a theoretical model of the RF-onl y collisional multipole ion guide, based on the calculation of frictio nal and random thermal interactions of ions with the buffer gas. It is shown on the basis of a hard-sphere model that a formalism describing Brownian motion in the RF electric field is applicable within the ene rgy and mass range of interest. Analytical formulae have been derived describing the lime and space characteristic the ion energy dissipatio n, the time-dependent space distribution of thermalized ions, and the asymptotic radial distribution of the ion beam.