Acceptance of the quadrupole mass filter in the upper stability island under biharmonic excitation

Dynamic characteristics of the quadrupole mass filter (QMF) with the parametric resonance excitation of ion oscillations by a low additional HF voltage are studied by numerical methods. The upper stability island formed by instability bands that follow the isolines of stability parameters of the unperturbed first stability band is considered. Isolines of characteristic indices on the island’s parameter plane are calculated. Ion trajectories at characteristic points near the x and y island boundaries, which have the form of beats, are presented. Parameters of the ellipses are determined as functions of the initial phase at which the ions enter the HF field. These parameters are shown to be periodic with a period of π with respect to the phase shift between the main and additional HF voltages. As a result, the QMF acceptance and, consequently, transmittance have maxima as functions of the phase shift. Therefore, to increase the QMF transmittance, the biharmonic signal must be synchronized.