Patent Grant
8395115
This invention relates to a multireflection time-of-flight (TOF) mass spectrometer.
Mass spectrometry is a well known analytical tool for identification and quantitative analysis of elements, compounds and so forth. The key qualities of a mass spectrometer are its resolving power, mass accuracy and sensitivity. One specific form of mass spectrometry, time-of-flight mass spectrometry (TOF-MS) involves accelerating ions in an electric field and then drifting them to a detector at a known distance. Ions of different mass to charge ratios (m/z) but having the same kinetic energy move at different velocities towards the detector and so separate according to their m/z.
The resolving power of TOF-MS is typically related to the flight length: the longer the distance between the location of ion packet formation and the detector, the greater the resolving power. To an extent, therefore, the resolution of a TOF-MS can be improved by maximizing the linear distance between the electric field and the detector. However, beyond a certain linear separation, practical problems arise as the instrument size increases, leading to increased cost, additional pumping requirements, and so forth.
To address this, so called multireflection time-of-flight mass spectrometry (MR TOF-MS) has been developed In a simplest embodiment of MR TOF-MS, two coaxial mirrors are provided (see, for example, U.S. Pat. No. 3,226,543, U.S. Pat. No. 6,013,913, U.S. Pat. No. 6,107,625 or WO-A-2002/103747). The problem with such an arrangement is that it severely limits the mass range that can be analyzed. This is because, as the ions of different m/z separate, the initial single pulse of ions becomes a train of pulses whose duration depends on the flight length they have travelled and the range of m/z ions within the train. On increasing separation this train of pulses separates to such an extent that ions at the front of the train reach around to the back of the train, and ion mixing begins which complicates m/z analysis of those ions. Consequently in such coaxial multireflection analysers, either the flight path length or the range of m/z must be limited for meaningful analysis to be possible or, alternatively, the overlapping information has to be deconvoluted by processing means. To achieve high resolving power, a long flight path length is required, and consequently the mass range of ions in the analyser must be restricted.
Multireflection ion mirrors for TOF-MS that addressed this limited mass range are described in GB-A-2,080,021 to Wollnik. Here, each mirror provides a single reflection and is functionally independent of the other mirrors. Although the arrangement of Wollnik addresses the limited mass range of other prior art devices, it does not offer a practical solution which could implement the large number of ion mirrors in the case where a large ion incidence angle provides higher resolution.
SU-A-1,725,289 describes a TOF-MS with two opposed planar ion mirrors that allows for repeated reflections in a direction generally transverse to a drift direction (Y). Unlimited beam divergence in that drift (Y) direction limits the usefulness of this design with modern ion sources (electrospray, MALDI etc).
The problem of defocussing in a drift direction is addressed by Verentchikov et al in WO-A-2005/001878. Here, as in other prior art, the reflectors are extended in the shift direction. Because of the limited focussing in this plane, multiple planar lenses are inserted orthogonally to the drift direction (Y) so as repeatedly to refocus the ion beam as it spreads in that Y direction. Nonetheless, the amount of refocussing in that drift direction remains relatively weak (compared to the focusing in the other directions). Moreover, the presence of the planar lenses in the middle of the mirror assembly complicates the practical realization of the device, since, for example, it is then difficult to locate an ion detector and an ion source in the same plane (which is normally coincident with the plane of time of flight focussing of the mirrors). This in turn necessitates an additional isochronous ion transfer as shown in, for example, US-A-2006/0214100. It is also costly due to the inclusion of multiple additional components.
Against this background, there is provided a method of reflecting ions in a multireflection time of flight mass spectrometer comprising:
Thus embodiments of the present invention, in its first aspect, provide for a MR TOF MS wherein ions move across a minor axis (Y) (such as, for example, a short side) of an ion mirror thereof as they undergo reflection within the ion mirror. This is in contrast to prior art arrangements such as, for example, the ion mirror arrangement of the above referenced Verentchikov publication, in which ions have a “shift direction” which is across a major axis of the ion mirror.
By generating a drift direction across the short or minor axis of the ion mirror, multiple ion mirrors can be stacked adjacent to one another with a relatively limited (shallow) angle of reflection within each mirror. Thus a large path length through a MR TOF MS can be created whilst adjacent mirrors can be shielded from one another by the presence of the mirror electrodes themselves. Furthermore, space charge effects are reduced.
Although, throughout the description, cartesian coordinate axes X, Y and Z are employed, it is to be understood that this is merely for ease of explanation and that the absolute orientation of the MR TOF MS is not important. Moreover, in defining the longitudinal axis to be generally in the direction of TOF separation it is recognized that the ions actually have a mean path through the ion mirror that is not parallel with the electrodes thereof at all times. Thus the longitudinal direction is simply intended to identify the cartesian direction which lies orthogonal to the sectional axes.
In a particularly preferred embodiment of this aspect of the present invention a voltage may be applied to the electrodes so as to create an electric field which causes ions to cross the plane of symmetry at least three times. In other words, ions described a “gamma” shape viewed in a plane containing the longitudinal and minor axes of the ion mirror.
The electric field of the ion mirror may be arranged to enhance spatial focussing by causing the ions to undergo spatial compression at least once (and preferably twice) during passage through the ion mirror.
In one particularly preferred embodiment, the ion mirror forms part of a stack of ion mirrors together constituting a first ion mirror arrangement. A second ion mirror arrangement is also provided, opposed to the first ion mirror arrangement. Ions are directed into the first ion mirror of the first mirror arrangement where they reflect back towards the second ion mirror arrangement, and are then reflected into a second ion mirror of the first ion mirror arrangement, back to the second ion mirror arrangement and so forth. Thus ions describe a series of “gamma” shaped loops within the first ion mirror arrangement, being reflected back each time by the second ion mirror arrangement. In this way, a “shift” direction in the direction of the minor axis of each ion mirror of the first ion mirror arrangement is established. Spatial focussing within each ion mirror of the first ion mirror arrangement obviates the need to have spatial focussing means elsewhere which is a significant drawback of the Verentchikov arrangement described above.
In one alternative, the second ion mirror arrangement likewise comprises a plurality of (for example, four) ion mirrors, each opposed to a corresponding ion mirror within the first ion mirror arrangement. In an alternative embodiment, however, the second ion mirror arrangement has a plane of symmetry containing a longitudinal axis generally perpendicular to a plane of reflection of the second ion mirror arrangement, and a minor axis of the cross section of the second ion mirror arrangement, and ions intersect that plane of symmetry of the second ion mirror arrangement as they reflect within it. This plane of symmetry of the second ion mirror arrangement is, preferably, perpendicular to the plane of symmetry defined by the longitudinal and minor axes of each ion mirror in the first ion mirror arrangement.
It has been discovered that, optimally, four ion mirrors are preferable within the first ion mirror arrangement. Four ion mirrors appears to optimise the degree of TOF focussing.
It is possible to arrange for ions having passed through the first and second ion mirror arrangements in zig-zag fashion to be detected upon their exit. Alternatively, ions may be passed to a further ion processing device such as a fragmentation chamber or the like. Furthermore, ions may be reflected back through the MR TOF MS and, most preferably, reflected once again in the forward direction to make a total of three passes through the MR TOF MS. Because of the difference in time of flight of ions of different mass to charge ratios, increasing the number of passes through the device beyond three leads to an undesirably small mass range of analysis, in a similar manner to that described in relation to the coaxial mirror arrangement of the prior art.
In accordance with a second aspect of the present invention, there is provided a method of reflecting ions in a multireflection time of flight mass spectrometer comprising:
In accordance with another aspect of the present invention, there is provided a method of reflecting ions in a multireflection time of flight mass spectrometer comprising: providing a first ion mirror arrangement including at least one ion mirror which has a longitudinal axis generally perpendicular with a plane of reflection of ions within that at least one ion mirror; the or each ion mirror further having electrodes define a cross section with a first, minor axis and a second, major axis each orthogonal to the longitudinal axis of the, or the respective, ion mirror; providing a second ion mirror arrangement including at least one ion mirror which has a longitudinal axis generally perpendicular with a plane of reflection of ions within that at least one ion mirror; the or each ion mirror further having electrodes define a cross section with a first, minor axis and a second, major axis each orthogonal to the longitudinal axis of the, or the respective, ion mirror, wherein the or each ion mirror of the first ion mirror arrangement has a plane of symmetry which contains the longitudinal and major axes thereof, wherein the or each ion mirror of the second ion mirror arrangement likewise has a plane of symmetry which contains the longitudinal and major axes thereof, wherein the first and second ion mirror arrangements are arranged in opposition to each other so that ions may pass between them, and wherein the plane of symmetry of the or each ion mirror of the first ion mirror arrangement intersects the plane of symmetry of the or each ion mirror of the second ion mirror arrangement; the method comprising:
The invention also extends to a multireflection time of flight mass spectrometer (MR TOF MS) comprising:
In accordance with another aspect of the present invention there is provided a multi-reflection time of flight arrangement, having a first Z-axis which lies generally in the direction of time of flight, the arrangement comprising:
In accordance with yet another aspect of the present invention, there is provided a multi-reflection time of flight mass analyzer comprising:
Further preferred embodiments and advantages will be apparent from the description which follows, and the claims.
The present invention may be put into practice in a number of ways and some embodiments will now be described by way of example only and with reference to the accompanying figures in which:
It will be noted that the first ion mirror arrangement 10 comprises, in the preferred embodiment of
While the mirrors appear from
Furthermore, while the Figure shows the Type 2 mirror to be rotated by 90° with respect to the Type 1 mirror, this is also not a requirement of the invention. Other degrees of rotation are contemplated in this invention.
The intention is to provide inclined and preferably orthogonal mirror arrangements which cooperate in the generation of separated temporal and spatial foci. The simplest embodiment of the apparatus of the invention has orthogonal mirror arrangements.
Each ion mirror of the first ion mirror arrangement has two planes of symmetry, a first containing the X and Z axes 400, 200, and a second containing the Y and Z axes. It is the first plane of symmetry, in the XZ direction, that is of most relevance for the ion mirrors in the first ion mirror arrangement 10, as will be explained in further detail in connection with
Finally with regard to
Referring now to
Ions continue generally in the direction that they enter the first ion mirror 10a since the first part of the ion mirror 10a in the longitudinal direction is a field free region without electrodes 47. Approximately one third of the way into the ion mirror (that is, approximately one third of the distance between the entrance slot 35a and the plane at which reflection occurs further along the longitudinal axis), ions enter an electric field established by a plurality of electrodes 37.
The electric field has the effect of spatially focussing the ion for a first time at a saddle point 38. The ions then continue in a direction generally parallel with the longitudinal axis of the ion mirror 10a before being reflected back at a turning point 45 defining a plane of reflection. It is at this point 45, where the ions change direction, that they intersect the plane of symmetry in the XZ plane for a second time.
The ions are then spatially focussed for a second time at a second saddle point 39 and then carry on again in a direction generally parallel with the longitudinal axis of the ion mirror 10a, before exiting the electric field of the ion mirror 10a into the field free region 47. The ions are deflected before leaving the electric field of the ion mirror 10a so that they once more have a component of movement in the Y direction. Thus they intersect the plane of symmetry in the XZ plane of the ion mirror 10a for a third and final time, again generally in the region of the elongate slot 35a as they pass back out of the ion mirror 10a.
Thus the shape described by the ions may be likened, generally, to the Greek “gamma” and ions intersect the plane of symmetry three times.
As an advantage and important effect the flight path is arranged such that a projection of the flight path onto the plane containing the longitudinal direction (Z) and the minor (Y) direction crosses over itself once for each entry into one of the first mirrors 10.
Having passed back through the elongate aperture 35a, ions continue moving right to left in
Following the second reflection in the second ion mirror arrangement 20, ions travel generally in a straight line back towards the first ion mirror arrangement 10 where they enter an elongate slot 35b of a second ion mirror 10b of the first ion mirror arrangement 10 which is adjacent the first ion mirror 10a of it, but whose longitudinal axis is displaced in the Y direction. The second ion mirror 10b is preferably of a identical construction to the first ion mirror 10a and thus has a set of electrodes extending part way along the longitudinal axis to provide an electric field for reflection of ions entering the second ion mirror 10b.
Ions again describe the “gamma” shape through the second ion mirror 10b so that they intersect the plane of symmetry of the second ion mirror 10b three times and so that ions leaving the second ion mirror 10b do so in a direction that has a component in the Y direction again.
Ions then pass back into the second ion mirror arrangement 20 where they are reflected at an angle to the longitudinal axis and thus continue with a component in the Y direction downwards (when viewed in the orientation of
The second mirror arrangement 20 reduces spatial dispersion of ions in a second direction orthogonal or at least at an angle to the focusing direction of the mirror arrangement 10. Preferably the second mirror arrangement 20 provides focusing in that second direction.
It is to be understood that the preferred configuration has the first mirror assembly orthogonal to the second in the sense that the respective other mirror assembly does not affect the behaviour of the former in its main focusing direction.
It is not necessary that the Type 1 and Type 2 mirrors are orthogonal.
Thus the arrangement of
The flight path may be increased still further by employing a fourth deflector 42 instead of the third deflector 41. The fourth deflector straightens up the path of the ions but keeps them generally in the YZ plane (in contrast to third deflector 41 which deflects ions up out of the YZ plane for detection at second detector 50)—see the upper part of
Instead of the first and/or second detectors 50, 51, as the case may be, ions may instead be removed from the plane of transmission through the MR TOF MS in the X direction to another stage of mass analysis (not shown in the Figures). For example, a fragmentation device may be situated out of the plane of
A mass spectrometer incorporating the invention can comprise a first mass selector, which can be a multipole, an ion trap, or a time of flight instrument, including an embodiment of the invention, or an ion mobility device and any known collision, fragmentation or reaction device and a further mass analyzer which can preferably be an embodiment of the invention or—especially when the first mass analyzer is an embodiment of the invention—another mass analyzer, like a reflectron TOF or an ion trapping mass analyzer, e.g. an RF-ion trap, or an electrostatic trap or any type of FT/MS. Both mass analyzers can have separate detection means. Alternatively a low cost version could have detection means only after the second mass analyzer.
When the analyzer is not to be used re-entrant, as described above, also a combination of two embodiments of the invention can be advantageous. Operation modes include full MS1, as well as MS2 or MSn in the known fashions, as well as the wide and narrow mass range detection modes disclosed in this description. Advantageously an apparatus of the invention incorporates a chromatograph and an atmospheric pressure ion source or a laser desorption ion source.
Although the ion mirrors 10a-10d of the first ion mirror arrangement 10 as shown in
Preferably the major axes of the first set of mirrors (Type 1) and the second set of mirrors (Type 2) are different to each other.
As shown in the figures, the mirrors preferably comprise elongated electrodes or electrode elements in the shape of rods or plates which are arranged along the respective major axis of the mirror. The mirrors can be closed at the minor sides with similar electrode arrangements to eliminate fringing fields. These closing elements could also be PCBs which mimic the ideal field as found in the centre of the arrangements. However the mirrors can be open at the minor sides if those sides are sufficiently far from the path of the ion beam.
For non planar ion mirrors, electrodes may be formed by stamping or electrochemical etching. A preferred implementation employs flat plates on its edges to minimise fringing fields, so as to constitute a planar mirror. The flat plates are located, in preference, at least one mirror height away from the ion trajectories, and preferably more than 1.5 to 2 mirror heights.
The second ion mirror arrangement 20 may likewise be a single planar mirror (as shown in
Though focussing of this planar lens 60 is unlikely to be as strong as the arrangement of
As with the arrangement of
Although the arrangements of
Alternatively, as in the embodiment of
Only those parts of the system 100 that are relevant to an understanding of the invention are shown in
In use, ions generated in the ion source 110 pass through the lens 120, and into the fragmentation cell 130. Here they may be fragmented or not depending upon the ions being analysed and the user's choice. They then pass via second lens 140 into the linear trap 150 where they are captured and cooled. Some crude mass selection may also take place within the linear trap 150. Ion packets are then ejected generally in a direction the curved axis of elongation of the linear trap, as is described in the above referenced GB 0626025.1, and are focussed downstream of the trap 150. They then pass into the second ion mirror arrangement 20 and continue onwards as described above in connection with
After one, two or three passages through the MR TOF MS, ions may be deflected out of the plane of the drawing such as for example by deflector 41 deflecting ions to detector 50 out of the plane of the paper.
One specific embodiment of the Type 2 mirror is shown in XZ section in
Typically the 1-pass mode will allow quick low resolution mass analysis, 3-pass mode will provide higher resolution analysis over a mass range that approximately matches the mass range of an RF-ion trap operated at a fixed frequency and the higher pass modes providing high resolution “zoom” modes of operation of a smaller mass range.
An injector trap 210 is preferably (but not necessarily) oriented parallel to one of the transversal directions and parallel to the elongation direction of at least one of the mirror sets. Advantageously it can be positioned outside the plane of ion movement, decoupling its properties from the longitudinal motion.
The injector trap 210 may be a curved non-linear RF ion trap such as that disclosed in the applicant's co-pending application published as WO 2008 081334, the contents of which are incorporated herein by reference.
Ions can enter the injector trap directly from an ion source, or through a first mass analyzer and an optional first reaction device which could also be part of the first mass analyzer.
In this configuration a single detector 290 can be used for all single- and multi-pass analyzing modes.
Y deflectors 221, 222, 223 organize entry, reflection and exit of ions in this device as shown in the figure.
Preferably in this configuration the detector element 290 is again parallel to the injector trap 210 and a transversal main direction 230. The detector element 290 can be in the plane of ion movement or out of plane.
While the Type 1 and Type 2 mirrors illustrated in the figures suggest that they are closed on three sides, this is not necessary.
It is preferable to sustain a pressure lower than around 10−9 . . . 10−8 mbar within this system, preferably using split flow turbomolecular pumps. The preferable overall flight length of an MR TOF MS in accordance with preferred embodiments lies in the range of 10 to 200 meters, with an overall length of the system being between about 0.5 to 1 meter. The average ion acceleration is preferably in the range of 1 to 20 kv, 2 kv being used in the arrangements of
The arrangements thus described provide a large increase in the path length relative to a single reflection time of flight mass spectrometer, but at the same time enhance spatial focussing, improved shielding of ion packets from each other to minimize space charge effects, and provide a simplified ion injection scheme due to the removal of spatial conflict between the ion source and the fringing fields of an ion mirror.
While
There are two X-focus points per complete passage. This means that if the entry beam into mirror 20 is parallel, it will focus the beam in X at the turning point of the next mirror 10 (say 10a). The beam crosses over in X at its turning point in Z in mirror 10a, and comes back out divergent again, mirrors 10 not having any X-focusing action. It enters mirror 20 and is brought parallel by that mirror. It travels parallel into mirror 10b, comes out parallel from 10b and then enters 20 again. Mirror 20 makes it focus at the turning point in mirror 10c. It crosses over, returns divergent to mirror 20 and is again brought parallel by mirror 20.
There are ten Y-focus points per complete passage as shown in
The mirror system depicted schematically in
The mirror system produces focal points in X and Y that are not coincident with the time focal points. This has benefits for the detector, as it spreads the ion beam over a larger surface, whist during its extended passage through the instrument it has been contained in X and Y, and not allowed to diverge so as to be too large to detect.
Also the ions are not focused for the majority of their passage, reducing space charge effects, especially as the focus points in X are never the same as those in Y, giving line foci, never point foci.
An odd number of passes through the mirror system is beneficial, because of the action of the Y-deflectors 221, 222, 223 in the embodiment of
When operating in 1-pass mode, the action of Y-deflector 223 cancels that of Y-deflector 221.
When operating in 3, 5, 7 . . . -pass mode, the action of Y-deflector 222 cancels itself out.
When operating in 3, 5, 7 . . . -pass mode the action of Y-deflector 221 cancels itself out except for the first action, which is cancelled by the final action before detection of Y-deflector 223.
In the specific example where a single passage of flight through the mirror system gives about 4 meters of flight, typical resolutions achieved are approximately 20 k for 1 pass, 60 k for 3 passes and 100 k for 5 passes.
This embodiment, as illustrated in
The injector 210 is displaced in X so that it does not interfere with the ion beam path when performing more than one pass of the mirror system, and ions emitted from the injector are deflected into the Z-Y plane by an X-deflector. The detector is shown not displaced but having its centre plane lying in the Z-Y plane in this embodiment. Alternatively it may be out of the Z-Y plane, displaced in X in the same or opposite direction to the displacement of the injector 210 and collimator 220.
In this arrangement, an additional X deflector is required (not shown in
The cancelling effect of the Y-deflectors 221, 222, 223 means the detector 290 lies perpendicular to the ion beam at best time-focus, and is not tilted. A single detector can be used when odd numbers of passes are performed. For these reasons this arrangement is preferred over that of
The collimator 220 comprises an entry lens and two “button” lenses (not shown for clarity) contained in a shielding enclosure. The collimator is coupled to the ion injector and is also out of the Z-Y plane. The injector and collimator produce a beam of ions suitable for injection into the mirror system, the beam being tilted with respect to the Z-Y plane, intersecting with it in the vicinity of the X-deflector 240. The X deflector deflects the ion beam into the plane of the mirror system.
To switch from 1-pass mode to multiple pass mode, Y deflector 222 is energised so that it deflects the ion beam along the trajectory 250. Mirror 20 sends the beam back through Y deflector 222 and back through the mirror system. Y deflector 221 is energized so that it deflects the ion beam along trajectory 260. The beam then passes back through the mirror system substantially along the same trajectory as on the first forward pass. This deflection arrangement can be used one or more times to increase the flight path through the mirror system, the beam ultimately reaching detector 290.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0725066.5 | Dec 2007 | GB | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/GB2008/004231 | 12/22/2008 | WO | 00 | 9/30/2010 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2009/081143 | 7/2/2009 | WO | A |
| Number | Name | Date | Kind |
|---|---|---|---|
| 6013913 | Hanson | Jan 2000 | A |
| 6657190 | Hanson et al. | Dec 2003 | B2 |
| 20010011703 | Franzen | Aug 2001 | A1 |
| 20050242279 | Verentchikov | Nov 2005 | A1 |
| 20050258364 | Whitehouse et al. | Nov 2005 | A1 |
| 20070176090 | Verentchikov | Aug 2007 | A1 |
| Number | Date | Country |
|---|---|---|
| 2 080 021 | Jan 1982 | GB |
| WO 2005001878 | Jan 2005 | WO |
| WO 2006102430 | Sep 2006 | WO |
| WO 2007044696 | Apr 2007 | WO |
| WO 2007104992 | Sep 2007 | WO |
| WO 2008047891 | Apr 2008 | WO |
| WO 2008071921 | Jun 2008 | WO |
| Entry |
|---|
| Wollnik et al. “Time of Flight Mass Spectrometers With Multiply Reflected Ion Trajectories”, International Journal of Mass Spectrometry and Ion Processes, vol. 96, No. 3, 1990, pp. 267-274. |
| Wollnick et al. “Time of Flight Mass Spectrometers With Multiply Reflected Ion Trajectories”, International Journal of Mass Spectrometry and Ion Processes, vol. 96 No. 3, 1990, pp. 267-274. |
| Wollnik et al., “Time-Of-Flight Mass Spectrometers with Multiply Reflected Ion Trajectories,” Int. J. Mass Spectrom. and Ion Processes, 96 (3), 1990, pp. 267-274. |
| H. Wollnik, “Energy-isochronous time-of-flight mass analyzers,” Int. J. Mass Spectrom. Ion Processes 131 (1994) 387-407. |
| Number | Date | Country | |
|---|---|---|---|
| 20110017907 A1 | Jan 2011 | US |
