RF quadrupole systems with potential gradients
View Patent ↗The invention relates to two-dimensional quadrupole systems along whose axis an axial DC field is superimposed. The invention involves coating the hyperbolic or cylindrical surfaces of quadrupole systems with thin insulating layers and metal films thereupon and generating axial potential gradients or saddle ramps using appropriate electrical supply of DC potentials and superimposed RF voltages to the metal films. Systems of this type can be used in a plurality of ways, ranging from mass filters with high transmission to fragmentation cells with extremely low ion losses.
1. RF quadrupole system made of lengthy electrodes,
wherein the electrodes consist of a material with good electrical conductivity, carrying at their surface a thin insulating layer covered by a thin conductive layer, and wherein for each of the electrodes, the electrodes and the ends of their thin conductive layers are connected to different DC potentials superimposed by the same phase of the RF voltage, the phases of the superimposed RF voltage changing in turn from electrode to electrode.
2. RF quadrupole system according to claim 1 , wherein the thin conductive layers are each electrically connected to the corresponding electrode beneath at least at one distinct location.
3. RF quadrupole system according to claim 1 , wherein the thin conductive layers on the electrodes have a maximum thickness of ten micrometers.
4. RF quadrupole system according to one of the claims 1 , wherein the longitudinal resistances of the thin conductive layers are between one and a hundred kiloohms.
5. RF quadrupole system according to one of the claims 1 , wherein parts of the surface of the electrodes have a hyperbolic shape, said surfaces being positioned diagonally opposite each other in the quadrupole system.
6. RF quadrupole system according to claim 5 , wherein only the hyperbolically shaped parts of the surface are coated with insulated thin conductive layers.
7. RF quadrupole system according to claim 1 , wherein the DC potentials applied to the electrodes and to the ends of the thin conductive layers, are supplied by means of center taps on separate secondary windings of an RF transformer.
8. RF quadrupole system according to claim 1 , wherein the DC potentials are adjustable.
9. RF quadrupole system according to claim 1 for selecting ions according to their mass-to-charge ratio,
wherein
a) a first set of DC voltages of opposite polarity is superimposed on the two phases of the RF voltages in order to select ion species in a preset range of mass-to-charge ratios,
b) the thin conductive layers are connected to the electrodes beneath at one location, and
c) further DC potentials are applied along the thin conductive layers, said potentials attenuate the first set of DC voltages in the injection region for ions, the attenuation disappearing in the shape of a ramp into the interior of the quadrupole system.
10. RF quadrupole system according to claim 9 , wherein in the ejection region, a ramp for collecting the selected ions in the axis of the quadrupole system is set up by means of a third set of DC potentials which gradually attenuate the first set of DC voltages.
11. RF quadrupole system according to claim 9 , wherein a damping gas maintains a damping pressure to damp the transverse oscillations of ions.