Method of mass separating ions and mass separator

Abstract

A method of separating ions according to their time of flight is provided comprising: a. providing an analyzer comprising two opposing ion mirrors, each mirror comprising inner and outer field-defining electrode systems elongated along an analyzer axis with the outer field-defining electrode system surrounding the inner field-defining electrode system and creating therebetween an analyzer volume; b. injecting ions into the analyzer volume or creating ions within the analyzer volume so that they separate according to their time of flight as they travel along a main flight path while undergoing a plurality of axial oscillations in the direction of the analyzer axis and a plurality of radial oscillations while orbiting about one or more inner field-defining electrodes; c. the plurality of axial oscillations and plurality of radial oscillations causing the separated ions to intercept an exit port after a predetermined number of orbits. Also provided is an analyzer for performing the method, comprising: the two opposing ion mirrors which abut at a first plane, wherein the outer field-defining electrode system of one mirror comprises two sections, the sections abutting at a second plane, comprising a first section between the first plane and the second plane, and a second section adjacent the first section and wherein the first section has at least a portion which extends radially from the analyzer axis a greater extent than an adjacent portion of the second section at the second plane.

Claims

1. A method of separating ions according to their time of flight comprising: a. providing an analyser comprising two opposing ion mirrors, each mirror comprising inner and outer field-defining electrode systems elongated along an analyser axis with the outer field-defining electrode system surrounding the inner field-defining electrode system and creating therebetween an analyser volume; b. injecting ions into the analyser volume or creating ions within the analyser volume so that they separate according to their time of flight as they travel along a main flight path while undergoing a plurality of axial oscillations in the direction of the analyser axis and a plurality of radial oscillations whilst orbiting about at least one inner field-defining electrode; c. the plurality of axial oscillations and plurality of radial oscillations causing the separated ions to intercept an exit port after a predetermined number of orbits, whereby the separated ions pass through the exit port and, d. detecting the separated ions after they pass through the exit port and leave the analyzer volume, wherein the ions are detected directly after they pass through the exit port or following further processing after they pass through the exit port.

2. The method of claim 1 wherein the analyser comprises two opposing electrostatic ion mirrors.

3. The method of claim 1 wherein the exit port comprises an aperture in the outer field-defining electrode structure of one of the mirrors.

4. The method of claim 1 wherein the analyser further comprises an entry port which comprises an aperture in the outer field-defining electrode structure of one of the mirrors.

5. The method of claim 4 wherein the entry port also comprises the exit port.

6. The method of claim 1 wherein the exit port is within the analyser volume and is connected to an ion optical transmission device located at least partially within the analyser volume for transporting the ion beam out of the analyser volume.

7. The method of claim 1 further comprising an entry port, the entry port being within the analyser volume and connected to an ion optical transmission device located at least partially within the analyser volume for transporting the ion beam into the analyser volume.

8. The method of claim 1 wherein the ions reach a turning point within the ion mirrors, the turning point lying upon a turning plane and wherein the exit port lies closer to the turning plane than to a plane at which the mirrors abut each other.

9. The method of claim 8 wherein the exit port lies substantially on the turning plane.

10. The method of claim 8 wherein an entry port lies substantially on the turning plane.

11. The method of claim 1 wherein the axial oscillation frequency is ? and the radial oscillation frequency is ?.sub.r and the ratio ?.sub.r/? lies between 0.5 and 3, or between 0.85 and 1.2.

12. The method of claim 1 wherein the angular oscillation frequency is ?.sub.? and the axial oscillation frequency is ?, and ?.sub.?>?/2.sup.1/2.

13. The method of claim 1 wherein the analyser comprises set of electrodes which when energised produces three-dimensional perturbations to the electric field within one or both the ion mirrors so as to induce arcuate focusing of ions when they pass through the perturbed electric field.

14. The method of claim 13, wherein the analyser comprises a plurality of the sets of electrodes and wherein some of the sets of electrodes have electrical potentials applied to them so that ions passing in the vicinity of the said some of the sets of electrodes are directed to a second main flight path having a different average radius than the main flight path.

15. The method of claim 1 wherein the opposing ion mirrors produce substantially linear opposing electrostatic fields.

16. The method of claim 1 wherein downstream of the exit port is located an ion gate for selecting ions of at least one range of narrow m/z.

17. The method of claim 16 wherein downstream of the ion gate is located a fragmentor for fragmenting the ions selected by the ion gate and downstream of the fragmentor is located a mass analyser for mass analysing the fragmented ions.

18. The method of claim 1 wherein a detector is located downstream of the exit port.

19. The method of claim 1 wherein an external storage device is located upstream of an entry port, the external storage device comprising an RF or electrostatic trap, the external storage device being used to inject ions into the analyser through the entry port.

Description

DESCRIPTION OF FIGURES

(1) FIG. 1 illustrates the coordinate system used to describe features of the present invention.

(2) FIG. 2 shows a schematic cross-sectional view of the inner and outer field defining electrode structures of the two opposing mirrors for a preferred embodiment of the invention.

(3) FIG. 3 shows schematic views of an arcuate lens system within an analyser of the present invention.

(4) FIG. 4 shows a schematic cross-sectional view of an analyser of the present invention.

(5) FIG. 5 shows a schematic instrumental layout including the analyser of the present invention.

DETAILED DESCRIPTION

(6) In order to more fully understand the invention, various embodiments of the invention will now be described by way of examples only and with reference to the Figures. The embodiments described are not limiting on the scope of the invention.

(7) One preferred embodiment of the present invention utilises the quadro-logarithmic potential distribution described by equation (1) as the main analyser field. FIG. 2 is a schematic cross sectional side view of the electrode structures for such a preferred embodiment. Analyser 10 comprises inner and outer field-defining electrode systems, 20, 30 respectively, of two opposing mirrors 40, 50. The inner and outer field-defining electrode systems in this embodiment are constructed of gold-coated glass. However, various materials may be used to construct these electrode systems: e.g. Invar; glass (zerodur, borosilicate etc) coated with metal; molybdenum; stainless steel and the like. The inner field-defining electrode system 20 is of spindle-like shape and the outer field-defining electrode system 30 is of barrel-like shape which annularly surrounds the inner field-defining electrode system 20. The inner field-defining electrode systems 20 and outer field-defining electrode systems 30 of both mirrors are in this example single-piece electrodes, the pair of inner electrodes 20 for the two mirrors abutting and electrically connected at the z=0 plane, and the pair of outer electrodes 30 for the two mirrors also abutting and electrically connected at the z=0 plane, 90. In this example the inner field-defining electrode systems 20 of both mirrors are formed from a single electrode also referred to herein by the reference 20 and the outer field-defining electrode systems 30 of both mirrors are formed from a single electrode also referred to herein by the reference 30. The inner and outer field-defining electrode systems 20, 30 of both mirrors are shaped so that when a set of potentials is applied to the electrode systems, a quadro-logarithmic potential distribution is formed within the analyser volume located between the inner and outer field-defining electrode systems, i.e. within region 60. The quadro-logarithmic potential distribution formed results in each mirror 40, 50 having a substantially linear electric field along z, the fields of the mirrors opposing each other along z. The shapes of electrode systems 20 and 30 are calculated using equation (1), with the knowledge that the electrode surfaces themselves form equipotentials of the quadro-logarithmic form. Values for the constants k, C and R.sub.m are chosen and the equation solved for one of the variables r or z as a function of the other variable z or r. A value for one of the variables r or z is chosen at a given value of the other variable z or r for each of the inner and outer electrodes and the solved equation is used to generate the dimensions of the inner and outer electrodes 20 and 30 at other values of r and z, defining the inner and outer field-defining electrode system shapes.

(8) For illustration, in one example of an analyser as shown schematically in FIG. 2, the analyser has the following parameters. The z length (i.e. length in the z direction) of the electrodes 20, 30 is 380 mm, i.e. +/?190 mm about the z=0 plane. The maximum radius of the inner surface of the outer electrode 30 lies at z=0 and is 140.0 mm. The maximum radius of the outer surface of the inner electrode 20 also lies at z=0 and is 97.0 mm. The outer electrode 30 has a potential of 0 V and the inner electrode 20 has a potential of ?2060.7 V in order to generate the main analyser electrical field in the analyser volume under the influence of which the charged particles will fly through the analyser volume as herein described. The voltages given herein are for the case of analysing positive ions. It will be appreciated that the opposite voltages will be needed in the case of analysing negative ions. The values of the constants of equation (1) are: k=1.54*10.sup.5 V/m.sup.2, R.sub.m=296.3 mm, C=0.0. Ions enter the analyser and start upon the main flight path at radius 100 mm and z=?157.3 mm.

(9) The inner and outer field-defining electrode systems 20, 30 of both mirrors are concentric in the example shown in FIG. 2, and also concentric with the analyser axis z 100. The two mirrors 40, 50 constitute two halves of the analyser 10. A radial axis is shown at the z=0 plane 90. The analyser is symmetrical about the z=0 plane. For a TOF analyser of this size able to achieve high mass resolving power such as 50,000, the alignment of the mirror axes with each other should be to within a few hundred microns in displacement and between 0.1-0.2 degrees in angle. In this example, the accuracy of shape of the electrodes is within 10 microns. Ions would travel on a stable flight path through the analyser even at much higher misalignment but the mass resolving power would reduce.

(10) Analyser 10 of FIG. 2 has entry port 70 in the outer field-defining electrode system of mirror 50, and exit port 80 in the outer field-defining electrode system of mirror 50. In this preferred embodiment exit port 80 and entry port 70 comprise the same aperture in the outer field-defining electrode system of mirror 50. Ions enter the analyser volume 60 through entry port 70 along trajectory 112. The main flight path within analyser 10 is an eccentric helix envelope 110 having a minimum radius r1 and a maximum radius r2 from the analyser axis 100. The maximum radius r2 of main flight path envelope 110 is close to the inner surface of outer field-defining electrode 30 at four points in the cross-sectional view of the figure. One of those points lies at entry port 70 and exit port 80. The eccentric helix envelope 110 would, if the ion beam followed the main flight path for sufficient time, strike the inner surface of the outer field-defining electrode of one or other of the mirrors 40, 50. However the trajectory parameters of the ion beam on entry are chosen so that the ion beam extends to its maximum radius r2 at locations closer to the z=0 plane at all times along the flight path until the ions reach exit port 80 and ions following the main flight path do not collide with the inner surface of the outer field-defining electrode. On reaching exit port 80 the ions pass through the exit port 80 and leave the analyser volume 60 along trajectory 114. In this example, r1 is approximately 100 mm, r2 is 140 mm and the beam extends to a maximum z dimension of 157 mm. The ion beam undergoes repeated oscillations in the direction of the z axis as it reflects from mirror 40 to mirror 50 and back again. Each oscillation in the direction of the z axis is simple harmonic motion.

(11) In a particular embodiment of this example, a beam of ions following the main flight path has an arcuate velocity corresponding to 3000 eV kinetic energy and no axial velocity upon entering the analyser through entry port 70. The maximum total beam energy reaches 4908.1 eV. In this particular embodiment, after 36 full oscillations along z (equal to 72 passes across the z=0 plane), the beam travels an effective path length of approximately 35.6 m in the analyser axial direction, which is the direction of time of flight separation of the ions, before reaching its starting point once again. This is due to the particles travelling the z length of the cylindrical envelope 110 twice (i.e. back and forth) for each full oscillation along z (i.e. a distance per oscillation of 157 mm?2=314 mm but an effective distance of 157 mm?2?=988 mm). For 36 full oscillations, the total effective length travelled is therefore 988 mm?36=35.6 m. The beam orbits around the z axis just over once (i.e. 5 degrees over) per reflection from one of the mirrors, i.e. just over twice (i.e. 10 degrees over) per full oscillation along the z axis. During this travel ion beam approaches so closely to the outer electrode that a significant proportion of the beam could be lost or scattered in this particular embodiment of the example. To avoid this, the analyser further comprises arcuate lenses as will be further described. The arcuate lenses are formed from sets of electrodes; a set may consist of a single electrode. To prevent the ion beam approaching too close to the outer electrodes of the mirrors 30, when the ion beam approaches a first arcuate lens, the electrode(s) of the first lens are energised to deflect the ion beam onto a second main flight path, the second main flight path having a smaller average radius than the average radius of the main flight path, so that, for example, r1 is reduced from 100 mm to 99 mm. The ions then proceed to oscillate from one ion mirror to the other without approaching too closely the outer electrode 30 of the mirrors, during which ion separation occurs. During this time all arcuate focusing lenses are energised to produce localised perturbed electric fields which provide arcuate focusing. Finally, upon reaching the last arcuate lens the electrode(s) of the last arcuate lens are energised to deflect the ion beam back onto the main flight path.

(12) A further example (Example B) of the invention utilises a similar analyser to that described above (Example A), but alternative values for some constants, dimensions and potentials are used. Table 1 shows the constants, dimensions and potentials which differ between the two examples, all other values being the same for both examples and being as detailed above.

(13) TABLE-US-00001 TABLE 1 Parameter Example A Example B Maximum radius of the outer surface 97.0 mm 94.5 mm of the inner electrode Outer electrode potential 0 V 0 V Inner electrode potential ?2060.74 V ?1976 V k 1.54 * 10.sup.5 V/m.sup.2 5.4 * 10.sup.5 V/m.sup.2 R.sub.m 296.3 mm 179.0 mm Maximum distance of the main 157 mm 77.3 mm flight path from the z = 0 plane Total effective length of flight path 35.6 m 17.5 m Potential of the inner belt electrode ?2050 V ?1966 V assembly Potential of the outer belt electrode ?1683 V ?1288 V assembly Inner radius of the outer belt 103 mm 106 mm Belt electrode assembly z length 44 mm 50 mm Offset distance of arcuate lenses 3.05 mm 3.2 mm from the z = 0 plane

(14) As previously described, in the absence of the action of the arcuate lenses, whilst travelling upon the main flight path, the beam is confined radially but is unconstrained in its arcuate divergence within the analyser. Without arcuate focusing with ion beams having significant arcuate beam divergence only a very limited path length within the analyser is possible without substantial beam broadening, causing the attendant problems of ejection and detection as already described. The lens electrodes are mounted within the belt electrode assemblies upon insulators which thereby insulate the lens electrodes from the belt electrode assemblies. In other embodiments, the lens electrodes can be part of the belt electrode assembly.

(15) The electrical potentials applied to the belt electrode assemblies may be varied independently of the potentials upon the inner and outer field-defining electrode systems or the lens electrodes.

(16) The spatial spread of the ions of interest in the arcuate direction ? should not exceed the diameter of the lens electrodes of the arcuate lenses so that large high-order aberrations are not induced. This imposes a lower limit upon the potential applied to the lens electrodes. Large potentials applied to the lens electrodes should also be avoided so that distortions of the main analyser field are not produced. The arcuate lenses also affect the ion beam trajectory in the radial direction to some extent, introducing some beam broadening in the radial direction, larger beam broadening occurring to those ions that start their trajectories with larger initial displacements radially.

(17) Electrode assemblies to support arcuate focusing lenses may be positioned anywhere near the main flight path within the analyser. A preferred embodiment is shown schematically in FIG. 3. In this embodiment a single belt electrode assembly 670 that supports arcuate lenses 675 is located adjacent the main flight path at one of the turning points. FIG. 3 shows both a side view cross section of the analyser and a view along the z axis of the belt electrode assembly 670 with arcuate lens electrodes 675 equally spaced about the analyser axis z. Only eight arcuate lens electrodes 675 are shown in this example; in other embodiments there may be more or less; preferably there would be one gap between adjacent arcuate lens electrodes for each full oscillation of the main flight path along the analyser axis z, so that arcuate focusing of the beam occurs each time the beam reaches the turning point adjacent the belt electrode assembly. The beam envelope in this embodiment is an ellipse 680 having minimum radius r1 and maximum radius r2. Entry and exit ports are not shown in the figure, but may comprise a single or a pair of apertures in the outer field-defining electrode system of one or both the mirrors. Inner field-defining electrode systems of both mirrors 600 are surrounded by outer field-defining electrode structures of both mirrors 610. The belt electrode assembly 670 supporting the arcuate lenses 675 comprises a disc shaped plate with a central aperture through which passes the end of the inner field-defining electrode system 600. Electrode tracks 671 are mounted upon the belt electrode assembly 670, set in insulation. These electrode tracks 671 are each given an appropriate electrical bias to reduce distortion of the main analyser field in the vicinity of the belt electrode assembly 670.

(18) FIG. 4 shows a further preferred embodiment of the present invention in schematic cross-sectional form. Analyser 400 comprises two opposing mirrors 410 and 420 which abut at a first plane p1, each mirror comprising inner field-defining electrode systems 430, 440 and outer field-defining electrode systems 450, 460 elongated along an analyser axis z. Outer field-defining electrode system 450 of mirror 410 comprises two sections, the sections abutting at a second plane p2. The two sections comprise a first section 452 between plane p1 and plane p2 and a second section 454 adjacent the first section. The first section 452 has a portion 453 which extends radially from the analyser axis z a greater extent than an adjacent portion 455 of the second section at the second plane p2. A radial gap 456 is thereby provided through which ions may enter and exit. The radial gap 456 provides an exit port. Where it is desired to introduce ions from a pulsed ion source into the analyser, radial gap 456 also provides an entry port. In this embodiment the radial gap 456 extends all the way around the analyser axis and hence the first section of the outer field-defining electrode system is of larger diameter than the second section of the outer field-defining electrode system at the second plane p2.

(19) Analysers used with methods of the present invention are able to operate at high resolving powers, such as 20,000 RP to 100,000 RP. Analysers of the present invention may be used in various instrumental configurations. A preferred instrumental layout 700 is depicted schematically in FIG. 5. An analyser according to the present invention 720 comprises an entry and an exit port (not shown). Upstream of the analyser 720 is an injector comprising an external storage device 710. External storage device 710 injects ions 715 into analyser 720 through the entry port. Analyser 720 separates at least some of the injected ions according to their mass to charge ratio and the separated train of ions 725 leave the analyser 720 through the exit port. Separated ions 725 are directed to an ion gate 730 which is switched to select ions of one or more ranges of m/z 735 to proceed on to fragmentor 740. Fragmentor 740 is operated to fragment ions 735 forming fragmented ion beam 745, which passes on to mass analyser 750 and fragmented ions 745 are mass analysed.

(20) As used herein, including in the claims, unless the context indicates otherwise, singular forms of the terms herein are to be construed as including the plural form and vice versa. For instance, unless the context indicates otherwise, a singular reference herein including in the claims, such as a or an means one or more.

(21) Throughout the description and claims of this specification, the words comprise, including, having and contain and variations of the words, for example comprising and comprises etc, mean including but not limited to, and are not intended to (and do not) exclude other components.

(22) It will be appreciated that variations to the foregoing embodiments of the invention can be made while still falling within the scope of the invention. Each feature disclosed in this specification, unless stated otherwise, may be replaced by alternative features serving the same, equivalent or similar purpose. Thus, unless stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

(23) The use of any and all examples, or exemplary language (for instance, such as, for example and like language) provided herein, is intended merely to better illustrate the invention and does not indicate a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.