Method of generating electric field for manipulating charged particles

Abstract

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 is disclosed. The method comprises providing first electrodes of different lengths, supplying different voltages to these electrodes and arranging grounded electrodes between the first electrodes in order to form the desired axial potential profile.

Claims

1. 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 for manipulating the charged particles; arranging at least a first plurality of electrodes along the longitudinal axis of the device, wherein the lengths of the electrodes in the direction along the longitudinal axis of the device vary as a function of the distance along the longitudinal axis of the device; connecting one or more first DC voltage supplies to said first plurality of electrodes, wherein the one or more DC voltage supplies are configured to apply one or more DC voltages to the first plurality of electrodes in use; arranging a second plurality of electrodes along the longitudinal axis of the device, wherein one of the second plurality of electrodes is arranged between each longitudinally adjacent pair of electrodes in the first plurality of electrodes; connecting one or more second DC voltage supplies to said second plurality of electrodes, wherein said one or more second DC voltage supplies are configured to maintain each of the second plurality of electrodes at a DC voltage in use; and selecting said lengths of the electrodes in said first plurality of electrodes, the voltages applied to the first and second plurality of electrodes and the locations of said electrodes along the longitudinal axis of the device so that said electrical potential profile is established along the longitudinal axis of the device in use; wherein said one or more first DC voltage supplies and/or said one or more second DC voltage supplies are configured to be pulsed on and off for pulsing the electrical potential profile on and off.

2. The method of claim 1, wherein in use the electrical potential profile varies in a non-linear manner along the longitudinal axis of the device; or wherein in use the electrical potential profile varies along the axis of the device as a quadratic function or a higher order function.

3. The method of claim 1, wherein the length of each electrode in the second plurality of electrodes is selected so that longitudinally adjacent electrodes of the first plurality of electrodes are spaced apart from each other along the longitudinal axis by a distance such that a substantially smooth axial electric field is generated within the device in use.

4. The method of claim 1, wherein the first and second electrodes are arranged directly adjacent to each other so as to form a substantially continuous surface along the longitudinal axis of the device.

5. The method of claim 1, wherein the one or more first voltage supplies are configured to maintain each of the first plurality of electrodes at the same voltage in use, and wherein this voltage is different to the voltage(s) applied to the second plurality of electrodes by the second voltage supply.

6. The method of claim 1, wherein the first plurality of electrodes consists of electrodes that are arranged sequentially along the longitudinal axis of the device, and wherein the voltages applied to these electrodes vary linearly as a function of the position of the electrode within the sequence.

7. The method of claim 1, wherein the first plurality of electrodes consists of electrodes that are arranged sequentially along the longitudinal axis of the device, and wherein the voltages applied to these electrodes vary in a quadratic manner as a function of the position of the electrode within the sequence.

8. The method of claim 1, wherein the first plurality of electrodes consists of electrodes that are arranged sequentially along the longitudinal axis of the device, and wherein the lengths of these electrodes vary linearly as a function of the position of the electrode within the sequence.

9. The method of claim 1, wherein the first plurality of electrodes consists of electrodes that are arranged sequentially along the longitudinal axis of the device, and wherein the lengths of these electrodes vary in a quadratic manner as a function of the position of the electrode within the sequence.

10. The method of claim 1, wherein the length of any given electrode in the first plurality of electrodes combined with the length of an adjacent electrode of the second plurality of electrodes is constant at any point along the device.

11. The method of claim 1, wherein the number of electrodes in said first plurality of electrodes is 5.

12. The method of claim 1, wherein at least x electrodes in said first plurality of electrodes have different lengths, wherein x is selected from the group consisting of:>2; >3; >4; >5; >6; >7; >8; >9; >10; >15; >20; >25; >30; >35; >40; >45; >50; >60; >70; >80; >90; and >100; and/or wherein at least y electrodes in said second plurality of electrodes have different lengths, wherein y is selected from the group consisting of:>2; >3; >4; >5; >6; >7; >8; >9; >10; >15; >20; >25; >30; >35; >40; >45; >50; >60; >70; >80; >90; and >100.

13. A mass spectrometer or ion mobility spectrometer comprising a device formed according to claim 1 wherein the charged particles are ions.

14. A device for manipulating charged particles using an axial electric field as they travel along a longitudinal axis of the device, said device comprising: a first plurality of electrodes arranged along the longitudinal axis of the device, wherein the lengths of the electrodes in the direction along the longitudinal axis of the device vary as a function of the distance along the longitudinal axis of the device; one or more first DC voltage supplies connected to said first plurality of electrodes, wherein the one or more DC voltage supplies are configured to apply one or more DC voltages to the first plurality of electrodes in use; a second plurality of electrodes arranged along the longitudinal axis of the device, wherein one of the second plurality of electrodes is arranged between each longitudinally adjacent pair of electrodes in the first plurality of electrodes; one or more second DC voltage supplies connected to said second plurality of electrodes, wherein the DC voltage supply is configured to maintain each of the second plurality of electrodes at a DC voltage in use; wherein the first and second plurality of electrodes are arranged along the longitudinal axis of the device and the first and second voltage supplies are selected such that an electric potential profile is established along the longitudinal axis of the device in use; and wherein said one or more first DC voltage supplies and/or said one or more second DC voltage supplies are configured to be pulsed on and off for pulsing the electrical potential profile on and off.

15. The device of claim 14, wherein the device is an ion mirror, or an acceleration region or reflectron of a Time of Flight mass analyser.

16. The device of claim 15, wherein the device is a Time of Flight mass analyser, wherein the device is configured so that ions enter the device orthogonal to the longitudinal axis, and wherein the device is configured to pulse or establish said electric potential profile along the entire length of the longitudinal axis of the device such that ions are accelerated along the longitudinal axis and separate according to their mass to charge ratios.

17. A method of manipulating charged particles, or a method of mass spectrometry or ion mobility spectrometry comprising: providing the device or spectrometer of claim 14; applying said one or more voltages to the first plurality of electrodes with said one or more first voltage supplies, and applying said one or more voltages to the second plurality of electrodes with said one or more second voltage supplies, such that a non-linear electric potential profile is established along a longitudinal axis of the device; and manipulating charged particles using the electric potential profile as they travel along the longitudinal axis of the device.

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 according to a preferred embodiment of 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 an embodiment of the present invention; and

(5) FIG. 4 shows a schematic of the electrode structure and voltages that may be applied to the electrodes in another embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(6) In order to illustrate the present invention the simple case of the so called perfectron will now be described. 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.

(7) FIG. 1 shows a preferred embodiment of a perfectron on the right hand side of the vertical dashed line. 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 6 of the device, wherein the front end of the device is arranged at the vertical dashed line. The second set of electrodes 2 is connected to the ion mirror potential and comprises electrodes 2 that become progressively longer in the longitudinal direction of the device as one moves away from the front end 6 of the device. The lengths of the electrodes 2 increase as a quadratic function of their distances from the front end 6 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.

(8) 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 in FIG. 1) 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 mirrors the device on the right side of the vertical dashed line. 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.

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

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

(11) FIG. 2C shows the potential maintained along the device at a radius of 2 cm from the central axis z, due to the voltages applied to the first and second sets of electrodes 2,4. 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.

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

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

(14) FIG. 3 shows another embodiment of a device having a first set of electrodes 4 and a second set of N electrodes 2. The set of curved, dashed lines indicate that the number of electrodes in the device may be greater than the number shown in FIG. 3. The electrodes in the device alternate between electrodes in the first set 4 and electrodes in the second set 2. The electrodes 2,4 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 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 N volts. 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.

(15) 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 preferred embodiment enables a quadratic electric field to be established along the device using a linear voltage divider comprising only resistors of the same value.

(16) FIG. 4 shows an embodiment 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. A quadratic axial electric field is formed within the device, as described above with respect to FIG. 3. The embodiment of FIG. 4 is particularly advantageous in the event that the axial electric field is desired to be pulsed on and off.

(17) The technique of the present invention may be referred to as Electrode Width Modulation (EWM) in analogy to pulse width modulation techniques employed in electronic power converters, except that in the present invention the modulation occurs spatially in terms of the width of the electrodes (i.e. length along the device) rather than temporally.

(18) 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 electrodes to the desired length to provide the desired potential profile along the device. The technique of the present invention is therefore more accurate than the conventional techniques, which rely upon using resistive or capacitive dividers of different values 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 all the electrodes in a particular set of electrodes may be connected to the same voltage output in the preferred embodiment of the present invention, the device 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.

(19) The present invention has general applicability to the creation of any electrostatic or pulsed 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.

(20) 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.

(21) For example, 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.