GB2301704A - Introducing ions into a high-vacuum chamber, e.g. of a mass spectrometer - Google Patents

Abstract

The invention consists of using an RF multipole ion guide 8 in a differential pumping stage 7. The ion guide ends at the wall 9 of the high-vacuum chamber 13 and is terminated there by a lens. In the wall of the high-vacuum chamber, there is only a very small hole through which primarily only thermalized ions can pass. This hole, together with apertures 10, 11, defines the lens. The voltages at the lens can be utilized to switch passage of ions into the chamber 13 on and off. The movement of the ions is automatically damped in the ion guide due to the residual gas pressure in the differential pump stage. The source of ions may be an electrospray source, as shown, or an ICP source. The high-vacuum chamber may house a mass spectrometer, e.g. a quadrupole ion trap, a quadrupole filter, a time-of-flight spectrometer or an ICR Spectrometer.

Description

Guidance of ions through a differential pump stage The invention relates to methods and devices for the generating ions a high-vacuum area.

It is often advantageous to generate ions for mass spectrometric analyses outside of the mass spectrometer and to transfer them into the high vacuum of the mass spectrometer. The advantages are a much greater ionization yield than with ionization in the vacuum and greatly reduced contamination of the mass spectrometer since the vapor of the unionized substance need not be introduced into the high-vacuum system.

Most successful among vacuum-external ion sources is the electrospray (ESI), with which substances of extremely high molecular weight can be ionized. Also the ion sources for ionization by an inductively coupled plasma (ICP), which are used for inorganic analysis, belong to this group. Finally there is chemical ionization at atmospheric pressure (APCI) with ionization of reactant gases through corona discharges or beta emitters which are used for the analysis of pollutants in air, but also for the ionization of other substances which are present as diluted vapors in air. The development of other types of vacuum-external ion sources can be expected.

Vacuum-externally generated ions are usually introduced into the vacuum either through minute apertures of about 30 to 300 micrometers in diameter or through capillaries about 500 micrometers inside diameter. Common to both types of introduction is that, together with the ions passing into the vacuum of the mass spectrometer, there is also a large amount of ambient gas. This gas must be removed by differential pump stages. Most often, two such pump stages are used with two chambers in front of the high-vacuum chamber of the mass spectrometer. The chambers must be connected to one another by only very small openings, and the ions must be transferred through these small openings. All the apertures are usually arranged in a straight line.

The apertures between the pump chambers generally take the form of skimmers. The skimmers are cone-shaped dents with a central, very small opening at the tip. The cone envelopes are intended to deflect the impinging gas molecules toward the outside so that a largely laminar flow of gas is maintained between the apertures in the axis, but thins down from chamber to chamber.

The gas expands adiabatically from the entrance aperture or the end aperture of the capillary into the chamber of the first pump stage in which there is usually a pressure between 0.1 and 10 millibars. Through gas friction the ions are accelerated into a conical space angle. A portion of the ions can be transferred into the chamber of the next differential pump stage through a skimmer placed relatively close to the aperture, and by a weak electrical focusing field. However this portion is small since scattering effects and a wide distribution of energy make good focusing impossible.

The ions are caught in the second differential pump stage, in which there is a residual gas pressure of 104 to 10-2 millibars, usually by stationary ion-optic DC fields and guided to the minute opening in a second skimmer which separates this second differential pump stage from the high vacuum. A portion of the ions then passes into the high-vacuum chamber. The yield is usually small since the ions have both a large angle of distribution and a wide distribution of initial velocity and are still subject to considerable scattering due to collisions with residual gas molecules.

It may become possible in the future to use finer capillaries of about 20 centimeters in length and about 200 micrometers inside diameter with sufficient transfer efficiency for the ions, transporting much less gas into the vacuum system. Even then a differential pump stage would still be necessary. This would be identical to the second differential pump stage in the above arrangement.

For guidance of the ions to the mass spectrometer after their entrance into the high-vacuum chamber, stationary lens systems and RF multipole based ion guides are both known. The latter generally extend from the skimmer aperture in the wall up to the entrance of the mass spectrometer. Multipole systems are advantageous here since they can also capture ions with spatial divergence and a wide scatter of kinetic energies.

Recently an apparatus from Analytica of Branford Inc. (USA) has appeared on the market which represents a major advance. The ions are already collected in the final (second) differential pump stage using a linear RF hexapole field, and transferred through the differential pump stage and on through a wall opening to the high vacuum, and finally to the mass spectrometer. The advantage of this system is that, due to the residual gas in the differential pump chamber, the kinetic energies of the ions are already damped.

The ion guide from Analytica of Branford described above considerably improves the transfer of ions to the mass spectrometer. However it still has disadvantages. The wall opening to the high-vacuum chamber must be relatively large and the rod arrangement including the isolating holders for the rods also brings more residual gas to this opening. In this way, a relatively large quantity of residual gas enters the high-vacuum chamber. This either requires a large and costly pumping system for the high-vacuum chamber, or the pressure in the second differential pump chamber must be kept very low.

Both requirements must be seen as disadvantages. High requirements of the pumping system are reflected in the price of such a pump device. Low pressure does not lead to the desired damping of kinetic energy in the ions. Additionally the price paid for low pressure is a smaller opening in the first skimmer, which reduces transfer efficiency for the ions at this point.

The invention seeks to find a method and device for transferring ions to a mass spectrometer from a continuously operating vacuum-external ion source with a high yield, without increasing the demands on the pumping system. If such ions are deprived of their kinetic energy, the ions sustain a reduction in their phase space (the phase space is defined as a six dimensional space which is generated from the maximum location and pulse coordinates of the ion assembly), and thus provide good ion-optic preconditions for further focusing, and for analyses in the mass spectrometer. It is also important when using storage mass spectrometers to be able to store the ions temporarily so that all ions are available within a short filling period.

The invention involves the use of an RF multipole field, which may be of known form and generated in the space between extended, parallel pole rods. The ion guide ends at the wall of the high-vacuum chamber and is terminated there by a switchable lens. In the wall of the high-vacuum chamber, there is only a very small hole through which primarily only thermalized ions (i.e., ions which have been slowed down to thermal energies), can pass. The voltages at the lens can be utilized to switch passage on and off. The movement of the ions is automatically damped in the ion guide due to the residual gas pressure in the differential pump stage. Their phase space is reduced by the damping and the precondition for further transmission by ion-optical means is improved.Efficient guidance of ions through the prevacuum area of the differential pump stage is possible, and further guidance of the ions, due to the improved vacuum, is possible with any ion-optical means.

In accordance with the invention an multipole RF ion guide is located in the final differential pump chamber before the high-vacuum chamber. In contrast to previously known designs, the multipole RF ion guide ends at the wall of the high-vacuum chamber and is terminated there by a switchable drawing lens. The RF applied to the multipole RF ion guide may be a two-phase or multi-phase alternating voltage, depending on the kind of rod system. The wall of the high-vacuum chamber thereby forms the first apertured diaphragm of the drawing lens, and this apertured diaphragm only has a very small opening. The ion guide is provided with reflecting electrical fields on both sides by lowering the axis potential with reference to the entrance and exit apertures. In this way, the ions are trapped in the ion guide. Their movement is damped in the residual gas of the differential pump stage; the ions assemble themselves in a thread-like area exactly along the axis of the ion guide.

Through the very small hole in the wall of the high-vacuum chamber, centered exactly at the axis of the ion guide, and by the generation of a protruding weak electrical suction field of the lens, the calmed ions on the axis of the ion guide are drawn into the high-vacuum chamber.

Since the undamped, freshly arriving ions remain almost exclusively off the axis, their passage is obstructed and they are reflected back into the ion guide.

Through the small wall opening, passage of the residual gas into the high-vacuum chamber is limited in the desired manner. The calmed ions are transferred almost completely however, having for the most part a very narrow spatial and energy distribution. Their six dimensional phase space, generated from spatial and pulse co-ordinates, is small. Therefore they can be guided on very easily by ion-optical means.

The ion guide can consist of RF quadrupole, hexapole or octopole fields which are generated in a known way by a bipolar RF voltage between four, six or eight pole rods. Larger multipole fields can also be used, but are somewhat disadvantageous since the calmed ions are no longer assembled in the center. It can however also be advantageous to generate an RF five-pole field between five pole rods, operated by a five-phase RF voltage.

The generation of a weak suction field near the hole is caused by a voltage across the second apertured diaphragm of the lens which is arranged behind the hole. With the help of a third additional apertured diaphragm, an Einzel lens can be formed in a known way. The lens can also be designed as a drawing lens by providing an asymmetrical voltage on both outer electrodes. The ions then leave the lens with a defined kinetic energy, thus speeding up the transfer.

By switching the potentials on the lens, passage of the ions through the wall can be switched on and off. In this way intermediate storage of the ions is possible. If the mass spectrometer does not need any ions, the ion supply can be switched off without losing any ions formed in the meantime. This is especially important for ion trap mass spectrometers which have two operation phases, an analysis and a filling phase, whereby ions are only admitted during the filling phase.

As is already known for such ion guides, undesirable ions can be removed from the storing ion guide before they pass on into the mass spectrometer. This can be achieved by means of the mass cutoff limit which is a property of every RF multipole field, by the use of superimposed DC fields, and/or by means of superimposed frequency mixtures which excite the secular frequencies of undesired ions and thus eliminate them from intermediate storage.

The ion guide made up of multipole fields between rod-shaped poles has, as is known in every quadrupole filter, an automatic cutoff limit for lightweight ions. Ions having an m/z ratio lower than a defined (adjustable) value have no stable trajectories in the ion guide and are eliminated before they reach the mass spectrometer in the high-vacuum chamber.

If additional DC fields are superposed on the ion guide, the mass range of the ions to be stored can be further reduced, even reduced to one single mass.

If a mixture of frequencies is superposed on the ion guide rods, the secular frequencies of the ions can be excited and the ion oscillations increased so much that the ions are removed from the storage region. A frequency gap in the frequency mixture can be selected in such a way that the ions which it is desired to retain are not excited and are therefore not removed. The frequency mixture can be mixed into the storage RF, the excited field is then a hexapole field (or, according to the type of rod system, a different multipole field). This arrangement has, however, only limited effectiveness since the hexapole field disappears on the axis.

If however the excitation frequency mixture lies on two facing rod pairs or on two facing rod triples, a largely dipolar shape of the field is thus generated and the undesirable ions are very effectively removed.

However, the ion trajectories become more unstable due to both the DC fields as well as the applied frequency mixtures, so that desired ions may also be eliminated by collisions with residual gas molecules. It is therefore practical to install a second ion guide in the highvacuum chamber and to superimpose the DC voltage or the frequency mixture only here, since a good vacuum is available here with very low residual gas pressure.

When coupling the ion source with chromatographic or electrophoretic separation methods in the substance feeder, a further advantage of this arrangement results. Ions from individual substance peaks can be stored for a longer period of time and be analyzed mass spectrometrically portion-wise by adapting the outflow rate of the ions to the requirements of the mass spectrometer.

In storage mass spectrometers, for example ion traps, the ions can be analyzed in consecutive filling and analysis periods. Thus it is possible to first measure a normal mass spectrum and, in continuing steps, to generate daughter ion spectra of all types of ions appearing. Here a further advantage of intermediate storage is that the length of time for filling can be greatly reduced and therefore a more rapid sequence in the scanning of spectra is achieved.

However, even in continuously operating mass spectrometers, for example in triple quadrupole mass spectrometers ("Triple Quads") for the analysis of daughter ion spectra, control of the outflow rate can be very beneficial.

It has proven to be favorable in experiments to use a hexapole arrangement for the ion guide.

The hexapole field focuses the ions well enough without crowding them together too strongly against their space charge. The hexapole field is also largely unaffected by slight maladjustments of the rods. A quadrupole field can also be used, however it crowds the ions strongly together and cannot store as many ions as the hexapole field; it must also be adjusted more carefully. An octopole field leads to a storage with low ion density on the axis. The ions, due to the effect of the space charge, find themselves near the rods on the surface of a cylinder with very few ions inside the cylinder. For this reason, injection into the ion trap through a small hole is no longer favorably possible. A very good arrangement for the ion guide is also the five-phase RF field between five pole rods.

A number of preferred embodiments of the invention will now be described in detail with reference to the accompanying drawings, in which: Figure 1 shows an ion guide which takes the form of a hexapole.

Figure 2 shows an arrangement according to this invention with an external electrospray ion source and a mass spectrometer with a quadrupole ion trap, and Figure 3 shows a further example a mass spectrometer with a quadrupole ion trap which has an external ICP (inductively coupled plasma) ion source.

The polarity of the applicable two-pole RF voltage in figure 1 is indicated on the end surfaces of the rods. The holders for the rods are not shown for reasons of clarity.

Figure 2 shows an arrangement according to this invention with an external electrospray ion source and mass spectrometer with a quadrupole ion trap. The arrangement includes a supply tank (1) containing a liquid which is sprayed by means of an electrical voltage between a minute spray capillary (2) and the end surface of the entrance capillary (3). The ions enter through the entrance capillary (3) together with ambient air into the differentially pumped first pump chamber (4), which is connected to a first pump (not shown) by a conduit (16).

The ions are accelerated toward the skimmer (5) and enter through the opening in the skimmer (5), located in a partition (6), into the second chamber (7) of the differential evacuation system. Chamber (7) is connected via conduit (17) to a vacuum pump. The ions are accepted by the ion guide (8) which may, for example, be as illustrated in Figure 1, and which ends on the wall (9) between the second differential pump chamber (7) and the highvacuum chamber (13). In the wall (9) there is a small opening which forms an "Einzel lens" together with apertures (10) and (11). The drawing potential at the middle aperture (10) extends through the hole in wall (9) and removes thermalized ions by suction. The ions entering into the high-vacuum chamber (13) are led through a second ion guide (12) to the ion trap mass spectrometer which consists of end cap electrodes (14) and ring electrodes (15).

The high-vacuum chamber (13) is evacuated by a pump nozzle (18).

Figure 3 illustrates a further example of a device in accordance with the invention. In Figure 3, a quadrupole mass spectrometer has an external ICP (inductively coupled plasma) ion source. A solution with the analysis substances is sprayed in a spray apparatus (20) and excited by a microwave coil (22) into a plasma cloud (21). Through a minute opening (23) in the water-cooled wall of the first differential pump stage (4), a small stream of molecules and ions from the plasma cloud (21) enters into the first differential pump stage (4), in which adiabatic expansion of the gas plasma takes place on the skimmer (5). Part of the ions can successfully pass through the opening in the skimmer (5) and are accepted by the RF ion guide (8).The thermalized ions are removed by suction from the lens (9, leo, 11) and brought directly into the quadrupole filter (24), in which only one type of ion respectively is allowed to pass in a known manner and is measured in the Detector (25).

The embodiment described above refers to an RF quadrupole ion trap operated as part of a mass spectrometer. However, the invention may, of course be utilised in other systems in which it is desired to introduce into a high-vacuum chamber ions which are created outside the chamber. Such methods and in particular other ways of using the RF ion trap, or other types of ion traps, are well known to those skilled in the art. An RF quadrupole ion trap consists of a ring electrode (15) and two axially arranged end cap electrodes (14). Filling with ions occurs through a hole in one of the end caps.

An ion trap mass spectrometer is only filled with ions for a short time. Generally a damping period then follows in which the ions are collected in a small cloud in the center of the ion trap. If a normal mass spectrum is to be scanned, there then follows a period in which the ions are ejected mass by mass out of the ion trap and measured with a measuring device. The ejection generally occurs through the end cap of the ion trap which faces the injection end cap. For other operating modes, for example MS/MS, further periods of ion isolation and fragmentation are inserted. The filling period is therefore generally short compared with the total time of other periods. The ions generated in the ion source in the other periods are usually rejected and are lost for the analysis. By means of the present invention it is possible to store such ions temporarily and use them for analysis.

The embodiment described above employs an electrospray ion source (1, 2) outside of the vacuum housing of the mass spectrometer. The invention is not however limited to this type of ion generation. The ions are obtained in an electrospray ion source (1) through the spraying of fine droplets of a liquid in air (or nitrogen) out of a fine capillary (2) under the influence of a strong electrical field, whereby the droplets vaporize and leave behind their charge on released molecules. In this way, very large molecules can be ionized easily.

The ions from this ion source are usually introduced via a capillary (3) with an inside diameter of about 0.5 millimeters and a length of about 100 millimeters into the vacuum of a mass spectrometer. They are entrained by the simultaneously inflowing air (or by another gas fed to the area around the entrance) by gas friction. A differential pump device with two intermediate stages (4 and 7) takes over the evacuation of the resulting gas. The ions entering through the capillary are accelerated in the first chamber (4) of the differential pump device within the adiabatically expanding gas jet and pulled by an electrical field toward the facing opening of a gas skimmer (5). The gas skimmer (5) is a conical tip with a central hole, whereby the exterior cone wall deflects the inflowing gas toward the outside.The opening of the gas skimmer leads the ions, now with much less accompanying gas, into the second chamber (7) of the differential pump device.

Directly behind the opening of the skimmer (5), the RF ion guide (8) begins. This consists preferably of a linear hexapole arrangement (Figure 1), which here consists of six thin, straight rods, that are uniformly arranged around the circumference of a cylinder. It is however nevertheless possible to use a curved ion guide with curved pole rods, to eliminate neutral gas especially well, for example. The rods are provided with an RF voltage, in which the phase between neighboring poles alternates respectively. The rods are fastened at several points by isolating devices.

The especially favorable embodiment has 70 millimeter long rods each of one millimeter diameter, and the enclosed cylindrical guide space has a diameter of 2.5 millimeters. The ion guide is therefore very slender. Experience shows that the ions which pass through a 1.2 millimeter diameter skimmer hole, are accepted by this ion guide practically without loss, if their mass is above the cutoff limit. This unusually good acceptance rate is primarily due to the gas dynamic situation at the input opening.

With a frequency of about 4 megahertz and a voltage of about 300 volts, all singly charged ions with masses about 30 atomic units are focused in the ion guide. Lighter ions escape the ion guide. Using higher voltages for lower frequencies, the cutoff limit for the ion masses can be raised to any desired value.

The ion guide (8) leads from the opening in the gas skimmer (5), which is arranged as part of the wall (6) between the first (4) and second chamber (7), through this second chamber to the small opening in the wall (9) which represents the first apertured diaphragm of a drawing lens (9, 10, 11), which encloses the ion guide according to this invention.

By changing the axis potential of the ion guide (8) relative to the potentials of the skimmer (5) and the wall (9), the ion guide (8) can be used as storage for ions of one polarity, either for positive or negative ions. The axis potential is identical to the zero potential of the RF voltage on the rods of the ion guide. The stored ions constantly run back and forth in the ion guide (8) and are braked by collisions with the residual gas in the chamber (7). Since they attain a speed of about 500 to 1,000 meters per second or more during the adiabatic acceleration phase, they first run the length of the ion guide several times per millisecond.

Their radio oscillation in the ion guide is dependent upon the injection angle. At a residual gas pressure of about 10-3 millibar, the ions are thermalized in a few milliseconds.

The calmed ions assemble in the axis of the ion guide. Their longitudinal movement is also braked to thermal speeds.

Using a drawing voltage at the central lens aperture (10), a potential protruding into the ion guide occurs through the opening (9), which can be adjusted in such a way that only ions near the axis can leave the ion guide. In this way, non-thermalized ions are kept back for the most part. The ions passing through are, in the case of this example, transported through a second ion guide on toward the mass spectrometer. The wall of the ion trap end cap (14) has an injection hole for the ions with a diameter of 1.5 millimeters.

By changing the potential on the central lens diaphragm (10), the stored ions can be made to either discharge into the ion trap through the second ion guide, or to remain stored in the first ion guide.

The ion source can particularly be coupled with devices for sample separation, for example with capillary electrophoresis. Capillary electrophoresis then delivers time-delayed substance aliquots of brief duration in great concentration. The temporary storage of the ions can then be applied especially favorably in order to preserve the ions of a substance for several fillings of the ion trap, whereby numerous MS/MS analyses of daughter ion spectra of different parent ions would be possible. Even MS/MS/MS analyses with granddaughter ion spectra may be performed; the latter are of special interest for the amino acid sequence analysis of proteins. The electrophoresis run can easily be interrupted for longer lasting analysis by switching off the electrophoresis voltage intermittently.

RF quadrupole ion traps need not necessarily form part of an ion trap mass spectrometer. For example, they may also serve to collect ions for time-of-flight spectrometers, to concentrate them in a dense cloud, and then outpulse them into the flight path of the time-of-flight spectrometer. It is also then possible first to isolate or also to fragment certain desired ions in the usual way in the ion trap before outpulsing the ions, thereby obtaining MS/MS measurements in time-of-flight spectrometers. The advantage of the time-of-flight spectrometer is its large mass range and rapid spectral scanning.

The transfer of ions from an ion source to an ion cyclotron resonance mass spectrometer can also be carried out advantageously with storage ion guides. The ICR spectrometer is also subject to similar operating rates such a RF quadrupole ion trap, thus the storage capability of the ion guide in the analysis phase is of enormous advantage. Even the thermalization of ions has an advantageous effect. The ion guide does not generally reach the storage cell of the spectrometer, so that the magnetic field takes over further guidance of the ions here.

In a second example, the coupling of an ICP ion source with its microwave plasma may be carried out using this invention, for example in a simple quadrupole mass spectrometer. A solution of the analysis substance is sprayed through spray apparatus (20) and is excited by the microwave coil (22) into a plasma. Since the plasma cloud (21) does not demonstrate a very high ion density, it is practical to suction in a very large volume of plasma into the vacuum. This is best done through an opening (23) of about 0.3 millimeters in a water-cooled wall of the differential pump stage (4). The large amount of inflowing gas is then evacuated using a very large fore-pump via the pump conduit (16). A proportion of the ions proceed through the small opening of about 0.8 millimeters diameter in the skimmer (5) into the second stage (7) of the differential pump unit. Here the ions from the ion guide (8) are captured and thennalized. Due to the Einzel lens (9, 10, 11), which terminates the ion guide according to this invention, the thermalized ions are injected directly into the quadrupole system (24). Here they are filtered down to ions of one mass in the usual way and measured in the detector (25). The quadrupole system can consecutively measure all masses and therefore scan complete mass spectra, or it can be especially adjusted for the detection of only certain masses (Selected Ion Monitoring).

Claims (19)

Claims

1. A method for the transfer of ions to a high-vacuum chamber of a vacuum system from an ion source outside the vacuum system, using a multipole RF ion guide with rod shaped poles for a part of the transfer path, wherein at least one differential pumping stage is employed between the ion source and the high-vacuum chamber, the differential pumping stage having an entrance opening and an exit opening for the ions, wherein the RF ion guide is provided in the final differential pumping stage before the high-vacuum chamber, and wherein the exit end of the RF ion guide is provided with a lens which essentially allows the passage through into the high-vacuum chamber only of thermalized ions, and reflects non- thermalized ions.

2. A method as claimed in Claim 1, wherein the exit opening of the last differential pump stage is part of the lens.

3. A method as claimed in Claim 2, wherein the ion lens is switchable between the states of ion passage for thennalized ions and ion reflection, so that ions in the RF ion guide can be stored temporarily.

4. A method as claimed in any one of the preceding claims, wherein the said final differential pumping stage having the RF ion guide is operated at a gas pressure of from 104 to 10-2 millibar.

5. A method as claimed in any one of the preceding claims, wherein the frequency and voltage for the RF ion guide are such that undesired ions with a mass-to-charge ratio which is too low have no stable trajectories in the storage RF ion guide and therefore escape therefrom.

6. A method as claimed in any one of the preceding claims, wherein a DC voltage is superimposed on the RF ion guide in addition to the RF voltage, so that only ions within a desired range of mass-to-charge ratios are stored.

7. A method as claimed in any one of the previous claims, wherein the RF ion guide is a hexapole rod system.

8. A method as claimed in any one of claims 1 to 6, wherein the RF ion guide is a pentapole rod system with a five-phase RF voltage applied to its five rod-shaped poles.

9. A method as claimed in any one of claims 1 to 8, wherein the RF ion guide extends between the entrance opening of the said final differential pumping stage and the exit opening,

10. Apparatus for supplying ions in a high-vacuum chamber of a vacuum system which apparatus comprises a vacuum system including a high-vacuum chamber an ion source outside the vacuum system, at least one differential pumping chamber between the ion trap and the high-vacuum chamber, the differential pumping chamber having an entrance opening for the ions and an exit opening for the ions, and a multipolar RF ion guide for transfer of ions, wherein the RF ion guide is located in the last differential pump chamber before the high-vacuum chamber and is provided with a switching lens at its exit end.

11. Apparatus as claimed in Claim 10, wherein the said RF ion guide extends from the entrance opening for the ions to the exit opening for the ions.

12. Apparatus as claimed in Claim 11, wherein the exit opening forms of part of the switching lens.

13. Apparatus as claimed in any one of Claims 10 to 12, wherein the switching lens is a drawing lens.

14. Apparatus as claimed in any one of Claims 10 to 13, wherein the ion lens is switchable between the states of ion passage for thermalized ions and ion reflection, so that ions in the RF ion guide can be stored temporarily.

15. Apparatus as claimed in any one of Claims 10 to 14, including means for superimposing a DC voltage on the RF ion guide in addition to the RF voltage, so that only ions within a desired range of mass-to-charge ratios are stored.

16. Apparatus as claimed in any one of Claims 10 to 15, wherein the RF ion guide is a hexapole rod system.

17. Apparatus as claimed in any one of Claims 10 to 15, wherein the RF ion guide is a pentapole rod system with a five-phase RF voltage applied to its five rod-shaped poles.

18. A method for the transfer of ions to a high-vacuum chamber of a vacuum system, substantially as hereinbefore described with reference to Figure 2 or Figure 3 of the accompanying drawings.

19. Apparatus for supplying ions in a high-vacuum chamber of a vacuum system, substantially as hereinbefore described with reference to Figure 2 or Figure 3 of the accompanying drawings.