Abstract
The main features of the stability diagram for a quadrupole mass filter can be understood qualitatively using two concepts: (i) moderate RF voltages stabilize a trajectory; (ii) large RF voltages destabilize a trajectory. The trajectories of the ions traveling in the quadrupole are readily displayed pictorially for any set of operating conditions. These are computed as a straightforward student exercise, integrating numerically the equations of motion of the ion in the quadrupole electric field. The outcome of these trajectories-whether the ion is transmitted or deflected-allows us to map the stability diagram. The stability diagram reveals how the quadrupole acts as a mass filter and delivers a mass spectrum. The characteristic features of the trajectories are complicated and varied; and they are obtained for each region of the stability diagram. Explanations for some of these features are developed by examining the forces on the ion as it proceeds through the quadrupole.