Method of generating electric field for manipulating charged particles

Abstract

A device for manipulating charged particles using an axial electric field as they travel along a longitudinal axis of the device is disclosed. The method comprises providing an outer electrode for generating an electric field and providing a plurality of inner electrodes that are separated by gaps of different lengths. The electric field generated by the outer electrode penetrates the gaps between the inner electrodes and the gaps are selected such that the desired potential profile is arranged along the longitudinal axis in order to manipulate the charged particles in the desired manner.

Claims

1. A time of flight mass analyser comprising a time of flight region for manipulating ions using an axial electric field as they travel along a longitudinal axis of the time of flight region, said time of flight region comprising: at least one outer electrode that extends continuously along at least a portion of the length of the time of flight region; a first voltage supply connected to said at least one outer electrode for supplying a first voltage to the at least one outer electrode in use; at least one set of a plurality of three or more inner electrodes or inner electrode portions arranged between the at least one outer electrode and said longitudinal axis along which the ions travel in use; wherein the inner electrodes or inner electrode portions are spaced apart along the length of the time of flight region so as to provide gaps between the inner electrodes or inner electrode portions; wherein the gaps have lengths in the longitudinal direction of the time of flight region, and wherein the lengths of the gaps vary as a function of the position of the gaps along the length of the time of flight region; and a second voltage supply connected to said plurality of inner electrodes or inner electrode portions, wherein the second voltage supply is configured to maintain at least some of the inner electrodes or inner electrode portions at a second voltage that is different to said first voltage.

2. The mass analyser of claim 1, wherein the inner electrodes or inner electrode portions and the at least one outer electrode are arranged and configured, and the first and second voltages are selected, such that in use an electric field generated by the at least one outer electrode penetrates through the gaps between the inner electrodes or inner electrode portions so as to provide an electrical potential profile along said longitudinal axis for manipulating said ions.

3. The mass analyser of claim 2, wherein said electrical potential profile varies progressively along said longitudinal axis in a continuous manner.

4. The mass analyser of claim 2, wherein the first and/or second voltage supply is configured to be pulsed on and off such that said electrical potential profile is pulsed on and off in use.

5. The mass analyser of claim 1, wherein the inner electrodes or inner electrode portions are arranged sequentially along the length of the time of flight region, and wherein the lengths of these electrodes or electrode portions vary linearly or quadratically as a function of the position of the electrode within the sequence; and/or wherein the gaps between the inner electrodes or inner electrode portions are arranged sequentially along the length of the time of flight region, and wherein the lengths of these gaps vary linearly or quadratically as a function of the position of the gap within the sequence.

6. The mass analyser of claim 1, wherein the at least one outer electrode is one of: substantially planar; rod shaped; or cylindrical and arranged around the longitudinal axis; and/or wherein each of the inner electrodes or inner electrode portions is one of: substantially planar; rod shaped; or cylindrical and arranged around the longitudinal axis.

7. The mass analyser of claim 1, wherein the surface of the at least one outer electrode that is facing the longitudinal axis is substantially parallel to said longitudinal axis.

8. The mass analyser of claim 1, wherein the inner electrodes or inner electrode portions are arranged along an axis that is substantially parallel to said longitudinal axis.

9. The mass analyser of claim 1, wherein the surface of the at least one outer electrode that is facing the longitudinal axis is arranged at an angle to the longitudinal axis such that one end of the outer electrode is further from the longitudinal axis than the other end of the outer electrode.

10. The mass analyser of claim 1, wherein the at least one outer electrode has an inner surface facing the longitudinal axis, and wherein the radial distance of said surface from the longitudinal axis varies as a function of position along the longitudinal axis.

11. The mass analyser of claim 10, wherein the inner surface of the at least one outer electrode is curved, stepped or non-linear.

12. The mass analyser of claim 1, wherein the first and/or second voltage supplies are DC voltage supplies such that the electrodes are maintained at DC voltages in use; and/or wherein the electrical potential profile is an electrostatic potential profile.

13. The mass analyser of claim 1, wherein only DC potentials are applied to said at least one outer electrode and/or to said at least one set of inner electrodes or inner electrode portions.

14. A method of mass analysing ions comprising using a mass analyser as claimed in claim 11, the method comprising: applying said first voltage to said at least one outer electrode and applying said second voltage to said at least one set of inner electrodes or inner electrode portions so that an electric field is generated by said at least one outer electrode which penetrates the gaps between the inner electrodes or inner electrode portions so as to form an electrical potential profile along the longitudinal axis which manipulates the ions.

15. The method of claim 14, wherein the electric field generated by the at least one outer electrode penetrates through the gaps between the inner electrodes or inner electrode portions so as to provide an electrical potential profile along said longitudinal axis for manipulating said ions; and wherein the electrical potential profile varies in a non-linear manner along the longitudinal axis of the time of flight region; or wherein the electrical potential profile varies along the longitudinal axis of the time of flight region as a quadratic function or a higher order function.

16. A method of manufacturing a time of flight mass analyser comprising a time of flight region for manipulating ions using an axial electric field as they travel along a longitudinal axis of the time of flight region, said method comprising: selecting an electrical potential profile desired to be established along the longitudinal axis of the time of flight region in use for manipulating the ions; providing at least one outer electrode that extends continuously along at least a portion of the length of the time of flight region; connecting a first voltage supply to said at least one outer electrode for supplying a first voltage to the at least one outer electrode in use; providing at least one set of a plurality of three or more inner electrodes or inner electrode portions between the at least one outer electrode and said longitudinal axis along which the ions travel; wherein the inner electrodes or inner electrode portions are spaced apart along the length of the time of flight region so as to provide gaps between the inner electrodes or inner electrode portions; wherein the gaps have lengths in the longitudinal direction of the time of flight region, and wherein the lengths of the gaps vary as a function of the position of the gaps along the length of the time of flight region; connecting a second voltage supply to said plurality of inner electrodes or inner electrode portions, wherein the second voltage supply is configured to maintain at least some of the inner electrodes or inner electrode portions at a second voltage in use, wherein the second voltage is different to said first voltage; and selecting the lengths of the gaps between the inner electrodes or inner electrode portions, selecting the first voltage and selecting the second voltage such that an electric field generated by the at least one outer electrode, in use, penetrates the gaps between the inner electrodes or inner electrode portions to provide said electrical potential profile along said longitudinal axis.

17. A device for manipulating charged particles using an axial electric field as they travel along a longitudinal axis of the device, said device comprising: at least one outer electrode that extends continuously along at least a portion of the length of the device; a first voltage supply connected to said at least one outer electrode for supplying a first voltage to the at least one outer electrode in use; at least one set of a plurality of three or more inner electrodes or inner electrode portions arranged between the at least one outer electrode and said longitudinal axis along which the charged particles travel in use; wherein the inner electrodes or inner electrode portions are spaced apart along the length of the device so as to provide gaps between the inner electrodes or inner electrode portions, wherein the lengths of the gaps vary as a function of the position of the gaps along the length of the device; and a second voltage supply connected to said plurality of three or more inner electrodes or inner electrode portions, wherein the second voltage supply is configured to maintain at least some of the inner electrodes or inner electrode portions at a second voltage that is different to said first voltage.

18. A mass spectrometer or ion mobility spectrometer comprising a device according to claim 17.

19. A method of manipulating charged particles comprising using a device as claimed in claim 17, the method comprising: applying said first voltage to said at least one outer electrode and applying said second voltage to said at least one set of inner electrodes or inner electrode portions so that an electric field is generated by said at least one outer electrode which penetrates the gaps between the inner electrodes or inner electrode portions so as to form an electrical potential profile along the longitudinal axis which manipulates the charged particles.

20. A method of mass spectrometry or ion mobility spectrometry comprising the method of manipulating charged particles claimed in claim 19, wherein the method comprises analysing the charged particles to determine their mass or ion mobility.

21. A method of manufacturing a device for manipulating charged particles using an axial electric field as they travel along a longitudinal axis of the device, said method comprising: selecting an electrical potential profile desired to be established along the longitudinal axis of the device in use for manipulating the charged particles; providing at least one outer electrode that extends continuously along at least a portion of the length of the device; connecting a first voltage supply to said at least one outer electrode for supplying a first voltage to the at least one outer electrode in use; providing at least one set of a plurality of three or more of inner electrodes or inner electrode portions between the at least one outer electrode and said longitudinal axis along which the charged particles travel; wherein the inner electrodes or inner electrode portions are spaced apart along the length of the device so as to provide gaps between the inner electrodes or inner electrode portions, wherein the lengths of the gaps vary as a function of the position of the gaps along the length of the device; connecting a second voltage supply to said plurality of inner electrodes or inner electrode portions, wherein the second voltage supply is configured to maintain at least some of the inner electrodes or inner electrode portions at a second voltage in use, wherein the second voltage is different to said first voltage; and selecting the lengths of the gaps between the inner electrodes or inner electrode portions, selecting the first voltage and selecting the second voltage such that an electric field generated by the at least one outer electrode, in use, penetrates the gaps between the inner electrodes or inner electrode portions to provide said electrical potential profile along said longitudinal axis.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Various embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings, in which:

(2) FIG. 1 shows a schematic of a device not according the present invention;

(3) FIGS. 2A to 2D show the potential profiles maintained along the device of FIG. 1 at different radial positions within the device;

(4) FIG. 3 shows a schematic of the electrode structure and voltages that may be applied to the electrodes in the arrangement of FIG. 1;

(5) FIG. 4 shows a schematic of the electrode structure and voltages that may be applied to the electrodes in another arrangement not forming part of the present invention;

(6) FIG. 5A shows a preferred embodiment of the present invention having parallel outer electrodes, and FIG. 5B shows the potential profile along the device of FIG. 5A;

(7) FIG. 6 shows a portion of the device of FIG. 5A;

(8) FIG. 7 shows another preferred embodiment of the present invention having non-parallel outer electrodes;

(9) FIG. 8 shows another preferred embodiment of the present invention having curved outer electrodes;

(10) FIG. 9 shows an embodiment of an inner electrode of the preferred device; and

(11) FIG. 10 shows an embodiment of an outer electrode of the preferred device.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(12) Arrangements not forming part of the present invention, although helpful for understanding the invention, will first be described with reference to FIGS. 1 to 4.

(13) FIG. 1 shows a perfectron on the right hand side of the vertical dashed line 6. A perfectron is a cylindrical device having a parabolic potential function arranged along the length of its central axis and having defined potential surfaces at the front and rear ends of the device. The perfectron comprising two sets of concentric ring electrodes 2,4 arranged along a longitudinal axis of the device and having front and rear equipotential surfaces. Alternate electrodes in the device form the first set of electrodes 4 and are connected a ground potential. The electrodes in this set become progressively shorter in the longitudinal direction of the device as one moves away from the front end of the device, wherein the front end of the device is arranged at the vertical dashed line 6. The second set of electrodes 2 is connected to the ion mirror potential and comprises electrodes that become progressively longer in the longitudinal direction of the device as one moves away from the front end of the device. The lengths of the electrodes increase as a quadratic function of their distances from the front end of the device. In order to eliminate boundary condition effects of the device and to examine the true behaviour of the device, a mirror image of the device is considered to be arranged on the left hand side of the vertical dashed line 6.

(14) FIGS. 2A to 2D show simulations of the electrical potential along the device (i.e. within the arrangement on the right side of the vertical dashed line 6 in FIG. 1) as a function of distance z along the device, for different radial positions within the device. The simulations assume that the device has a radius of 3 cm and a length of 20 cm. The simulation also assumes that the arrangement on the left side of the vertical dashed line 6 mirrors the device on the right side of the vertical dashed line 6. The simulation assumes that the pitch of the electrodes along the length of the device is 2 cm (i.e. ten electrodes between the entrance and exit electrodes) and that the electrodes vary in length from 0.025 to 10 mm. The simulation assumes that the first set of electrodes 4 are maintained at ground potential and that each electrode in the second set of electrodes 2 is maintained at 200 V.

(15) FIG. 2A shows the potential profile maintained along the central axis of the device due to the voltages applied to the first and second sets of electrodes 4,2. It can be seen that the potential profile along the central axis of the device is quadratic.

(16) FIG. 2B shows the potential profile maintained along the device at a radius of 1 cm from the central axis, due to the voltages applied to the first and second sets of electrodes 4,2. It can be seen that the potential profile along the device at this radius is substantially quadratic.

(17) FIG. 2C shows the potential profile maintained along the device at a radius of 2 cm from the central axis, due to the voltages applied to the first and second sets of electrodes 4,2. It can be seen that the potential profile along the device at this radius follows a generally quadratic pattern, although there is a significant ripple in the potential function due to the electrode structure.

(18) FIG. 2D shows the potential profile maintained along the device at a radius of 2.9 cm from the central axis, due to the voltages applied to the first and second sets of electrodes 4,2. It can be seen that the potential profile along the device at this radius is significantly distorted from the desired quadratic function.

(19) FIGS. 2A to 2D illustrate that the electrode structure can be used to generate a quadratic potential along the device for manipulating ions using only two voltages, i.e. ground voltage and 200 V. This is achieved by varying the lengths of the electrodes in the second set of electrodes 2.

(20) FIG. 3 shows another device having a first set of electrodes 4 and a second set of N electrodes 2. The electrodes in the device alternate between electrodes in the first set 4 and electrodes in the second set 2. The electrodes are arranged directly adjacent to each other so as to form a continuous, flush surface. The first set of electrodes 4 are electrically grounded and decrease in length from the right side to left side of the device. The electrodes in the second set of electrodes 2 increase in length from the right side of the device to the left side of the device. The electrodes 2 increase in length in a linear manner as a function of their distance from the right side of the device. The voltages applied to the second set of electrodes 2 increase from the right side of the device to the left side of the device. The voltages increase in a linear manner such that the nth electrode of the second set of electrodes 2 is maintained at a voltage that is a multiple of n times the voltage that the n=1 electrode is maintained at. A linear divider formed from a plurality of resistors having the same resistance is used to supply the second set of electrodes 2 with the different voltages.

(21) The effect of linearly increasing the length of the electrodes in the second set of electrodes 2 and linearly increasing the voltages applied to these electrodes results in a quadratic axial electric field being generated along the device. The quadratic electric field increases in amplitude in the same direction along the device that the voltages and lengths of the electrodes increase. It will therefore be appreciated that the device enables a quadratic electric field to be established along the device using a linear voltage divider comprising only resistors of the same value.

(22) FIG. 4 shows a device that is substantially the same as that of FIG. 3 except that the voltage divider uses capacitors of the same capacitance value, rather than resistors, in order to form the voltage gradient along the second set of electrodes 2. A quadratic axial electric field is formed within the device, as described above with respect to FIG. 3. The device of FIG. 4 is particularly advantageous in the event that the axial electric field is desired to be pulsed on and off.

(23) FIGS. 5 to 10 show schematics of embodiments of the present invention.

(24) FIG. 5A shows a device according to a first embodiment of the present invention comprising two continuous outer electrodes 8 and two sets of inner electrodes 10 arranged between the outer electrodes 8. Each set of inner electrodes 10 is arranged along an axis parallel to the central axis of the device. The electrodes in each set of inner electrodes 10 are spaced apart in a direction along the axis such that gaps are provided between adjacent pairs of the inner electrodes 10. The lengths of the gaps between the inner electrodes 10 vary as a function of position along the device. This allows the desired axial electric potential to be maintained along the central axis, as will be described in more detail below. In this embodiment, the lengths of the gaps increase from the left to the right of the device.

(25) A first DC voltage V1, e.g. 200 V is applied to the outer electrodes 8. The inner electrodes 10 are each maintained at a second voltage V2, which is preferably ground potential. An electric field is generated by applying the first voltage V1 to the outer electrodes 8 and this electric field penetrates through the gaps in the adjacent inner electrodes 10 so as to form a superimposed electric field along the central axis of the device. As the lengths of the gaps between the inner electrodes 10 vary along the length of the device, the amount of electric field penetration through the inner electrodes 10 also varies along the length of the device. It will therefore be appreciated that the electric field along the central axis of the device can be selected by selecting the position and lengths of the gaps between the inner electrodes 10. In the example shown in FIG. 5A the gaps between the inner electrodes 10 increase in length quadratically as a function of position along the device. This results in a substantially quadratic electrical potential being created along the length z of the device, as shown in FIG. 5B. In use, charged particles travel along a longitudinal axis arranged between the two sets of inner electrodes 10 and are manipulated by the axial potential profile D.

(26) It will be appreciated that axial potential profiles other than quadratic potential profiles may be created by varying the positions and lengths of the gaps in different ways.

(27) FIG. 6 shows a portion of a length of the device of FIG. 5 in order to illustrate the parameters that may be varied in order to achieve the desired potential profile along the central axis of the device. As previously described, the length of each gap W between adjacent pairs of inner electrodes 10 may be varied in order to alter the amount of electric field penetration from the adjacent outer electrode 8 and hence alter the potential at the central axis of the device. The smaller the length W of the gap, the less field penetration there is through the inner electrodes 10. The distance S between each outer electrode 8 and the gap between the inner electrodes 10 may be varied in order to alter the amount of electric field penetration from the adjacent outer electrode 8 and hence alter the potential at the central axis of the device. The thickness t of the gap, as determined in the radial direction from the central axis, may be varied in order to alter the amount of electric field penetration from the adjacent outer electrode 8 and hence alter the potential at the central axis of the device. In the illustrated embodiment the thickness t of the gap corresponds to the thickness of the inner electrodes 10 on either side of the gap. The greater the thickness t of the gap, the less field penetration there is through the inner electrodes 10. The distance H of the inner electrodes 10 from the central axis of the device may be varied in order to alter the potential at the central axis of the device.

(28) FIG. 7 shows another embodiment of the present invention that is the same as that of FIGS. 5 and 6, except that each of the outer electrodes 8 is arranged at an angle relative to the central axis and to the axes along which the inner electrodes 10 are arranged. As described in relation to FIG. 6, varying the distance between an outer electrode 8 and the gap between the adjacent inner electrodes 10 causes the electrical potential at a corresponding axial position along the central axis to vary. Accordingly, by providing angled outer electrodes 8 the distance between each outer electrode 8 and the gaps between the adjacent inner electrodes 10 varies as a function of the position along the length of the device. Angling the outer electrodes 8 therefore controls the amount of electric field penetration through the gaps in the inner electrodes 10.

(29) FIG. 8 shows another embodiment of the present invention that is the same as that of FIGS. 5 and 6, except that each of the outer electrodes 8 are profiled differently. In the embodiment of FIG. 8, each outer electrode 8 has a curved surface facing the central axis such that the radial distance of said surface from the central axis (and adjacent inner electrodes 10) varies as a function of position along the longitudinal axis. As described in relation to FIG. 6, varying the distance between an outer electrode 8 and the gap between the adjacent inner electrodes 10 causes the electrical potential at a corresponding axial position along the central axis to vary. Accordingly, by providing outer electrodes 8 having curved surfaces the distance between each outer electrode 8 and the gaps between the adjacent inner electrodes 10 varies as a function of the position along the length of the device. The curved surfaces of the outer electrodes 8 therefore control the amount of electric field penetration through the gaps in the inner electrodes 10.

(30) Each set of inner electrodes 10 has been described as being formed from a plurality of discrete electrodes. However, it is contemplated a plurality of inner electrode portions may be used instead, wherein the electrode portions are portions of the same electrode that are spaced apart along the length of the device by providing apertures in the single electrode. FIG. 9 shows a schematic of such an embodiment.

(31) FIG. 9 shows a single electrode 10 that may be used to form each set of inner electrode portions. The single electrode has a plurality of apertures (i.e. slots) 12 formed therein which define a plurality of electrode portions 14 between the apertures 12. The widths of the apertures (i.e. the dimension in the longitudinal direction of device) vary along the length of the electrode 10. The apertured electrode 10 may be arranged in the device such that the electrode portions 14 between the apertures 12 correspond to the inner electrodes 10 of the previously described embodiments and the apertures 12 correspond to the gaps between the inner electrodes 10. In this embodiment, the inner electrode 10 is a flat plate or sheet electrode, although it is contemplated that that electrode 10 could be curved around the central axis (e.g. cylindrical) or, less preferably, could be curved along the length of the device.

(32) FIG. 10 shows an embodiment of one of the outer electrodes 8 as viewed in the x-z plane. In this embodiment the electrode 8 is a solid, continuous electrode.

(33) Each inner 10,10 electrode and/or outer electrode 8 of the present invention may be a rectilinear electrode.

(34) The accuracy of the electric field that can be achieved according to the present invention is greater than that of conventional techniques since it is relatively easy to precisely machine the inner electrodes 10 to the desired lengths (or inner electrode portions 14) and/or provide the desired gaps between the inner electrodes (or inner electrode portions 14) in order to provide the desired potential profile along the device. The technique of the present invention is more accurate and simple than the conventional techniques, which rely upon using resistive or capacitive dividers of different values and electrical insulators between electrodes in order to provide a voltage profile along the electrodes. This is particularly the case when trying to achieve higher order potential functions which deviate from commercially available preferred values. Furthermore, as few different voltages are required to be applied to the device it is ideally suited to the rapid pulsing of electric fields which require support over large physical volumes, for example, such as those found in orthogonal acceleration TOF technology.

(35) The present invention has general applicability to the creation of any electrostatic field, provided that the boundary conditions are known. For example, the present invention may be used to generate a hyperlogarithmic field along the length of the device. This may be useful in devices such as, for example, orthogonal acceleration TOF devices.

(36) Although the present invention has been described with reference to preferred 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.

(37) For example, although two outer electrodes and two sets of inner electrodes have been described in relation to the illustrated embodiments, it is contemplated that the outer electrodes could be formed from a single cylinder or tube electrode that surrounds the central axis. Alternatively, or additionally, the inner electrodes could be formed from ring or tubular shaped electrodes that extend around central axis, rather than being formed from two sets of electrodes. For example, the electrode 10 may be a cylindrical or tubular electrode.

(38) Preferably, the device described in the above embodiments is a time of flight region of a time of flight mass analyser.

(39) Although it is preferred that the device of the present invention is for manipulating ions in a mass spectrometer, it is also contemplated that the device be used for manipulating charged particles in other applications. Examples of such other applications are the manipulation of electrons in electron microscopes, electron spectrometers or other devices.