Abstract
A multi-reflecting, time-of-flight (MR-TOF) mass spectrometer including two quasi-planar electrostatic ion mirrors extended along drill direction (Z) and formed of parallel electrodes, separated by a field free region. The MR-TOF includes a pulsed ion source to release ion packets at a small angle to X-direction which is orthogonal to the drill direction Z. Ion packets are reflected between ion mirrors and drill along the drift direction. The mirrors are arranged to provide time-of-flight focusing ion packets on the receiver. The MR-TOF mirrors provide spatial focusing M the Y-direction orthogonal to both drift direction Z and on injection direction X. In a preferred embodiment, at least one mirror has a feature providing periodic spatial focusing of ion packets in the drift Z-direction.
Claims
1. A multi-reflecting time-of-flight mass spectrometer comprising: two quasi-planar electrostatic ion mirrors extended along a drift Z-direction and formed of parallel electrodes, wherein said mirrors are separated by a field-free region; a pulsed ion source to release ion packets at a small angle to an X-direction which is orthogonal to the drift Z-direction, such that the ion packets are reflected between the ion mirrors and drift along the drift Z-direction direction; a receiver to receive the ion packets; wherein said mirrors are positioned to provide time-of-flight focusing on said receiver and provide spatial focusing in a Y-direction orthogonal to both the drift Z-direction and the ion injection X-direction; wherein at least one of said mirrors has a periodic feature providing modulation of electrostatic field along the drift Z-direction for the purpose of periodic spatial focusing of the ion packets in the Z-direction; and wherein said periodic feature comprises at least one of the following: at least one mirror electrode having an opening varying in height in the Y-direction; at least one mirror electrode with varying width along the X-direction; or a set of periodic lenses incorporated into an internal electrode of at least one of said mirrors.
2. The multi-reflecting time-of-flight mass spectrometer as defined in claim 1 and further including at least one end deflector for reverting ion path in the drift direction.
3. The multi-reflecting time-of-flight mass spectrometer as defined in claim 1 and further including at least one isochronous curved interface between said pulsed ion source and said receiver.
4. The multi-reflecting time-of-flight mass spectrometer as defined in claim 1 and further including at least two lenses in the field-free region.
5. The multi-reflecting time-of-flight mass spectrometer as defined in claim 1, wherein at least one of said mirrors comprises at least four electrodes with at least one electrode having attracting potential applied thereto to provide said time-of-flight focusing and said spatial focusing in the Y-direction.
6. The multi-reflecting time-of-flight mass spectrometer as defined in claim 1, wherein said periodic feature comprises a set of auxiliary electrodes incorporated into at least one mirror electrode and wherein a potential of the auxiliary electrodes varies periodically in the Z-direction.
7. The multi-reflecting time-of-flight mass spectrometer as defined in claim 1, wherein said periodic feature has a period equal N*Z/2, where N is an integer number and Z is an advance in the drift direction of an ion jigsaw trajectory per reflection.
8. The multi-reflecting time-of-flight mass spectrometer as defined in claim 1, wherein said periodic feature has a period equal to integer number of periods of an jigsaw trajectory.
9. A method of time-of-flight analysis comprising the steps of: forming packets of analyzed ions; passing ion packets between two parallel and quasi-planar ion mirrors extended along a drift Z-direction while retaining relatively small velocity component of the ion packets along the Z-direction such that the ion packets move along a jigsaw ion trajectory; receiving ions at a receiver; focusing the ion packets in time and spatially focused in direction Y; spatially and periodically modulating an electrostatic field within at least one mirror in order to provide for spatial focusing of the ion packets along the Z-direction; applying an end potential to an end of a single mask window electrode disposed between the two ion mirrors; and applying a main potential to a center of the mask window, wherein the end potential is different than the main potential to produce a deflecting field at the end of the mask window.
10. The method as defined in claim 9 and further comprising a step of reverting the direction of ion drift at the edges of an analyzer.
11. The method as defined in claim 9 and further comprising injection of ion packets via a curved isochronous interface.
12. The method as defined in claim 9 and further comprising spatial focusing of ion packets within a drift space between said mirrors by at least two lenses.
13. The method as defined in claim 9, wherein said step of periodically modulating electrostatic field within at least one of said mirrors comprises a step of spatial modulation of the shape of at least one mirror electrode.
14. The method as defined in claim 9, wherein said step of periodically modulating electrostatic field within at least one of said ion mirrors comprises a step of introducing periodic field of auxiliary electrodes.
15. The method as defined in claim 9, wherein the period of said modulation equals to N*Z/2, where N is an integer number and Z is an advance in the drift direction of said ion jigsaw trajectory per reflection.
16. The method as defined in claim 9, wherein said step of forming ion packets includes step of ion accumulation of ions coming from a continuous ion source.
17. The method as defined in claim 9, wherein the strength of periodic focusing in the Z-direction is adjustable.
18. The method as defined in claim 9, wherein spatial focusing of the ion packets along the Z-direction is done by a periodic feature, the periodic feature comprising at least one of the following: at least one mirror electrode with an opening varying in height in the Y-direction; at least one mirror electrode with varying width along an X-direction; or a set of periodic lenses incorporated into an internal electrode of at least one of said mirrors.
19. A method of time-of-flight analysis comprising the steps of: forming packets of analyzed ions; passing ion packets between two parallel and quasi-planar ion mirrors extended along a drift Z-direction while retaining relatively small velocity component of the ion packets along the Z-direction such that the ion packets move along a jigsaw ion trajectory; receiving ions at a receiver; focusing the ion packets in time and spatially focused in direction Y; spatially and periodically modulating an electrostatic field within at least one mirror in order to provide for spatial focusing of the ion packets along the Z-direction by a periodic feature, the periodic feature comprising at least one of the following: at least one mirror electrode with an opening varying in height in the Y-direction; at least one mirror electrode with varying width along an X-direction; or a set of periodic lenses incorporated into an internal electrode of at least one of said mirrors.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) FIGS. 1A and 1B show a prior art MR-TOF MS;
(2) FIG. 2 shows a prior art planar MR-TOF MS;
(3) FIG. 3 is a schematic view of a prior art planar MR-TOF MS with periodic lenses;
(4) FIG. 4A is a top view of a preferred embodiment of a quasi-planar ion mirror with spatial field modulation achieved by a mask electrode located between two mirror electrodes;
(5) FIG. 4B is a side elevational view of the auxiliary electrode shown in FIG. 4A;
(6) FIG. 4C is a perspective view of preferred embodiment of a quasi-planar ion mirror with spatial field modulation achieved by a mask electrode located between two mirror electrodes;
(7) FIG. 4D is a top plan view of a preferred embodiment of a quasi-planar TOF MS with a stable confinement of a narrow ion beam with reverting Z-direction of ions by an end deflector;
(8) FIG. 5 is a top plan view of a preferred embodiment of the quasi-planar TOF MS with reverting Z-direction of ions by a deflecting field created by mask electrodes split into several parts with different potentials;
(9) FIG. 6A is a plan view illustrating an initially parallel ion beam, created by an orthogonal accelerator and elongated in the Z-direction, in another preferred embodiment of a quasi-planar TOF MS with Z-focusing of ion bunches with the aid of a periodic mask electrode embedded into one ion mirror;
(10) FIG. 6B is a plan view illustrating the transport of an ion beam, created by an orthogonal accelerator, elongated in the Z-direction and having realistic angular and energy spread, in a quasi-planar TOF MS with Z-focusing of ion bunches with the aid of a periodic mask electrode embedded into one ion mirror;
(11) FIG. 7A is a schematic view of an embodiment of quasi-planar MR-TOF MS of the invention, with lenses being formed by additional electrodes incorporated into ion mirror electrodes and having the period of half of the period of ion jig-saw motion;
(12) FIG. 7B is a schematic view of an embodiment of quasi-planar MR-TOF MS of the invention, with lenses being formed by additional electrodes incorporated into ion mirror electrodes and having the period of quarter of the period of ion jig-saw motion;
(13) FIG. 8A is a schematic view of an embodiment in which a set of periodic lenses is added within the field free region to further increase ion focusing in Z-direction provided by additional electrodes located between the mirror electrodes;
(14) FIG. 8B is a schematic view of an embodiment in which a set of periodic lenses is added within the field free region to further increase ion focusing in Z-direction provided by additional electrodes implemented into the mirror electrodes;
(15) FIG. 9A is a schematic view of an embodiment in which the modulating electrostatic filed of the ion mirror is achieved by geometrical modulation of at least one mirror electrode;
(16) FIGS. 9B and 9C are schematic views showing the modulation of the electric field by periodically varying electrode thickness (9B) and by periodically varying window height (9C); and
(17) FIG. 10 is a schematic diagram showing a system with an external ion source made of an ion trap and an external collision cell followed by a second TOF mass analyzer.
DETAILED DESCRIPTION OF THE EMBODIMENTS
(18) The present invention relates generally to the area of mass-spectroscopic analysis and, more particularly, is concerned with the apparatus, including a multi-reflecting time-of-flight mass spectrometer (MR TOF MS). Specifically, the invention improves resolution and sensitivity of a planar and gridless MR-TOF MS by incorporating a slight periodic modulation of the mirror electrostatic field. Because of improved spatial and time focusing, the MR-TOF MS of the invention has a wider acceptance and confident confinement of ion beam along an extended folded ion path. As a result, the MR-TOF MS of the invention can be efficiently coupled to continuous ion sources via an ion storage device, thus saving on duty cycle of ion sampling.
(19) FIGS. 1A and 1B show a multi-reflecting time-of-flight mass spectrometer (MR-TOF MS) of prior art, by Wollnik et.al., GB patent No 2080021 (FIG. 3 and FIG. 4 of the GB patent). In a time-of-flight mass spectrometer ions of different masses and energies are emitted by a source 12. The flight path of ions to a collector 20 is folded by arranging for multiple reflections of the ions by mirrors R1, R2, . . . Rn. The mirrors are such that the ion flight time is independent of ion energy. FIGS. 1A and 1B show two geometrical arrangements of multiple axially symmetric ion mirrors. In both arrangements ion mirrors are located in two parallel planes I and II and are aligned along the surface of the ion path. In one arrangement, this surface is a plane (FIG. 1A) and in another one it is a cylinder 22 (FIG. 1B). Note that ions travel at an angle to the optical axis of the ion mirrors, which induces additional time-of-flight aberrations and thus considerably complicates achieving high resolution.
(20) FIG. 2 shows a folded path MR-TOF MS of a prototype by Nazarenko et.al., described in Russian patent SU1725289. The MR-TOF MS comprises two gridless electrostatic mirrors, each composed of three electrodes 3, 4 and 5 for one mirror, and 6, 7 and 8 for another mirror. Each electrode is made of a pair of parallel plates a and b, symmetric with respect to the central plane XZ. A source 1 and receiver 2 are located in the drift space between the said ion mirrors. The mirrors provide multiple ion reflections. Number of reflections is adjusted by moving the ion source along the X-axis relative to the detector. The patent describes a type of ion focusing which is achieved on every ion turn, achieving a spatial ion focusing in the Y-direction and a second order time of flight focusing,: with respect to ion energy.
(21) Note that the FIG. 2 structure provides no ion focusing in the shift direction (i.e., Z-axis), thus essentially limiting the number of reflection cycles. It also does not provide time-of-flight focusing with respect to spatial ion spread in Y-direction. Therefore, the MR-TOF MS of the prototype fails in delivering wide acceptance of the analyzer and, thus, an ability of working with real ion sources.
(22) FIG. 3 is a schematic view of a planar MR-TOF MS with prior art periodic lenses by the present inventors. The spectrometer comprises two parallel and planar ion mirrors. Each mirror is formed from 4 electrodes 11 having a shape of rectangular frames, substantially elongated in the drift Z-direction. Far away from the mirror Z-edges the electric field is planar, i.e. depends on X and Y and is independent on Z. Mirrors are separated by a field free region 13. A set of periodic lenses 15 is placed within the field free region. Ions pulses are ejected from a source 1 at small angle to the X-axis. Ion packets get reflected between mirrors while slowly drifting in Z-direction. The angle is selected such that the advance in Z-direction per reflection coincides with the period of the periodic lens. The lens enforces ion motion along the jigsaw trajectory. End-deflectors 17 allow reverting ion motion. The far-end deflector is set static. After passing the defector, ions are directed along another jigsaw trajectory towards the ion receiver 2, commonly a time-of-flight detector, such as microchannel plates (MCP) or secondary electron multiplier (SEM).
(23) FIG. 4 shows one preferred embodiment of a quasi-planar MR-TOF MS of the present invention. In this embodiment, a periodic field structure in the Z-direction is formed by auxiliary electrodes 30 with periodic windows 31 (also denoted here as mask windows) located between two adjacent mirror electrodes 32 and 34, as shown in FIG. 4A-4C. The Y-height of the mask windows 31 is preferably equal to the Y-opening of the mirror electrodes. The spacing of the mask windows 31 in the Z-direction is equal to Z ion advance per one mirror reflection and is comparable to Y-opening of ion mirrors. The potential applied to the mask electrodes is slightly different as compared to the middle potential between two adjacent mirror electrodes, so that a weak periodic focusing field is created in Z-direction. FIG. 4C shows trajectories of ions with realistic angular (0.4 deg) and energy spread (5%).
(24) In operation (FIG. 4D), narrow ion bunches in the Z-direction are formed by a pulsed ion converter like a linear ion trap source or a double orthogonal injection device (WO2007044696, the disclosure of which is incorporated herein by reference). The latter forms ion packets extended in the Y-direction but which are narrow in the Z-direction. These ion bunches are injected into the time-of-flight analyzer with the aid of a set of defectors or a curved isochronous interface, such as disclosed in WO2006102430, the disclosure of which is incorporated herein by reference. The packets are ejected within the drawing plane and at a small angle to axis X, such that ion advance Z per one reflection in the mirror coincides with the period of spatial modulation of the electric field in the ion mirror. Inside the analyzer, ions move along jig-saw trajectories being periodically reflected by the ion mirrors 34 which provide for time focusing as well as for spatial focusing in the Y-direction. Passing through mask electrodes 30, ions are focused by periodic field in the Z-direction. The preferable focal length of mask electrode lenses in X-direction equals to half period of the jig-saw motion. After reaching the end of the analyzer, ions are preferably turned back either by a deflector, such as disclosed in WO200411S19593, the disclosure of which is incorporated herein by reference. Alternatively, the drift direction of ion packets is reverted by a deflector incorporated into the ion mirror as described below. Ions, after passing through the analyzer (forth and back in Z-direction) are ejected onto the detector or another receiver with the aid of a set of deflectors or a curved isochronous interface.
(25) FIG. 5 shows an alternative way of reflecting ion in Z-direction after reaching the far end (in Z-direction) of the analyzer. The ion mirror structure of the FIG. 5 embodiment is generally similar to the FIGS. 4A-4C embodiment with the following noted difference. Reflection is performed by a weak deflecting field created by the end mask window 40 split into two parts 41, 42 with a different potential applied to the end part of the window. In general, cutting the mask into multiple parts and applying slightly different potentials to these parts allows gradually changing the drift angle within the analyzer.
(26) FIGS. 6A and 6B show another option of the preferred embodiment wherein the analyzer tolerates on packets which are long in the Z-direction. Again, ion focusing in the Z-direction is performed by the auxiliary electrodes 50 with periodic windows 51. However, in this case, the size of the mask windows 51 is essentially larger compared to the Y-window of mirror electrodes. Ion bunches elongated in the Z-direction are formed by an orthogonal accelerator positioned between the mirrors. After acceleration, ion packets move along the jig-saw path. Preferably, the mask is implemented within one mirror only and the step of the mask windows is equal to the period 2Z of the ion motion in the Z-direction, as shown in FIG. 6. Alternatively, masks are implemented at both mirrors, as in FIG. 4, and the position of the windows in the masks in opposite mirrors is shifted in the Z-direction by Z. After passing through the analyzer, ions are received by a detector 54. The potential at the mask(s) is preferably adjusted to provide for the initially parallel mono-energetic ion beam after several reflections, for example, at half of the flight path length as shown in FIG. 6A. The optimal adjustment of the potential compromises small time of flight aberrations caused by the mask and confinement of ions with a realistic angular and energy spread along all the flight path, as shown in FIG. 6B.
(27) FIG. 7A shows a schematic of another embodiment of quasi-planar MR-TOF MS of the present invention, with periodic lenses 60 being formed by additional electrodes incorporated into ion mirror electrodes, here into the internal electrodes, next to field free region. The lens period of in FIG. 7A equals to the half period of ion jig-saw motion (one lens per reflection). Alternatively, as shown in FIG. 7B, the period of the lenses 62 can be equal to a quarter of the period of the ion jig-saw motion (two lenses per reflection).
(28) FIG. 8 shows yet another embodiment in which a set of periodic lenses 70 is added within the field free region to further increase ion focusing in the Z-direction provided by additional electrodes located either between the mirror electrodes, as in FIG. 8A, or implemented into the mirror electrodes 72, as in FIG. 8B. The set of periodic lenses in the field-free space can be replaced by a set of beam restricting masks which prevents hitting the detector by ions occasionally under-focused or over-focused by periodic fields of quasi-planar mirrors and thus coming to the detector after having a different number of reflections.
(29) FIG. 9A shows yet another embodiment in which modulating electrostatic filed of the ion mirror is achieved by geometrical modulation of at least one mirror electrode. FIG. 9B shows modulation of electric field by periodically varying electrode thickness. FIG. 9C shows modulation of electric field by periodically varying window height. Since potentials of electrodes are fixed to provide best time-of-flight and spatial focusing, the geometrical modulation causes a fixed strength of ion focusing in the Z-direction for each chosen geometrical modulation. The strength of modulation should he chosen as a compromised between the acceptance and resolution of the analyzer.
(30) FIG. 10 shows an arrangement with an external ion source made of ion trap 80 and with external collision cell followed by a second TOF mass analyzer 90. The external devices are coupled to MRT via an isochronous curved interface 85. Such arrangements of tandem TOF instruments are described in applications WO2003US13262 and WO2004008481.
(31) The drawing presents several different setups described in prior applications by the present inventors. A single stage TOF MS employs ion trap for accumulation of ions coining from continuous ion sources. Ion packets are ejected into the analyzer via curved field interface 85. After passing twice (forth and back) through the analyzer, ions pass through the second leg of isochronous interface and impinge upon a common TOF detector (not shown in the drawing).
(32) In the case of running the instrument as a high throughput tandem mass spectrometer, the detector is replaced by rapid collision cell, followed by a fast second TOF spectrometer. While parent ions are separated in time in the MR-TOF MS, the fragments are rapidly formed and analyzed for each ion species in a time. This allows so-called parallel MS-MS analysis for multiple parent ions without introducing additional ion losses, usually related to scanning in other types of tandem instruments.
(33) In case of running the instrument as a high resolution tandem, ions are periodically ejected from the axial trap into the MRT analyzer. Single ion specie is time selected and get injected back into the axial trap, this time working as a fragmentation cell. The fragments are collisional dampened in the gaseous cell and get ejected back into the same MRT analyzer for analysis of fragment masses.
(34) The above description is considered that of the preferred embodiments only. Modifications of the invention will occur to those skilled in the art and to those who make or use the invention. Therefore, it is understood that the embodiment shown in the drawings and described above is merely for illustrative purposes and not intended to limit the scope of the invention, which is defined by the following claims as interpreted according to the principles of patent law, including the doctrine of equivalents.
