Influence of octupole field on quadrupole mass filter performance in the second stability zone

TL;DR

This study investigates how radial asymmetry, especially octupole field components, affects the performance of quadrupole mass filters in the second stability zone, revealing ways to optimize resolution and transmission.

Contribution

It provides the first stability diagram for radially asymmetric QMFs in the second stability zone and analyzes how asymmetry influences performance parameters.

Findings

Radial asymmetry shifts the stability apex systematically.

Octupole components scale with asymmetry and depend on DC polarity.

Optimal resolution and transmission are achievable through controlled asymmetry.

Abstract

Radial asymmetry in a quadrupole mass filter (QMF), introduced by symmetric displacement of a diagonally opposite rod pair, modifies the confinement potential and alters the ion stability characteristics. In this work, the influence of such radial asymmetry on QMF operation in the second stability zone is investigated through simulations. Using an existing potential formulation for a radially asymmetric QMF, the stability diagram in the second stability zone is extracted for the first time, revealing a systematic shift of the stability apex with the asymmetry parameter. Radial asymmetry introduces additional multipole components, notably an octupole term whose magnitude scales with the asymmetry parameter and whose sign depends on the DC polarity applied to the displaced rods. Transmission simulations show that the transmission peak shifts in accordance with the displaced stability apex, while the resolution exhibits a strong dependence on both the asymmetry parameter and the DC polarity. Comparable maximum resolution is obtained for inward and outward displacements under suitable polarity configurations, with outward displacement providing higher transmission efficiency due to an increased effective aperture. These results demonstrate that controlled radial asymmetry provides an effective means to tailor the resolution and transmission characteristics of QMFs operating in the second stability zone.