Most ion guides consist of multipole structures, extending longitudinally, having rod-shaped pole pieces. Their disadvantage is that they do not actively drive the ions forward without complicated additional measures. For this reason, ion guides consisting of stacked diaphragms with circular apertures (known as “stacked rings”) are sometimes used for special purposes; an axial DC or modulated potential gradient permits the ions to be driven forward actively. Examples of this include ion funnels used to capture the ions from a gas flowing into the vacuum, collision cells with diaphragms of constant internal diameter and with active forward drive (“ion tunnels”), and ion packeting equipment using a traveling wave field to provide a forward drive of an ion beam with a desired time profile of ion density.
For example, U.S. Pat. No. 6,107,628 (R. D. Smith and S. A. Shaffer) elucidates an arrangement of an ion funnel in which ions are extracted from a stream of gas and are directed to the output opening that leads to the next differential pump stage. The ion yield is significantly greater than when simple skimmer diaphragms are used. This ion funnel represents a special case of the general description of ion guides in U.S. Pat. No. 5,572,035 (J. Franzen), in which, among other embodiments, arrangements of stacked rings have already been described, operated at radio frequency voltages (RF) with an axial direct current (DC) potential gradient and with both cylindrical or conical internal open space.
Diaphragm stacks in the form of ion funnels are being used more and more frequently instead of the gas skimmers usually applied. The ion funnel consists of a package of coaxially arranged diaphragms having circular apertures, in which the diameters of the circular holes diminishes increasingly toward the central exit hole that leads into the next chamber. The diaphragms are stacked with relatively small spaces between the apertured diaphragms. This creates the shape of a funnel inside the stack of diaphragms. Gas with entrained ions from an ion source that is external to the vacuum is blown through an inlet opening into the vacuum system, or through an inlet capillary, into the open ion funnel. The wall of the ion funnel is highly permeable to gas, as it is formed from the faces of the apertured diaphragms with the open spaces between them. The gas escapes through the spaces between the apertured diaphragms, and is removed by a vacuum pump. Only very little gas enters the next chamber of the differential pump system through the small exit opening.
Both phases of an RF voltage (several hundred kilohertz up to several megahertz; a few hundred volts) are applied alternately to the apertured diaphragms. This repels the ions from the inner funnel wall. The method of operation and the effect of this repelling pseudopotential are described in detail in the quoted patent specification, U.S. Pat. No. 5,572,035. The ions are thus prevented from being drawn away through the intermediate spaces between the apertured diaphragms by the escaping gas stream. The ions are separated out. In addition, a graded DC voltage (a few tens of volts in total) is applied to the apertured diaphragms to create a potential gradient along the axis of the stack of diaphragms. This forces the mobile ions through the highly rarefied gas in the ion funnel toward the exit hole.
The system of annular diaphragms, including the ion funnel, has the advantage of actively driving the ions forward to the exit of the annular diaphragm system. They have, however, the disadvantage that, even in the presence of a cooling damping gas, the ions are not collected along the axis of the annular diaphragm system, since the pseudo-force that repels the ions only exerts its effect close to the outer wall of the cylinder or cone created by the diaphragm openings. The ions therefore fill the entire internal space of the cylinder or cone. If this space is densely filled with ions, the Coulomb repulsion (“space charge effect”) will even increasingly drive the ions against the pseudopotential wall, whereas the part of the internal space that is close to the axis has a lower ion density.
The repelling effect of the walls around the interior space is, moreover, different for ions with different specific masses. “Specific mass” here refers to the ratio of mass to charge. For heavy ions (ions with high specific masses) the ions are only reflected when close to the wall, whereas lighter ions are reflected at a greater distance from the wall.
The embodiment of the ion funnel that has become familiar is particularly disadvantageous from this point of view. The published embodiment has the disadvantage that only a relatively narrow band of specific masses passes through. If the diaphragm openings at the final exit are very small, the pseudopotential from the walls of the narrow channel overlap, and the rise in the overlapping pseudopotential reflects light ions back into the funnel; they cannot leave the funnel. Additionally, the pseudopotential along the axis of the ion funnel displays ripples with potential wells, which collect ions inside and can only be emptied if the axial potential gradient has a certain minimum value. On the other hand, too much gas will enter into the next differential pump stage if the diaphragm openings at the final exit of the funnel are too large. If large diaphragm openings are followed by an extraction lens with narrow openings to extract the ions from the ion funnel, the heavy ions cannot be extracted if the space charge is high, since they will be driven outward to the walls of the funnel and escape the drawing field of the extraction lens, which can only effectively extract ions out from the axis.
One solution that is already known is an ion funnel that consists of annular diaphragms each of which is divided into four quadrants, wherein the four quadrants of each annular diaphragm alternately carry the two phases of the RF voltage. The next annular diaphragm then carries the phases of the RF voltage crosswise. Manufacture of this quadrant funnel is, however, extraordinarily difficult and expensive.
The individual guiding elements of the mass spectrometer through which the ions are to pass generally have very sharply defined acceptance cross sections for incoming ions, different as well for the distribution of directions as for the energy distribution of the ions available in the ion beam. The beam cross section in particular can generate high or low transmission of ions into the next section. For example, the literature describes a very narrow, elliptical acceptance cross section for a quadrupole filter. The narrow acceptance cross section extends between the two pole pieces that carry the DC voltage that attracts the ions. In contrast, a time-of-flight mass spectrometer with orthogonal injection of the ions into an ion pulser requires a very narrow ion beam, close to the axis, and with the most homogeneous distribution of directions and energies that can be achieved. These requirements cannot be satisfied by the stack of annular diaphragms constructed in the manner known at present, even though the possibility of actively driving the ions in the axial direction is a strong factor in favor of the use of diaphragm stacks.