Radial asymmetry in quadrupole mass filters: stability, multipole fields and resolution enhancement

Abstract

This study examines the effects of radial asymmetry in a linear quadrupole mass filter with circular rods, introduced either by a change in electrode radii or by displacement of a diametrically opposite pair. A radial potential model is developed to account for the resulting geometric deviations, enabling analysis of the first stability region specific to the quadrupole component. The transmission characteristics of such asymmetric configurations are systematically examined, revealing a linear shift in the transmission peak along the q-axis as a function of the asymmetry parameter. This behavior is interpreted through modifications observed in the stability diagram. Furthermore, increased asymmetry is shown to broaden the transmission contours and generally reduce resolution. Notably, a `magic' asymmetry parameter of -0.02 corresponding to electrode displacement, yields enhanced resolution compared to the symmetric case for a fixed rod-to-field radius ratio. An empirical relationship is established between the resolution and the combined contribution of the coefficients of octupole and dodecapole potential components, highlighting the critical role of higher-order field effects in performance optimization.