Correction of the ion front tilt in a time-of-flight (TOF) mass spectrometer

Abstract

Device for correcting the inclination of a time-of-flight (TOF) ion beam front, comprising the following: at least one electrode which, when supplied with a voltage, defines a substantially equipotential channel, wherein the channel extends in a longitudinal direction Z TC extends, with the canal continuing in a first, longer path along a first transverse axis X TC , perpendicular to the longitudinal direction Z TC is defined, and in a second, shorter path along a second transverse axis Y TC extends perpendicular to both the first axis X TC as well as the longitudinal axis Z TC stands, where the ratio of the first, longer path along the first axis X TC to the second, shorter path along a second axis Y TC at least 2; where the length of the channel in the longitudinal direction Z TC according to the transverse position in the X direction TCperpendicular to the longitudinal direction Z TC of the channel varies, so that the ions at a first transverse position X TC The ion beam has a different time to traverse the channel of at least one electrode than the ions at a second, different transverse position X. TC of the ion beam.

Description

Field of the invention

[0001] This invention relates to the correction of the tilt angle of the ion front in a time of flight (TOF) mass spectrometer. Background of the invention

[0002] Time-of-flight (TOF) mass spectrometers with ion impact detectors utilize the property that the travel time of an ion in an electrostatic field is proportional to the square root of the ion's mass. The ions are simultaneously ejected from an ion source (e.g., an orthogonal accelerator or a radiofrequency ion trap), accelerated to a desired energy, and, after traveling a specified distance, impact an ion detector (e.g., a microchannel plate). While the travel distance is essentially the same for all ions, the arrival time of the ions is used to determine the mass-to-charge ratio (m / q), which is later used for ion identification.

[0003] The accuracy of the mass-to-charge measurement and the quality of the mass separation depend on the spread of the travel times for ions with the same mass-to-charge ratio (m / q). This spread is caused by different initial conditions, coordinates, and velocities, as well as the limited ability of a mass spectrometer to focus ion bunches in time—that is, to bring ions with the same m / q to a detector simultaneously, regardless of their initial conditions.

[0004] Time focusing with respect to ion energy is typically achieved with one or more electrostatic mirrors, as in reflectron-type mass analyzers (Mamyrin BA et al., Sov. Phys.-JETP, 37, pp. 45-48, 1973). Time focusing with respect to initial coordinates and velocities can be achieved by various means. In the earliest grating reflectrons, a uniform electrostatic field was used for ion reflection, which guaranteed time-of-flight independence from the lateral initial coordinates and velocities. In more sophisticated mass analyzers with gratingless ion mirrors, the field configuration is specifically designed to eliminate the major space-time aberrations. Such configurations have been described for axially symmetric mirrors (H. Wollnik and A. Casares, Int. J. of Mass Spectrom. 227 (2), 217-222, 2003) and planar mirrors (Yavor M., et al., Physics Procedia 1, pp.391-400, 2008). Electrostatic sectors—to focus ions both spatially and temporally—were also used (Satoh T., J. Mass Spectrom. Soc. Jpn., 57 (5), pp. 363-369, 2009).

[0005] In all these arrangements, the ion beams diverge spatially as they move away from an ion source, and their transverse dimension can reach several millimeters when they impact a detector. A spatially spread ion beam is also beneficial for reducing space-charge effects and preventing detector saturation. The latter is particularly important for microchannel plate (MCP) detectors and dynode detectors. A negative effect of a wide ion impact area is that it imposes particularly stringent requirements for detector alignment with respect to the incident ion beam. In fact, for an ion beam with a width of 10 mm, even a small misalignment of a detector (for example, by one angular degree) results in a difference of ~0.17 mm in ion impact times.If the total travel distance of the ions is 1 meter, this discrepancy limits the mass resolution of the mass analyzer by the value R = 1 meter / 0.170 mm / 2 ≈ 3000, which is usually unacceptable.

[0006] The problem of detector alignment is exacerbated by the fact that an actual TOF front (a location, usually a plane but sometimes a curved surface, where ions with different lateral initial conditions arrive simultaneously) is affected by misalignments of other ion optical elements, e.g., the ion source and / or mirrors, as well as by factors such as fringe electric fields and stray magnetic fields in the instrument's environment, all of which are difficult to predict. As a result, accurate alignment of an ion detector and the TOF front is a difficult technical challenge.

[0007] A number of solutions have been proposed to overcome the problems listed above. US Pat. No. 5,654,544 A (Dresch) discloses the precise mechanical control of an ion detector to adjust its ion-sensitive plane to the current TOF front of an incoming ion beam. However, such a method is difficult to implement because the moving parts require an activator for their precise adjustment.

[0008] Electrically controlled methods are preferred because they allow precise adjustment during operation of the mass spectrometer. In US 2017 / 0098533 A1 (Stewart et al.), it was proposed to use a dipolar electric field to rotate the TOF front and align it with an ion impact detector. The position and orientation of the detector are fixed. This method uses a property of the dipolar transverse electric field to tilt the TOF front in a direction opposite to that of deflection. The effect is based on the velocity difference for ions passing near a positive pole and for ions passing close to a negative pole of a dipolar electrostatic element. This difference creates a correlation between the ion position and the arrival time at a detector located immediately behind the dipolar element.

[0009] US 7,772,547 B2 (Verentchikov, see Fig. 3 and Fig. 4) and US 9,136,102 B2 (Grinfeld et al., see Fig. 11A and Fig. 11B) also disclose a rotation of the TOF front using a dipolar electric field for preparing the ion beam before it enters a TOF mass analyzer.

[0010] A limitation of a dipolar field TOF front correction device is that this field is never perfectly uniform, leading to significant and unavoidable distortions at the input and output of the electrostatic dipolar element. The presence of the surface of an equipotential detector in the immediate vicinity of a dipolar element also contributes to such field disturbances. Due to field imperfections, the net time-of-flight correction is not exactly linear with respect to the ion's input coordinate, resulting in TOF front distortion.

[0011] US 2014 / 0054454 A1 (Hoyes et al.) proposes correcting the misalignment of the TOF front using a system of planar meshes that are angled to each other and biased with different acceleration or deceleration potentials. Ion bunches cross all the meshes sequentially. If the distance between two adjacent meshes and their inclination to each other are small enough, the electric field between the meshes is quasi-uniform and varies linearly in the direction of the inclination. The time required for a given ion to cross the stack of meshes varies depending on where the ion enters the stack. The TOF front is therefore rotated to align with the detector. However, passing through multiple meshes results in significant ion losses and scattering effects.Furthermore, ion collisions with the mesh wires lead to ion fragmentation and possible sputtering of the mesh material; the charged and neutral fragments can impinge on the detector and generate false peaks.

[0012] Against this background, the present invention proposes solutions to the problems associated with the inclination of the time-of-flight front.

[0013] CN 103165395 A discloses a device for adjusting the movement section of an ion beam. The device for adjusting the movement section of the ion beam mainly consists of a pre-electrode, a counter electrode a, a counter electrode b, and a post-electrode. The pre-electrode and post-electrode serve to reduce the outward permeation of the electric fields of the counter electrode a and the counter electrode b and to supply different voltages to the counter electrode a and the counter electrode b.

[0014] DE 11 2013 000 722 T5 discloses a multiple reflection mass spectrometer comprising two ion optical mirrors, each mirror having a length extending substantially along a drift direction (Y) of ions, each mirror being opposite the other in an X-direction and having a space therebetween, the X-direction being orthogonal to the drift direction (Y), and the mirrors having continuous, unsplit electrode structures. Summary of the invention

[0015] According to a first aspect of the present invention, there is provided an apparatus for correcting the tilt of a time-of-flight (TOF) ion beam front as claimed in claim 1.

[0016] The correction device contains one or more electrodes, preferably a stack of several electrodes, with channels. An electrode as described realizes a region of substantially equal electrical potential within its channel. If the total ion energy per unit charge is U0, and the potential of the kth electrode is U k is the kinetic energy of the ion U0 - U k per unit charge when the ion flies within the channel.

[0017] The time required for the ion to pass through a channel of length L k in the direction of movement Z of the ion is Tk=Lk(m / 2q)1 / 2 / (U0−Uk)1 / 2.

[0018] At least one electrode of the correction device has a channel length that varies in a direction transverse to the Z-axis, i.e. the channel length ΔZ = L k(X,Y) varies depending on the position X, Y of the ion when it enters the channel. As a result, the time T for a particular ion to cross the stack of electrodes depends on the transverse position of the ion (X TC ,Y TC ), which causes a difference in flight time that compensates for the flight time error.

[0019] In many practical implementations, the tilt of the TOF front in a first direction is more significant than its tilt in a second, orthogonal direction, for example, when the second direction is perpendicular to a symmetry plane of the mass analyzer. In this case, only the tilt in the first direction needs to be considered and can be ignored in the orthogonal direction. This is also the case when the ion bunch is more extended in one direction than in the other, and the second direction is therefore more tolerant of the tilt. Both situations are, for example, typical for TOF mass analyzers with planar ion mirrors, as described in Soviet patent SU 1725289 A1 (Nazarenko L.M., et al.), US patent 7,385,187 B2 (Verentchikov et al.), or US patent 9,136,102 B2 (Grinfeld D., Makarov A.).When the ion beam arrives at a detector, its width in the direction perpendicular to the symmetry plane of the ion mirrors is relatively small, while the spread along the mirrors is high. Correction of the TOF front is most important in the direction where the ion beam is wider.

[0020] In contrast to the arrangement shown in the above-mentioned US 2014 / 0054454 A1, the TOF ion beam front tilt correction device of the present invention does not require a mesh for adjusting the ion beam front tilt.

[0021] In a preferred embodiment, the ion beam is in the X ı -Direction of the plane perpendicular to the axis of motion of the ion Z ı spread, compared to the beam dimension in the Y ı-direction of this orthogonal plane. According to the above discussion, the device for correcting the inclination of the ion beam front is then preferably configured to correct only the inclination in this X ı -direction, taking into account any inclination in the Y ı -direction is ignored because it contributes less to the TOF error.

[0022] In this case, the angle γ of the rotation of the TOF front, which is performed by the device for correcting the tilt of the ion beam front, can be mathematically determined only in terms of the X TC -axis can be expressed as: tanγ=(XTC)=T′vz=∑k=1KU0U0−UkLk′(XTC) where (prime) is a derivative with respect to the coordinate X TC is, and v z = (2qU0 / m) 1 / 2 is the speed with which an ion enters the stack of K-electrodes with the voltages U1 ... U K occurs.

[0023] In further preferred embodiments, the means for correcting the tilt of the TOF ion beam front may comprise a stack of K-electrodes arranged along the Z TC -longitudinal axis, each electrode defining a channel, wherein the channel defined by each electrode is at least partially aligned with the others, so that the ions in the ion beam entering a first, upstream electrode are able to pass through the plurality of spaced electrodes via their at least partially aligned channels and exit the TOF ion beam front tilt correction device, wherein the beam front angle relative to the Z TC -axis. In this case, the expression (1) presented above for T k be generalized; the total time to cross the stack of K-electrodes is then T=m2q∑k=1KLkU0−Uk where L kis the length of the channel in the k-th electrode.

[0024] The or each channel preferably has a generally rectangular section in the planes perpendicular to the Z TC -direction. The shorter dimension (in the example above, the Y TC -direction) of the or each channel is sufficiently large to accommodate the transverse width of the ion bundles in the ion beam.

[0025] Alternatively, the electrode (or some / all of the electrodes, if multiple) comprises two parts of equal potential, spaced apart and substantially parallel to each other. The gap between the parts of equal potential forms the channel through which the ion beam travels.

[0026] In preferred embodiments in which the means for correcting the tilt of the TOF ion beam front comprises a plurality of electrodes arranged in a stack, there are narrow gaps between the adjacent electrodes.

[0027] An ion impact detector may preferably be located downstream of the TOF ion beam front tilt correction device.

[0028] In some embodiments, the electrode is wedge-shaped, wherein the electrode has a first opening in a plane perpendicular to the Z TC -axis in an X TC -Y TC -plane and a second opening spaced from the first opening and formed in a second plane that is inclined relative to the plane of the first opening. In other embodiments, the planes of both the first and second openings are relative to the X TC -Y TC-plane. In the example above, where the ion beam is in the X ı -direction relative to the Y I -direction, the plane of the second opening of the channel defined by the electrode can be the Y TC -axis, but at an angle α to the X TC -axis. The channel length in the Z TC -direction is therefore an essentially linear function of the transverse coordinate X TC , and dL k / dX TC is a constant. In such embodiments, the correction of the TOF front is described by a uniform rotation by the angle γ.

[0029] However, in other embodiments, either the first opening, the second opening, or both of the at least one electrode may be curved. For example, the first opening may be planar (e.g., in the X TC -Y TC -plane perpendicular to the Z TC -axis), while the second opening again the YTC -axis, but a curved line in the X TC -Z TC -planes. Then the function dL k (X TC ) / dX TC nonlinear. Such an embodiment is capable, for example, of correcting a curved distortion of the TOF beam front.

[0030] In preferred embodiments, the device for correcting the tilt of the TOF ion beam front comprises a first and a second electrode arranged adjacent to each other in the Z TC -direction. Each electrode may have a first opening in a plane perpendicular to the Z TC -axis in an X TC -Y TC -plane and a second opening which is spaced from the first opening and is formed either in a second plane inclined relative to the plane of the first opening or which has a Y TC -axis opening with a curved line in the X TC -Z TC-planes. In both cases, the second openings are arranged opposite one another. In the case that the second openings each define an inclined plane, the inclination angle of the plane of the second opening in a first of the electrodes may be formed at an angle +α, while the inclination angle of the plane of the opposite second opening in the second of the electrodes may be formed at an angle -α. In the case that the second openings are each curved, the second opening in the first of the electrodes may be generally convex, while the second, opposite opening in the second of the electrodes may be generally concave.

[0031] The or each electrode can be supplied with acceleration or deceleration voltages U kbe electrically biased, which can be adjusted during operation or maintenance to rectify the TOF fronts of incident ion beams and align them with a detection surface of an ion detector, e.g., a microchannel plate.

[0032] Further aspects of the invention provide an ion detection system as recited in claim 16 and a TOF mass spectrometer comprising such an ion detection system as defined in claim 18.

[0033] In yet another aspect of the invention, a method for correcting the tilt of the ion beam front in a time-of-flight (TOF) mass spectrometer according to claim 19 is provided.

[0034] According to a further aspect of the invention, a device for correcting the inclination of a time-of-flight (TOF) ion beam front is provided, comprising at least one electrode which, when supplied with a voltage, defines a substantially equipotential channel, the channel extending in a longitudinal direction Z which is generally parallel to the direction of movement of the ions in the ion beam, and in a direction X perpendicular to this longitudinal direction Z; wherein the length of the channel in the longitudinal direction Z varies according to the transverse position in the direction X perpendicular to the direction of movement of the ions in the channel, so that the ions at a first transverse position X in the ion beam require a different time to traverse the channel of the at least one electrode than the ions in a second, different transverse position X of the ion beam.

[0035] In yet another aspect of the invention, a method for correcting the tilt of an ion beam front in a time-of-flight (TOF) mass spectrometer is provided, comprising (a) generating, in an ion source, an ion beam having a beam axis Z along a direction of travel in the TOF mass spectrometer, the ion beam having a width and a height in an XY plane perpendicular to the Z axis; (b) directing the ion beam toward an ion detector at a location in the TOF mass spectrometer downstream of the ion source;and (c) directing the ion beam by a TOF ion beam front tilt corrector located between the ion source and the ion detector, the TOF ion beam front tilt corrector comprising at least one electrode defining a channel extending in the Z-axis and also in the XY plane, the length of the channel in the Z-axis direction varying according to the position in the channel in the orthogonal XY plane;wherein the method further comprises applying a voltage to the at least one electrode of the device for correcting the tilt of the TOF ion beam front so as to create a substantially equipotential channel defined by the electrode, whereby the ions in the ion beam at different locations in the XY plane are exposed to the substantially same potential in the electrode channel for different periods of time as they traverse the channel so as to change the location of the plane of the ion beam front relative to the Z; TC -axis as the ions pass through the TOF ion beam front tilt correction device.

[0036] Further preferred features are set out in the dependent claims. Short description of the drawings

[0037] The invention may be carried out in a number of ways, and some specific embodiments will now be described by way of example only and with reference to the accompanying drawings. Fig. Figure 1 is a schematic representation of a time-of-flight (TOF) mass spectrometer embodying an aspect of the invention and including means for correcting the tilt of the TOF ion beam front; Fig. 2 is a perspective view of an ion detection system including an ion detector and a device for correcting the tilt of the TOF ion beam front according to a first embodiment of the invention; Fig. 3 a sectional top view of the ion detection system of Fig. 2; Fig. 4 is a sectional plan view of an ion detection system including an ion detector and a device for correcting the tilt of the TOF ion beam front according to a second embodiment of the invention; Fig. 5 is a sectional plan view of an ion detection system including an ion detector and a device for correcting the tilt of the TOF ion beam front according to a third embodiment of the invention; Fig. 6a the lines of equal potential of the electric field of a tilt correction device with a first rectangular cross-section of the electrodes; Fig. 6b the lines of equal potential of the electric field of a tilt correction device with a second rectangular cross-section of the electrodes; and Fig. 6c the lines of equal potential of the electric field of a tilt correction device with a circular cross-section of the electrodes. Detailed description of preferred embodiments

[0038] With reference first to Fig. 1 shows a schematic representation of a TOF mass spectrometer 1 constructed according to one aspect of the present invention. Fig. Spectrometer 1 shown in Figure 1 is of the “reflectron” type.

[0039] The TOF mass spectrometer 1 consists of a pulsed ion source 10, an ion mirror 20, a time-resolved ion impact detector 35, and a device for correcting the inclination of the TOF ion beam front 40, which is located between the ion mirror 30 and the ion impact detector 35. The ion source 10 and the ion impact detector 35 are formed in an XY plane (the Y direction points in the plane of the side in Fig. 1). The ions come from the ion source 10 as a sequence of pulses with a beam axis Z ı , which has a relatively wide cross-sectional profile in an X ı -Direction perpendicular to the beam axis Z ı , which is almost parallel to the X-axis of the XY plane, relative to the Y ı -Direction perpendicular to X ı and the direction of the beam axis Z ı In other words, in the example shown, the cross section of each ion pulse can be elliptical, for example, with a major axis in the X ı -direction and a minor axis of the ellipse in the Y ı -Direction.

[0040] The spectrometer 1 defines a Z-axis, which is perpendicular to the X- and Y-axis. The ions in each pulse can leave the ion source 10 as an ion beam 30 formed from a sequence of pulses. Typically, the beam axis Z deviates ıonly by a small angle from the Z-direction. The ion beam 30 moves in the direction of the beam axis Z ı , which is arranged at an acute angle to the longitudinal axis (+Z direction), to the downstream ion mirror 20, wherein the ion pulses are reflected by the ion mirror 20 and travel back in a direction at an acute angle to the longitudinal axis (-Z direction). The ions pass through the TOF ion beam front tilt correction device 40 (described in more detail below) and then impact the ion impact detector as ion bunches separated in flight time according to their mass-to-charge ratio m / z.

[0041] The ions leave the ion source 10 simultaneously (although along the X ı -direction and to a lesser degree in the Y ı -direction). The plane of the beam front is in Fig. 1 as a sequence of lines labeled 50a. The plane of the beam front as it leaves the ion source 10 is thus located in the XY plane, orthogonal to the plane shown in Fig. 1. As described in the Background section above, it is desirable that the plane of the beam front remains perpendicular to the Z axis until the ions impact the ion impact detector 35. Misalignments of the ion optical components (e.g., in the ion mirror 20 and in other optical components such as lenses, in Fig. 1 not shown) as well as field disturbances, e.g., fringe fields, can have a non-uniform effect on the ion velocities in the ion beam 30. This causes the beam front to arrive at the ion impact detector 35 tilted at a non-zero angle in the XZ plane relative to the plane of the detector, such that ions of the same pulse and with the same m / z arrive at the ion impact detector at different times, depending on their lateral position across the beam width. Since the mass-to-charge ratio is related to the detected time of flight, the result is that the mass resolution and accuracy are reduced because the time of impact of a particular pulse on the ion impact detector is spread out.

[0042] The forward movement of the beam front as a result of misalignments, disturbances and other electromechanical factors is Fig. 1, the ions move through the ion mirror 20 to the TOF ion beam front tilt correction device 40. The plane of the beam front 50b of the ion beam 30 is initially perpendicular to the Z-axis upon exiting the ion source 10. However, as the ions enter the ion mirror 20, the beam front begins to tilt in the XZ plane (represented by the dashed line 50c), and as the ions move through the ion mirror 20 and out the other side to the TOF ion beam front tilt correction device 40, the tilt in this XZ plane becomes even more significant (see the dotted lines 50d, 50e, 50f, which each represent the tilt of the ion beam front).

[0043] The purpose of the TOF ion beam front tilt correction device 40 is to correct the tilt in the introduced ion beam front as the ions pass through the TOF mass spectrometer 1. As shown in Fig. 1, the angle of the beam front is adjusted at a location immediately upstream of the TOF ion beam front tilt correction device 40 (indicated by the dotted line 50f) by the passage of the ions in the ion beam 30 through the TOF ion beam front tilt correction device 40, so that the beam front at a point immediately downstream of the TOF ion beam front tilt correction device 40 again lies in an XY plane perpendicular to the (-)Z direction in the TOF mass spectrometer 1. This is indicated by the dotted line 60 in Fig. 1. The ions then impact the ion impact detector 35 simultaneously across the X-direction extension of the ion beam 30, so that the total impact time of the ions in a given pulse is minimized.

[0044] After describing the general arrangement of a TOF mass spectrometer 1, which comprises a TOF ion beam front inclination correction device 40 for correcting the angle of the ion beam front 30, some examples of specific TOF ion beam front inclination correction devices with respect to the Fig. 2 to 5.

[0045] The most common distortion of the TOF front is a tilt, where the ion impact time depends linearly on the transverse coordinate X. A device for correcting the tilt of the TOF ion beam front 40, which is suitable for correcting such a linear tilt arising during the passage of the ions through the TOF mass spectrometer 1, is described in Fig. 2. The device for correcting the inclination of the TOF ion beam front 40 comprises four electrodes 100, 110, 120 and 130, which are arranged along the longitudinal direction Z TCand which have an outer surface. Preferably, the longitudinal direction is almost parallel to the Z-axis. Preferably, the angle between the longitudinal direction Z TC and the (-Z) axis is less than 5°, especially less than 2°. The optimal angle is less than 0.1°. Each electrode has a channel located in the X TC and Y TC -directions and is defined by the inner surface of the electrode. The X TC - and Y TC -directions are perpendicular to each other and lie in the X TC -Y TC -plane which is perpendicular to the longitudinal direction Z TC the device for correcting the tilt of the TOF ion beam front. The length of the channel in the X TC -direction (the first axis X TC ) relative to the length of the channel in the Y TC -direction (the second axis Y TC) is extended to accommodate the expansion of the ion beam 30 in any direction due to its cross-sectional profile. The ratio of the first, longer length along a first axis X TC to the second, shorter length along a second axis Y TC is at least 2. Preferably the ratio is between 2 and 10, more preferably between 2.4 and 7 and most preferably between 2.7 and 5. As in Fig. 2, the first and fourth electrodes 100, 130 are generally rectangular and define a channel with inlet and outlet openings arranged in the Z TC -direction, but generally lie in parallel planes (each plane being perpendicular to the Z TC-direction). The first and fourth electrodes 100, 130 form the outer electrodes of the group. Between the outer electrodes are a second and a third electrode 110, 120. These electrodes are generally wedge-shaped, in the X TC -Z TC -plane. In particular, the second electrode 110 is located downstream of the first electrode 100 and has an inlet opening lying in a plane perpendicular to the Z-direction. The second electrode also has a TC -direction, but in a direction relative to the Z TC -axis inclined plane.

[0046] The third electrode 120 also has an inlet and an outlet opening. However, the inlet opening of the third electrode 120 is at an angle to the Z TC-direction. The angle of inclination is preferably equal to the angle of inclination of the exit opening of the second electrode 110, but with the opposite sign: that is, if the angle of the exit opening of the second electrode 110 is +α relative to the Z TC -direction, then the angle of the entrance opening of the third electrode 120 is defined as -α.

[0047] Thus, the second and third electrodes form a pair of internal electrodes, and there is a mirror symmetry between the pair of internal electrodes in a plane parallel to the exit opening of the second electrode 110 and the entrance opening of the third electrode 120.

[0048] The channels of each of the four electrodes 100, 110, 120 and 130 of the TOF ion beam front tilt correction device 40 are each arranged relative to each other in both the X TC - as well as the Y TC-direction so that the ions are able to pass through the TOF ion beam front tilt correction device 40 from front to back without being obstructed by the electrodes themselves. Although in Fig. 2 the openings and the channels of the electrodes are each fully aligned, it is of course not necessary that the openings are all exactly along a single axis, the longitudinal direction Z TC , except that they are substantially aligned to allow a direct line of sight through the TOF ion beam front tilt correction device 40.

[0049] Preferably - as mentioned above - the electrodes have a channel which is in the X TC -direction longer than in the Y TC -direction, with the wider cross section of the ion beam in the X ı-direction is taken into account. Embodiments of the invention are particularly suitable in which the cross section of the ion beam in the X ı -direction 2 times, preferably 4 times and most preferably 7 times larger than the cross section in the Y i -direction of the ion beam. Preferably, the inner surface and / or the outer surface of at least one of the electrodes of the device for correcting the inclination of the ion beam comprises parallel planes in the X TC -Z TC -plane. It is desirable that for at least one of the electrodes of the ion beam tilt correction device, the inner surface and / or the outer surface additionally form parallel planes in the Y TC -Z TC -plane, so that in particular the electrode - or at least the channel of the electrode - has a rectangular cross-section in the X TC -Y TC-plane. Then an inlet or outlet opening of such an electrode can be rectangular, whether inclined at a constant angle or not.

[0050] In a preferred embodiment of the device for correcting the inclination of the ion beam, the inner surface and / or the outer surface of each of the electrodes of the device for correcting the inclination of the ion beam comprise parallel planes in the X TC -Z TC -plane. In a particularly preferred embodiment of the device for correcting the inclination of the ion beam, the inner surface and / or the outer surface of each of the electrodes of the device for correcting the inclination of the ion beam additionally comprise parallel planes in the Y TC -Z TC -plane, so that in particular each electrode - or at least the channel of each electrode - has a rectangular cross-section in the X TC -Y TC-plane. Then an inlet or outlet opening of such an electrode can be rectangular, whether inclined at a constant angle or not.

[0051] A power supply (in Fig. 2 not shown) provides a potential for the electrodes 100, 110, 120, and 130 of the TOF ion beam front tilt correction device 40. The voltage applied to each electrode is different in use. This results in ions having different travel times when passing through the TOF ion beam front tilt correction device 40, depending on the X TC coordinate of the ions as they enter the TOF ion beam front tilt correction device 40.

[0052] Fig. 3 shows a top view in the X TC -Z TC -Plane of the device for correcting the inclination of the TOF ion beam front 40 of Fig. 2. The ions with essentially the same kinetic energies per unit charge U0 enter the TOF ion beam front tilt correction device 40 at different X TC -coordinates in the incident ion beam 30, as shown by trajectories 31, 32, and 33. The ion beam front 50f is the plane crossed by all of the ions in the beam 30 simultaneously at a time t = t1. The beam front 50f is inclined at an angle θ with respect to the ion beam detector 35. Without correction, the ions would reach the detector with the following time difference: ΔT=(m / 2qU0)1 / 2ΔXTCtanθ, where ΔX TC is the difference of the entry coordinates, m is the mass of the ions, and q is their charge.

[0053] The potentials applied to the electrodes 100, 110, 120, and 130 of the TOF ion beam front tilt correction device 40 are U1 to U4, respectively. When passing through a channel of one of the electrodes, an ion is accelerated or decelerated depending on the sign of the electrode's potential. Accordingly, the time required for a single ion to pass through the stack is given by equation (3) above, where the lengths of the wedge-shaped electrodes depend linearly on X. TC as L2 = L 20 - X TC tana and L3 = L 30 + X TC tana; α is the wedge angle and L 20 and L 30 are constants.

[0054] The flight time difference between two ion orbits 31 and 33, which are ΔX TC are separated in the transverse direction, then ΔT=m2q(1U0−U2−1U0−U3)ΔXTCtanα which causes the TOF front to rotate by an angle γ, which is expressed as: tanγ=(U0U0−U2−U0U0−U3) tanα

[0055] The choice of the electrode voltages U2 and U3 to satisfy the equation y = - θ compensates for the initial misalignment of the TOF beam front and causes the ions in the beam front to hit the detector simultaneously.

[0056] A side effect of correcting the TOF front is a deflection of a bunch of ions in the beam in a direction opposite to the rotation of the front. However, if the required correction is small, i.e., tan γ < 1, the additional effect on the travel time can be ignored, since the increase is a constant multiplied by (tan γ). 2 is.

[0057] Fig. 4 shows a top view in the X TC -Z TC-Plane of a second, alternative embodiment of a TOF ion beam front tilt correction device 40 according to the present invention. The TOF ion beam front tilt correction device 40 generalizes the concept explained above for the case where the geometry and electrostatics of the TOF mass spectrometer introduce a nonlinear shift in the beam front direction, such that it is curved, as shown by the dotted line 50f' following the trajectories 31', 32', and 33'.

[0058] As with the TOF ion beam front tilt correction device 40 of the Fig. 2 and Fig. 3, a first and a fourth electrode 100, 130 form a pair of outer electrodes, which are rectangular cuboids with inlet and outlet openings in parallel planes and which define a channel between them. The two middle electrodes 110', 120' are again similar to the middle electrodes 110, 120 in the Fig. 2 and Fig. 3, but the opposite surfaces do not form flat surfaces in a plane which is parallel with respect to the Z TC -direction, but instead form curved surfaces. The exit opening of the second electrode 110' is in the example of Fig. 4 is generally concave in shape, while the inlet opening of the third electrode 120' is generally convex. A curved line of symmetry extends equidistantly between the outlet opening of the second electrode 110' and the inlet opening of the third electrode 120'.

[0059] Again, different voltages U0 to U4 are applied to the successive electrodes, whose openings are aligned as described above in connection with the Fig. 2 and Fig. 3 described.

[0060] The arrangement of Fig. 4 corrects the curved beam front 50f' into a straight beam front 60.

[0061] Fig. Figure 5 shows a schematic top view of another preferred embodiment of a device for correcting the tilt of the TOF ion beam front 40, which is combined with a post-accelerator. The post-accelerator increases the kinetic energy of the ions as they impact the ion impact detector 35.

[0062] In the embodiment of Fig. 5, the post-accelerator is designed as a plurality of electrodes, each of which has aligned channels and is supplied with increasingly negative voltages. In the exemplary arrangement of Fig. 5 forms the fourth electrode 130 ( Fig. 2, Fig. 3 and Fig. 4), which represents one of the outer electrodes of the TOF ion beam front tilt correction device 40, is a first of the post-accelerator electrodes and is supplied with a relatively lower voltage, such as -6 kV. A second of the post-accelerator electrodes is positioned downstream of the first post-accelerator electrode and is supplied with a more negative potential, such as -8 kV. The third and final post-accelerator electrode (in the specific example of Fig. 5) is located downstream of the second post-accelerator electrode and is supplied with a potential of -10 kV.

[0063] Positive ions entering the entrance opening of the first electrode 100 of the TOF ion beam front tilt correction device 40 with an acceleration voltage U0 = 4 kV are then further accelerated with 10 kV as they pass through the channels in the following middle electrodes 110, 120, the fourth electrode 130 of the TOF ion beam front tilt correction device 40 (which in the embodiment of Fig. 5 also forms the first of the post-accelerator electrodes of the ion beam) and passes through the second and third post-accelerator electrodes 140, 150. The potential applied to the ion impact detector 35 is equal to the potential applied to the third post-accelerator electrode 150, i.e., -10 kV in the present example. This means that there is no electric accelerating or decelerating field between the output of the TOF ion beam front tilt correction device 40 and the ion impact detector 35.

[0064] In the example of Fig. 5, the outlet openings of the middle electrodes 110 and 120 are inclined at an angle α = 10° to the Z direction.

[0065] The voltage U3 applied to the third electrode 120 (the second of the middle electrodes in the TOF ion beam front tilt correction device 40) can be selected to compensate for the initial misalignment θ of the TOF front. Table 1 shows the optimal value for U3 to compensate for a given misalignment θ. Table 1 Spannung der keilförmigen Elektrode U3, kV Kompensiertes θ, Grad -1 1,07 -2 1,85 -3 2,46 -4 2,96 -5 3,37 -6 3,71

[0066] Although some specific embodiments have been described, it is to be understood that these are provided for illustrative purposes only, and that various modifications or alternatives will occur to those skilled in the art. For example, the Fig. 1 illustrates the "reflectron" type TOF mass spectrometer, but it should be understood that this is merely exemplary, and that the invention is equally applicable to other forms of TOF mass spectrometer, such as a multiple reflection TOF (mr-TOF). In this case, the TOF ion beam front correction device may be positioned in front of the ion detector so as to correct the beam front tilt after the ions have been reflected multiple times between the mirrors in the mr-TOF, or alternatively, the TOF ion beam front correction device may be positioned in the flight path between the mirrors of the mr-TOF. In this case, the voltage applied to the electrodes of the TOF ion beam front correction device may be controlled by the system controller so as to correct the ion beam front angle each time ion bunches pass through the channels of the TOF ion beam front correction device.

[0067] Furthermore, the specific position of the TOF ion beam front tilt correction device 40 in the ion flight path from the ion source 10 to the ion impact detector 35 is not limited to the position specifically illustrated in the figures. It should be understood that the electromechanical effects on the direction of the ion beam front relative to the surface of the ion impact detector 35 are typically cumulative as the ions move through the TOF mass spectrometer, i.e., the total amount of tilt (expressed as angle θ) increases from a minimum at the ion source 10 to a maximum (if left uncorrected) at the ion impact detector 35.On this basis, it is desirable (though not essential) to position the TOF ion beam front tilt correction device 40 as close to the ion impact detector 35 as possible so that there is a minimal clearance for further ion beam front tilt after beam front correction in the TOF ion beam front tilt correction device 40 before the ion beam impacts the ion impact detector 35. It is not desirable to position the TOF ion beam front tilt correction device 40 between the ion source 10 and the ion mirror 20 in view of the degree of tilt caused by field disturbances, etc., within the ion mirror 20.

[0068] Finally, it is understood that although the embodiment of Fig. 5 includes a post-accelerator in the TOF ion beam front tilt correction device 40, the post-accelerator need not be part of the TOF ion beam front tilt correction device 40. The post-accelerator may instead be positioned between the TOF ion beam front tilt correction device 40 and the ion impact detector 35, but as a separate unit (with a relatively short or a relatively long flight distance between the TOF ion beam front tilt correction device 40 and the post-accelerator). Alternatively, the post-accelerator may be positioned upstream of the TOF ion beam front tilt correction device 40, either forming part of the correction device 40, or alternatively, again be physically separated from it by a relatively short or a relatively long distance.For example, the post-accelerator can be positioned between the ion mirror 20 and the TOF ion beam front tilt correction device 40 or between the ion source 10 and the ion mirror 20.

[0069] The device described herein for correcting the inclination of the ion beam front is specifically adapted to ion beams with a cross-section extending in one direction (X ı -direction). Due to the extension of the electrodes in the X TC -direction which is at least almost parallel to the X ı -direction of the ion beam when the ions pass through the device for correcting the tilt of the ion beam front, a correction of the tilt can be achieved in a precise manner across the entire beam. In particular, it is advantageous if the electrodes of the tilt correction device have parallel surfaces in the X TC -Z TC-plane. In the best case, a very accurate correction of the inclination can be achieved by a rectangular cross-section of the electrodes perpendicular to the longitudinal direction Z TC can be achieved.

[0070] The Fig. 6a, Fig. 6b and Fig. 6c show the lines of equal potential of the electric field for a tilt correction device with different cross-sections of the electrodes. In particular, the Fig. 6a and Fig. 6b shows the lines of equal potential of the electric field for a tilt correction device with electrodes having different rectangular cross-sections. Fig. 6a is the ratio of the first, longer path W along a first axis X TC to a second, shorter path H along a second axis Y TC 6.67. In Fig. 6b, the ratio of W:H is 3.33. The electric field exhibits a high degree of uniformity in each case, which prevents or significantly reduces the amount of distortion during tilt correction.

[0071] Fig. For comparison, Figure 6c shows a tilt correction device with electrodes with a circular cross-section. In this case, the electric field contains many disturbances.

Claims

[1] Apparatus for correcting the tilt of a time-of-flight (TOF) ion beam front, comprising: at least one electrode which, when supplied with a voltage, defines a substantially equipotential channel, wherein the channel extends in a longitudinal direction Z TC extends, wherein the channel continues in a first, longer path along a first transverse axis X TC , which are perpendicular to the longitudinal direction Z TC and in a second, shorter path along a second transverse axis Y TC which extends perpendicular to both the first axis X TC as well as the longitudinal axis Z TC where the ratio of the first, longer path along the first axis X TC to the second, shorter path along a second axis Y TC is at least 2; where the length of the channel in the longitudinal direction Z TC according to the transverse position in the X direction TCperpendicular to the longitudinal direction Z TC of the channel is varied so that the ions at a first transverse position X TC in the ion beam a different time to pass through the channel of the at least one electrode than the ions at a second, different transverse position X TC of the ion beam. [2] A TOF ion beam front correction device according to claim 1, wherein the ratio of the first longer path along the first transverse axis X TC to the second, shorter path along the second transverse axis Y TC is between 2 and 10. [3] A TOF ion beam front correction device according to claim 2, wherein the ratio of the first, longer path along the first transverse axis X TC to the second, shorter path along the second transverse axis Y TC is between 2.4 and 7. [4] A TOF ion beam front correction device according to claim 3, wherein the ratio of the first, longer path along the first transverse axis X TC to the second, shorter path along the second transverse axis Y TC is between 2.7 and 5. [5] Correction device for a TOF ion beam front according to one of claims 1 to 4, wherein the inner surface and / or the outer surface of the at least one electrode form parallel planes in the X TC -Z TC -level defined. [6] A TOF ion beam front correction device according to claim 5, wherein the at least one electrode or the channel of the at least one electrode has a rectangular cross-section in the X TC -Y TC -level. [7] Correction device for a TOF ion beam front according to one of claims 1 to 6, wherein the channel defined by the at least one electrode in the X TC -Z TC -plane is wedge-shaped. [8] A TOF ion beam front correction device according to any one of claims 1 to 7, wherein the channel has an ion inlet opening and an ion outlet opening extending in the longitudinal direction Z TC spaced apart, with both openings in planes parallel to the axis Y TC and are inclined to each other at an angle α (≠ 0). [9] A TOF ion beam front correction device according to claim 8, wherein α is between 10° and 50°, and preferably between 20° and 40°. [10] A TOF ion beam front correction device according to any one of claims 7 to 9, comprising first and second wedge-shaped electrodes positioned adjacent to each other such that the channels defined by the first and second wedge-shaped electrodes in the X TC - and Y TC -directions are aligned. [11] A TOF ion beam front correction device according to claim 10, wherein the ion exit opening of the first wedge-shaped electrode and the ion entry opening of the second wedge-shaped electrode are each located in mutually parallel planes. [12] A TOF ion beam front correction device according to any one of claims 1 to 7, wherein the channel has an ion entrance opening and an ion exit opening extending from the ion entrance opening in the longitudinal direction Z TC spaced, wherein the surface of at least one of these openings is in the Y TC -direction and is defined by a curved line in the X TC -Z TC -plane is defined to form a curved electrode surface. [13] A TOF ion beam front correction device according to claim 12, comprising first and second adjacent oppositely curved electrodes, the channels of which are arranged in the X TC - and Y TC-directions are aligned. [14] A TOF ion beam front correction device according to claim 13, wherein the ion exit opening of the first curved electrode and the ion entrance opening of the second curved electrode each define a curved surface, the distance between the curved surfaces of the ion entrance opening and the ion exit opening remaining substantially constant in the longitudinal direction Z, and the ion exit opening of the first curved electrode is opposite the ion entrance opening of the second curved electrode. [15] A TOF ion beam front tilt correction device according to any preceding claim, further comprising one or more electrodes defining a channel having a TC -Y TC -plane perpendicular to the longitudinal direction Z TC lying first opening and a spaced from the first opening in the direction, but also in the XTC -Y TC -plane perpendicular to the longitudinal direction Z TC lying second opening such that the planes of the first and second openings are parallel to each other. [16] An ion detection system comprising the TOF ion beam front tilt corrector according to any one of claims 1 to 15 and an ion impact detector disposed downstream of the TOF ion beam front tilt corrector. [17] The ion detection system of claim 16, wherein the means for correcting the tilt of the TOF ion beam front is positioned adjacent to the ion impact detector. [18] A TOF mass spectrometer comprising an ion source and the ion detection system according to claim 16 or 17. [19] A method for correcting the tilt of an ion beam front in a time-of-flight (TOF) mass spectrometer, comprising: (a) Generating, in an ion source, an ion beam with a beam axis Z ı along a direction of movement in the TOF mass spectrometer, wherein the ion beam has a width in a direction along a first transverse axis X ı in an X ı -Y ı -plane perpendicular to the Z ı -axis and a height in a direction along a second transverse axis Y ı perpendicular to the first transverse axis X ı in the X ı -Y ı -plane, wherein the width of the ion beam is greater than the height of the ion beam; (b) directing the ion beam toward an ion detector at a location in the TOF mass spectrometer downstream of the ion source; and (c) directing the ion beam by a TOF ion beam front tilt correction device located between the ion source and the ion detector, the TOF ion beam front tilt correction device comprising at least one electrode defining a channel extending longitudinally to a Z TC -axis and also in an X TC -Y TC -plane perpendicular to the Z TC -axis, with the length of the channel in the Z TC -Axis direction corresponding to the position in the channel in the vertical X TC -Y TC -plane varies and the channel follows a first, longer path along a first transverse axis X TC in the X TC -Y TC -plane and a second, shorter path along a second transverse axis Y TC perpendicular to the first transverse axis X TC in the X TC -Y TC-plane, whereby the ratio of the first, longer path along the first transverse axis X TC to the second, shorter path along a second transverse axis Y TC at least 2; the method further comprising: (d) applying a voltage to the at least one electrode of the device for correcting the inclination of the TOF ion beam front so as to create a substantially equipotential channel defined by the electrode, whereby the ions in the ion beam are arranged at different locations in the X TC -Y TC -plane are exposed to substantially the same potential in the electrode channel for different periods of time as they pass through the channel, so as to determine the location of the plane of the ion beam front relative to the Z TC -axis as the ions pass through the TOF ion beam front tilt correction device. [20] The method of claim 19, wherein the ratio of the first, longer path along the first transverse axis X TC to the second, shorter path along the second transverse axis Y TC is between 2 and 10. [21] The method of claim 20, wherein the ratio of the first, longer path along the first transverse axis X TC to the second, shorter path along the second transverse axis Y TC is between 2.4 and 7. [22] The method of claim 21, wherein the ratio of the first, longer path along the first transverse axis X TC to the second, shorter path along the second transverse axis Y TC is between 2.7 and 5.

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