Toroidal trapping geometry pulsed ion source

Abstract

An ion trap is disclosed comprising: a plurality of electrodes which define a toroidal or annular ion confining volume that extends around a central axis; a first device arranged and adapted to apply one or more DC voltages to said plurality of electrodes in order to generate a DC potential well which acts to confine ions in a radial direction within said toroidal or annular ion confining volume, wherein said radial direction is substantially perpendicular to said central axis; and a control system arranged and adapted to non-mass selectively eject ions from said toroidal or annular ion confining volume. The ion trap enables a large number of ions to be trapped and ejected simultaneously.

Claims

1. An ion trap comprising: a plurality of electrodes which define a toroidal or annular ion confining volume that extends around a central axis; a first device arranged and adapted to apply one or more DC voltages to said plurality of electrodes in order to generate a DC potential well which acts to confine ions in a radial direction within said toroidal or annular ion confining volume, wherein said radial direction is substantially perpendicular to said central axis; and a control system arranged and adapted to non-mass selectively eject ions from said toroidal or annular ion confining volume.

2. An ion trap comprising: two substantially parallel arrays of electrodes which are spaced apart so as to define a toroidal or annular ion confining volume therebetween; a device arranged and adapted to apply one or more DC voltages to said electrodes in order to generate a DC potential well which acts to confine ions within said toroidal or annular ion confining volume in a direction that is substantially parallel to said arrays, wherein each array comprises a plurality of electrodes arranged between two edges of the array, and wherein said DC potential well acts to confine ions in a direction between said two edges of each array; and a control system arranged and adapted to non-mass selectively eject ions from said toroidal or annular ion confining volume.

3. An ion trap as claimed in claim 2, wherein the electrodes in each array of electrodes are arranged in side-by side arrangement between two edges of the array, and wherein the electrodes are arranged parallel to the two edges of the array or extending in a direction between the two edges of the array; wherein said arrays of electrodes define a toroidal or annular ion confining volume that extends around a central axis of the device, and wherein said two edges of each array are a radially inner edge and a radially outer edge.

4. An ion trap as claimed in claim 2, wherein each of the parallel arrays of electrodes is tubular or conical and one of the arrays of electrodes is arranged concentrically within the other array of electrodes so as to define said toroidal or annular ion confining volume therebetween; and wherein at least one of: a) the central axes of the tubular or conical arrays are coaxial and the ion trap comprises a device for applying one or more RF or AC voltages to said electrodes in order to confine ions in a direction extending substantially radially from said central axes; b) each array of electrodes has a central axis and two edges, and said two edges are arranged at different locations along the central axis; c) the central axes of the tubular or conical arrays are coaxial and said DC potential well acts to confine ions in a direction extending along said central axes; and d) the tubular or conical arrays taper from a wider end to a narrower end, and wherein ions are ejected from said ion confining volume in a direction that is substantially perpendicular to a direction extending from one of said arrays to the other of said arrays, and in a direction that is from said wider end to said narrower end.

5. An ion trap as claimed in claim 2, wherein said plurality of electrodes comprises a plurality of closed loop, circular, annular, oval or elliptical electrodes.

6. An ion trap as claimed in claim 2, wherein said control system non-mass selectively ejects ions through an exit of the ion trap by removing said DC potential well, or removing a portion of the DC potential well between the ions and the exit, and then applying one or more electrical potentials to electrodes that drive the ions out of said ion confining volume and out of said exit.

7. An ion trap as claimed in claim 6, wherein said one or more electrical potentials form a DC potential gradient that drives the ions out of the exit.

8. An ion trap as claimed in claim 2, wherein substantially all ions within the ion trap are ejected from the ion trap at substantially the same time or in the same ion ejection pulse; or wherein ions having a range of different mass to charge ratios are ejected from the ion trap at substantially the same time or in the same ion ejection pulse; or wherein ions having a range of different mass to charge ratios are ejected from the ion trap at substantially the same time or in the same ion ejection pulse, wherein the ratio of the maximum mass to charge ratio ejected to the minimum mass to charge ratio ejected is selected from: >1.1; >1.2; >1.4; >1.6; >1.8; >2; >2.5; >3; >4; >5; or >10.

9. An ion trap as claimed in claim 2, wherein said control system is arranged and adapted to eject ions from said toroidal or annular ion confining volume so that said ions are focused to an ion volume which is smaller than said toroidal or annular ion confining volume.

10. An ion trap as claimed in claim 2, wherein said toroidal or annular trapping region extends around a central axis, and wherein said control system is arranged and adapted to cause ions which have been non-mass selectively ejected from said toroidal or annular ion confining volume to emerge axially from said ion trap along said central axis; and wherein at least one of: a) said trap comprises one or more deflection electrodes or one or more extraction electrodes arranged to cause ions to exit the ion trap along said central axis; and b) said toroidal or annular trapping region is arranged in a first plane, and wherein said one or more deflection electrodes is arranged on one side of said first plane or said one or more extraction electrodes is arranged on the other side of said first plane.

11. An ion trap as claimed in claim 2, wherein the plurality of electrodes comprises a first and second array of electrodes, wherein said first array of electrodes is arranged in a first inner conical arrangement and said second array of electrodes is arranged in a second outer conical arrangement.

12. An ion trap as claimed in claim 11, comprising a device arranged and adapted to apply a RF voltage to said first or second array of electrodes in order to generate a pseudo-potential well which acts to confine ions in a direction substantially perpendicular to the surface of said first inner conical arrangement or substantially perpendicular to the surface of said second outer conical arrangement within said ion trap; or wherein said device is arranged and adapted to apply a DC voltage to said first or second arrays of electrodes in order to generate the DC potential well which acts to confine ions in a direction parallel to the surface of said first inner conical arrangement or parallel to the surface of said second outer conical arrangement within said ion trap.

13. An ion trap as claimed in claim 2, wherein said control system is arranged and adapted to extract ions from said ion trap by either: (i) reducing or altering the amplitude of a DC or RF voltage applied to said plurality of electrodes; or (ii) lowering, removing or altering a DC potential well or a pseudo-potential well; or (iii) changing a DC potential well to an extractive DC potential.

14. An ion trap as claimed in claim 2, wherein during an ejection mode of operation ions ejected from said ion trap are caused to separate according to their mass, mass to charge ratio or time of flight.

15. An ion trap as claimed in claim 2, wherein said control system is arranged and adapted: (i) to pulse a DC electric field in order to cause ions to be ejected from said ion trap; or (ii) to apply one or more DC extraction potentials to said ion trap in order to cause ions to be ejected from said ion trap.

16. A mass or ion mobility spectrometer comprising an ion trap or a reaction or fragmentation device as claimed in claim 2.

17. A method of mass or ion mobility spectrometry comprising: trapping ions in a toroidal or annular ion confining volume that extends around a central axis; generating a DC potential well which acts to confine ions in a radial direction within said toroidal or annular ion confining volume, wherein said radial direction is substantially perpendicular to said central axis; and non-mass selectively ejecting ions from said toroidal ion confining volume.

18. A method of mass or ion mobility spectrometry comprising: trapping ions in a toroidal or annular ion confining volume defined between two spaced apart substantially parallel arrays of electrodes; generating a DC potential well which acts to confine ions within said toroidal or annular ion confining volume in a direction that is substantially parallel to said arrays, wherein each array comprises a plurality of electrodes arranged between two edges of the array, and wherein said DC potential well acts to confine ions in a direction between said two edges of each array; and non-mass selectively ejecting ions from said toroidal or annular ion confining volume.

19. A mass spectrometer comprising: a toroidal ion trap comprising a first array of electrodes and a second array of electrodes with a toroidal ion confining volume arranged therebetween; an ion-optical device arranged downstream of said toroidal ion trap; and a control system arranged and adapted to cause at least some of said ions to be non-mass selectively ejected from said toroidal ion confining volume into said ion-optical device.

20. A method of mass spectrometry comprising: trapping ions in a toroidal ion confining volume; and then non-mass selectively ejecting at least some of said ions from said ion confining volume into an ion-optical 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. 1A shows a toroidal ion trap according to an embodiment of the present invention and FIG. 1B shows the ion trap in cross-section during an ion trapping mode;

(3) FIG. 2A shows a plan view of the ion trap and FIG. 2B shows a cross-sectional view during an ion extraction mode; and

(4) FIG. 3A shows a cross-sectional view of an ion trap according to an alternative embodiment and FIG. 3B shows a perspective view of the trapping electrodes.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(5) FIG. 1A shows a perspective view of a device according to a preferred embodiment of the present invention. A toroidal ion trap is shown and comprises an upper planar electrode plate or array 1 and a corresponding lower planar electrode plate or array 2. The central axes of the electrode plates are aligned so as to form a central axis of the toroidal ion trap that extends in the y-direction. The electrode plates extend radially outwards from the central axis, in the radial direction r, in planes that are perpendicular to the central axis. The electrode plates 1,2 are preferably constructed from Printed Circuit Board (PCB) material. Each of the electrode plates 1,2 is preferably annular in shape and preferably has a hole at the centre, through which the central axis of the ion trap extends.

(6) In order to fill the ion trap with ions, an ion beam is preferably arranged to be incident upon the ion trap in a direction as indicated by arrow 3. This direction may be substantially perpendicular to the radial direction of the ion trap. The circumferentially open structure provided by the planar electrode plates 1,2 allows ions to be easily injected between the electrodes plates 1,2 and into one or more confining DC potential wells that are set up by the electrode plates, as will be described with reference to FIG. 1B. Ions are preferably injected into the ion trap in a direction that is substantially perpendicular to the radial direction of the ion trap, or substantially tangentially to the toroidal ion trapping volume, so that ions are preferably given the maximum time to cool or lose kinetic energy due to collisions with residual buffer gas present in the device as they enter the DC confining field.

(7) FIG. 1B shows a cross-sectional view in the (y,r) plane of the device shown in FIG. 1A. The inwardly facing sides of the upper and lower electrode plates 1,2 comprise annular electrodes 4. The annular electrodes extend around the central axis in a plane perpendicular to the central axis. Each plate preferably comprises a plurality of annular electrodes 4 having different radii from the central axis, wherein the annular electrodes 4 are concentrically arranged on the electrode plates 1,2. The electrodes 4 preferably form concentric strips which are attached to the PCB substrate. Radially adjacent annular electrodes 4 are preferably supplied with opposite phases of an alternating voltage that oscillates at radio frequency RF. Annular electrodes 4 at the same radial position on the electrode plates 1,2 are preferably supplied with the same phase of the RF voltage. The RF voltage serves to provide a pseudo-potential ion confinement field that confines ions in the y-direction, i.e. in a direction between the electrode plates 1,2.

(8) Ions are preferably confined in the radial direction r by application of DC confining voltages to the electrodes 4. The general form of the preferred DC confining potential is indicated on the plots of potential versus distance shown in FIG. 1B. The potentials is preferably substantially quadratic in the radial direction, with the minimum potential arranged between the inner and outer circumferential edges of the electrode plates 1,2. This may be achieved by applying minimum DC voltages to the radially centred electrodes 4 arranged on the electrode plates 1,2; applying progressively higher DC voltages to the electrodes 4 located at radial positions that progressively increase from the centred electrodes 4; and applying progressively higher DC voltages to the electrodes 4 located at radial positions that progressively decrease from the centred electrodes 4. It is contemplated that the DC potential may take any form, as long as there is at least one potential minima formed to confine ions radially in a torus about the central axis. During filling of the ion trap it may be advantageous to generate a radially asymmetric DC potential well such that the side of the potential well is shallower on the ion input side of the torus (i.e. radially outer side) as compared to the radially inner side of the torus.

(9) FIG. 2A shows a plan view of the device shown in FIGS. 1A and 1B. The direction of ion extraction is indicated by the arrows.

(10) FIG. 2B shows a cross-sectional view of the device in the y-r plane during rapid extraction of ions from the device. Once ions have been introduced into and trapped in the ion trap they are allowed to reduce in energy due to collisions with background buffer gas. The ions are then extracted by the device. In order to achieve this, the RF confining potential is preferably turned off or reduced, and a DC extraction potential is preferably applied so as to accelerate ions out of the trapping region towards a point at the centre of the device. The DC extraction potential is formed by applying DC potentials to the annular electrodes 4. The DC potentials applied to the electrodes 4 progressively increase with increasing radial position so as to create a potential gradient that accelerates the ions radially inwards. The general form of the extraction potential is shown in the plots of potential versus distance 8.

(11) The radial symmetry of the device preferably results in ions being accelerated to a single point at the centre of the device. An ion deflection electrode 6 is preferably arranged at the radial centre of the device and may extend through the aperture in one of the electrode plates 1. An electrical potential is applied to this deflection electrode so as to force ions away and cause the ions to move along the central axis y. Alternatively, or additionally, an extraction electrode 7 may be situated at the centre of the device, preferably outside of the electrode plates. An electrical potential is applied to this deflection electrode so as to attract ions to move along the along the central axis y. The potentials applied to the deflection and/or extraction electrodes 6,7 preferably result in ions being directed along the central axis y in a direction substantially orthogonal to the plane of the trapping device, i.e. the radial direction. The ions may advantageously separate by their time of flight during this extraction process, e.g. according to their mass to charge ratios or ion mobilities. The ions may then be ejected onto a detector or into a mass analyser, such as a Time of Flight mass analyser. Alternatively, the ions may be ejected into another device, such as an electrostatic ion trap.

(12) FIGS. 3A and 3B show views of an alternative embodiment wherein the parallel planar electrode plates 1,2 of FIGS. 1A to 2B are replaced by concentric conical or tubular electrode members 1,2.

(13) FIG. 3B shows a perspective view of the ion trap. A toroidal ion trap is shown and comprises an inner conical electrode member 1 surrounded by an outer conical electrode member 2. The central axes of the conical electrode members are aligned so as to form a central axis of the toroidal ion trap that extends in the y-direction. The conical electrode members 1,2 are preferably constructed from Printed Circuit Board (PCB) material.

(14) FIG. 3A shows a cross-sectional view in the y-r plane of the device shown in FIG. 3B. The radially outward facing side of the inner conical electrode member 1 comprises a plurality of annular electrodes 4 that extend circumferentially around the inner conical electrode member 1. As shown in FIG. 3A, different annular electrodes 4 are provided around the conical electrode member 1 at different axial positions along the central axis. The radially inward facing side of the outer conical electrode member 2 also comprises a plurality of annular electrodes 4 that extend circumferentially around the outer conical electrode member 2. Different annular electrodes 4 are provided around the conical electrode member 2 at different axial positions along the central axis. The electrodes 4 preferably form concentric strips which are attached to the PCB substrate.

(15) Adjacent annular electrodes 4 on any given conical electrode member 1,2 are preferably supplied with opposite phases of an alternating voltage that oscillates at radio frequency RF. The RF voltage serves to provide a pseudo-potential ion confinement field that confines ions in a first direction between the conical electrode members 1,2.

(16) Ions are preferably confined between the conical electrode members 1,2 in a second direction that is perpendicular to the direction extending between the conical electrode members 1,2 by application of DC confining voltages to the electrodes 4. The general form of the preferred DC confining potential is indicated on the plots of potential versus distance shown in FIG. 3A. The potential is preferably substantially quadratic in the second direction, with the minimum potential arranged between the upper and lower edges of the conical electrode members 1,2. It is contemplated that the DC potential may take any form, as long as there is at least one potential minima formed to confine ions radially in a torus about the central axis. During filling of the ion trap it may be advantageous to generate an asymmetric DC potential well such that the side of the potential well is shallower on the ion input side as compared to the other side.

(17) According to this embodiment the conical electrode members 1,2 are preferably angled relative to the central axis so as to form concentric cone like structures. Ions may be injected and extracted in similar manners to those described above in relation to the embodiment shown in FIGS. 1 and 2. However, an advantage of the angled cone like configuration is that when the ions are ejected from different positions around the circumference of the torus, the ions are directed towards the same focal point arranged along the central axis of the device. This is shown by the arrows in FIG. 3A. Ions will be focused to substantially the same point in space without the need for deflection or extraction electrodes. The distance from the centre of the trapping structure to this focal point can be selected by selecting the angle shown in FIG. 3A, i.e. the angle between the second direction and the central axis.

(18) 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.

(19) For example, the electrode structure need not be circular around the central axis, but may take the form of other shapes.

(20) It is contemplated that the device may be used as a reaction or fragmentation cell.

(21) Although a DC confining well has been described having only one minima, it is contemplated that more than one DC confining well may be provided.