A collisional focusing ion guide for coupling aa atmospheric pressure ion source to a mass spectrometer

The formation of a beam of ions produced by an ion source working at high background gas pressure is a very important process in any gas-dynamic interface. While undergoing expansion into vacuum the ions acquire large velocity and spatial dispersions. Recent experiments have shown that it is possible to obtain large improvements in the beam quality by subsequently passing the beam through an RF multipole at an intermediate pressure. The multipole RF field constrains the ions close to the axis, while the necessary dissipative forces are provided by collisions with the gas. Presented here is a theoretical model of the RF-only collisional multipole ion guide, based on the calculation of frictional 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 energy and mass range of interest. Analytical formulae have been derived describing the lime and space characteristic the ion energy dissipation, the time-dependent space distribution of thermalized ions, and the asymptotic radial distribution of the ion beam.