Rf-only mass spectrometer with auxiliary excitation

Abstract

An RF-only quadrupole mass spectrometer in which an auxiliary RF field (dipole or quadrupole) excites selected ions at low q (typically between 0.20 and 0.89). The excited ions experience radial excursions, but many do not experience excursions sufficient to strike the rods, and are therefore transmitted and acquire increased kinetic energy in the fringing fields at the exit ends of the rods. The excited ions may therefore be selected from the non-excited ions either by their increased axial kinetic energy, or by their radial dispersion, or both, and are detected for analysis, producing sharp peaks of high sensitivity at low q, while retaining the high acceptance and other advantages of RF-only mass spectrometers. Operated in converse fashion, a high quality notch filter can be achieved. In the notch filter, an auxiliary RF field again excites ions and notches some of them out by causing them to strike the rods. Ions which are not notched out but which are nevertheless excited by the auxiliary field are discriminated against, again either by reason of their higher axial kinetic energy or their spatial dispersion or both, so that only ions on axis are detected, producing a high resolution high sensitivity notch filter.