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The rf-only multipole is the key technology for efficiently transporting ions generated under atmospheric pressure conditions into the high vacuum region of a mass spectrometer. A detailed review of the physics of multipoles andtheir use as narrow band pass mass filters is availableelsewhere.1,2 Multipole devices consist of two sets of polished, isolated rod pairs set in a parallel geometry within aninsulating spacer. rf waveforms, which are 180° out of phasewith respect to adjacent rods, are applied to each rod set. Inmost configurations, a single dc bias potential can also beapplied to the rod pairs so that a convenient zero potentialcan be defined elsewhere within the instrument. Within themultipole, ions are radially focused as they pass along theaxis of the device. Transport through the multipole dependson an ion’s initial velocity, space charge effects, and the effect of any externally applied field. While moving along theaxis of the multipole, ions undergo inelastic collisions withthe surrounding buffer gas. The net result is a collimated ionbeam at the exit of the device which displays a reduced average kinetic energy and narrowed energy distribution compared to the initial conditions of the ions as they are introduced into the device.3
