Patent ID: 6897438

Claim:
A quadrupole electrode system for connection to a voltage supply means for providing an at least partially-AC potential difference within the quadrupole electrode system, the quadrupole electrode system comprising: (a) a central axis; (b) a first pair of rods, wherein each rod in the first pair of rods is spaced from and extends alongside the central axis; (c) a second pair of rods, wherein each rod in the second pair of rods is spaced from and extends alongside the central axis; and (d) a voltage connection means for connecting at least one of the first pair of rods and the second pair of rods to the voltage supply means to provide the at least partially-AC potential difference between the first pair of rods and the second pair of rods; wherein, at any point along the central axis, an associated plane orthogonal to the central axis intersects the central axis, intersects the first pair of rods at an associated first pair of cross sections, and intersects the second pair of rods at an associated second pair of cross sections; the associated first pair of cross sections are substantially symmetrically distributed about the central axis and are bisected by a first axis orthogonal to the central axis and passing through a center of each rod in the first pair of rods; the associated second pair of cross sections are substantially symmetrically distributed about the central axis and are bisected by a second axis orthogonal to the central axis and passing through a center of each rod in the second pair of rods; the associated first pair of cross sections and the associated second pair of cross sections are substantially asymmetric under a ninety degree rotation about the central axis; and, the first axis and the second axis are substantially orthogonal and intersect at the central axis; such that in use the first pair of rods and the second pair of rods are operable, when the at least partially-AC potential difference is provided by the voltage supply means and the voltage connection means to at least one of the first pair of rods and the second pair of rods, to generate a two-dimensional substantially quadrupole field having a quadrupole harmonic with amplitude A 2 , an octopole harmonic with amplitude A 4 , and a hexadecapole harmonic with amplitude A 8 , wherein A 8 is less than A 4 , and A 4 is greater than 1% of A 2 .