Patent Grant
8115165
The specification relates generally to mass spectrometers and specifically to a mass spectrometer and method for performing high resolution mass spectrometry.
In conventional mass spectrometer systems, it is challenging to achieve high mass resolution of a precursor ion for MS/MS. While electrostatic traps have been used to achieve high mass resolution, this is done at the expense of first exciting ions within the trap and then ejecting ions that are not of interest, until only ions of interest remain. The remaining ions of interest can be subsequently ejected from the electrostatic trap, fragmented and analyzed. If it is desired to analyze other ions, the electrostatic trap must be refilled and other ions of interested selected in a similar manner. While these techniques can achieve high mass resolution, they are generally very wasteful of sample, which can often be available in limited quantities.
A first aspect of the present specification provides a method of operating a mass spectrometer. The mass spectrometer comprises an electrostatic trap and a mass analyzer. The electrostatic trap comprises an entrance end, an exit end, an entrance end ion mirror and an exit end ion mirror, a central field-free region, and a longitudinal axis. The mass analyzer is enabled to receive ions from the exit end. The method comprises admitting ions into the electrostatic trap via the entrance end. The method further comprises trapping ions in the electrostatic trap, the ions oscillating between the entrance end ion mirror and the exit end ion mirror along the longitudinal axis. The method further comprises waiting for the ions to separate into bunches of ions of different m/z values via the oscillating. The method further comprises exciting a given bunch of ions of a given m/z value along the longitudinal axis until at least a portion of the given bunch overcomes a barrier field at the exit end ion mirror, thereby exiting the electrostatic trap, leaving behind remaining ions in the electrostatic trap. The method further comprises analyzing at least a portion of the given bunch via the mass analyzer.
Exciting the subset of the ions can comprise applying an oscillating electric field along the longitudinal axis in phase with an oscillation of the given bunch along the longitudinal axis. The oscillating electric field can be applied between a pair of at least one of ring electrodes, grid electrodes and aperture containing plate electrodes, located in the central field-free region. The oscillating electric field can be applied in at least one of the entrance end ion mirror and the exit end ion mirror.
The method can further comprise compensating each of the entrance end ion mirror and the exit end ion mirror to maintain timing and phase of the oscillating during the exciting.
The method can further comprise fragmenting at least a portion of the given bunch in a fragmentation module prior to the analyzing. The method can further comprise decelerating the given bunch prior to the fragmenting. Decelerating can occur via at least one of a decelerating lens, a decelerating electric field, and an ion focussing field.
The method can further comprise: exciting a second given bunch of the remaining ions of a second given m/z value along the longitudinal axis until at least a portion of the second given bunch overcomes the barrier field at the exit end ion mirror, thereby exiting the electrostatic trap, leaving behind further remaining ions in the electrostatic trap; and analyzing at least a portion of the second given bunch via the mass analyzer.
A second aspect of the specification provides a mass spectrometer. The mass spectrometer comprises an electrostatic trap and a mass analyzer. The electrostatic trap comprises an entrance end, an exit end, an entrance end ion mirror and an exit end ion mirror, a central field-free region, and a longitudinal axis. The electrostatic trap is enabled to: admit ions therein via the entrance end; trap ions therein such that the ions oscillate between the entrance end ion mirror and the exit end ion mirror along the longitudinal axis; wait for the ions to separate into bunches of ions of different m/z values via the oscillating; and excite a given bunch of ions of a given m/z value along the longitudinal axis until at least a portion of the given bunch overcomes a barrier field at the exit end ion mirror, thereby exiting the electrostatic trap, leaving behind remaining ions in the electrostatic trap. The mass analyzer is enabled to receive ions from the exit end and analyze at least a portion of the given bunch.
To excite the subset of the ions, the electrostatic trap is further enabled to apply an oscillating electric field along the longitudinal axis in phase with an oscillation of the given bunch along the longitudinal axis. The electrostatic trap can further comprise a pair of at least one of ring electrodes, grid electrodes and aperture containing plate electrodes located in the central field-free region, the oscillating electric field applied there between. The electrostatic trap further can be further enabled to apply the oscillating electric field in at least one of the entrance end ion mirror and the exit end ion mirror.
The electrostatic trap further can be further enabled to compensate each of the entrance end ion mirror and the exit end ion mirror to maintain timing and phase of the oscillating when the given bunch of ions is excited
The mass spectrometer can further comprise a fragmentation module enabled to fragment at least a portion of the given bunch prior to the given bunch being analyzes at the mass analyzer. The mass spectrometer can be further enabled to decelerate the given bunch prior to fragmenting the given bunch. The mass spectrometer can further comprise at least one of a decelerating lens, a decelerating electric field apparatus, and an ion focussing field apparatus for decelerating the given bunch prior to the fragmenting the given bunch.
The electrostatic trap can be further enabled to: excite a second given bunch of the remaining ions of a second given m/z value along the longitudinal axis until at least a portion of the second given bunch overcomes the barrier field at the exit end ion mirror, thereby exiting the electrostatic trap, leaving behind further remaining ions in the electrostatic trap at least a portion of the second given bunch analyzed via the mass analyzer.
Embodiments are described with reference to the following figures, in which:
In operation, ionisable materials are introduced into ion source 120. Ion source 120 generally ionises the ionisable materials to produce precursor ions which are transferred to ion optics 130 (also identified as Q0, indicative that ion optics 130 take no part in the mass analysis). Precursor ions are transferred from ion optics 130 to EST 140 which selectively and sequentially filters precursor ions, in a manner described below. Selected precursor ions can then be transferred to fragmentation module 150 (also identified as q2) for fragmentation, to form respective product ions. Product ions are subsequently transferred to mass analyzer 160 for mass analysis, resulting in production of product ion spectra.
Furthermore, while not depicted, mass spectrometer 100 can comprise any suitable number of vacuum pumps to provide a suitable vacuum in ion source 120, ion optics 130, EST 140, fragmentation module 150 and/or mass analyzer 160. It is understood that in some embodiments a vacuum differential can be created between certain elements of mass spectrometer 100: for example a vacuum differential is generally applied between ion source 120 and ion optics 130, such that ion source 120 is at atmospheric pressure and ion optics 130 are under vacuum. While also not depicted, mass spectrometer 100 can further comprise any suitable number of connectors, power sources, RF (radio-frequency) power sources, DC (direct current) power sources, gas sources (e.g. for ion source 120 and/or fragmentation module 150), and any other suitable components for enabling operation of mass spectrometer 100.
Ion source 120 comprises any suitable ion source for ionising ionisable materials. Ion source 120 can include, but is not limited to, an electrospray ion source, an ion spray ion source, a corona discharge device, and the like. In these embodiments, ion source 120 can be connected to a mass separation system (not depicted), such as a liquid chromatography system, enabled to dispense (e.g. elute) ionisable to ion source 120 in any suitable manner.
In specific non-limiting embodiments, ion source 120 can comprise a matrix-assisted laser desorption/ionisation (MALDI) ion source, and samples of ionisable materials are first dispensed onto a MALDI plate, which can generally comprise a translation stage. Correspondingly, ion source 120 is enabled to receive the ionisable materials via the MALDI plate, which can be inserted into the MALDI ion source, and ionise the samples of ionisable materials in any suitable order. In these embodiments, any suitable number of MALDI plates with any suitable number of samples dispensed there upon can be prepared prior to inserting them into the MALDI ion source.
Precursor ions produced at ion source 120 are transferred to ion optics 130, for example via a vacuum differential and/or a suitable electric field(s). Ion optics 130 can generally comprise any suitable multipole including, but limited to, a quadrupole rod set. Ion optics 130 are generally enabled to cool and focus precursor ions, and can further serve as an interface between ion source 120, at atmospheric pressure, and subsequent lower pressure vacuum modules of mass spectrometer 100.
Precursor ions are then transferred to EST 140, for example via any suitable vacuum differential and/or a suitable electric field(s), EST 140 enabled to selectively and sequentially filter ions, which are transferred to fragmentation module 150.
Attention is now directed to
EST 140 further comprises electrodes 250, which can comprise, in some non-limiting embodiments, a pair of ring electrodes, depicted in cross-section in
EST 140 can further comprise any number of suitable electrodes (not depicted) for causing ions to oscillate between ion mirrors 230, 240 via the application of suitable static and time varying electric fields (e.g. including, but not limited to, RF (radio-frequency) and sinusoidal fields, etc.). In particular, ions of different mass-to-charge (m/z) ratios will oscillate with a unique period of oscillation (though not necessarily sinusoidally). Hence, once ions are admitted into EST 140, after a period of time, ions separate into bunches of ions of different (m/z) values as they oscillate. This is generally depicted in
A bunch of ions of (m/z)1 will pass a given point within EST 140, for example a midpoint M between electrodes 250, at times t11, t12, t13, . . . , where the difference between each successive time is constant (i.e. t13−t12=t12−t11). Bunches of ions of (m/z)2 will pass the same given point M at times t21, t22, t23, . . . where the difference between each successive time is also a constant (i.e. t23−t22=t22−t21). Bunches of ions of (m/z)3 will pass the same given point M at times t31, t32, t33, . . . where the difference between each successive time is constant (i.e. t33−t32=t32−t31). Furthermore, t33−t32≠t22−t21≠t12−t11 as each bunch of ions of (m/z)1, (m/z)2, and (m/z)3, are oscillating at different frequencies f1, f2, and f3. Hence, each bunch of ions of (m/z)1, (m/z)2, and (m/z)3 will pass given point M at different times
A bunch of ions of a given (m/z)n can then be selectively excited by applying excitation field E via electrodes 250, at particular times tn1, tn2, tn3 . . . when the bunch of ions of the given (m/z)n are passing a given point M (e.g. ions of (m/z)3 in
In some embodiments, however, such an oscillating electric field can be applied in at least one of entrance end ion mirror 230 and exit end ion mirror 240. In these embodiments, electrodes 250 are not present in EST 140. For example, a small perturbation can be applied to the back of one or both ions mirrors 230, 240 in phase with a selected bunch of ions of mass (m/z)n. Such a perturbation selectively energizes the selected bunch if ions such that the barrier field at exit end ion mirror 240 can be overcome.
Furthermore, in some embodiments, each of entrance end ion mirror 239 and exit end ion mirror 240 can be compensated to maintain timing and phase of oscillating ions when excited.
In any event, a given bunch of ions of a given m/z value can be excited along longitudinal axis 225 until at least a portion of the given bunch overcomes a barrier field at exit end ion mirror 240, the given bunch of ions thereby exiting EST 140, leaving behind remaining ions in EST 140. Such a situation is depicted in
In any event, the given bunch of ions of (m/z)n (or portion thereof) can then be transmitted to fragmentation module 150 for fragmentation such that fragmented respective product ions are produced. In some embodiments, the given bunch of ions of (m/z)n are decelerated prior to fragmentation in order to control and select the energy of fragmentation. In these embodiments, decelerating can occur via at least one of a decelerating lens (not depicted), a decelerating electric field (applied via a decelerating field apparatus, not depicted), and an ion focussing field (applied via an ion focussing field apparatus, not depicted), between EST 140 and fragmentation module 150. In some embodiments, fragmentation module 150 can be operated in alternating low energy fragmentation and high energy fragmentation modes to first identify precursor (i.e. parent) ions and associated respective product (i.e. child) ions of each mass range.
Once fragmented, product ions are transferred to mass analyzer 160 for analysis and production of product ion spectra. Mass analyzer 160 can comprise any suitable mass spectrometer module including, but not limited to, a time of flight (TOF) mass spectrometry module, a quadrupole mass spectrometry module and the like.
Once the given bunch of ions of (m/z)n has been fragmented and analyzed, successive bunches of ions of (m/z)m, can be excited in EST 140 in a similar manner, each fragmented and analyzed in turn. Thus ions in EST 140 are not discarded and sample is used efficiently.
Attention is now directed to
At step 510 ions are admitted into EST 140 via entrance end 210. It is understood that ions are produced in ion source 120 and are transferred to EST 140 via ion optics 130.
At step 520 ions are trapped in EST 140, ions oscillating between entrance end ion mirror 230 and exit end ion mirror 249 along longitudinal axis 225 by application of suitable electric fields in ion mirrors 230, 240 and between ion mirrors 230, 240.
At step 530, ions separate into bunches of ions of different m/z values via the oscillating, as described above. In particular, a suitable period of time elapses such that ions separate into bunches of different m/z values due to the respective different oscillation frequencies.
At step 540, a given bunch of ions of a given m/z value are excited along longitudinal axis 225 until at least a portion of the given bunch overcomes a barrier field at exit end ion mirror 240, thereby exiting EST 140, leaving behind remaining ions in EST 140. Excitation occurs via application of excitation field E between electrodes 250 when the given bunch of ions passes given point M. Application of excitation field E can occur when the given bunch of ions is travelling in either direction between ion mirrors 230, 240. Furthermore, the remaining ions in EST 140 continue to oscillate between ion mirrors 230, 240. Alternatively, a perturbation can be applied at one or both of ion mirrors 230, 240, as described above, to excite the given bunch of ions of the given m/z.
In some embodiments, at step 550, once the given bunch of ions have excited EST 140, the given bunch of ions can be fragmented in fragmentation module 150 prior to analysis of at least a portion of the given bunch ions via mass analyzer 160, at step 560.
At step 570 it can be determined if more ions in EST 140 are to be selected and analyzed. If not, method 500 ends at step 580. Ions remaining in EST 140 can be flushed and ions of a new sample can be introduced into EST 140, as desired.
However, if it is determined at step 570 that more ions in EST 140 are to be selected and analyzed, steps 540 through 560 are repeated such that a second given bunch of remaining ions of a second given m/z value are excited along longitudinal axis 225 until at least a portion of the second given bunch overcomes the barrier field at exit end ion mirror 240, thereby exiting EST 140, leaving behind further remaining ions in EST 140. The second given bunch can then be fragmented (if desired) and analyzing said mass analyzer 160. Steps 540 to 570 can be repeated any suitable number of times until ions no ions of interest remain in EST 140.
In general, then, by operating EST 140 in a mode where a period of time elapses to allow ions to oscillate into bunches if ions of different (m/z), and then selectively excited a bunch of ions of interest until it overcomes the barrier field at exit end ion mirror 240, very high mass resolutions can be achieved. Furthermore, as ions remaining in EST 140 are not discarded, at least a second bunch of ions can be similarly excited, transferred from EST 140 and analyzed, such that ions from samples of limited quantities are not wasted.
Those skilled in the art will appreciate that in some embodiments, the functionality of mass spectrometer 100 can be implemented using pre-programmed hardware or firmware elements (e.g., application specific integrated circuits (ASICs), electrically erasable programmable read-only memories (EEPROMs), etc.), or other related components. In other embodiments, the functionality of mass spectrometer 100 can be achieved using a computing apparatus that has access to a code memory (not shown) which stores computer-readable program code for operation of the computing apparatus. The computer-readable program code could be stored on a computer readable storage medium which is fixed, tangible and readable directly by these components, (e.g., removable diskette, CD-ROM, ROM, fixed disk, USB drive). Alternatively, the computer-readable program code could be stored remotely but transmittable to these components via a modem or other interface device connected to a network (including, without limitation, the Internet) over a transmission medium. The transmission medium can be either a non-wireless medium (e.g., optical and/or digital and/or analog communications lines) or a wireless medium (e.g., microwave, infrared, free-space optical or other transmission schemes) or a combination thereof.
Persons skilled in the art will appreciate that there are yet more alternative implementations and modifications possible for implementing the embodiments, and that the above implementations and examples are only illustrations of one or more embodiments. The scope, therefore, is only to be limited by the claims appended hereto.
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