STACKED-RING ELECTROSTATIC ION GUIDE

In 1969 Bahr, Gerlich, and Teloy introduced an rf device that consiste d of a stack of ring electrodes, with charge sign alternation between neighboring rings, to store or transport ions. Here we propose to oper ate such a device with electrostatic potentials rather than rf potenti als: ions that move axially along the center of the guide are thereby subjected to an oscillating electrical potential similar to the sinuso idal rf potential in familiar rf-only multipole ion guides. The oscill ating potential of the stacked-ring static ion guide focuses ions by e xerting a field gradient force on the ions so as to push ions toward t he central axis where the field is weakest. The stacked-ring ion guide produces an effectively static ''pseudopotential'' that is much steep er at the edge (potential varies as e(r)) compared to a quadrupole or octupole guide (for which the potential varies as r(2) or r(6), where r is radial position) and that is much flatter near the center of the guide (for potentially higher ion flux). Advantages of the new ion gui de include static rather than rf potential, low electrical noise, a la rge field-free region near the central axis of the guide, and simple m echanical construction. A disadvantage of the stacked-ring ion guide i s that high ion axial kinetic energy is required; ions with axial kine tic energy that is too low may be trapped in the shallow pseudopotenti al well between adjacent ring electrodes.