Influence of the 6th and 10th spatial harmonics on the peak shape of a quadrupole mass filter with round rods

The influence of the ratio of the rod radius, r , to field radius, r 0 , on the peak shape for a linear quadrupole mass filter constructed with round rods has been investigated. The expansion of the potential in multipoles, $\phi _N,\Phi (x, y) = \sum_{N = 0}^{\infty} A_N \phi_N/r_{0}^{N}$ , has been considered, and the peak shape and resolution have been determined by numerical calculation of ion trajectories in quadrupoles with different ratios, r/r 0 . Geometries that make the dodecapole term ( A 6 ) zero ( r/r 0  = 1.14511) do not give the best performance because the contribution of the 20‐pole term, A 10 , must be considered. The optimum ratio is r/r 0  ≈ 1.13. With this ratio the dodecapole term ( A 6 ) is ca. 1 × 10 −3 , but its effects are partially compensated by the A 10 term which has similar magnitude, but opposite sign. Copyright © 2002 John Wiley & Sons, Ltd.