Orthogonal acceleration coaxial cylinder mass analyser

Abstract

A mass analyzer is disclosed comprising an annular ion guide comprising a first annular ion guide section and a second annular ion guide section, wherein the annular ion guide comprises: (i) an inner cylindrical electrode arrangement which is axially segmented and comprises a plurality of first electrodes and (ii) an outer cylindrical electrode arrangement which is axially segmented and comprises a plurality of second electrodes. Ions are introduced into the first annular ion guide section so that the ions form substantially stable circular orbits. Ions are orthogonally accelerated from the first annular ion guide section into the second annular ion guide section and one or more parabolic DC potentials are maintained along a portion of the second annular ion guide section so that ions undergo simple harmonic motion. An inductive ion detector is arranged and adapted to detect ions within the second annular ion guide section.

Claims

1. A mass analyser comprising: an annular ion guide having a longitudinal axis and comprising a first annular ion guide section and a second annular ion guide section, wherein said annular ion guide comprises: (i) an inner cylindrical electrode arrangement which is axially segmented and comprises a plurality of first electrodes; and (ii) an outer cylindrical electrode arrangement which is axially segmented and comprises a plurality of second electrodes; a first device arranged and adapted to introduce ions into said first annular ion guide section so that said ions form substantially stable circular orbits within said first annular ion guide section about said longitudinal axis; a second device arranged and adapted to orthogonally accelerate ions from said substantially stable circular orbits within the first annular ion guide section into said second annular ion guide section such that ions follow substantially spiral paths as they pass through the second annular ion guide section; a device arranged and adapted to maintain one or more parabolic DC potentials along a portion of said second annular ion guide section so that ions undergo simple harmonic motion; and an inductive ion detector arranged and adapted to detect ions within said second annular ion guide section.

2. A mass analyser as claimed in claim 1, wherein said ion detector is arranged and adapted to detect ions within said second annular ion guide section in a non-destructive manner.

3. A mass analyser as claimed in claim 1, wherein said ion detector comprises one or more cylindrical, ring or annular pick-up electrodes.

4. A mass analyser as claimed in claim 3, wherein said one or more pick-up electrodes are arranged along an outer section of said second annular ion guide section.

5. A mass analyser as claimed in claim 3, wherein said one or more pick-up electrodes are connected to an amplifier or a differential amplifier which is arranged and adapted to amplify a signal induced in said one or more pick-up electrodes.

6. A mass analyser as claimed in claim 3, wherein said one or more pick-up electrodes are arranged substantially centrally within said second annular ion guide section.

7. A mass analyser as claimed in any preceding claim, wherein said second annular ion guide section further comprises one or more second pick-up electrodes arranged along an inner section of said second annular ion guide section.

8. A mass analyser as claimed in claim 1, wherein said ions which are orthogonally accelerated are arranged to be spatially focused to an isochronous plane which is substantially perpendicular to said longitudinal axis.

9. A mass analyser as claimed in claim 1, wherein said ion detector is arranged and adapted to detect ions as said ions undergo multiple axial passes through said second annular ion guide section.

10. A mass analyser as claimed in claim 1, wherein said second device is arranged and adapted to apply a pulsed axial electric field.

11. A mass analyser as claimed in claim 10, wherein said second device is further arranged and adapted to apply a pulsed radial electric field at substantially the same time as said pulsed axial electric field.

12. A mass analyser as claimed in claim 1, wherein said second device is arranged and adapted to orthogonally accelerate said ions so that time of flight dispersion occurs only in a longitudinal direction.

13. A mass analyser as claimed in claim 12, wherein an annular time of flight ion guiding region is formed between said inner cylindrical electrode arrangement and said outer cylindrical electrode arrangement.

14. A mass analyser as claimed in claim 1, further comprising a device arranged and adapted to apply DC potentials to said inner cylindrical electrode arrangement and/or said outer cylindrical electrode arrangement in order to maintain a radial DC potential which acts to confine ions radially within said annular ion guide.

15. A mass analyser as claimed in claim 1, further comprising a control system arranged and adapted: (i) to apply one or more first voltages to one or more of said first electrodes so that ions located in said first annular ion guide section precess or move in orbits about said inner cylindrical electrode arrangement; and then (ii) to apply one or more second voltages to one or more of said first electrodes and/or said second electrodes so that ions are orthogonally accelerated into said second annular ion guide section so that ions pass along spiral paths through said second annular ion guide section in a first axial direction; (iii) to apply one or more third voltages to one or more of said second electrodes so that ions are reflected back in a second axial direction which is opposed to said first axial direction and wherein optionally said ions are caused to oscillate axially; and (iv) to determine the mass to charge ratio of ions passing through or oscillating axially within said second annular ion guide section.

16. A mass analyser as claimed in claim 1, wherein said second device is arranged and adapted to apply a potential difference across said first annular ion guide section so that ions are orthogonally accelerated out of said first annular ion guide section and pass into said second annular ion guide section.

17. A mass analyser as claimed in claim 1, wherein ions having different mass to charge ratios follow substantially different spiral paths through said annular ion guide or said second annular ion guide section.

18. A mass analyser as claimed in claim 1, wherein electrodes in said first annular ion guide section are segmented so that at least a first electric field sector and a second electric field sector are formed in use.

19. A mass analyser as claimed in claim 18, further comprising a control system arranged and adapted at a first time T1 to inject ions substantially tangentially into said first electric field sector whilst maintaining a substantially zero radial electric field in said first electric field sector so that said ions experience a substantially field free region whilst being injected into said first annular ion guide section; wherein said control system is further arranged and adapted to maintain a radial electric field in said second electric field sector so that at a second later time T2 ions pass from said first electric field sector into said second electric field sector and become radially confined; wherein said control system is further arranged and adapted at a third time T3, wherein T3>T1, to cause a radial electric field to be maintained in said first electric field sector so that as ions pass from said second electric field sector into said first electric field sector said ions continue to be radially confined and form substantially stable circular orbits within said first annular ion guide section; and wherein said second device is arranged and adapted to orthogonally accelerate ions from said first annular ion guide section into said second annular ion guide section at a fourth time T4, wherein T4>T3.

20. A method of mass analysing ions comprising: providing an annular ion guide having a longitudinal axis and comprising a first annular ion guide section and a second annular ion guide section, wherein said annular ion guide comprises: (i) an inner cylindrical electrode arrangement which is axially segmented and comprises a plurality of first electrodes; and (ii) an outer cylindrical electrode arrangement which is axially segmented and comprises a plurality of second electrodes; introducing ions into said first annular ion guide section so that said ions form substantially stable circular orbits within said first annular ion guide section about said longitudinal axis; orthogonally accelerating ions from said substantially stable circular orbits within the first annular ion guide section into said second annular ion guide section such that ions follow substantially spiral paths as they pass through the second annular ion guide section: maintaining one or more parabolic DC potentials along a portion of said second annular ion guide section so that ions undergo simple harmonic motion; and inductively detecting ions within said second annular ion guide section.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Various embodiments will now be described, by way of example only, together with other arrangements given for illustrative purposes only and with reference to the accompanying drawings in which:

(2) FIG. 1 shows the principle of spatial focusing;

(3) FIG. 2 shows ion velocity and phase space ellipses;

(4) FIG. 3 illustrates Liouville's theorem;

(5) FIG. 4 shows a W-shaped Time of Flight region in a conventional Time of Flight mass analyser;

(6) FIG. 5 shows an end view of a mass analyser according to an embodiment;

(7) FIG. 6A shows ions confined in a stable orbit and FIG. 6B shows a pulsed voltage being applied to a grid which is placed inside the analyser between the two cylinders;

(8) FIG. 7A shows an illustrative arrangement and FIG. 7B shows an embodiment;

(9) FIG. 8A shows an arrangement wherein ions are initially confined, FIG. 8B shows a along a parabolic potential according to an embodiment and FIG. 8D shows an arrangement wherein the ions are transmitted to an ion detector;

(10) FIG. 9A shows ions being confined initially, FIG. 9B shows ions being orthogonally accelerated, FIG. 9C shows ions being detected by an ion detector located at the exit of a field free region, FIG. 9D shows an embodiment wherein ions experience a parabolic potential, FIG. 9E shows an embodiment wherein ions oscillate within a parabolic potential and FIG. 9F shows an arrangement wherein ions are transmitted to an ion detector located at the exit of a field free region;

(11) FIG. 10 shows evolution of phase space in pre push state with beam stop;

(12) FIG. 11 shows a further embodiment comprising a gridless geometry with pulsed voltages shown as dotted lines;

(13) FIG. 12 shows a schematic of the geometry of a mass analyser which was modelled;

(14) FIG. 13 shows a view of the co-axial geometry of a mass analyser which was modelled;

(15) FIG. 14 shows a comparison of ion peaks due to an analytic system and a mass analyser according to an illustrative arrangement;

(16) FIG. 15 shows a comparison of time of flight peaks due to an analytic system and a mass analyser according to an illustrative arrangement;

(17) FIG. 16A shows the trajectory of an ion injected into an annular ion guiding region without scanning the internal field and FIG. 16B shows the trajectory of an ion injected into an annular ion guiding region with a higher energy and also without scanning the internal field;

(18) FIG. 17A shows a preferred method of injecting ions into the annular ion guiding region by splitting the injection region into a first and second sector and ensuring that ions initially experience a field free region when they are injected into the first sector and FIG. 17B shows the resulting ion trajectories after ions have moved from the first sector into the second sector and a radial field is restored in the first sector;

(19) FIG. 18A shows ions which have been injected into the mass analyser separating rotationally and FIG. 18B shows ions which have been injected into the mass analyser separating rotationally at a later time;

(20) FIG. 19 shows an inductive ion detector arrangement according to an embodiment; and

(21) FIG. 20 shows a signal which is induced in the inductive ion detector due to the axial oscillation of ions according to an embodiment.

DETAILED DESCRIPTION

(22) Aspects of an embodiment will first be described with reference to FIGS. 4-18. An embodiment includes the provision of an inductive ion detector system which will be described in more detail with reference to FIGS. 19 and 20.

(23) FIG. 5 illustrates an embodiment wherein a mass analyser is provided comprising two coaxial cylindrical electrodes with an annular ion guiding volume therebetween.

(24) Ions are confined radially between two coaxial cylinders held at different potentials Vouter and Vinner. The ion beam (which may be a packet of ions containing the different mass to charge ratio species to be analysed) approaches the outer cylinder where either a hole or a gap through which the ion beam may pass may be provided.

(25) Ions entering the annular ion guiding volume may form stable circular orbits by increasing the field between inner and outer cylinders as the beam is entering the device. In the absence of any other fields once inside the cylinders the ions may remain in orbit but will disperse in the axial direction according to their initial axial velocities. This is shown in FIG. 6A.

(26) Referring now to FIG. 6B, once the ions are confined in stable orbit a pulsed voltage may be applied to a grid which may be placed inside the analyser between the two cylinders. In order to create the electric field functions required to achieve spatial focussing the inner and outer cylinders may be segmented and different voltages may be applied to each of the inner and outer segmented electrodes.

(27) FIG. 6A shows how the inner and outer cylindrical electrodes may be axially segmented according to an embodiment. The inner and outer cylindrical electrodes may be axially segmented in all of the embodiments described below, although for ease of illustration only some of the following drawings may omit the axial segmentation.

(28) Referring again to FIG. 6B, the ions may continue to rotate around the central electrode set but at the same time may begin to move along the axis of the mass analyser in a substantially helical manner.

(29) It should be understood that time of flight dispersion only occurs in the axial direction and that the ions are confined radially to prevent transmission losses. As a result, the two coordinates in the preferred cylindrical device are decoupled in their behaviour.

(30) FIG. 6B illustrates a simple embodiment with four principal planes P1,P2,P3,P4 which are directly analogous to the principal planes in a Wiley McLaren Time of Flight mass analyser as shown in FIG. 1. Spatial focussing is achieved in the same principle. However, in the geometry according to an embodiment as shown in FIG. 6B, the stable orbits in the radial direction prevent losses due to beam divergence and grid scattering at the grid boundaries.

(31) A further advantage of the geometry according to an embodiment is that when coupled to a pulsed packet of ions incoming to the spectrometer the entire ion packet may be captured into a stable orbit and utilised. If ions are stored in an upstream RF device between spectrometer acquisition cycles (pushes) then essentially a 100% duty cycle is potentially achievable with the preferred geometry.

(32) According to the embodiment shown in FIGS. 6A and 6B ions may still be lost due to collisions with grid electrodes but an embodiment advantageously has higher transmission than conventional arrangements.

(33) Other embodiments are also contemplated and will be described below which do not utilise grid electrodes and which are therefore even more advantageous compared with conventional arrangements.

(34) It should be noted that the application of an orthogonal acceleration electric field or pulsing electric field after stable radial orbits are achieved is an important distinction over other known forms of mass analysers.

(35) In particular, electrostatic mass analysers are known wherein a packet of ions from outside the device is pulsed into the mass analyser using deflection devices to change the direction of the beam to the axis of the spectrometer. Such deflection devices cause aberrations in the time of flight and distortions in the isochronous plane. These aberrations limit the resolution of such devices such that very long flight times are required before high resolutions can be achieved.

(36) A particular advantage of an embodiment is that the application of the acceleration field after stable orbits are achieved negates the need for deflection devices and enables resolution performances to be achieved in timescales similar to conventional orthogonal acceleration Time of Flight mass analysers.

(37) It is an advantage of radially confined coaxial cylinder mass analysers according to embodiments that long flight paths are possible without losses due to beam divergence losses. As such an embodiment is ideally suited to multipass mass analyser geometries.

(38) Various multipass geometries are contemplated and according to preferred embodiments contain the minimum number of grids to reduce losses at the principal planes.

(39) FIGS. 7A and 7B show an illustrative geometry. According to this embodiment ions are injected and stabilised into one side of an axially symmetric device before a parabolic potential is applied along the length of the mass analyser. The parabolic potential acts to accelerate the ions towards the centre of the spectrometer. The form of parabolic potential well may allows the ions to oscillate back and forth exhibiting simple harmonic motion. The more passes that the ions experience before detection the greater the resolution of the instrument. According to an embodiment ions are detected using an inductive ion detector.

(40) Advantageously, ions may be stored in an upstream ion trap. Ions may be mass selectively ejected from the ion trap to sequentially release known mass ranges of ions to the analyser while storing others in the population. In this way a high resolution mass spectrum covering the entire mass range may stitched together from segments of the smaller acquired mass range.

(41) The evolution of phase space illustrated in FIG. 7B shows that the isochronous plane is found in the centre of the device substantially at the bottom of the potential well. In fact there is a small deviation from the bottom which is a function of the inclination of the initial phase space ellipse but this is a small effect.

(42) According to other less preferred embodiments the ion detector may be placed or located in a region of the instrument where there is no axial field present.

(43) According to an embodiment the ion detector may be located in an axial field free region of the instrument as will now be discussed with reference to FIGS. 8A-8D. FIGS. 8A-8D show an embodiment incorporating a combination of a Wiley McLaren and parabolic potential well sections. Each of FIGS. 8A-8D illustrate a different time in the acquisition cycle of the instrument.

(44) Ions may be extracted from a coaxial geometry mass analyser according to an embodiment and incorporating a two field Wiley McLaren type source. The ions are orthogonally accelerated into a field free region and pass along through the field free region. The ions then experience a parabolic potential gradient (half a well) as shown in FIGS. 8A and 8B.

(45) While ions are inside the parabolic section as shown in FIG. 8B, the other half of the well may be switched ON as shown in FIG. 8C and the ions may be allowed to oscillate for a desired number of times to increase the effective flight path of the instrument. According to an arrangement ions may be ejected towards an ion detector as shown in FIG. 8D. However, according to an embodiment ions are detected using an inductive ion detector.

(46) It will be noted from FIG. 8D that the isochronous plane is no longer at the base of the potential well. This is due to the amount of field free region required to bring the ion beam into isochronous spatial focus being exactly the distance between P3 and the detector in this case. Only half of the field free region is taken up on its outward trip to the right hand half of the parabolic potential well. When this half is switched OFF the ions of interest fly the remaining required field free region to be brought into isochronous spatial focus. It is the combination of a geometry that allows a portion of field free region along with a parabolic potential well allowing simple harmonic motion that makes such a multipass instrument possible. Without such a field free region there would be nowhere to position the detector without distortion of the electric fields.

(47) If a higher degree of spatial focussing is required then the pulsed parabolic potential well may be contained in a field free region of a reflectron mass analyser. This further embodiment will now be described with reference to FIGS. 9A-9F.

(48) The principle of operation according to this arrangement is similar to that described above with reference to FIGS. 8A-8D, but also included is a single pass mode as shown in FIGS. 9A-9C which does not include the pulsing of the parabolic potential well. Such a mode of operation is particularly useful when faster acquisition at lower resolution is required. The higher degree of spatial focussing enables the very highest possible resolution to be achieved for the lowest number of passes of the potential well.

(49) As will be understood by those skilled in the art, the mass range of the mass analyser will reduce with the number of round trips made of the harmonic potential well. If the analyser is traversed a number of times N then the available mass range reduces with this value in the relation:
m.sub.max/m.sub.min=(N/(N1)).sup.2(1)

(50) This could be seen as a disadvantage but the reduced mass range may be exploited by optimising the phase space conditions of the beam entering the analyser prior to acceleration. Generally ion beams are conditioned by a combination of RF focussing elements such as lenses and grids. Optimisation of initial conditions involves confining the beam closely to the optic axis. Most often the beam is confined tightly to the optic axis by using an RF only quadrupole but this device has a strong mass dependence in its focussing action. This means that while ions of a certain mass may be effectively squeezed to the optic axis, ions of higher mass are less strongly confined and ions of lower mass may be unstable in the device or pick up excessive energy from the RF field.

(51) Accordingly, the transmission of a limited mass range to the analyser determined by the number of round trips enables optimisation of phase space characteristics for the masses contained within the reduced mass range for best possible instrument resolution.

(52) WO 2011/154731 (Micromass) describes how an ion beam may be expanded to optimise phase space conditions in a conventional two stage Wiley McLaren instrument. WO 2011/154731 discloses how the limiting turn around time aberration in a properly expanded beam scales with the acceleration potential difference seen by the beam rather than the electric field in that region.

(53) An embodiment allows for perfect aberration free beam expansion by allowing the packet of ions that have been injected into the analyser to rotate around the central electrode for as long as desired before orthogonal acceleration. The analyser is entirely field free in the axial direction before the acceleration pulse is applied. This allows free expansion due to the ions initial velocity. The process is essentially like having a variable flight distance from the transfer optics to the mass analyser. As the ions rotate and expand the phase space ellipse becomes more elongated and the beam picks up more of the acceleration potential when it is applied. So long as the analyser has a good enough spatial focussing characteristic then resolution will improve as the beam is allowed to expand. By prudent placing of an aperture plate (or beam stop) within the mass analyser acceleration region the maximum size that the beam can axially expand to may be limited to the spatial focussing characteristic of the analyser. If once the position of the beam stop is reached the ions are allowed to rotate further prior to acceleration, the phase space will take the form of a truncated ellipse getting thinner in the velocity direction the longer the rotation takes. This is illustrated in FIG. 10.

(54) By varying the delay time greater resolutions may be reached at the expense of some ion losses. This may be thought of as analogous to the technique of delayed extraction in MALDI instruments whereby the ions are allowed to leave the target plate and adopt positions correlated with their initial velocity in the ion source prior to extraction into analyser. The correlation of ion velocity and position is very high due to the desorption event being defined by a plane. The delayed extraction according to various embodiments does not have such complete position/velocity correlation but nevertheless high degrees of ion focussing can be achieved and can be further optimised for the mass range of interest being injected into the analyser i.e. the delay time could be set to allow the central mass in the injected range to just reach the position of the beam stop (i.e. fill to the level before spatial focussing degrades the resolution) before extraction takes place.

(55) U.S. Pat. No. 546,495 (Cornish) discloses using a curved field reflectron to bring ions of wide kinetic energy difference created by post source decay (PSD) in MALDI Time of Flight mass analyser instruments. According to an embodiment such an arrangement may be utilised to give a first acceleration stage with good spatial focussing and the field free region necessary for suitable positioning of the ion detector.

(56) As mentioned above, further embodiments are contemplated wherein no grid electrodes are utilised. The radial confinement afforded by the stable orbit means that the ions adopt a narrow range of radial positions. This means that it is possible to make the entire system gridless and still maintain good spatial focussing while avoiding the disturbance in the axial electric fields and ion losses that these elements introduce. Gridless mass analysers without the radial stability of the present embodiments suffer from the defocusing effect of the electric fields caused by overfilling of the ion optical elements ultimately limiting device sensitivity and resolution.

(57) An example of a gridless electrode arrangement according to an embodiment is shown in FIG. 11. In this case the electric potentials to be pulsed are shown as dotted lines but the order and nature of their pulsing and the phase space evolution is similar to that described with reference to FIGS. 8A-8D. Elimination of the grids has a further advantage in that is simplifies the method of construction as the device may consist of two concentric segmented cylinders assembled independently rather than having common mechanical parts (the grids) in contact with both outer and inner assemblies within the internal space between the two.

(58) Modelling of a coaxial mass analyser according to an embodiment was performed. Results from an analytic system were compared with SIMION calculated results for a coaxial mass analyser geometry according to an embodiment.

(59) FIG. 12 shows the mass analyser geometry used for the modelling where the mean ion start plane is at the centre of the pusher region of length L1=40 mm. The voltage V1 equals 1000 V.

(60) The acceleration region L2 was set at 50 mm and voltage V2 was set at 5000 V. The various regions are bounded by grids while the parabolic regions are not grid bounded. The distance Lp was modelled as being 99 mm and Vp was set at 10,000 V. The left hand parabola (LHP) needs to be ramped up (after ions are in the right hand parabola (RHP), and the RHP needs to be ramped down while the ions are in the LHP after the desired number of passes have occurred.

(61) In the python model the total field free distance is a variable that can be solved while in the SIMION simulation the ions are recorded at a fixed detector plane distance. These ions can then be imported into the python model and can be solved for a variable field free region, hence both approaches can be brought into focus.

(62) FIG. 13 shows the co-axial geometry used in the SIMION modelling. The radius of the inner cylinder Rin was set at 10 mm and the outer cylinder radius Rout was set at 20 mm. Accordingly, Rgap equals 10 mm.

(63) The axial electrode segments were 1 mm wide with 1 mm gaps therebetween. Grids were modelled as being located between segments and voltages were modelled as being applied to give linear voltage drops across the first two regions and quadratic potentials in the parabolic regions.

(64) A potential difference was applied between the inner and the outer cylinders to give radial confinement. In the results presented +650 V was applied to the outer cylinder and the inner cylinder is at the same potential as the grids.

(65) For singly charged ions having a mass to charge ratio of 500 with 500 eV of radial KE and +650 V being applied to the outer cylinder gives good radial confinement. Significant radial KE is required to retain confinement within the parabolic regions which give radial divergence.

(66) For the first system the initial ion conditions were 1 mm position delta (+/0.5 mm), Gaussian velocity spread with a 5 m/s standard deviation, no initial ion drift, 8 passes through the parabolic regions (1 pass is into then back out of a single parabola) and 10 kV on the parabolas. The results are shown in FIG. 14.

(67) The total FFR is 1203 mm for the analytic system with 70.712 s drift time. For the mass analyser according to an embodiment the FFR is 1619 mm with a 79.617 s drift time. The resolution performance of the mass analyser according to an embodiment is comparable with the analytic system.

(68) If the initial phase space is set smaller and more passes through the parabolas are allowed then the resolution according to an embodiment is improved. For this system the initial ion conditions were 0.2 mm position delta (+/0.1 mm), Gaussian velocity spread with a 1 m/s standard deviation, no initial ion drift, 32 passes through the parabolic regions (1 pass is into then back out of a single parabola) and 10 kV on the parabolas.

(69) The analytic system had a FFR of 1203 mm whereas the FFR according to the preferred system was 1630 mm. The resolution of the analytic system was 189,000 compared with 170,000 resolution for system according to an embodiment.

(70) It will be appreciated that a mass analyser having a potential resolution of 170,000 represents a very significant advance in performance compared with current state of the art commercial Time of Flight mass analysers.

(71) Although the analytic and SIMION systems are not in exact agreement it is apparent that an embodiment is able to achieve about 90% of the analytic resolution. The flight time for the analytic system was 191 s whereas the flight time for an embodiment was 200 s as shown in FIG. 15. In both cases the flight time is not excessively long (12 GHz TDC detector).

(72) Method of Ion Injection into Co-Axial Cylinder TOF

(73) A less desired method of injecting ions into the spectrometer so that they achieve a stable trajectory has been shown and described above with reference to FIG. 5. According to this less desirable embodiment stable trajectories may be achieved by reducing the voltage on the inner electrode with respect to the outer electrode as the ions enter the device. This approach requires a packet of ions of limited temporal distribution to be pulsed into the device. Ions injected in this way adopt a range of radial positions that have a slight mass dependency. This is not ideal since it is required that all ions experience the same overall fields in the axial direction as they traverse the mass analyser in order to achieve the highest possible resolution.

(74) If ions are simply injected into a pair of coaxial cylinders through a small hole without scanning the internal field then no stable trajectories are achieved and the injected ions will always describe a trajectory that ends up outside the space between the concentric cylinders. Two examples of such trajectories are shown in FIGS. 16A and 16B.

(75) In FIG. 16A an ion is injected at an energy such that it would describe a circular trajectory halfway between the inner and outer cylinders if it were to find itself instantaneous created at such a position and with its velocity component entirely tangential to both cylinders. It can be seen that this ion is completely unstable quickly striking the inner cylinder after only about a quarter of one revolution.

(76) In FIG. 16B the ion is injected at higher energy and is still unstable although it survives for about one and a half revolutions before it strikes the outer cylinder.

(77) So it is desirable to find a way to inject ions into the instrument such that the fill factor is minimised and with little or no mass dependence on radial position within the device once the ions are in stable orbits.

(78) The segmented coaxial cylinder geometry which is utilised according to an embodiment enables different voltages to be applied to different segments and different portions of such segments as required. According to an embodiment the acceleration region of the Time of Flight analyser is divided into two sectors. This allows control of the radial confining field with respect to sector angle and time. By pulsing the voltage to an angular portion of either the inner or outer cylinder the confining radial field may be pulsed ON or OFF.

(79) FIG. 17A shows how in an embodiment the device is split into two regions or sectors. With reference to the dials of a clock face the first region or sector (which extends from 12:00 o'clock around to 3:00 o'clock) is separated from the rest of the electrodes (which extend from 3:00 o'clock clockwise around to 12:00 o'clock).

(80) FIG. 17A shows lines of equipotential and shows how ions that are injected at the top of the device from the right will experience a substantially field free flight in the first sector before they are deflected into the main radial sector in an anticlockwise direction. As the field is essentially static at this point mono energetic ions of differing mass take the same trajectory. This will be understood by those skilled in the art since this is a fundamental principle of electrostatics.

(81) Whilst the ions are traversing around the main sector the small sector may be switched up to the same voltage as the main sector such that a continuous radial trapping field is created by the time the ions complete the circuit (see FIG. 17B). Such a scheme allows ion packets that are relatively long temporally to be injected into the device giving the mass analyser a high duty cycle of operation.

(82) An embodiment is therefore particularly advantageous in that it enables ions to be injected into the instrument such that the fill factor is minimised and with effectively zero mass dependence on radial position within the device once the ions are in stable orbits.

(83) One of the problems with known multipass Time of Flight mass analysers is that it is difficult to determine the number of passes that a particular species of ion has traversed when detected. It is known to seek to address this problem by injecting a limited mass range into the mass analyser so that such aliasing is impossible. If a shorter temporal packet of ions is injected into the analyser then it may be possible to determine the mass by retaining the angular position of the ion packet when it strikes the detector.

(84) With reference to FIGS. 18A-B three ions M1, M2, and M3 (where M1>M2>M3) may be injected into the mass analyser in a compact temporal packet. Immediately after injection in FIG. 18A it can be seen that the different masses have begun to separate rotationally. With a detector that retains angular information it is possible to predict the change in angle as each of the ions traverse the analyser. The combination of time of flight and angular position is enough to unequivocally determine the mass to charge (and therefore the number of roundtrips of the analyser) in certain cases. This extra angular information allows larger mass ranges to be injected into the analyser at any one time, so reducing the number of different spectra to be stitched together to cover the entire mass range.

(85) Inductive Ion Detector

(86) According to an embodiment an inductive detection method is used for detecting the ion beam (rather than a destructive detection method) in order to acquire a signal.

(87) In the case of known electrostatic ion traps the lack of a true isochronous plane and the splitting of the outer electrodes of the device yields a signal which is substantially triangular in nature for each mass.

(88) FIG. 19 shows an inductive ion detector according to an embodiment. Ions may be caused to oscillate axially backwards and forwards and according to one embodiment ions may undergo simple harmonic motion. The DC potential profile along the axial direction of the ion detection region may have a parabolic profile.

(89) One or more central outer electrodes may be arranged and adapted to operate as one or more pick-up electrodes. Accordingly, one of the field defining electrodes may acts as a pick-up electrode and may be attached to an amplifier in order to amplify the signal induced in the one or more pick-up electrodes.

(90) The pick-up electrode detects or has induced within the pick-up electrode an induced voltage as an ion beam approaches the pick-up electrode and passes the pick-up electrode. As the ion beam moves away from the pick-up electrode the induced signal is very small.

(91) The induced signal quickly grows as an ion beam approaches the pick-up electrode and equally rapidly decays as the ion beam moves away from the pick-up electrode with the result that a series of pulses associated with each pass of ions past the pick-up electrode may be detected as shown in FIG. 20.

(92) According to a further embodiment an additional pick-up electrode may be used and a differential amplifier may then be used which advantageously helps in elimination of common mode noise.

(93) According to an embodiment a narrower central detection electrode (or electrodes) may be used that may allows advantage to be taken of the isochronous plane.

(94) The transient signal which may be induced in the pick-up electrode(s) may be sparser but more concentrated in nature than is the case with corresponding transient signals generated with other known arrangements. The induced signal is broader than would be achieved by a destructive time of flight ion detector placed exactly at the isochronous plane due to the nature of the induced signal and also due to the fact that as the ion detection method is non-destructive then multiple measurements can be made.

(95) Appropriate signal processing of the transients yields a resolution per unit time value that is significantly greater than that currently achieved with known electrostatic analysers. It is apparent, therefore, that the non-destructive induction or inductive ion detection method according to an embodiment is particularly advantageous compared to known arrangements. A significant improvement in resolution per unit time compared with other known arrangements is achieved since the transient signal is detected at an isochronous plane and has a higher frequency (and hence a higher information content) than a distributed signal.

(96) The improvement in the resolution (per unit time) achievable according to an embodiment is inversely proportional to the temporal filling factor of the signal i.e. a signal occupying 10% of the oscillation period improves resolution per unit time by a factor of ten.

(97) The mass analyser which utilises an inductive ion detector according to an embodiment therefore represents a significant advance in the field of mass spectrometry.

(98) 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.