Stacked-ring electrostatic ion guide.

In 1969 Bahr, Gerlich, and Teloy introduced an rf device that consisted of a stack of ring electrodes, with charge sign alternation between neighboring rings, to store or transport ions. Here we propose to operate such a device with electrostatic potentials rather than rf potentials: ions that move axially along the center of the guide are thereby subjected to an oscillating electrical potential similar to the sinusoidal rf potential in familiar rf-only multipole ion guides. The oscillating potential of the stacked-ring static ion guide focuses ions by exerting a field gradient force on the ions so as to push ions toward the central axis where the field is weakest. The stacked-ring ion guide produces an effectively static "pseudopotential" that is much steeper 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 guide include static rather than rf potential, low electrical noise, a large field-free region near the central axis of the guide, and simple mechanical construction. A disadvantage of the stacked-ring ion guide is that high ion axial kinetic energy is required; ions with axial kinetic energy that is too low may be trapped in the shallow pseudopotential well between adjacent ring electrodes.