Abstract
An ion guide system includes an ion guide with pole rods, a device for laterally introducing an ion species, and a mass spectrometer for analyzing product ions of reactions between different ion species. The device is configured and positioned such that an RF field with at least two-fold rotational symmetry with respect to the axis is generated. The device includes shortened pole rods and/or further electrodes. The pole rods and the further electrodes have at least two-fold rotational symmetry. The symmetry of the RF field allows ions to travel straight ahead through the ion guide with no hindrance. Such arrangements are particularly suitable for bringing together largely loss-free positive and negative ion species for reacting them. The reactions may be used to fragment multiply charged biopolymer ions by electron transfer or to remove excess charges of multiply charged biopolymer ions.
Claims
1. An ion guide system, comprising: an octopole rod set having four opposing rod pairs, of which two opposing rod pairs are shortened at one end thereof such that RF fields with a two-fold rotational symmetry around an axis of the octopole rod set are generated; and a deflective member located at said end of the octopole rod set opposing the two shortened rod pairs, the deflective member being configured and positioned for allowing passage of ions along the axis of the octopole rod set, and being supplied with DC voltages which deflect ions that are introduced laterally through a gap created by the shortening of two adjacent rods at said end of the octopole rod set.
2. A mass spectrometer for the acquisition of mass spectra of reaction products from reactions of ions of a first ion species with ions of a second ion species, comprising: an ion source for producing ions of the first ion species; an ion guide system, comprising an octopole rod set having four opposing rod pairs, of which two opposing rod pairs are shortened at one end thereof such that RF fields with a two-fold rotational symmetry around an axis of the octopole rod set are generated, and a deflective member located at said end of the octopole rod set opposing the two shortened rod pairs, the deflective member being configured and positioned for allowing passage of ions of the first ion species along the axis of the octopole rod set, and being supplied with DC voltages which deflect ions that are introduced laterally through a gap created by the shortening of two adjacent rods at said end of the octopole rod set; an ion source to produce the ions of the second ion species, which are fed to the gap between ion guide and deflective member; and a mass analyzer for acquiring the mass spectra of the reaction products.
3. The ion guide system of claim 1, wherein the deflective member is one of a ring diaphragm and a grid arrangement.
4. The mass spectrometer of claim 2, wherein the deflective member is one of a ring diaphragm and a grid arrangement.
Description
BRIEF DESCRIPTION OF THE DRAWING
(1) FIG. 1 shows a prior art ion-feeding quadrupole rod system according to U.S. Pat. No. 7,196,326 which joins a continuous basic quadrupole rod system laterally at an angle; the transmission of ions in the straight ahead direction is greatly disturbed by the asymmetric field;
(2) FIG. 2 is a greatly simplified schematic representation of an ion trap mass spectrometer according to U.S. Pat. No. 7,456,397 for the reaction between positive and negative ions, with an electrospray ion source (1, 2) for the production of multiply positively charged analyte ions, an ion source for negative reactant ions (8) and a 3D ion trap (11, 12, 13); the basic ion guide (9) takes the form of an octopole rod system and can guide both positive and negative ions to the ion trap; the negative reactant ions from the ion source (8) are fed in through a gap (7) in the octopole rod system, which is formed by two shortened neighboring pole rods, and are reflected by a DC voltage at the ring diaphragm (6) into the octopole rod system (9); the potential distribution, however, is asymmetrically disturbed, albeit more weakly than in FIG. 1;
(3) FIG. 3 shows an improved embodiment of the octopole rod system from FIG. 2; the device for the lateral introduction essentially comprises a shortening of two opposing pairs of rods so that a symmetrical field distribution is generated for the transmission of ions in the straight ahead direction; the ions injected laterally (in the Figure, from the top) are deflected around the corner into the octopole rod system (9) by a DC voltage at the ring diaphragm (6); the ring diaphragm (6) is given by way of example only; other shapes for deflective elements connected to a DC voltage supply, which also allow passage of analyte ions in a longitudinal direction, such as grid arrangements, for instance, are also conceivable;
(4) FIGS. 4 and 5 illustrate an RF quadrupole ion guide whose device for the lateral introduction of ions has split pole rods (21/23) and (24/26) of the basic quadrupole rod system; electrode discs (22) and (25) are inserted into the gaps with an RF voltage of opposite polarity; a hexapole injection channel (27) is thus created between the electrode discs (22) and (25) on all four sides of the basic quadrupole ion guide; FIG. 5 shows the distribution of the pseudopotential in the center plane between two pairs of rods; this pseudopotential widens slightly in the center, but there is no potential in the axis deflecting the ions in a sideward direction; ions can move through the ion guide along the axis without any hindrance;
(5) FIG. 6 shows an arrangement for injecting ions through a hexapole rod system (54), (55) and (56) from above into a basic quadrupole rod system with the multiply split pole rod sequences (50, 51, 52, 53) and (57, 58, 59, 60); symmetrical to this, the example shows a bottom arrangement with the hexapole rod system (61), (62) and (63); extensions of the hexapole rods (55) and (61) (and their counterparts behind the axis) now replace the electrode discs (22) and (25) from FIG. 4; the pole rod segments (51), (52), (58) and (59) and their counterparts behind them can each be supplied with DC voltages in addition to the RF voltage, thus generating a transverse DC quadrupole field which can deflect the injected ions around the corner from the hexapole axis into the quadrupole axis;
(6) FIG. 7 depicts the distribution of the pseudopotential in the vertical center plane of the arrangement according to FIG. 6;
(7) FIG. 8 shows simulation results for the lateral injection of ions with the deflection by RF voltages and additional DC voltages at the pole segments (51), (52), (58) and (59); the deflection is highly efficient and loss-free if the injection energy and deflection voltages are chosen correctly; the ions can also be deflected with high efficiency and without losses from the straight ahead direction of the basic quadrupole rod system into one of the lateral hexapole systems if the ions possess a suitable kinetic energy;
(8) FIG. 9 shows a slightly modified device for the lateral injection of ions into a basic quadrupole ion guide; the basic quadrupole ion guide with the intermediate electrodes (72) supplied with voltages of opposite polarity, of which only the part behind the vertical center plane is shown here, is covered here by plates (75) with an opening through which the ions are injected into the hexapole channel; thus ion guides of any shape can be used outside to deliver the ions, including quadrupole ion guides, for example; here also, analyte ions can be efficiently deflected from the straight ahead direction into the lateral ion guide;
(9) FIG. 10 depicts the distribution of the pseudopotential in the arrangement according to FIG. 9;
(10) FIG. 11 illustrates how two lateral quadrupole ion guides (76) and (79) are connected, top and bottom, to an arrangement of a basic quadrupole ion guide (70-74) according to FIG. 8; different types of ion sources, ion reactors, ion sinks or ion measuring devices (77) and (81) can be connected to these lateral ion guides; the device (77) can, for example, simply be an ion source for producing negative ions which are to be introduced into the straight line quadrupole ion guide; it is also possible, however, to introduce analyte ions from the basic quadrupole ion guide into the top lateral quadrupole ion guide (76), where they are made to react with negative ions from the ion source (77); the product ions can then be returned into the basic quadrupole ion guide; if positive analyte ions are deflected into the bottom lateral quadrupole ion guide (79), they can be made to react with electrons from the electron source (81), for example; the product ions can then be returned into the basic quadrupole ion guide, where they are transmitted to a mass analyzer; in order to introduce the electrons into the quadrupole ion guide (79), the electron source (81) and the ion guide (79) are surrounded by a solenoid (80), which generates a magnetic field to guide the electrons in the axis of the ion guide;
(11) FIG. 12 is a schematic representation of a mass spectrometer according to an aspect of the invention; here, the ions generated with the spray capillary (31) of the ion source (30) are introduced together with ambient gas through the capillary (32) and into an ion funnel (33) made of coaxial ring diaphragms; the trumpet-shaped ion funnel (33) guides the ions into the basic ion guide (34) with two lateral ion injectors or ion extractors, which correspond to the lateral quadrupole ion guides from FIG. 11; several operating modes are possible with this arrangement, some of which are indicated in the description for FIG. 11; it is possible, for example, to produce negative reactant ions in the ion source (36) and, according to this invention, introduce them through a short ion guide into the basic quadrupole ion guide (34); both ion species are guided in succession through the mass analyzer (37), in which the desired, multiply charged positive analyte ions are selected, to the reaction cell (38), where they can react with each other in the desired way; the reaction products are mass selectively analyzed with high resolution in the time-of-flight mass spectrometer with pulser (40), reflector (42) and detector (43).
DETAILED DESCRIPTION OF THE INVENTION
(12) An aspect of the invention comprises removing/reducing asymmetric disturbances in the basic ion guide, which in some prior art are created by the wide openings for the lateral introduction. The electrodes of the basic ion guide around the lateral ion introduction have a multi-fold, at a least two-fold rotational symmetry in relation to the axis of the basic, ion guide. The supply of the associated RF voltages results in an RF field with rotational symmetry with corresponding symmetrical distribution of the pseudopotentials. The channel of the pseudopotential is only widened slightly in the longitudinal direction of the ion guide. No resistance is generated to the ions flowing in the straight ahead direction, nor is there any lateral deflection; this means that no analyte ions guided in the straight ahead direction are lost as long as they move close to the axis. The analyte ions can move by virtue of their injection energy, but also due to a motion of the damping gas in the ion guide, or in particular due to a combination of both.
(13) FIG. 2 schematically depicts the prior art for the introduction of ions from a special ion source 8 into a basic octopole ion guide 9. The introduction operates with only two shortened rods and a deflection by a DC voltage on the ring diaphragm 6. According to an aspect of the invention, two opposing rod pairs can be shortened, as depicted in FIG. 3. This shortening of two opposing pole rods corresponds to the symmetry requirement given above.
(14) As has already been described in the introduction, however, octopole rod systems are not capable of focusing the ions into a narrow beam in the axis, and so it is preferable to use quadrupole rod systems as basic ion guides in such mass spectrometers, in which the formation of fine ion beams close to the axis is important. One solution includes generating a hexapole injection channel into such a basic quadrupole rod system, as will be described below.
(15) FIG. 4 shows how the pole rods of a basic quadrupole rod system, of which only the pole rods 21/23 and 24/26 are visible here, are split, and how narrow electrodes 22 and 25 are symmetrically inserted into the gaps with an RF voltage in antiphase. Between each rod pair, hexapole field channels 27 are generated between the electrode discs 22 and 25. These field channels can be used as injection channels for ions. The injection channels are generated on all four sides of the basic quadrupole ion guide, between two pairs of rods in each case. As can be seen in the image of the pseudopotential distribution in FIG. 5, the channel of the pseudopotential in the basic quadrupole ion guide is only widened slightly. No resistance against the flow of the ions in the straight ahead direction is generated, nor is there any lateral deflection. The hexapole injection channel allows ions to be injected at right angles into the basic quadrupole rod system.
(16) In the embodiment of FIG. 6, the hexapole injection channel from FIG. 4 has been extended with a hexapole ion guide at the bottom and at the top. The quadrupole rod system includes the multiply split pole rods 50, 51, 52, 53 and 57, 58, 59, 60 and their counterparts behind them. The top hexapole ion guide is represented by the pole rods 54, 55 and 56 and their counterparts behind them, the bottom one by the pole rods 60, 61 and 62. The electrode discs 22 and 25 from FIG. 4, which are supplied with an RF in antiphase, are replaced by extensions of the hexapole rods 55 and 61, which project into the gaps between the pole rod segments 51 and 52 and the pole rod segments 58 and 59 into the quadrupole ion guide, supplied with an RF voltage which is in antiphase to the RF voltage of the pole rods. If the ions are to be deflected by 90 into the axis of the basic quadrupole rod system, appropriate DC voltages must be applied to the insulated segments 51, 52 and 58, 59 of the pole rods, as indicated in the illustration. This generates a transverse DC voltage quadrupole field, which deflects the injected ions from the hexapole axis around the corner into the quadrupole axis.
(17) FIG. 7 shows the distribution of the pseudopotential in the center plane through the axis of the arrangement according to FIG. 6; and FIG. 8 shows how the ions injected through the hexapole channel are accurately deflected in a transverse DC quadrupole field into the longitudinal direction of the basic ion guide. The figures are produced by computer simulations. In a similar way, it is also possible to extract ions from the basic quadrupole ion guide into one of the hexapole ion guides. The simulations show that the ions are guided very efficiently in each case.
(18) Since the lateral pole rods 54 and 56 and 60 and 62 of the two hexapole rod systems have practically no effect on the pseudopotential in the interior of the quadrupole ion guide from FIG. 6, they can be omitted for the unused hexapole rod system.
(19) FIG. 9 shows an embodiment with a slightly modified device for the lateral injection of ions into a quadrupole ion guide. The quadrupole ion guide with the round intermediate electrodes 72, which are supplied with voltages of opposite polarity, is covered at the top and bottom here by plates 75 with an opening through which the ions can be injected into the hexapole channel between the pole rods. Only the top plate 75 is labeled in the illustration. It is possible to mount ion guides of any shape on the outside of the cover plate 75 of this device to deliver the ions, including quadrupole ion guides, for example. Instead of lateral injection of ions into the basic ion guide, analyte ions can also be efficiently deflected from the straight ahead direction of the basic ion guide into the lateral ion guide.
(20) This arrangement from FIG. 9 is extended in FIG. 11 by two docked quadrupole ion guides 76 and 79. Different types of devices like ion sources, electron sources, reaction cells, or ion detectors 77 and 81 can, in turn, be connected to these ion guides.
(21) If the device 77 is a detector, for example, it can be used to occasionally measure the current of the analyte ions in the straight line quadrupole ion guide. This is necessary, for example, when the ion source is coupled to a chromatographic separation device, delivering substance ions in GC or LC peaks, and an ion storage device, such as a 3D ion trap as in FIG. 2, or an ICR cell, is to be filled as accurately as possible with a specified quantity of ions.
(22) The device 77 can also be an ion source for producing negative ions which are to be introduced into the basic quadrupole ion guide, where they are guided into a reaction cell. On the other hand, it is possible to introduce analyte ions from the basic quadrupole ion guide into the top lateral quadrupole ion guide 76, where they subjected to reactions with negative ions from the ion source 77; the product ions can then be guided back into the basic quadrupole ion guide.
(23) A special operating mode is shown in the bottom part of FIG. 11, where the device 81 represents an electron source. If positive analyte ions are deflected into the bottom quadrupole ion guide 79, they can be subjected to reactions with the electrons from the electron source 81 (ECD, electron capture dissociation). The product ions, for example fragment ions, which have been formed by electron capture dissociation, can then be guided back into the basic quadrupole ion guide, where they are transmitted to further components. In order to introduce the electrons into the lateral quadrupole ion guide 79, the electron source 81 and the ion guide 79 are surrounded by a solenoid 80, which generates a magnetic field of sufficient strength to guide the slow electrons in the axis of the ion guide. The fragmentation by ECD can especially be carried out in continuous flow, the analyte ions being introduced into the lateral quadrupole ion guide 81, reflected at the end and, on return, guided back into the basic quadrupole ion guide. This mode of operation requires the damping gas to have a low pressure so that the kinetic energy of the analyte ions is not reduced too greatly between introducing them into the reaction region 81 and returning them.
(24) In another aspect of the invention, a mass spectrometer for scanning reaction products from reactions between analyte and reactant ions with different charges. The mass spectrometer comprises the ion source for producing the analyte ions; an ion source for producing the reactant ions, which are introduced into the basic ion guide through the lateral inlet; a reaction cell, and a mass analyzer. A time-of-flight mass analyzer with orthogonal ion injection for acquiring the mass spectra of the reaction products is particularly favorable. The analyte ions, usually multiply charged positive ions, are best generated with an electrospray ion source.
(25) Reactions between ions only occur when ions of different polarities are mixed. As has been described in the Background, the reactions can be used for electron transfer dissociation (ETD), for charge reduction by proton transfer (PTR), and also for other types of product formation. Although there have been attempts to create the reactions continuously in a flow, the reactions are usually carried out with ions stored in reaction cells. The reaction cells can be designed as 3D ion traps, as depicted in FIG. 2, but often they are ion guides which are closed at both ends with pseudopotentials. The two ion species are successively, sometimes also simultaneously, introduced into these reaction cells from two different ion sources, often through basic ion guides which have lateral inlets for reactant ions.
(26) Arrangements according to FIG. 6 or 9 can also be used in order to make analyte ions and reactant ions react in the flow at the intersection of the two ion beams.
(27) The laterally introduced reactant ions are preferably produced in separate ion sources. This can occur in ion sources for chemical ionization, for example, which are able to produce both positive and negative ions. Ion sources for chemical ionization operate best at pressures of a few hundred pascal. Since pressures like this are found in the first pumping stage after the capillary inlet, these ion sources can be installed particularly well here. The ion sources for chemical ionization are known to those skilled in the art and do not need to be especially described here.
(28) Apart from chemical ionization, negative ions can also be formed by electron attachment. The ion sources 77 and 36 in the FIGS. 11 and 12 may be such electron attachment ion sources, for example. This type of ion source is likewise known to those skilled in the art and thus is not described here in detail.
(29) FIG. 12 is a schematic representation of an embodiment of a mass spectrometer having an electrospray ion source with housing 30 and spray capillary 31. The ions generated with the spray capillary 31 of the ion source 30 are introduced together with ambient gas through the inlet capillary 32 and into the vacuum system. An ion funnel 33 made of coaxial ring diaphragms captures the ions, separates them from most of the introduced gas, and feeds the ions into the basic ion guide 34, which is designed with two lateral inlets as shown in FIG. 11. It is possible, for example, to produce negative reactant ions in the ion source 36 and to introduce them into the basic quadrupole ion guide 34. The two ion species are guided successively through the mass filter 37, in which the desired, multiply charged positive analyte ions are selected, to the reaction cell 38, where they can react with each other in the desired way. The ionic reaction products are introduced as a fine ion beam into the time-of-flight mass spectrometer, accelerated orthogonally to the beam 41 in the pulser 40, reflected in the reflector 42 so as to focus the energy, and analyzed in the detector 43 with high mass resolution. Everyone skilled in the art knows how a time-of-flight mass analyzer operates and there is no need to describe it further here.
(30) The quadrupole rod system 34 with the docked ion guides and their ion sources, electron sources, reaction cells or ion detectors can be operated within the mass spectrometer in the various operating modes, which are described above. A different sequence of basic ion guide 34, mass filter 37 and reaction cell 38 can also be chosen if, for example, the parent ions are to be selected from the analyte ions before they are fragmented by ETD or ECD. The rod systems 34, 37 and 38, which here adjoin each other without transition, separated only by diaphragms, can also be separated from each other by further ion guides and vacuum stages and be operated in completely different pressure ranges.
(31) All ion guides are usually filled with damping gas, which causes the ions to collect near the axis of the system due to the effect of the pseudopotential. The slimmer the system, the better the collection. The pressure may range between 10.sup.3 and 10.sup.+1 pascal, a favorable pressure range is 0.1 to 1 pascal.
(32) A special application of this mass spectrometer offers the possibility of simply analyzing structures of biopolymer ions by ETD fragmentation. For this purpose, the mass spectrometer is connected to a separating system for dissolved substances, such as a nanoflow liquid chromatograph (nano-HPLC). The substances, eluating largely separated one after the other from the LC column, are ionized in the electrospray ion source so as to be predominantly multiply charged. They react with suitable negative ions of low electron affinity by fragmenting into fragment ions of the c-type, which produce an easily decipherable fragment ion spectrum. By periodically switching the supply of negative ions on and off, it is possible to alternately obtain spectra with and without fragmentation. By comparing the spectra, it is possible to determine which peaks of the fragment ion spectrum belong to the fragment ions, even if there is some overlap of substances.
(33) The mass spectrometer can contain a further device for fragmentation. In FIG. 12 the analyte ions selected in the mass filter 37 can also be injected after acceleration into the reaction cell 38, where they absorb small amounts of energy by collisions with the damping gas and can finally fragment. Comparing collisionally fragmented ions with fragment ions generated by electron transfer provides specific information concerning post-translational modifications of the biomolecular structure.
(34) Further applications for this type of mass spectrometer concern the analysis of substance mixtures with high molecular weights, which are usually multiply charged with a broad charge distribution in the electrospray ion source, and thus provide a mixture of peaks in the spectrum which is almost impossible to decipher. By removing the excess charges by proton stripping with suitable negative ions, it is possible to generate a mixture which consists virtually only of singly charged ions and is thus simple to interpret. A time-of-flight mass analyzer in particular is suitable for acquiring spectra with ions of high mass, the mass limit being limited only by the detector employed.
(35) Naturally it is also possible to use other types of mass analyzer instead of time-of-flight mass analyzers to acquire the spectra of product ions, such as ion cyclotron resonance mass spectrometers, 2D or 3D RF quadrupole ion traps or electrostatic ion traps according to the Kingdon principle, for example. At present, however, the time-of-flight mass analyzer seems to be the most favorable, in terms of value for money, for achieving high mass accuracy, high dynamic measurement range, high mass range and fast and flexibly adaptable measuring time.
(36) With knowledge of this invention, those skilled in the art will be able to design different types of arrangement of device components within mass spectrometers for the analysis of product ions from reactions between analyte ions and reactant ions without any loss in sensitivity. For instance, round pole rods have always been shown in the figures for reasons of simplicity. It is understood, however, that the invention can also be carried out with pole electrodes of other designs, while achieving the same advantageous effects.
(37) Although the present invention has been illustrated and described with respect to several preferred embodiments thereof, various changes, omissions and additions to the foul and detail thereof, may be made therein, without departing from the spirit and scope of the invention.
