Influence of Geometrical Asymmetry on the Performance of a Digitally Driven Quadrupole Mass Filter

Digital (rectangular-wave) mode has emerged as an attractive alternative to conventional sinusoidal operation in quadrupole mass filters because of its simplified electronics and enhanced operational flexibility. However, the influence of geometrical imperfections on digitally driven quadrupole mass filters and the resulting consequences for ion transmission and mass resolving power remain largely unexplored.

The transmission characteristics of a digitally driven quadrupole mass filter operating in the first stability zone were investigated using SIMION simulations. Controlled radial asymmetry was introduced through single-rod radius variation, single-rod displacement, diagonal rod-pair radius variation, and diagonal rod-pair displacement. Stability, transmission characteristics, and resolving power were evaluated for different degrees of asymmetry and for both initial high and low states of the applied rectangular waveform.

Radial asymmetry consistently degraded transmission efficiency and mass resolving power in all four asymmetric configurations, demonstrating the detrimental influence of geometry-induced higher order spatial harmonic field components. In addition, the performance was found to depend strongly on the initial state of the applied RF pulse, with significant changes in transmission profiles, peak widths, and resolving power. Under single-rod radius variation, the transmission peak splits for a specific initial pulse state and is correlated with a bifurcation of the stability region.

Geometrical asymmetry systematically degrades the performance of digitally driven quadrupole mass filters, while the initial pulse state introduces an additional operational parameter that influences transmission and resolving power. These findings establish practical constraints on maintaining symmetry in both rod geometry and spatial positioning, as well as on synchronizing ion injection with the driving RF pulse, and provide new insight into the interplay between higher order multipole fields and digital QMF operation.