Quadrupole mass filters with varying mathieu parameter

Abstract Quadrupole mass filters may have a q value (the Mathieu parameter) that varies as ions pass through owing to either non-parallel rods or to very rapid mass scanning either by voltage or by frequency variation. The acceptance of a mass filter of finite length is calculated under these conditions. Assuming full utilization of the available acceptance, peak shapes and peak positions are calculated. They are both functions of the rate of change of q . Varying q can lead to peak tailing. The orientation of the acceptance area in the phase plane varies for operating points near the stability boundary when there is a transition from stable to unstable conditions as the ions pass through the quadrupole. This could result in peak shape irregularities depending upon the matching of the ion source emittance to the analyser acceptance.