Dual mode mass spectrometer

Abstract

Disclosed herein is an ion analysis instrument comprising a Time of Flight (“TOF”) mass analyser comprising a reflectron. The instrument is operable in at least a first mode and a second mode, wherein in said first mode ions are caused to turn around at a first point in the reflectron and wherein in said second mode ions are caused to turn around at a second point in the reflectron such that the distance traveled by ions within the Time of Flight mass analyser is greater in the second mode than the distance traveled by ions within the Time of Flight mass analyser in the first mode. In this way, the operating modes can be selectively optimised for the analysis of ions of different masses.

Claims

1. An ion analysis instrument comprising: a Time of Flight (“TOF”) mass analyser comprising a reflectron, a voltage source configured to provide a first set of electric fields or potentials and a second, different set of electric fields or potentials; wherein the instrument is selectively operable in at least a first mode and a second mode, wherein in said first mode the first set of electric fields or potentials are applied to the reflectron such that ions are caused to turn around at a first point in the reflectron and wherein in said second mode the second, different set of electric fields or potentials are applied to the reflectron such that ions are caused to turn around at a second point in the reflectron such that the distance travelled by ions within the Time of Flight mass analyser is greater in the second mode than the distance travelled by ions within the Time of Flight mass analyser in the first mode; and a control system arranged and adapted to select between said first and second modes of operation based on the molecular weight, mass or mass to charge ratio, ion mobility or collision cross section of ions being analysed such that: a first set of ions having a molecular weight, mass or mass to charge ratio, ion mobility or collision cross section above a first value are analysed in said first mode rather than in said second mode; and a second set of ions having a molecular weight, mass or mass to charge ratio, ion mobility or collision cross section below the first value are analysed in said second mode rather than in said first mode.

2. An instrument as claimed in claim 1, wherein said reflectron is a multi-stage reflectron, and wherein in said first mode the first set of electric fields or potentials are applied to the reflectron such that ions are caused to turn around in a first stage of the reflectron and wherein in said second mode the second set of electric fields or potentials are applied to the reflectron such that ions are caused to turn around in a further stage of the reflectron.

3. An instrument as claimed in claim 1, wherein said Time of Flight mass analyser comprises a multi-pass or multi-turn Time of Flight mass analyser, and/or wherein said Time of Flight mass analyser comprises a plurality of reflectrons.

4. An instrument as claimed in claim 1, further comprising a separation or filtering device for separating or filtering ions or analyte material from which the ions derive according to one or more physico-chemical properties prior to their arrival at the Time of Flight mass analyser.

5. An instrument as claimed in claim 4, wherein said one or more physico-chemical properties include molecular weight, mass, mass to charge ratio, or a mass or mass to charge ratio correlated property such as ion mobility or collision cross section.

6. An instrument as claimed in claim 1, wherein the control system is further arranged and adapted to alternately record mass spectra in said first mode and in said second mode.

7. An instrument as claimed in claim 6, wherein said control system is arranged and adapted to repeatedly and/or automatically switch between said first mode of operation and said second mode of operation.

8. An instrument as claimed in claim 1, wherein said Time of Flight mass analyser comprises an acceleration region, and wherein ions are accelerated into the Time of Flight mass analyser by a pusher field applied at said acceleration region, wherein the pusher field is varied between the first and second modes.

9. A method of spectrometry performed using an ion analysis instrument comprising: a Time of Flight (“TOF”) mass analyser comprising a reflectron; and a voltage source configured to provide a first set of electric fields or potentials and a second, different set of electric fields or potentials; wherein the instrument is selectively operable in at least a first mode and a second mode, wherein in said first mode the first set of electric fields or potentials are applied to the reflectron such that ions are caused to turn around at a first point in the reflectron and wherein in said second mode the second, different set of electric fields or potentials are applied to the reflectron such that ions are caused to turn around at a second point in the reflectron such that the distance travelled by ions within the Time of Flight mass analyser is greater in the second mode than the distance travelled by ions within the Time of Flight mass analyser in the first mode; the method comprising: selectively analysing ions in said Time of Flight mass analyser using said first mode and/or using said second mode based on the molecular weight, mass or mass to charge ratio, ion mobility or collision cross-section of ions being analysed such that: a first set of ions having a molecular weight, mass or mass to charge ratio, ion mobility or collision cross section above a first value are analysed in said first mode, rather than in said second mode; and a second set of ions having a molecular weight, mass or mass to charge ratio, ion mobility or collision cross section below the first value are analysed in said second mode, rather than in said first mode.

10. A method as claimed in claim 9, comprising separating said ions or separating analyte material from which said ions are derived according to molecular weight, mass, mass to charge ratio, or a mass or mass to charge ratio correlated property prior to passing said ions to said Time of Flight mass analyser.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Various embodiments will now be described, by way of example only, and with reference to the accompanying drawings in which:

(2) FIG. 1 shows schematically a multi-stage reflectron Time of Flight mass spectrometer according to an embodiment operating in a first mode arranged for analysing ions having low molecular weights;

(3) FIG. 2 shows schematically the multi-stage reflectron Time of Flight mass spectrometer of FIG. 1 according to an embodiment and operating in a second mode arranged for analysing ions having high molecular weights;

(4) FIG. 3 shows a Time of Flight mass spectrometer with a three-stage reflectron according to an embodiment; and

(5) FIG. 4 shows schematically a Time of Flight mass spectrometer system with a four-stage reflectron according to an embodiment and illustrates the significantly reduced path length between the second and first modes of operation.

DETAILED DESCRIPTION

(6) Embodiments will now be described with particular reference to multi-stage reflectron based Time of Flight mass spectrometer systems. However, it should be understood that the teachings described herein may also be applied to various other suitable ion analysis instruments or mass spectrometer systems.

(7) Generally, the concepts described herein involve providing an ion analysis instrument wherein the path length that ions taken as they travel through the analysis instrument may be changed or controlled so as to improve the performance of the analyser for a certain mass, mass to charge ratio or molecular weight or a certain mass, mass to charge ratio or molecular weight range of ions. Thus, the path length in a first mode may be selected to improve the analysis of low molecular weight ions and the path length in a second mode may be selected to improve the analysis of high(er) molecular weight ions. That is, ion analysis instruments are disclosed that are selectively operable in at least two different modes wherein the distance traveled by ions within a Time of Flight mass analyser of the instrument is different in different modes, such that the distances traveled by ions in the different modes may be selected to be appropriate for ions having different molecular weights, or different ranges of molecular weights. The path lengths in the different modes may thus be chosen so as to substantially optimise or improve the analysis of ions having different molecular weights.

(8) In conventional Time of Flight mass analysers a single fixed path length is generally used to analyse both large and small ions, and this path length is generally chosen to be as long as possible to maximise the resolution of the Time of Flight mass analyser.

(9) Conventionally, no account is typically taken of potential sources of error or time of flight aberrations that become relevant for ions of different sizes.

(10) Whilst it may be beneficial for relatively small ions to travel a long distance within a Time of Flight mass analyser, the Applicant has recognised that for time of flight analysis of larger ions, it may be beneficial to reduce the distance traveled within the Time of Flight mass analyser (whilst still ensuring that the distance traveled is sufficient to separate the different ions). That is, the Applicant has recognised that it may be beneficial to arrange for larger ions to travel a shorter distance within the Time of Flight mass analyser compared with smaller ions. This approach goes against conventional thinking that all ions irrespective of their mass to charge ratio should be arranged to travel as far as possible in order to maximise the amount of separation.

(11) In particular, the Applicant has recognised that for larger ions the effects of collisions with background gas molecules within the Time of Flight mass analyser may become a dominant source of error which may outweigh any potential advantages associated with increasing the path length.

(12) On the other hand, for smaller ions, where there are typically fewer collisions with background gas molecules, the effects of these collisions may be less relevant, such that the potential advantages associated with the increased path length may be more important. It will be appreciated that there may generally be some compromise between these competing effects and the substantially optimum path length may be that which minimises the total error from all sources (for a given mass, mass to charge ratio, or molecular weight or range of mass, mass to charge ratio, or molecular weight). Suitable path lengths for a particular Time of Flight mass analyser may thus be determined e.g. using prior calibration experiments, or from theoretical considerations. In general, the optimum path length for larger ions may be shorter than the optimum path length for smaller ions.

(13) The average number of collisions, Nc, experienced by an ion within a time of flight region may e.g. be determined by mean free path calculations as:
Nc=k.Math.A.Math.P.Math.L  (1)
wherein A is the collision cross section of the ion of interest (in Å.sup.2, wherein 1 Angstrom (Å)=10.sup.−10 m), P is the pressure in mbar, L is the actual path length in meters that ion travels in the Time of Flight analyser (that is the total distance traveled, including any distance traveled into a reflectron, rather than an effective path length) and k is a constant of proportionality, here k=241 Å.sup.−2 m.sup.−1 mbar.sup.−1.

(14) The collision cross section (“CCS”) of an ion tends to increase with size, and so larger molecular ions generally have a larger collision cross section, and are therefore more likely to collide with the residual background gas molecules within the Time of Flight analyser than smaller ions. For instance, CCS may be approximated from the mass, m, of a species by the relationship CCS≈B.Math.m.sup.2/3, where B is some constant of proportionality. Thus, the CCS, if not already known or measured (e.g. by an ion mobility separation device), and hence probability of collision, may be estimated from the mass of the ions as may be determined using the Time of Flight mass analyser or some other mass analyser. For example, the Time of Flight mass analyser may be used to determine the mass to charge ratio of ions, and based on knowledge or determination of the charge state, the mass may thus also be determined.

(15) Collisions of analyte species with background gas molecules may lead to scattering, which may in turn result in a broadening of the spectral peaks. Thus, the ions' change in velocity upon colliding with background gas molecules is one source of error or aberration in the Time of Flight mass spectra. These collisions may also involve a release of energy e.g. due to dissociation of the ions. Various collisional processes involving a release of energy are known such as the so-called “Derrick” shift. Again, these processes represent a potential source of time of flight error as any change in energy will have an effect on the Time of Flight measurement, potentially distorting or broadening the peaks in the Time of Flight mass spectra. Empirical measurements indicate that the amount of scattering and the levels of chemical noise in the mass spectra (such as the percent valley “hump”) are almost directly related to the number of collisions.

(16) In view of the above, the Time of Flight mass analyser may be maintained under high vacuum conditions e.g. between around 10.sup.−5 to 10.sup.−8 mbar in order to reduce the average number of collisions as far as possible.

(17) However, for larger molecular weight ions, or ions having larger collision cross sections, the average number of collisions under typical Time of Flight mass analyser operating conditions may still be undesirably high. For example, for a large molecular weight ion such as a monoclonal antibody having a collision cross section of ˜7000 Å.sup.2, with a time of flight length of 2 m and a typical operating pressure of 10.sup.−8 mbar, then the mean number of collisions is around 3.4.

(18) In order to substantially avoid collisions in the Time of Flight mass analyser, it may be beneficial to reduce the pressure-path length product (PxL) by at least an order of magnitude. It can be seen from Eqn. 1 that the average number of collisions for a given ion is directly related to the pressure-path length product associated with the Time of Flight mass analyser. Therefore, reducing the pressure path length product by reducing the path length traveled by ions within the Time of Flight mass analyser may significantly reduce the number of collisions, and hence reduce the effects of these collisions on the mass spectra.

(19) As noted above, in conventional Time of Flight mass analysis the path length is generally maximised to increase the resolution and avoid other time of flight aberrations associated with short path lengths. However, as can be appreciated from Eqn. 1, for relatively large ions, the time of flight aberrations due to collisions with the background gas may be a major or dominant source of error in the time of flight spectra, and reducing the path length to compensate for this may outweigh any disadvantages associated with the “lost” path length. In general, there may be (for a given mass, mass to charge ratio, or molecular weight) some optimum or otherwise desired path length where the combined error from these competing effects is reduced as far as practical.

(20) On the other hand, species of lower molecular weights typically have a lower collision cross section and therefore experience fewer collisions with the background gas, so these collisions may be a less relevant source of error. For smaller ions, it may generally therefore be beneficial for the ions to travel a longer path length in order to maximise the resolution of the Time of Flight mass analyser. Furthermore, for species having lower molecular weights, reducing the path length may be detrimental to the spectral quality as other time of flight aberrations and sources of error may become more relevant as the flight time is reduced. Thus, for smaller ions, it may be beneficial to keep the path length as long as possible, to maximise the resolution of the Time of Flight analyser.

(21) Accordingly, the techniques described herein relate to an ion analysis instrument that is operable in two or more different modes, wherein the path length in the different modes is selected for the analysis of species of different molecular weights. For example, the instrument may be operable in a first mode having a relatively long path length suitable for the analysis of low molecular weight ions and further operable in a second or further mode having a relatively shorter path length suitable for the analysis of higher molecular weight ions.

(22) As explained above, the substantially optimum path length for a given mass, mass to charge ratio, or molecular weight (or range of mass, mass to charge ratio, or molecular weight) may result from a compromise between using a shorter path length to reduce the effect of collisions and using a longer path length to improve the resolution and reduce other sources of time of flight aberrations. In general, for smaller ions, the substantially optimum path length will be larger than for heavier ions. Thus, the path lengths used in the two modes may be selected based on path lengths that are determined to reduce the total time of flight aberrations for a certain mass, mass to charge ratio, or molecular weight (or range of mass, mass to charge ratio, or molecular weight). Suitable path lengths may be determined empirically, e.g. based on prior calibration experiments, and/or may be constrained by the dimensions of the Time of Flight analyser region. For example, in the first mode, which is arranged for the analysis of lower molecular weight ions, the Time of Flight analyser may be arranged so that the distance traveled by the ions is as long as possible, or at least as long as practical, given the size of the Time of Flight analyser. In the second mode, which is arranged for the analysis of higher molecular weight ions, the distance traveled by the ions may be constrained by the relative positions of the various stages or segments of the Time of Flight analyser. For example, and generally, the distance traveled by the ions within the Time of Flight analyser may be controlled by varying one or more electric fields or potentials that are applied to the various stages or segments of the Time of Flight analyser.

(23) By way of example, according to embodiments of the present disclosure, the Time of Flight analyser comprises a multi-stage reflectron wherein the electric fields or potentials applied to the stages of the reflectron may be varied between the different modes in order to control the distance traveled by ions into the reflectron. That is, in the first mode, the electric fields or potentials applied to the stages of the reflectron may be arranged to allow ions having sufficient energy to travel substantially the whole length of the reflectron (i.e. towards the final stage of the reflectron), whereas in the second mode, the electric fields or potentials applied to the stages of the reflectron may be arranged to force ions to turn around in the first, or an earlier, stage of the reflectron.

(24) FIG. 1 shows schematically a multi-stage or two-stage reflectron Time of Flight mass analyser operating in a first mode arranged for the analysis of low molecular weight ions. In the mode of operation illustrated in FIG. 1, the electric potentials Vr1 and Vr2 applied respectively to the first and second stages of the reflectron 20 are arranged so that ions turn around towards the end of the reflectron 20, and thus travel a relatively long distance within the Time of Flight analyser (path length in FIG. 1=a+b).

(25) FIG. 2 shows schematically the same multi-stage reflectron Time of Flight mass analyser as shown in FIG. 1 being operated in a second mode of operation arranged for the analysis of high molecular weight ions. Compared to the first mode of operation shown in FIG. 1, in the second mode of operation shown in FIG. 2, the electric potentials Vr1 and Vr2 applied to the first and second reflectron stages are raised so that ions are caused to turn around within the first stage of the reflectron, and thus the ions therefore travel a relatively shorter distance compared to that of FIG. 1 (path length in FIG. 2=x+y<a+b). Thus, according to various embodiments, the mass spectrometer is selectively operable in a first mode wherein ions are arranged to travel a relatively long distance within the Time of Flight mass analyser and a second mode where ions are arranged to travel a relatively shorter distance within the Time of Flight mass analyser.

(26) As shown in FIGS. 1 and 2, the distance traveled within the Time of Flight mass analyser, or the total path length, may generally be defined as the distance from the mid-point of a pusher region 10 to the ion reversal point in the reflectron plus the distance from the ion reversal point to the detector or detector plane 30. That is, the distance traveled within the Time of Flight mass analyser corresponds with the total actual distance traveled by the ions, rather than an effective path length. The actual distance traveled by the ions generally determines the average number of collisions experienced by the ions.

(27) FIGS. 1 and 2 show a multi-stage reflectron Time of Flight mass analyser according to various embodiments which are similar to that which might be found in currently available systems having a two-stage reflectron 20 where ions turn around between two grids held at respective potentials Vr1 and Vr2. However, by increasing the voltage on Vr1 in a second mode of operation ions are forced to turn around in the first stage of the reflectron such that the ion path length is reduced, as illustrated by FIG. 2.

(28) It will be appreciated that in both modes ions are caused to turn around in the reflectron, such that the reflectron may be used to focus the ions in both modes.

(29) In order to maintain spatial focussing when the path length is varied, the first time focus plane (“time focus 1”) 40 may also need to be varied. For example, as shown in FIG. 2, when the path length is reduced, the first time focus plane 40 may accordingly be extended away from the pusher region 10 by reducing the pusher field Vp in the pusher region 10.

(30) Reducing the pusher field Vp may cause an increase in turn-around time (another potential source of Time of Flight aberration), but the requirements for analytical mass resolving power for high molecular weight species are typically lower than for low molecular weight species, so the benefits of reducing the number of collisions may outweigh any increase in turn-around time so that the total or combined error is reduced. For high molecular weight species, the analytical mass resolving power may e.g. be limited to around the peak width of the isotope distribution envelope.

(31) As shown in FIGS. 1 and 2, the Time of Flight mass analyser may comprise a single detector 30 that is arranged to detect ions in each of the first and second modes. To facilitate using the same detector for both modes, the axial energy of the ions may also be adjusted between the first and second modes.

(32) Alternatively, in other embodiments, the mass analyser may comprise multiple detectors e.g. with different detectors being used to detect ions in the different modes. Using separate detectors may allow the detectors to be optimised independently for each mode to account for the axial energy of the ions, etc.

(33) Typically, the detector(s) 30 may be positioned at the or a spatial focus (e.g. of the multi-stage reflectron 20) to reduce time of flight aberrations due to the initial spatial and velocity distributions. The electric fields or potentials on the pusher electrode 10 and/or on the reflectron 20 may be arranged e.g. to focus ions to first or second order, using various known techniques.

(34) In use, the first mode may be used to analyse small or relatively small ions, e.g. those having low or relatively low mass, mass to charge ratio, molecular weight, ion mobility or collision cross section and the second mode may be used to analyse large or relatively large ions, e.g. those having high or relatively high mass, mass to charge ratio, molecular weight, ion mobility or collision cross section. The mass spectrometer may be arranged to select the operating mode based on prior knowledge or expectation or the mass, mass to charge ratio, molecular weight, ion mobility or collision cross section (range) of ions being analysed at a particular time. Alternatively, or additionally, the user may make this selection. For example, the operating mode may be selected based on an upstream mass or mass to charge selective correlated (e.g. ion mobility) separation or filtering, or based on the release of ions from a mass or mass to charge ratio selective ion trap.

(35) Provided that the flight time is long enough, the operating mode may also be switched during the course of recording a single time of flight spectra, e.g. such that the lightest and fastest ions within a particular ion packet are passed to the Time of Flight analyser at a given time are analysed in the first mode whereas slower and heavier ions from the same ion packet are analysed in the second mode.

(36) It is also contemplated that the mass spectrometer may repeatedly and/or cyclically alternate between the different operating modes, in use. Where this is combined with an upstream separation, adjacent (or closely spaced) spectra may be alternately recorded using the first and second modes such that ions arriving at the Time of Flight mass analyser sequentially in time are analysed in alternate modes. Where the switching rate between the different operating modes is fast enough so that spectra in both modes are recorded sufficiently closely together to sample a single eluting peak, ions within the single eluting peak may thus be recorded in both modes. The resulting spectra may be analysed during post-processing to select the peak(s) that are least distorted e.g. for low molecular weight ions, to select the peaks from the spectra obtained using the first mode.

(37) Although FIGS. 1 and 2 show first and second modes for respectively analysing low molecular weight and high molecular weight species, a skilled person will understand that the techniques described herein may also be extended to provide a mass spectrometer that is operable in three or more modes wherein each mode has a different path length (and is hence suitable for different ranges of mass, mass to charge ratio or molecular weight).

(38) Generally, the modes may be discrete modes, wherein the notional or maximum distance traveled by the ions in each mode is fixed. For example, the distance traveled in each mode may be controlled by arranging for ions to travel and/or turn around in different stages of the reflectron, as described above.

(39) Thus, although FIGS. 1 and 2 show a two-stage reflectron, the reflectron may in general have any number of stages, such as three or more.

(40) For example, FIG. 3 shows an example of a reflectron having three-stages, with respective grid potentials Vr1, Vr2 and Vr3. Providing additional stages in the reflectron may allow a further reduction in path length between the different modes. For example, the analyser shown in FIG. 3 has the same physical dimensions as that shown in FIGS. 1 and 2, but is provided with a further grid, located closer to the pusher region. Thus, this further grid defines the first stage of the reflectron shown in FIG. 3.

(41) By raising the potential Vr1 on this grid so that the ion reversal point is in this first reflectron stage, the path length may be significantly reduced compared to that in FIG. 2 (path length in FIG. 3=f+g<x+y<a+b).

(42) It will be appreciated that the use of additional reflectron stages may also facilitate providing additional modes of operation, as ions may be forced to turn around in any of the plurality of stages by appropriately adjusting the potentials on each of the stages. Thus, in a first mode, the potential on the first stage may be raised to force ions to turn around in a first stage of the reflectron, whereas in a further mode, the potentials may be adjusted to allow ions to pass through the first stage and turn around in a second, third or later stage of the reflectron.

(43) As another example, FIG. 4 shows a four-stage reflectron, and illustrates the different trajectories of ions in first and second modes. In the first mode shown in FIG. 4, which is arranged for the analysis of low molecular weight species, the potentials on the four stages Vr1,Vr2,Vr3,Vr4 are arranged so that the ions having sufficient axial energy turn around in the fourth stage and the ions have a relatively long path length. However, in the second mode shown in FIG. 4, the first stage potential Vr1 is raised so as to cause ions to turn around in the first stage, significantly reducing the path length.

(44) Various suitable reflectrons may be used to implement the techniques described herein. For example, although the reflectrons shown above comprise grids, it is equally contemplated that the reflectrons may be gridless. Similarly, the reflectrons may comprise one or more ion mirrors, as is illustrated in FIGS. 1-4, or may comprise one or more electrical and/or magnetic sector based or other reflectrons. Furthermore, it will be appreciated that the reflectron need not comprise a “multi-stage” reflectron, and the distance traveled by ions may be controlled by controlling the potential and/or electric field across a single reflectron stage, for instance, by adjusting the gradient or magnitude of the potential.

(45) The techniques described herein may also be applied to multi-turn or multi-pass instruments, e.g. having multiple reflectrons, wherein the ions are arranged to turn around multiple times as they pass between the ion pusher and the ion detector.

(46) The techniques used herein may generally be used with any suitable ion source arrangement. For example, the mass spectrometer may comprise a Matrix Assisted Laser Desorption Ionisation (“MALDI”) ion source.

(47) In the embodiments described above the pressure path length product may be varied between the modes by reducing the path length. It will also be appreciated that the pressure-path length product (and hence the average number of collisions) may alternatively or additionally be varied between the modes by varying the pressure. However, reducing the pressure further may be expensive and difficult to maintain for currently available mass spectrometer systems, particularly as there is typically already a fairly high vacuum (between around 10.sup.−5 to 10.sup.−8 mbar) in the analyser region.

(48) Although the present invention has been described with reference to various 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.