Performance of Quadrupole Mass Filter with Tapered and Flared Geometry

Abstract

The performance of a quadrupole mass filter (QMF) is highly sensitive to deviations from ideal electrode geometry. In this work, we investigate the effect of small inward and outward tilting of cylindrical rods on the resolution and transmission characteristics of a QMF. Such geometric perturbations introduce an axial variation in the radial confinement potential, resulting in Mathieu parameters that vary along the ion trajectory. To examine this effect, the ion stability diagram is computed using a Runge Kutta (RK45) method with axially-varying Mathieu parameters. The modified stability region exhibits shift and contraction depending on the magnitude and nature of rod inclination. The evolution of higher order field components, particularly the dodecapole term, is analyzed along the axial direction. Ion trajectory simulations are performed using SIMION to evaluate the corresponding changes in QMF transmission characteristics in the first stability zone of operation. While simulations at fixed operating conditions indicate a transmission resolution trade off at small tilting angles leading to resolution enhancement, analysis at constant peak transmission reveals that even slight deviations from the parallel configuration lead to a degradation in resolution. These results highlight the critical role of minute geometric imperfections in QMF operation and provide insights into tolerance limits and design optimization for improved mass filter performance.