Device for reacting analyte ions with electrons

Abstract

Ion-electron reaction device comprising: a first tubular magnet 2; a second tubular magnet 4 arranged coaxially with first tubular magnet; and a shim 10 located between the first and second tubular m

Description

CROSS-REFERENCE TO RELATED APPLICATION This application claims priority from and the benefit of United Kingdom patent application No. 2317276.0 filed on 10 November 2023. The entire contents of this application are incorporated herein by reference. FIELD OF THE INVENTION The present invention relates generally to a method of mass and / or ion mobility spectrometry in which analyte ions are reacted with electrons so as to produce charge-reduced analyte ions, fragment ions or other product ions. The present invention also provides a mass and / or mobility spectrometer configured to perform the method. BACKGROUND It is well known to conduct ion-electron reactions, in the field of mass spectrometry, during the analysis of analyte ions. Examples of such reactions include electron capture dissociation (ECD) and electron induced dissociation (EID). In order to perform ECD, multiply protonated analyte molecules (i.e. analyte ions) may be confined along with low energy electrons such that the analyte molecules and electrons react with each other so as to cause the analyte molecules to fragment into daughter ions. Mass analysis using electron-based fragmentation techniques such those described above are beneficial in that they may be more informative than using other fragmentation techniques, such as collisional induced dissociation (CID), since the product ions produced by the reactions retain labile post-translational modifications. Also, during the above-mentioned ion-electron interactions, some of the analyte molecules may not dissociate into fragment ions but may become charge-reduced instead. Such charge reduction may be useful for separating ions that would otherwise have overlapping charge states, thus enabling confident charge annotation. Also, electron-based fragmentation techniques can generate higher-quality (more complete) or complementary fragment information for polymers such as peptides. Although ion-electron devices are known, their design is typically complex and their operation can be challenging, e.g. due to the difficulty in confining the analyte ions and the low energy electrons in the same region for a sufficient time and with sufficient density for the reactions to be performed. For example, ECD devices are known that employ an RF electric field to confine the analyte ions, but this RF field increases the energy of the reactant electrons and so the probability of the analyte ions capturing the electrons is relatively low, and hence so is the probability of a reaction taking place. Furthermore, for devices where electrons are trapped, the additional energy they receive from the RF field causes them to be lost from the electron trapping fields. Other known reaction devices use strong magnetic fields in order to confine the electrons at low energy, but these also present difficulties in confining both the analyte ions and reactant electrons in the same region such that the reactions take place at the required rate. For example, difficulties may arise if the magnetic fields are too weak or are inhomogeneous. SUMMARY In a first aspect, the present invention provides an ion-electron reaction device comprising: a first tubular magnet; a second tubular magnet arranged coaxially with the first tubular magnet; and a shim located between the first and second tubular magnets; wherein the shim is arranged and configured such that the first and second tubular magnets provide a substantially homogenous magnetic field along a central axis of the device. The first and second tubular magnets are spaced apart in a direction along the central axis, and the shim is located axially between the magnets. Each of the first and second tubular magnets may be cylindrical magnets. The shim may be arranged and configured such that the first and second tubular magnets provide a substantially homogenous magnetic field within a continuous region that extends along: at least a portion of the central axis within the first tubular magnet; the entire central axis through the shim; and at least a portion of the central axis within the second tubular magnet. The homogenous magnetic field may have magnetic field lines that extend from the first tubular magnet to the second tubular magnet, within a volume arranged about the central axis, wherein the magnetic field lines are substantially linear and parallel to the central axis along at least 30 % of the length of the reaction device. Desirably, the homogenous magnetic field extends over a relatively large volume such that electrons may be confined in such a large volume. As such, the magnetic field lines may be substantially linear and parallel to the central axis along at least 40%, at least 50%, at least 60%, at least 70%, at least 80% or at least 90% of the length of the reaction device. By “length of the reaction device” it is meant the distance from an upstream end of the first tubular magnet to the downstream end of the second tubular magnet (i.e. between the ends of the two magnets that are facing away from each other). The shim is preferably in direct contact with the first tubular magnet and the second tubular magnet. The shim is preferably tubular, such as being cylindrical, and arranged coaxially with the first tubular magnet and the second tubular magnet. The shim is preferably ferromagnetic. The first tubular magnet may be arranged with its North magnetic pole adjacent to the shim and the second tubular magnet may be arranged with its South magnetic pole adjacent to the shim; or the first tubular magnet may be arranged with its South magnetic pole adjacent to the shim and the second tubular magnet may be arranged with its North magnetic pole adjacent to the shim. The first and second tubular magnets are arranged coaxially along an axis. The first and second magnets may each be axially magnetised such that its magnetic poles are at opposite ends of the magnet, in the direction along the central axis. Accordingly, the axial end of the first magnet that is closest to the second magnet preferably has the opposite magnetic pole to the pole at the axial end of the second magnet that is closest to the first magnet. That is, the first and second magnets are arranged North pole to South pole, or South pole to North pole, along the axis. The reaction device may comprise an electron generator arranged to generate and supply free electrons to a location where the homogenous magnetic field is located. The electron generator may generate free electrons outside of the tubular magnet and supply them to the location. Alternatively, the electron generator may generate free electrons inside of the tubular magnet at the location. The electron generator may comprise a voltage supply and a wire filament that are arranged and configured to generate the free electrons. The wire filament may be arranged such that it has a distal end arranged on the central axis; or the wire filament may be formed as a loop at its distal end, where the loop surrounds the central axis. The distal end or loop may be arranged on the central axis at an axial location between the first and second tubular magnets, such as substantially halfway between the magnets. The wire filament may pass through an aperture in the shim. An electrically insulating sheath may be provided between the wire filament and the shim for preventing an electrical current passing from the wire filament to the shim. The electrically insulating sheath may surround a portion of the filament that extends radially inwards of the inner wall of the shim. The reaction device may comprise a heat sink in contact with the shim for transferring heat from the wire filament out of the shim and to the heat sink. The filament may not pass through the shim, but may instead pass through an opening in the axially outer end of the first or second tubular magnet. The wire filament may be elongated and have a longitudinal axis that is arranged at an obtuse or acute angle to the central axis through the tubular magnet. The reaction device may be configured to receive analyte ions such that they travel substantially along the central axis. An ion source may therefore be provided for supplying the ions. The reaction device may comprise electrodes and one or more voltage supplies configured to apply one or more DC voltages to the electrodes so as to generate a DC electric field for confining electrons radially about, and / or axially along, the central axis; and / or may comprise electrodes and one or more voltage supplies configured to apply one or more DC and / or RF voltages to the electrodes so as to generate a DC and / or RF electric field for confining ions radially about, and / or axially along, the central axis. Although two tubular magnets have been described as being stacked with a shim therebetween, it is contemplated that more than two such tubular magnets may be stacked together to form the reaction device, where a shim is provided between each pair of adjacent magnets. In a second aspect, the present invention provides an ion-electron reaction device comprising: an elongated tubular magnet having a length of at least 5 mm; and an electron generator arranged to supply free electrons to a location within the tubular magnet; wherein either: (i) the electron generator is arranged to supply free electrons to a location within the tubular magnet that is substantially on a central, longitudinal axis of the tubular magnet, and wherein the tubular magnet has a ratio of length to inner radius of >3; or (ii) the electron generator is arranged to supply free electrons to a location within the tubular magnet that is radially outwards of a central, longitudinal axis of the tubular magnet, and wherein the tubular magnet has a ratio of length to inner radius of <3. The tubular magnet may be a cylindrical tube. It is contemplated that the tubular magnet may or may not have a length of at least 5 mm. The radial location within the tubular magnet to which the electrons are supplied may be selected based on the ratio of length to inner radius of the tubular magnet; and / or the ratio of length to inner radius of the tubular magnet that is used in the reaction device may be selected based on the radial location within the tubular magnet at which it is desired to supply electrons. According to option (i), the tubular magnet may have a ratio of length to inner radius of >3.2, >3.4, >3.6, >3.8, or >4. According to option (ii), the tubular magnet may have a ratio of length to inner radius of <2.8, <2.6, <2.4, <2.2, <2.0, <1.8, <1.6, <1.4, <1.2, or <1.0. The electron generator may be arranged to supply free electrons in the tubular magnet at a radial location at which the magnetic flux along the longitudinal axis is most homogenous. The electron generator may generate free electrons outside of the tubular magnet and supply them to the location within the tubular magnet. Alternatively, the electron generator may generate free electrons inside of the tubular magnet at the location within the tubular magnet. The tubular magnet may be configured and have a length such that a substantially linear homogenous magnetic field is provided along a central axis of the tubular magnet, wherein the homogenous magnetic field has magnetic field lines that extend from one end of the tubular magnet to the other end of the tubular magnet, within a volume arranged about the central axis, wherein the magnetic field lines are substantially linear and parallel to the central axis along at least 30 % of the length of the tubular magnet. Desirably the homogenous magnetic field extends over a large volume. As such, the magnetic field lines may be substantially linear and parallel to the central axis along at least 40%, at least 50%, at least 60%, at least 70%, at least 80% or at least 90% of the length of the tubular magnet. The electron generator may be arranged to generate and supply free electrons to a location where the homogenous magnetic field is located. The electron generator may comprise a voltage supply and a wire filament that are arranged and configured to generate the free electrons. The electron generator may comprise a voltage supply and a wire filament that are arranged and configured to generate free electrons on or around the central axis through the tubular magnet. The wire filament may be arranged such that it has a distal end arranged on the central axis; or the wire filament may be formed as a loop at its distal end, where the loop surrounds the central axis. The distal end or loop may be arranged on or around the central axis at an axial location between the axial ends of the tubular magnet, such as substantially half way along the tubular magnet. The wire filament may pass through an opening in an axial end of the tubular magnet. The wire filament may be elongated and have a longitudinal axis that is arranged at an obtuse or acute angle to the central axis through the tubular magnet. The reaction device may be configured to receive analyte ions such that they travel substantially along the central axis. An ion source may therefore be provided for supplying the ions. The reaction device may comprise electrodes and one or more voltage supplies configured to apply one or more DC voltages to the electrodes so as to generate a DC electric field for confining electrons radially about, and / or axially along, the central axis; and / or may comprise electrodes and one or more voltage supplies configured to apply one or more DC and / or RF voltages to the electrodes so as to generate a DC and / or RF electric field for confining ions radially about, and / or axially along, the central axis. The tubular magnet may be configured and have a length such that a substantially linear homogenous magnetic field is provided along the tubular magnet throughout an annular region that is arranged between the central axis and the inner wall of the tubular magnet; wherein the homogenous magnetic field has magnetic field lines that extend from one end of the tubular magnet to the other end of the tubular magnet, within the annular region, wherein the magnetic field lines are substantially linear and parallel to the central axis along at least 30 % of the length of the tubular magnet. The magnetic field lines (in the annular region) may be substantially linear and parallel to the central axis along at least 40%, at least 50%, at least 60%, at least 70%, at least 80% or at least 90% of the length of the tubular magnet. The reaction device may comprise an electron generator arranged to generate and supply the free electrons to the annular region. The electron generator may comprise a voltage supply and a wire filament that are arranged and configured to generate the free electrons. The wire filament may be arranged such that it has a distal end arranged in the annular region. The wire filament may be formed as a loop at its distal end. -6- The distal end or loop may be arranged in the annular region at an axial location between the axial ends of the tubular magnet, such as substantially halfway along the tubular magnet. The wire filament may pass through an opening in an axial end of the tubular magnet. The wire filament may be elongated and have a longitudinal axis that is arranged at an obtuse or acute angle to the central axis through the tubular magnet. The reaction device may be configured to receive analyte ions into the annular region such that the ions travel along an axis that does not intersect the wire filament. An ion source may therefore be provided for supplying the ions. The axis that the ions travel along may be arranged on the opposite side of the central axis through the magnet to that which the wire filament is located on. The reaction device may comprise a screen member positioned between the axis that the ions travel along and the wire filament for screening the analyte ions from an electric field generated by the wire filament. The screen member may be positioned along the central axis of the tubular magnet. The screen member may be non-ferromagnetic. The reaction device may comprise electrodes and one or more voltage supplies configured to apply one or more DC voltages to the electrodes so as to generate a DC electric field for confining electrons radially within, and / or axially along, the annular region; and / or may comprise electrodes and one or more voltage supplies configured to apply one or more DC and / or RF voltages to the electrodes so as to generate a DC and / or RF electric field for confining ions radially within, and / or axially along, the annular region. The elongated tubular magnet according to the second aspect of the present invention may be a single magnet. However, it is contemplated that the elongated tubular magnet may instead be formed from a plurality of tubular magnets that are stacked together with electrical insulators between them such that different voltages may be applied to the different magnets. Accordingly, in a third aspect, the present invention provides an ion-electron reaction device comprising: a plurality of tubular magnets that are stacked together coaxially with electrical insulators therebetween; and one or more voltage supplies configured to apply a voltage to one or more of the magnets. The one or more voltage supplies may be configured to apply different voltages to different ones of the magnets. The one or more voltage supplies may be configured to successively apply a DC voltage to successive ones of the magnets such that a DC potential travels along a central axis through the tubular magnets. The DC voltage may be applied such that the DC potential accelerates electrons and / ions within the reaction device. The one or more voltage supplies may be configured to apply one or more DC voltages to the magnets so as to generate a DC electric field for confining electrons radially about, and / or axially along, a central axis through the tubular magnets. The one or more voltage supplies may be configured to apply one or more DC and / or RF voltages to the magnets so as to generate a DC and / or RF electric field for confining ions radially about, and / or axially along, a central axis through the tubular magnets. The plurality of tubular magnets may be cylindrical magnets. The magnets may be stacked together with opposing magnetic poles adjacent to each other. The reaction device according to the third aspect of the invention may have any of the features described above in relation to the second aspect of the invention, except that the features described in relation to the tubular magnet of the second invention apply to the stack of tubular magnets instead. In a fourth aspect, the present invention provides an ion-electron reaction device comprising: a magnet having a substantially flat, planar surface; and an electron generator for supplying free electrons; wherein the reaction device is configured such that a magnetic field from the magnet confines the free electrons in a region adjacent to the planar surface. The magnet may be a bar magnet. The reaction device may be configured to receive analyte ions along an ion axis such that the ions pass into the region adjacent to the planar surface. The ion axis maybe arranged between the planar surface of the magnet and a first electrode that is spaced apart from the planar surface in a first direction. The reaction device may comprise a second electrode and a third electrode arranged on either side of the ion axis and between the bar magnet and the first electrode, wherein the second and third electrodes are spaced apart in a direction orthogonal to the first direction. The first and / or second and / or third electrodes may be non-ferromagnetic electrodes. The first and / or second and / or third electrodes may be planar electrodes. Alternatively, the first and / or second and / or third electrodes may have other configurations, such as being rod electrodes. The reaction device may comprise one or more voltage supplies configured to apply one or more DC or RF voltages to the first and / or second and / or third electrodes, and / or to the magnet, so as to generate a DC or RF electric field that confines analyte ions introduced into the reaction device adjacent to the planar surface of the magnet. The reaction device may comprise one or more voltage supplies configured to apply one or more DC voltage to the first and / or second and / or third electrodes, and / or to the magnet, so as to generate a DC electric field that confines electrons axially along the device. For example, any one of the first, second or third electrodes, and / or the magnet, may be axially segmented and different voltages may be applied to the different axial segments so as to achieve this. Alternatively, or additionally, the one or more voltage supplies may be configured to cause a DC potential to travel through the region, e.g. to accelerate ions or electrons therein. The electron generator may be arranged to generate and supply free electrons into the region. The electron generator may comprise a voltage supply and a wire filament that are arranged and configured to generate the free electrons. The wire filament may be arranged to generate electrons adjacent to the planar surface of the magnet. The wire filament may be arranged such that it has a distal end arranged adjacent to the planar surface. The wire filament may be formed as a loop at its distal end. The wire filament may extend through an opening into the reaction device, such as an opening through which ions enter or leave the device. Alternatively, the filament may extend through one of the electrodes or the magnet. The reaction device may be configured to receive analyte ions such that they travel substantially adjacent the planar surface. An ion source may therefore be provided for supplying the ions. In an embodiment, the present invention provides a mass spectrometer comprising: a reaction device as described above; and an ion source arranged for supplying analyte ions into the reaction device. The present invention also provides a method of mass spectrometry comprising: providing a mass spectrometer described above; supplying ions from the ion source into the reaction device; and providing electrons in the reaction device such that the electrons react with the ions. BRIEF DESCRIPTION OF THE DRAWINGS Various embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings in which: Fig. 1 illustrates a schematic of a reaction device according to an embodiment of the present invention comprising two magnets joined by a shim, and a filament assembly for generating electrons passing through an aperture in the shim; Fig. 2 illustrates a schematic of a reaction device according to another embodiment of the present invention, comprising a single elongated magnet and a filament for generating electrons passing into the reaction device via an exit aperture of the magnet; Fig. 3a illustrates a schematic of half of the reaction device of Fig. 2 having a ratio of length to inner radius of 4; and Fig. 3b illustrates a graph of the magnetic flux density along the length of the reaction device of Fig. 3a at different radial positions of the reaction device; Fig. 4a illustrates a schematic of half of the reaction device of Fig. 2 having a ratio of length to inner radius of 1.8; and Fig. 4b illustrates a graph of the flux density along the reaction device of Fig. 4a at different radial positions of the reaction device; Figs. 5a-5c illustrate a schematic of a reaction device according to another embodiment of the present invention, comprising a ring magnet and a filament for generating electrons arranged off-axis to the central axis of the magnet; Figs. 6a-6b illustrate a schematic of a reaction device according to another embodiment of the present invention comprising a bar magnet for confining the electrons; Fig. 7 illustrates a schematic of a reaction device according to another embodiment of the present invention comprising a plurality of stacked ring magnets; and Fig. 8 shows a schematic of a mass spectrometer according to an embodiment of the present invention. DETAILED DESCRIPTION Embodiments of the present invention provide an ion-electron reaction device for reacting analyte ions with electrons. The analyte ions may be positively charged ions. The electrons may be confined within the reaction device without using any RF electric fields. For example, the electrons may be confined using only magnetic fields. The analyte ions may also be trapped within the reaction device whilst they react with the electrons. Alternatively, the analyte ions may simply be transmitted through the reaction device in which the electrons are trapped, such that the reactions may take place without trapping the analyte ions within the reaction device. The reaction device may be used to react the analyte ions with the electrons so as to cause charge reduction of the analyte ions and / or to cause dissociation of the analyte ions into fragment ions (e.g. ECD or EID). Fig. 1 illustrates a schematic of a reaction device according to an embodiment of the present invention. The reaction device comprises a first magnet 2 and a second magnet 4, the magnets being spaced apart in a first dimension. The magnets may be permanent magnets and may be substantially the same, e.g. the same form or shape. For example, the magnets may be tubular (e.g. cylindrical) magnets that are arranged coaxially along a central axis extending in the first dimension. The first and second magnets may each be axially magnetised such that its magnetic poles are at opposite ends of the magnet (in the direction along the central axis). The axial end of the first magnet that is closest to the second magnet preferably has the opposite magnetic pole to the pole at the axial end of the second magnet that is closest to the first magnet. That is, the first and second magnets are arranged North pole to South pole, or South pole to North pole. This allows the magnetic fields of the individual magnets to be aligned in the same direction, which enables magnetic field lines to extend substantially parallel to the central axis over a relatively long length of the reaction device, as will be described further below. A shim 10 is positioned axially between, and preferably directly contacting, the magnets. The shim may be the same form or shape as the magnets. For example, the shim may be tubular (e.g. cylindrical) and arranged coaxially with the magnets along the central axis. The shim is formed from a soft ferromagnetic material such as steel. The shim is arranged and configured so as to increase the homogeneity of the magnetic field produced by the magnets in the volume about the central axis of the reaction device. The shim is arranged and configured such that the magnetic field lines from the upstream axial end of the first magnet 2 pass through the reaction device to the downstream axial end of the second magnet 4 (or vice versa), in the volume about the central axis of the reaction device. As such, the magnetic field lines within the volume located about the central axis of the reaction device are substantially parallel to the central axis over the majority of the length of the reaction device. This is beneficial for keeping electrons radially captured so that they react with the analyte ions, as will be discussed below. As illustrated in Fig. 1, the inclusion of the ferromagnetic shim between the magnets causes the magnetic field lines to be substantially parallel to the central axis over a relatively large volume and length of the reaction device. The magnetic field lines are substantially parallel to the central axis even at radial distances that are relatively far from the central axis. Thus, the shim is configured to provide a homogenous magnetic field along the central axis. The reaction device is configured to introduce electrons into the magnetic field within the reaction device such that the electrons are confined by the magnetic field. The reaction device may comprise a thermal filament 12 for supplying free electrons into the reaction device. The filament may extend into the magnetic field region within the reaction device such that electrons are released by the filament into the magnetic field region. As such, the filament may comprise a non-ferromagnetic material suitable for producing free electrons. The filament is preferably configured to release the electrons into a homogenous region of the magnetic field, such as at a location substantially at the central axis of the reaction device. For example, the distal end of the filament may be arranged at the central axis of the reaction region. The filament may be a substantially straight wire with a loop 14 at its distal end. As will be discussed below, the addition of a loop allows at least some of the analyte ions to pass through the loop, and therefore through the reaction device, without being impeded by the filament. A voltage supply 16 connected to the filament may be configured to provide a current to the filament, in order to heat the distal end of the filament and cause electrons to be released. As such, the filament may generate low energy free electrons, ideally having a kinetic energy close to thermal energy for ECD or higher for EID. As discussed above, the distal end of the filament may be positioned on the central axis of the reaction device so that the electrons become confined by the magnetic field. More specifically, the electrons spiral around the magnetic field lines that are arranged substantially parallel to the central axis. As such, the electrons are radially confined within the reaction device to a volume arranged about the central axis. In order to provide the filament to the interior of the reaction device, the shim may comprise an aperture through which the filament passes. The filament may be positioned orthogonal to the central axis. The aperture may be machined into the shim, which is relatively easy as the shim is preferably steel. Also, the shim may be configured to conduct heat away from the filament and away from the magnets, which is beneficial as magnets tend to lose their strength at high temperatures. This may be facilitated by providing a heat sink in direct contact with the shim. An electrically insulating sheath may be provided around the filament so as to prevent electric current passing from the filament to the shim. The sheath may extend beyond the radially inner wall of the shim. The reaction device may also have electrodes and voltage supplies (not shown) arranged and configured to generate a DC electric field within the reaction device that helps to confine the electrons and / or guide or focus the ions and / or products derived from the ions through the device. For example, the reaction device may comprise a plurality of electrodes that are spaced along the first dimension and voltage supplies configured to apply DC voltages to these electrodes so as to generate the DC electric field that confines the electrons radially and / or axially (i.e. in the first dimension) within the reaction device. For electrically conductive magnets, such as Nickel coated Samarium Cobalt or Neodynium Iron Cobalt, the magnets may also be used as electrodes. The reaction device includes an analyte entrance, which may be an aperture, for receiving analyte ions 18 into the reaction device such that the analyte ions are transmitted into the region in which the electrons are confined, thereby causing the analyte ions to react with the electrons and dissociate to produce fragment ions and / or produce other product ions such as charge-reduced analyte ions. As described above, the electrons may be introduced into the reaction device by a filament having a looped portion. The analyte ions may be introduced into the reaction device along an axis that passes through the looped portion (e.g. along the central axis), such that the filament does not interfere with the passage of at least some of the ions through the reaction device. The reaction device also includes an exit, which may be an aperture, for allowing the fragment or product ions to exit the reaction device. The entrance and exit may be arranged such that the axis through these extends along the first dimension through the region in which the electrons are trapped, e.g. along the central axis of the reaction device. As such, the analyte ions can be urged into the entrance with sufficient energy in the first dimension such that the fragment and / or product ions derived therefrom pass out of the exit. The reaction device may include a plurality of electrodes (not shown) for axially and / or radially confining the analyte ions and / or electrons within the device. For example, the plurality of electrodes may form an ion guide within the reaction device. One or more DC voltages may be applied to the plurality of electrodes so as to radially confine the analyte ions in the dimensions orthogonal to the first dimension. Alternatively, or additionally, an RF voltage may be applied to the electrodes in order to increase the electron energy for EID, or to confine the ions and / or electrons. However, for ECD it is preferred that the electrodes are DC-only electrodes, i.e. that an RF voltage is not applied to the electrodes, so that the trapped electrons do not become excited by an RF electric field. Fig. 2 shows another embodiment that operates in the same manner as that described in relation to Fig. 1, except wherein the first magnet 2, second magnet 4, and shim 10 are replaced with a single tubular magnet 20. The magnet 20 is elongated in the first dimension and may be cylindrical. The filament may be received through the entrance aperture or the exit aperture, in which case the filament may extend into the reaction device at an angle to the central axis so that the majority of the filament is not arranged at the central axis and hence does not to interfere with the passage of the analyte, fragment and / or product ions. The distal end of the filament may have a looped portion, as described above. The distal end of the filament may be located at any position along the central axis of the reaction device, although it is desirably located substantially halfway along the length of the reaction device. It is alternatively contemplated that the filament may extend through the magnet into the reaction region, rather than through the entrance or exit aperture, although this is less preferred. In operation, electrons are generated within the reaction device along the central axis using the filament. The electrons become radially trapped as they spiral around the magnetic field lines, as described above, i.e. along the central axis. The density of the trapped electrons eventually builds up to a density that is sufficient to provide reactions with the analyte, such as ECD reactions. Analyte ions are introduced into the reaction device through the entrance aperture, also along the central axis. As the analyte ions pass through the reaction device they may react with the electrons so as to produce fragment ions or product ions such as charge-reduced analyte ions. The fragment or product ions, along with analyte ions that have not reacted with the electrons, then pass out of the reaction device via the exit aperture to be mass analysed by a mass analyser downstream of the reaction device. Fig. 3a illustrates a schematic of the upper half of the reaction device of Fig. 2, wherein the length of the magnet 20 is 20 mm and the inner radius of the magnet is 5mm, i.e. having a ratio of length to inner radius of 20:5 (i.e. a ratio of 4). The region between the central axis and the inner diameter of the magnet is depicted as being divided by five longitudinal lines, each of which represents a different radial position in the magnet. Fig. 3b shows the magnetic flux density along the length of the magnet of Fig. 3a at the different radial positions indicated by the longitudinal lines in Fig. 3a. Specifically, the first peak on the left in Fig. 3b represents the magnetic flux density along the first longitudinal line in Fig. 3a (i.e. along the central axis), the second peak represents the magnetic flux along the adjacent longitudinal line that is radially adjacent to the first longitudinal line, and so on. As demonstrated in Fig. 3b, the magnetic flux density along the central axis is more homogenous than the magnetic flux density at the other radial positions that are away from the central axis. This is due to the magnet being relatively long compared to its inner radius, i.e. having a relatively high ratio of length to inner radius. In this embodiment, the magnet has a length of 20 mm and an inner radius of 5 mm. However, in other embodiments the magnet may have different lengths and / or inner radii and still provide a relatively high ratio of length to inner radius. For example, the magnet may have a length in the first dimension of at least 5mm, but optionally not exceeding 100mm. As such relatively high ratios provide a relatively homogenous magnetic flux along the central axis. A magnet having a relatively high ratio may be used in embodiments in which the electrons are injected into the reaction device, or generated within the reaction device, at a location along the central axis, such as is shown in Fig. 2. This helps to confine the electrons more effectively. Fig. 4a illustrates another schematic of the upper half of the reaction device of Fig. 2, except wherein the length of the magnet 20 is 20 mm and the inner radius of the magnet is 11 mm, i.e. having a ratio of length to inner radius of 20:11 (i.e. a ratio of 1.8). Here, the region between the central axis and the inner diameter of the magnet is depicted as being divided by eleven longitudinal lines, each of which represents a different radial position in the magnet. Fig. 4b shows the magnetic flux density along the length of the magnet of Fig. 4a at the different radial positions indicated by the longitudinal lines in Fig. 4a. In a similar manner to Fig. 3b, the first peak on the left in Fig. 4b represents the magnetic flux density along the first longitudinal line in Fig. 4a (i.e. at the central axis), the second peak represents the magnetic flux along the adjacent longitudinal line that is radially adjacent to the first longitudinal line, and so on. As demonstrated in Fig. 4b, the magnetic flux density along the reaction device is most homogenous in the ninth peak, i.e. at the radial position indicated by the ninth longitudinal line in Fig. 4a. It has been found that, as the ratio of the length of the magnet to the inner radius of the magnet decreases (e.g. below a ratio of 3:1), the magnetic field along the reaction device becomes more homogenous at radial positions that are further from the central axis. For example, it has been found that for a magnet having a ratio of length to inner radius of 2.2, the magnetic field is most homogenous at a radial position that is located a distance of 55% of the inner radius away from the central axis. In contrast, for the magnet shown in Figs. 4a-4b, which has a ratio of length to inner radius of 1.8 (i.e. 20:11), the magnetic field is most homogenous at a radial position that is located a distance of 72% of the inner radius away from the central axis. This being the case, when the magnet has a relatively low ratio of length to inner radius, the electrons may not be introduced into the reaction device, or generated within the reaction device, at a location along the central axis. Instead the electrons may be introduced into the reaction device, or generated within the reaction device, at a radial position within the magnet that is off the central axis. For example, the electrons may be introduced or generated at a radial location where the magnetic flux along the device is most homogenous. This helps to confine the electrons more effectively. The electrons may be introduced or generated at a radial location that is selected based on the ratio of the length to inner radius of the magnet. Alternatively, the ratio of length to inner radius of the magnet may be selected based on the position at which or electrons are desired to be introduced or generated. In these embodiments that have a relatively low ratio of length to inner radius, it is still preferable that the magnet has a minimum length, in order to provide a relatively large electron confinement / reaction region. For example, the magnet may have a length of at least 5 mm. Figs. 5a-5c show views of an embodiment which is the same as that described in relation to Fig. 2, except that the electrons are introduced into the reaction device at a location that is radially outwards of the central axis. In order to do this, the distal end of the filament is located at a position that is radially outwards of the central axis of the reaction device. As described above, the magnetic field of the magnet is stronger and more homogenous closer to the radially inner wall of the magnet than along the central axis for a magnet having a relatively low ratio of length to inner radius (e.g. for a ratio less than 3). As such, in this embodiment electrons can be better confined by the magnet, even at relatively high electron energies. Also, as the magnetic field lines converge at each axial end of the magnet, this provides axial trapping of the electrons within the reaction region. As in the other embodiments described above, the electrons spiral around the magnetic field lines and hence move back and forth in the first dimension. The electrons are also able to drift circumferentially around the central axis, at a substantially constant radial distance from the central axis, as shown in Figs. 5b and 5c. The electrons therefore remain radially confined within the magnet in a relatively large annular volume. The analyte ions may be introduced into the reaction device at any position such that they interact with the electrons that are trapped in the annular region. However, as the electrons are trapped in an annular region this enables the analyte ions to be easily introduced into the reaction device along an axis that does not intersect the electron filament. For example, as shown in Figs. 5a and 5c, the analyte ions 18 may introduced into the reaction device along an axis that is on the opposite side of the central axis to where the distal end of the filament is located. This allows the filament to be positioned within the reaction device without interfering with the introduction of the analyte ions. This is the case even if the filament extends into the reaction device along an axis parallel to the central axis. With reference to Fig. 5c, a screen member 22 may be positioned between the distal end of the filament and the path along which the analyte ions travel so as to screen the analyte ions from the electric field generated by the filament. For example, the screen member may be located along the central axis of the reaction device. The screen member may have any form or shape. For example, the screen member may be an elongated rod or plate member. The screen member may be formed from any non-ferromagnetic material. Figs. 6a-6b illustrate a schematic of a reaction device according to another embodiment of the present invention. The reaction device comprises an elongated bar magnet 24, which may be a permanent magnet. The bar magnet may be planar or another rectilinear shape. The bar magnet comprises a North pole at one axial end and a South pole at the other axial end. The reaction device also comprises a filament 12 for generating electrons, which may have any of the features of the filaments described in relation to the earlier embodiments, such as the looped portion 14 through which analyte ions 18 are able to pass. As can be seen in Fig. 6a, the distal end of the filament is located relatively close to the bar magnet such that the electrons 8 generated therefrom spiral along its magnetic field lines and hence are trapped relative to the magnet in a corresponding manner to that described above in relation to Figs. 5a-5c (except the bar magnet causes the electrons to be trapped in a curved plane rather than in an annular volume). The analyte ions 18 are then passed through the region in which the electrons are trapped, which may be through a looped end of the filament or along an axis that does not pass through the filament. The reaction device may also comprise electrodes for focussing or otherwise guiding the ions and / or electrons. For example, the reaction device may comprise a first electrode 26, a second electrode 28, and a third electrode 30 which, together with the magnet 24, form a conduit through which analyte ions are passed. The third electrode may have a planar surface spaced apart from and facing the magnet. The first and second electrodes may be located between the magnet and the third electrode. The first and second electrodes may be planar with their major surfaces positioned spaced apart from, and e.g. parallel to, each other. The electrodes may be formed from any suitable non-ferromagnetic material. Any of one the first, second or third electrodes may comprise an aperture through which the filament passes. However, in other embodiments, the filament may pass into the reaction region through the entrance aperture or the exit aperture that the analyte ions pass into or out of, respectively. The distal end of the filament may be positioned along the central axis of the reaction device or at a position closer to the bar magnet. A current is then passed through the distal end of the filament, as described above, causing electrons to be released from the distal end and become trapped along the magnetic field lines. Analyte ions are introduced into the reaction device and DC voltages are applied to the first, second and third electrodes so as to generate a DC electric field that guides the analyte ions through the region in which the electrons are located. A voltage may also be applied to the magnet 24 so as to help guide the ions, e.g. such that the magnet and electrodes 26-30 provide a quadrupolar ion guide. It is contemplated that the electrodes may be segmented in a direction along the central axis and different voltages applied thereto so as to drive ions through the reaction device. Additionally, or alternatively, the electrodes may be angled relative to each other or the magnet so as to drive ions through the reaction device. Although the electrodes are depicted as planar electrodes, they may alternatively have other geometries, such as being rod electrodes. Fig. 7 shows another embodiment that is the same as that described in relation to Fig. 2, except that the magnet is formed from a plurality of magnets 32 that are stacked together in the first dimension, with electrical insulators between at least some of the magnets such that different DC or RF voltages can be applied to the different magnets. Some of the adjacent magnets may not be electrically insulated from each other such that they are maintained at the same electrical potential. Each of the plurality of magnets may be identical in form or shape. The reaction device may also comprise a filament (not shown) for generating electrons, which may have any of the features of the filaments described in relation to the earlier embodiments, such as the looped portion through which analyte ions are able to pass. The filament may be provided to an interior of the reaction device through an aperture in one of the electrical insulators that is arranged between adjacent magnets, e.g. in a similar manner to Fig. 1. The magnets are arranged such that any pair of directly adjacent magnets have opposite magnetic poles at their ends that face each other. As such, the magnetic field generated by the plurality of magnets is substantially the same as a single, elongated magnet. However, voltages may be applied to the magnets, e.g. so as to control the motion of the ions therein. For example, voltages may be applied to the magnets such that they operate as electrostatic lenses. That is, the plurality of magnets may be configured to focus the analyte ions as they pass through the reaction device. The reaction device may have voltage supplies configured to apply different DC voltages to different respective (electrically insulated) magnets of the plurality of magnets so as to generate an electric field that focuses the analyte ions in the first dimension and / or confines the analyte ions in the dimensions orthogonal to the first dimension. Different voltage supplies may be configured to apply different DC voltages to different respective (electrically insulated) magnets of the plurality of magnets so as to generate one or more quadratic potentials so as to trap analyte ions and / or electrons axially within the reaction device. In this way, the plurality of magnets may be operated like an ion and / or electron trap (e.g. a Penning trap). By trapping the analyte ions, together with the electrons, the efficiency of ion-electron reactions can be improved. Additionally, or alternatively, different voltage supplies may be configured to successively apply a transient voltage to successive (electrically insulated) magnets of the plurality of magnets so as to repeatedly travel a DC potential along the reaction device, e.g. to increase the energy of the trapped electrons or to urge ions through the reaction device. This might be desired, for example, during EID. Additionally, or alternatively, an RF voltage may be applied to the magnets, e.g. so as to increase the energy of the electrons and / or confine ions. The reaction device described herein is desirably held at a sub-atmospheric pressure e.g. such that the rate of collisions between the background gas and the electrons is relatively low. However, the gas pressure within the reaction device may also be sufficiently high to help reduce the thermal energy of the electrons trapped therein and / or the thermal energy of the analyte ions introduced to the reaction device. As such, the gas pressure in this region may be between 10~7 and 10~1 mbar. The gas may be an inert gas. Fig. 8 shows a schematic of an embodiment of a mass spectrometer that includes a reaction device as described herein. The spectrometer comprises an ion source 70 and the reaction device 72. The ion source may be an electrospray ionisation (ESI) ion source. Any other type of ion source may be used instead of an ESI ion source, although ion sources that generate analyte ions having high charge states are preferred. The analyte ions generated in the ion source, or ions derived therefrom, are transmitted into the reaction device. The ions may be filtered, e.g. in a quadrupole mass filter 74, and / or separated by mobility in an ion mobility separator 76 prior to entering the reaction device. The quadrupole mass filter 74 and the ion mobility separator 76 may be provided in any order. The ions that are transmitted into the reaction device are reacted with the electrons therein, e.g. via ECD or EID reactions, so as to produce fragment or product ions such as charge-reduced analyte ions. These ions may be ejected or released from the reaction device and transmitted downstream for analysis. For example, these ions may be analysed so as to determine their ion mobilities through an ion mobility separator 78 and / or to determine their mass to charge ratios using a mass analyser 80 downstream of the reaction device. Although embodiments have been described in which the trapped electrons are reacted with analyte ions, it is contemplated that the trapped electrons may alternatively or additionally react with the background gas molecules in the reaction device so as to ionise these gas molecules and form radical or non-radical cations and anions, e.g. via electron impact ionisation (El). Based on knowledge of what the gas comprises, the type of cations and anions that are generated may be predictable and hence the true mass to charge ratio of such ions may be known. As such, these cations or anions may be mass analysed by the same mass analyser that is used to mass analyse the fragment or product ions and hence may be used as calibration or lock-mass ions for calibration of the mass analyser, i.e. for correcting the mass to charge ratios of the ions detected by the mass analyser. Alternatively, the radical or non-radical cations and anions may be trapped in the reaction region and react with the analyte ions, e.g. by ETD. Embodiments have been described in which the analyte ions are of the opposite polarity to the electrons so as to produce the reactions. For example, the reaction may be ECD or EID in which positive analyte ions react with negative electrons. However, the invention also applies to reactions between analyte ions that have the same polarity as the electrons, e.g. to electron detachment dissociation (EDD) reactions between negative analyte ions and higher energy electrons. Although the present invention has been described with reference to various embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as set forth in the accompanying claims.

Claims

1. An ion-electron reaction device comprising:a first tubular magnet;a second tubular magnet arranged coaxially with the first tubular magnet; and a shim located between the first and second tubular magnets;wherein the shim is arranged and configured such that the first and second tubular magnets provide a substantially homogenous magnetic field along a central axis of the device.

2. The reaction device of claim 1, wherein the homogenous magnetic field has magnetic field lines that extend from the first tubular magnet to the second tubular magnet, within a volume arranged about the central axis, wherein said magnetic field lines are substantially linear and parallel to the central axis along at least 30 % of the length of the reaction device.

3. The reaction device of claim 1 or 2, wherein the first tubular magnet is arranged with its North magnetic pole adjacent to the shim and the second tubular magnet is arranged with its South magnetic pole adjacent to the shim; or the first tubular magnet is arranged with its South magnetic pole adjacent to the shim and the second tubular magnet is arranged with its North magnetic pole adjacent to the shim.

4. The reaction device of claim 1, 2 or 3, comprising an electron generator arranged to generate and supply free electrons to a location where said homogenous magnetic field is located.

5. The reaction device of claim 4, wherein the electron generator comprises a voltage supply and a wire filament that are arranged and configured to generate said free electrons.

6. The reaction device of claim 5, wherein the wire filament is arranged such that it has a distal end arranged on the central axis; or wherein the wire filament is formed as a loop at its distal end, where the loop surrounds the central axis.

7. The reaction device of claim 5 or 6, wherein the wire filament passes through an aperture in the shim.

8. The reaction device of any one of claims 5-7, comprising a heat sink in contact with the shim for transferring heat from the wire filament out of the shim and to the heat sink.

9. The reaction device of any preceding claim, wherein the reaction device is configured to receive analyte ions such that they travel substantially along the central axis.

10. The reaction device of any preceding claim, comprising electrodes and one or more voltage supplies configured to apply one or more DC voltages to the electrodes so as to generate a DC electric field for confining electrons radially about, and / or axially along, the central axis; and / orcomprising electrodes and one or more voltage supplies configured to apply one or more DC and / or RF voltages to the electrodes so as to generate a DC and / or RF electric field for confining ions radially about, and / or axially along, the central axis.

11. An ion-electron reaction device comprising:an elongated tubular magnet having a length of at least 5 mm; andan electron generator arranged to supply free electrons to a location within said tubular magnet;wherein either:(i) the electron generator is arranged to supply free electrons to a location within the tubular magnet that is substantially on a central, longitudinal axis of the tubular magnet, and wherein the tubular magnet has a ratio of length to inner radius of >3; or(ii) the electron generator is arranged to supply free electrons to a location within the tubular magnet that is radially outwards of a central, longitudinal axis of the tubular magnet, and wherein the tubular magnet has a ratio of length to inner radius of <3.

12. The reaction device of claim 11, wherein the tubular magnet is configured and has a length such that a substantially linear homogenous magnetic field is provided along a central axis of the tubular magnet, wherein the homogenous magnetic field has magnetic field lines that extend from one end of the tubular magnet to the other end of the tubular magnet, within a volume arranged about the central axis, wherein said magnetic field lines are substantially linear and parallel to the central axis along at least 30 % of the length of the tubular magnet.

13. The reaction device of claim 11 or 12, wherein the electron generator comprises a voltage supply and a wire filament that are arranged and configured to generate free electrons on or around a central axis through the tubular magnet.

14. The reaction device of claim 11, 12 or 13, wherein the tubular magnet is configured and has a length such that a substantially linear homogenous magnetic field is provided along the magnet throughout an annular region that is arranged between the central axis and the inner wall of the tubular magnet; wherein the homogenous magnetic field has magnetic field lines that extend from one end of the tubular magnet to the other end of the tubular magnet, within the annular region, wherein said magnetic field lines aresubstantially linear and parallel to a central axis along at least 30 % of the length of the tubular magnet.

15. The reaction device of claim 14, comprising an electron generator having a voltage supply and a wire filament that are arranged and configured to generate free electrons in said annular region.

16. The reaction device of claim 15, wherein the reaction device is configured to receive analyte ions into the annular region such that the ions travel along an axis that does not intersect the wire filament.

17. The reaction device of claim 16, wherein the axis that the ions travel along is arranged on the opposite side of the central axis through the magnet to that which the wire filament is located on.

18. The reaction device of claim 16 or 17, comprising a screen member positioned between the axis that the ions travel along and the wire filament for screening the analyte ions from an electric field generated by the wire filament.

19. An ion-electron reaction device comprising:a plurality of tubular magnets that are stacked together coaxially with electrical insulators therebetween; andone or more voltage supplies configured to apply a voltage to one or more of the magnets.

20. An ion-electron reaction device comprising:a magnet having a substantially flat, planar surface; andan electron generator for supplying free electrons;wherein the reaction device is configured such that a magnetic field from the magnet confines the free electrons in a region adjacent to the planar surface.

21. The reaction device of claim 20, wherein the magnet is a bar magnet.

22. A mass spectrometer comprising:a reaction device as claimed in any preceding claim; andan ion source arranged for supplying analyte ions into the reaction device.

23. A method of mass spectrometry comprising:providing a mass spectrometer as claimed in claim 22;supplying ions from the ion source into the reaction device; andproviding electrons in the reaction device such that the electrons react with the ions.AMENDMENTS TO THE CLAIMS HAVE BEEN FILED AS FOLLOWS:Claims:5 1. An ion-electron reaction device comprising:a first tubular magnet;a second tubular magnet arranged coaxially with the first tubular magnet; anda shim located between the first and second tubular magnets;wherein the shim is arranged and configured such that the first and second tubular10 magnets provide a substantially homogenous magnetic field along a central axis of the device; andwherein the first tubular magnet is arranged with its North magnetic pole adjacent to the shim and the second tubular magnet is arranged with its South magnetic pole adjacent to the shim; or the first tubular magnet is arranged with its South magnetic pole adjacent to15 the shim and the second tubular magnet is arranged with its North magnetic pole adjacent to the shim.

2. The reaction device of claim 1, comprising an electron generator arranged to generate and supply free electrons to a location where said homogenous magnetic field is 20 located.

3. The reaction device of claim 2, wherein the electron generator comprises a voltage supply and a wire filament that are arranged and configured to generate said free electrons.

254. The reaction device of claim 3, wherein the wire filament is arranged such that it has a distal end arranged on the central axis; or wherein the wire filament is formed as a loop at its distal end, where the loop surrounds the central axis.30 5. The reaction device of claim 3 or 4, wherein the wire filament passes through anaperture in the shim.

6. The reaction device of any one of claims 3-5, comprising a heat sink in contact with the shim for transferring heat from the wire filament out of the shim and to the heat sink.

357. The reaction device of any preceding claim, wherein the reaction device is configured to receive analyte ions such that they travel substantially along the central axis.

8. The reaction device of any preceding claim, comprising electrodes and one or more 40 voltage supplies configured to apply one or more DC voltages to the electrodes so as to generate a DC electric field for confining electrons radially about, and / or axially along, the central axis; and / orng electrodes and one or more voltage supplies configured to apply one or more DC and / or RF voltages to the electrodes so as to generate a DC and / or RF electric field for confining ions radially about, and / or axially along, the central axis.5 9. A mass spectrometer comprising:a reaction device as claimed in any preceding claim; andan ion source arranged for supplying analyte ions into the reaction device.

10. A method of mass spectrometry comprising:10 providing a mass spectrometer as claimed in claim 9;supplying ions from the ion source into the reaction device; and providing electrons in the reaction device such that the electrons react with the ions.LDCM

Images