Charge detection mass spectrometry
阅读说明：本技术 电荷检测质谱分析 (Charge detection mass spectrometry ) 是由 基思·理查森 杰弗里·马克·布朗 大卫·J.·兰格里奇 于 2019-02-22 设计创作，主要内容包括：本文公开了用于执行电荷检测质谱分析(CDMS)的各种方法和设备。具体地说,公开了用于监测来自CDMS装置的检测器信号以确定所述CDMS装置的离子阱(10)中存在多少个离子的技术。例如,如果不存在离子,则可以提前终止测量。类似地,如果存在多于一个离子,则可以提前终止所述测量,或者可以从所述阱(10)中移除离子,直到留下仅单个离子。还提供了用于增加所述阱(10)中有单个离子的概率的技术。还提供了一种用于衰减离子束的技术。(Various methods and apparatus for performing Charge Detection Mass Spectrometry (CDMS) are disclosed herein. In particular, techniques are disclosed for monitoring detector signals from a CDMS apparatus to determine how many ions are present in an ion trap (10) of the CDMS apparatus. For example, if no ions are present, the measurement may be terminated prematurely. Similarly, if there is more than one ion, the measurement may be terminated prematurely, or ions may be removed from the trap (10) until only a single ion is left. Techniques for increasing the probability of a single ion being in the trap (10) are also provided. A technique for attenuating an ion beam is also provided.)
1. A method of charge detection mass spectrometry comprising:
monitoring, within an ion trap of a charge detection mass analysis device, a detector signal from a charge detector of the charge detection mass analysis device during a first ion capture event to determine how many ions are present within the ion trap during the first ion capture event.
2. The method of claim 1, further comprising: terminating the first ion capture event and/or initiating a second ion capture event when it is determined that ions are not present within the ion trap during the first ion capture event.
3. The method of claim 1 or 2, further comprising: terminating the first ion capture event and/or initiating a second ion capture event when it is determined that more than one ion is present within the ion trap during the first ion capture event.
4. The method of any preceding claim, further comprising: ejecting or otherwise removing one or more of the ions from the ion trap when it is determined that more than one ion is present within the ion trap during the first ion trapping event.
5. The method of claim 4, comprising ejecting or otherwise removing all of the ions from the ion trap and initiating a second ion trapping event.
6. The method of claim 4, comprising ejecting or otherwise removing one or more of the ions from the ion trap such that only a single ion remains within the ion trap.
7. The method of any preceding claim, wherein the number of ions present within the ion trap of the charge detection mass analysis device is determined based on the number of masses recorded in a profile by the charge detection mass analysis device and/or based on the total charge detected by the charge detection mass analysis device.
8. A method according to any preceding claim, wherein the geometry of the ion trap is configured such that when more than one ion is present, the ion trajectory becomes unstable, thereby causing the ejection of all but one ion.
9. A charge detection mass spectrometry apparatus comprising:
an ion trap for containing one or more ions to be analysed;
one or more charge detectors within the ion trap for determining the charge of the one or more ions to be analysed; and
control circuitry to monitor detector signals from the one or more charge detectors during a first ion capture event to determine how many ions are present within the ion trap during the first ion capture event.
10. A charge detection mass spectrometry apparatus comprising:
an ion guide for confining a plurality of ions, wherein the ion guide comprises a plurality of ion traps, and wherein the geometry of each ion trap is configured such that when more than one ion is present, the ion trajectory becomes unstable, causing all but one ion to be ejected from the ion trap,
such that when a plurality of ions are passed to the charge detection mass analysis device, the plurality of ions distribute themselves among the plurality of ion traps such that no more than one ion is present in any one of the ion traps.
11. A method of charge detection mass spectrometry comprising:
passing a plurality of ions to be analyzed to the charge detection mass spectrometry apparatus of claim 10.
12. A charge detection mass spectrometry apparatus comprising:
a plurality of charge detection mass spectrometry devices; and
ion optics for selectively or sequentially delivering a respective plurality of ions to be analysed to the plurality of charge detection mass spectrometry apparatus.
13. A method of charge detection mass spectrometry comprising:
selectively or sequentially delivering a plurality of ions to a respective plurality of charge detecting mass analysis devices such that a single ion is delivered to each of the charge detecting mass analysis devices; and
the ions are analyzed within the respective ion traps.
14. A charge detection mass spectrometry apparatus comprising:
an ion trap for containing one or more ions to be analysed; and
a charge detector within the ion trap for determining the charge of the one or more ions to be analysed,
wherein the ion trap is configured such that ion trajectories diverge away from the charge detector such that when a plurality of ions are simultaneously present within the ion trap, the ions fan out away from the charge detector to reduce space charge interactions between the plurality of ions.
15. A charge detection mass spectrometry apparatus comprising:
an ion trap for containing one or more ions to be analysed; and
a plurality of charge detectors within the ion trap for determining the charge of the one or more ions to be analyzed.
16. The charge detection mass spectrometry apparatus of claim 15, wherein the ion trap has a multi-pass geometry, or wherein the ion trap has a cyclic or folded flight path geometry.
17. A method or apparatus according to any preceding claim, wherein a substantially second order potential is applied to the or each ion trap such that ions undergo substantially harmonic motion within the ion trap.
18. A charge detection mass spectrometry apparatus comprising:
an ion trap for containing one or more ions to be analysed; and
one or more charge detectors within the ion trap for determining the charge of the one or more ions to be analysed,
wherein a substantially second order potential is applied to the ion trap such that ions undergo substantially harmonic motion within the ion trap.
19. A method or apparatus according to any preceding claim, wherein the signal from the charge detection mass spectrometry apparatus is processed using a forward fitting and/or bayesian signal processing technique.
20. A method of charge detection mass spectrometry comprising:
obtaining one or more signals from a charge detector of a charge detection mass spectrometry apparatus; and
processing the one or more signals using forward fitting and/or bayesian signal processing techniques to extract charge values for one or more ions within the charge detection mass analysis device.
21. An ion beam attenuation apparatus, comprising:
a first ion beam attenuator operable in a high ion transport mode or a low ion transport mode to selectively attenuate an ion beam, wherein an output of the first ion beam attenuator passes through a first gas-filled region;
a second ion beam attenuator operable in a high ion transport mode or a low ion transport mode to selectively attenuate an ion beam; and
control circuitry configured to:
repeatedly switching the first ion beam attenuator between the high ion transport mode and the low ion transport mode to produce a first non-continuous ion beam at the output of the first ion beam attenuator, wherein the first non-continuous ion beam passes through the gas-filled region before reaching the second ion beam attenuator and is thereby converted to a substantially continuous ion beam; and is
Repeatedly switching the second ion beam attenuator between the high ion transport mode and the low ion transport mode to produce a second discontinuous ion beam at an output of the second ion beam attenuator.
22. The apparatus of claim 21, wherein the output of the second ion beam attenuator passes through a second gas-filled region to produce a substantially continuous attenuated ion beam.
23. A method of attenuating an ion beam, comprising:
passing the ion beam to a first ion beam attenuator and repeatedly switching the first ion beam attenuator between a high ion transport mode and a low ion transport mode to produce a first discontinuous ion beam at an output of the first ion beam attenuator;
passing the first non-continuous ion beam through a gas-filled region to convert a first attenuated ion beam to a substantially continuous attenuated ion beam; and
passing the substantially continuous ion beam to a second ion beam attenuator, and repeatedly switching the second ion beam attenuator between a high ion transport mode and a low ion transport mode to produce a second discontinuous ion beam at an output of the second ion beam attenuator.
24. The method of claim 23, further comprising passing a second attenuated ion beam through a second gas-filled region to produce a substantially continuous attenuated ion beam.
25. The apparatus of claim 21 or 22 or the method of claim 23 or 24, wherein the first and/or second ion beam attenuator comprises one or more electrostatic lenses.
Technical Field
The present invention relates generally to methods of mass spectrometry, and in particular to methods and apparatus for performing charge detection mass spectrometry. A method and apparatus for attenuating an ion beam is also provided.
Background
Charge Detection Mass Spectrometry (CDMS) is a technique in which the mass of an individual ion is determined by simultaneously measuring the mass-to-charge ratio (m/z) and the charge of the ion. Thus, this method can avoid the need to resolve multiple charge states associated with conventional mass spectrometry methods, especially where electrospray ionization is used. Examples of CDMS techniques are described in Keifer et al, "Charge Detection Mass Spectrometry with Almost Perfect Charge Accuracy" (Charge Detection Mass Spectrometry with Almost Perfect Charge Access), "analytical chemistry (anal. chem.)" 2015,87, 10330-.
Disclosure of Invention
According to a first aspect, there is provided a method of charge detection mass spectrometry, the method comprising: monitoring, within an ion trap of a charge detection mass analysis device, a detector signal from a charge detector of the charge detection mass analysis device during a first ion capture event to determine how many ions are present within the ion trap during the first ion capture event.
The method may further comprise: terminating the first ion capture event and/or initiating a second ion capture event when it is determined that ions are not present within the ion trap during the first ion capture event.
The method may additionally or alternatively comprise: terminating the first ion capture event and/or initiating a second ion capture event when it is determined that more than one ion is present within the ion trap during the first ion capture event.
In an embodiment, when it is determined that more than one ion is present within the ion trap during the first ion trapping event, the method may comprise ejecting or otherwise removing one or more of the ions from the ion trap. For example, the method may comprise ejecting or otherwise removing all of the ions from the ion trap and initiating a second ion trapping event. However, it is also contemplated that the method may include ejecting or otherwise removing less than all of the ions from the ion trap. For example, the method may comprise ejecting or otherwise removing one or more of the ions from the ion trap such that (or until) only a single ion remains within the ion trap.
For example, the number of ions present within the ion trap of the charge detection mass analysis device may be determined based on the number of masses recorded in the profile by the charge detection mass analysis device and/or based on the total charge detected by the charge detection mass analysis device. In an embodiment, the number of ions present within the ion trap is determined by analyzing a transient detector signal from the charge detector. For example, in an embodiment, the determination may be made in less than about 1 second of time to initiate the ion capture event, such as in about 0.5 seconds. In an embodiment, the determination may be made within 0.2 seconds or within 0.1 seconds.
In any of the embodiments of the method of the first aspect, the method is typically performed using a charge detection mass spectrometry apparatus. Charge detection mass spectrometry apparatus may generally comprise an ion trap for containing one or more ions to be analysed and (at least one) charge detector(s) located within the ion trap for determining the charge of the one or more ions to be analysed. The charge detector may comprise one or more charge detection electrodes. The charge detection mass spectrometry apparatus may further comprise control circuitry for processing signals obtained, for example, from the charge detector. The charge detection mass spectrometry apparatus may generally comprise part of a mass spectrometer. Accordingly, various ion guiding or manipulation components of the mass spectrometer may be provided upstream and/or downstream of the charge detection mass analysis device.
Thus, according to a second aspect, there is provided a charge detection mass spectrometry apparatus comprising: an ion trap for containing one or more ions to be analysed; one or more charge detectors within the ion trap for determining the charge of the one or more ions to be analysed; and control circuitry to monitor detector signals from the one or more charge detectors during a first ion capture event to determine how many ions are present within the ion trap during the first ion capture event.
To the extent not mutually inconsistent, the present invention in the second aspect may incorporate any or all of the features described in relation to the first aspect of the invention, and vice versa. Thus, the apparatus may comprise suitable means or circuitry for performing any one of the steps of the methods or inventions described herein, even if not explicitly stated herein.
In particular, when it is determined that ions are not present within the ion trap during the first ion capture event, the control circuitry may be configured to terminate the first ion capture event and/or initiate a second ion capture event.
Additionally or alternatively, the control circuitry may be configured to terminate the first ion capture event and/or initiate a second ion capture event when it is determined that more than one ion is present within the ion trap during the first ion capture event.
In an embodiment, when it is determined that more than one ion is present within the ion trap during the first ion trapping event, the control circuitry may be configured to eject or otherwise remove one or more of the ions from the ion trap. For example, the control circuitry may cause all ions to be ejected or otherwise removed from the ion trap and then initiate a second ion trapping event. However, it is also contemplated that less than all of the ions may be ejected (removed) from the ion trap. For example, the control circuitry may be configured to eject or otherwise remove one or more of the ions from the ion trap such that only a single ion remains within the ion trap.
Suitable signal processing circuitry may be used to determine the number of ions present within the ion trap of the charge detection mass analysis apparatus. The signal processing circuitry may for example be configured to analyse the (transient) signal in substantially real time to determine how many ions are present within the ion trap during the first ion trapping event.
In an embodiment, the geometry of the ion trap may be configured such that when more than one ion is present, the ion trajectory becomes unstable, causing ejection of all but one ion. In this way, the ion trap may be configured to eject one or more ions naturally when more than one ion is present within the ion trap during the first ion trapping period.
In an embodiment, a plurality of charge detection mass spectrometry apparatus are provided. Each charge detection mass analysis device may comprise an ion trap and one or more charge detectors, and thus each charge detection mass analysis device may be capable of performing independent measurements. Multiple charge detection mass spectrometry devices can then be used to perform simultaneous or parallel measurements.
For example, in some embodiments, a plurality of such charge detection mass spectrometry devices can be arranged within an ion guide. It is alternatively contemplated that a charge detection mass analysis device may be provided that includes a plurality of ion traps or ion trapping regions positioned within an ion guide, each ion trap or ion trapping region having an associated one or more charge detectors.
In this case, the charge detection mass spectrometry apparatus may be arranged to increase the likelihood of (only) a single ion being within the ion trap (or trapping region). For example, each of the ion traps may be configured such that when more than one ion is present, the ion trajectory becomes unstable, causing ejection of all but one ion. At the same time, the ion guide may provide an overall (radial) confinement of the ions. Thus, when multiple ions are injected into the ion guide, the ions may naturally distribute themselves between the multiple ion traps (trapping regions) due to space charge effects, and in embodiments, such that no more than one ion is present in any one of the ion traps (trapping regions).
The method of the first aspect described above may be implemented in such an apparatus. In such cases, the method may comprise monitoring detector signals from each (or any) of the charge detection mass analysis devices to determine how many ions are present within each (or any) ion trap. However, it is believed that this device is novel and inventive by its own characteristics.
Thus, according to a further aspect, there is provided a charge detection mass spectrometry apparatus comprising: an ion guide for confining a plurality of ions, wherein the ion guide comprises a plurality of ion traps, and wherein the geometry of each ion trap is configured such that when more than one ion is present, the ion trajectory becomes unstable, causing all but one ion to be ejected from the ion trap, such that when a plurality of ions are passed to the charge detection mass analysis device, the plurality of ions distribute themselves between the plurality of ion traps such that no more than one ion is present in any one of the ion traps. The ion guide may comprise any suitable ion guide. For example, in an embodiment, the ion guide may comprise a stacked ring ion guide, but other arrangements will of course be possible. According to a related aspect, there is provided a method of charge detection mass spectrometry, the method comprising: a plurality of ions to be analysed are passed to a charge detection mass spectrometry apparatus according to this further aspect.
In some embodiments, a plurality of independent charge detection mass spectrometry devices may be used, each charge detection mass spectrometry device comprising an ion trap and one or more charge detectors. Upstream ion optical means, such as a lens or beam splitter arrangement, may then be provided to selectively or sequentially pass the plurality of ions to be analysed to the respective ion traps of the charge detection mass analysis apparatus. Thus, this arrangement may allow multiplexing (interleaving) measurements to be performed, thereby improving the duty cycle. This may be used in combination with the method of the first aspect or the apparatus of the further aspect described above. That is, the detector signal from each of a plurality of charge detecting mass analysis devices may be monitored to determine how many ions are present within each device. However, it is also believed that this device is novel and inventive by its own characteristics.
Thus, according to a further aspect, there is provided a charge detection mass spectrometry apparatus comprising: a plurality of charge detection mass spectrometry devices; and ion optics for selectively or sequentially delivering a respective plurality of ions to be analysed to the plurality of charge detection mass spectrometry apparatus. Each charge detection mass spectrometry device comprises an ion trap and one or more charge detectors for detecting ions within the ion trap such that each ion trap is capable of performing independent measurements. The ion optical means may be provided separately from and upstream of the charge detection mass analysis means. However, it is also envisaged that the ion optics may be integrated as part of a single charge detection mass analysis apparatus comprising a plurality of ion traps and ion optics for selectively or sequentially delivering a respective plurality of ions to be analysed to the plurality of ion traps. According to a related aspect, there is provided a method of charge detection mass spectrometry, the method comprising: selectively or sequentially transferring the plurality of ions to a respective plurality of ion traps such that a single ion is transferred to each of the ion traps; and analyzing the ions within the respective ion traps.
In embodiments, a plurality of charge detection mass spectrometry devices can be configured in a microfabricated array. In this way, hundreds of devices operating in parallel can be provided, allowing spectra to be generated at much higher rates. Each trap may then contain zero, one or more than one ion, depending on the mechanism used to fill the trap. In such cases, data from a trap containing zero or more ions may be discarded. Thus, in an embodiment, multiple charge detection mass spectrometry apparatus are provided in parallel, and measurements from any apparatus that gives no signal (no ions) or poor signal (ions) can then be discarded during signal processing.
In an embodiment, one or more charge detection mass spectrometry devices are used to measure a single ion. For example, in an embodiment of the first aspect, as described above, when it is detected that this is not the case, the measurement may be terminated, or the device operation adjusted accordingly. Embodiments are therefore directed to methods of single ion charge detection mass spectrometry. However, in other embodiments, multiple ions may be measured simultaneously using a single charge detection mass spectrometry apparatus. That is, multiple ions may be present simultaneously within a single ion trap of a charge detection mass spectrometry apparatus. In this case, to minimise interference between ions, the ion trap geometry and electric field may be arranged such that the ion trajectories diverge away from the charge detector, such that when multiple ions are present simultaneously within the ion trap, the ions diverge from each other as they move away from the charge detector. That is, when ions are not passing through or are not passing through a charge detector, the trajectories of the ions are such that the ions can be separated from each other. For example, the ion trajectories may define a "dumbbell" or "H" shape such that all ions may pass through the central charge detector, but then diverge as they move away from the charge detector. In this way, the effect of space charge interactions can be reduced. For example, the charge detector may be positioned in the centre of the trap, with the ion trajectory being set such that the ions have a maximum velocity as they pass through the charge detector. However, away from the ion detector, at the extreme where the ions of the trajectories move relatively slowly and are therefore most susceptible to space charge effects, the trajectories may be designed to move the ions away from each other.
Thus, according to a further aspect, there is provided a charge detection mass spectrometry apparatus comprising: an ion trap for containing one or more ions to be analysed; and a charge detector within the ion trap for determining the charge of the one or more ions to be analysed, wherein the ion trap is configured such that ion trajectories diverge away from the charge detector such that when a plurality of ions are simultaneously present within the ion trap, the ions fan out away from each other away from the charge detector to reduce space charge interactions between the plurality of ions.
The one or more charge detection mass spectrometry devices according to any of the above aspects or embodiments may generally comprise one or more charge detector electrodes. In some embodiments, only a single charge detector is provided, which may comprise a single electrode, for example in the form of a metal cylinder. However, other arrangements will of course be possible. For example, in other embodiments, a charge detection mass spectrometry apparatus can include a plurality of charge detectors (each charge detector including one or more electrodes).
According to yet another aspect, there is provided a charge detection mass spectrometry apparatus comprising: an ion trap for containing one or more ions to be analysed; and a plurality of charge detectors within the ion trap for determining the charge of the one or more ions to be analysed. The ion trap may have a multi-pass geometry, or may have a cyclic or folded flight path geometry.
In an embodiment, according to any of the aspects described herein, a substantially second order potential may be applied to the ion trap(s) of the charge detection mass analysis device such that the ions undergo substantially harmonic motion within the ion trap.
Indeed, according to another aspect, there is provided a charge detection mass spectrometry apparatus comprising: an ion trap for containing one or more ions to be analysed; and one or more charge detectors within the ion trap for determining the charge of the one or more ions to be analysed, wherein a substantially secondary potential is applied to the ion trap such that ions undergo substantially harmonic motion within the ion trap.
In an embodiment, the signal obtained from the charge detection mass spectrometry apparatus may be processed using forward fitting and/or bayesian signal processing techniques. Indeed, according to another aspect, there is provided a method of charge detection mass spectrometry comprising: obtaining one or more signals from a charge detector of a charge detection mass spectrometry apparatus; and processing the one or more signals using a forward fitting and/or bayesian signal processing technique to extract charge values of one or more ions within the charge detection mass analysis device.
The ion beam may be attenuated before being passed to the charge detection mass analysis apparatus according to any of the above aspects or embodiments. In this way, the ion flux delivered into the charge detection mass analysis device can be controlled (reduced) to reduce the likelihood of more than one ion being present in a given trap during a single ion capture event. Any suitable ion beam attenuating device may be used. However, in an embodiment, the ion beam attenuation apparatus comprises a plurality of ion beam attenuators each operable to transmit substantially 100% of the ions (high transmission (or low attenuation) state) or to transmit substantially 0% of the ions (low transmission (or high attenuation) state).
Each beam attenuator may be arranged to switch alternately between a high ion transport state and a low ion transport state such that a continuous ion beam passing through the beam attenuator is effectively switched off to produce a discontinuous attenuated ion beam. The resulting attenuated ion beam may then be homogenized and converted back into a substantially continuous ion beam by passing the attenuated ion beam through a gas-filled region (such as an ion guide or generally a gas cell) in which interactions between ions and gas molecules effectively disperse the ions in a dispersive manner.
To improve attenuation, a plurality of ion beam attenuators may be arranged in series, wherein the attenuated ion beam output from each ion beam attenuator passes through a respective gas fill region(s) to produce a substantially continuous ion beam for input to the next ion beam attenuator in the series (and so on, wherein more than two ion beam attenuators are provided) to produce a plurality of attenuated outputs.
The plurality of ion beam attenuators may be arranged successively one after the other in an alternating sequence of one or more ion beam attenuators and one or more gas filled regions (gas chambers). However, other arrangements will of course be possible.
In this way, the incoming ion beam can therefore be easily attenuated as it passes through a series of ion beam attenuators to reliably produce very low fluxes. It should be understood that the ion beam attenuation apparatus may also be used in other applications and is not limited to use in combination with a charge detection mass spectrometry detection apparatus. For example, there are various applications where it may be desirable to reliably reduce ion flux. In general, the ion beam attenuation apparatus may be used in any experiment where it is desirable to controllably reduce ion flux. For example, the ion beam attenuation apparatus may be disposed upstream of any suitable ion trap to avoid overfilling the trap. A specific example thereof may be an ion trap that supplies ions to the ion mobility separation means. As another example, an ion beam attenuation device may be provided as part of the detector system (or may be disposed upstream of the detector system) to avoid detector saturation. A further example would be to control the ion flux into the reaction chamber in order to optimize the efficiency of the ion-molecule or ion-ion reaction. However, various other arrangements would of course be possible.
Thus, according to yet another aspect, there is provided an ion beam attenuation apparatus comprising: a first ion beam attenuator operable in a high ion transport mode or a low ion transport mode for selectively attenuating an ion beam, wherein an output of the first ion beam attenuator passes through a first gas-filled region; a second ion beam attenuator operable in a high ion transport mode or a low ion transport mode to selectively attenuate an ion beam; and control circuitry configured to: repeatedly switching the first ion beam attenuator between the high ion transport mode and the low ion transport mode to produce a first non-continuous ion beam at the output of the first ion beam attenuator, wherein the first non-continuous ion beam passes through the gas-filled region before reaching the second ion beam attenuator and is thereby converted to a substantially continuous ion beam; and repeatedly switching the second ion beam attenuator between the high ion transport mode and the low ion transport mode to produce a second discontinuous ion beam at an output of the second ion beam attenuator.
According to a related aspect, there is provided a method of attenuating an ion beam, the method comprising: passing the ion beam to a first ion beam attenuator and repeatedly switching the first ion beam attenuator between a high ion transport mode and a low ion transport mode to produce a first discontinuous ion beam at an output of the first ion beam attenuator; passing the first non-continuous ion beam through a gas-filled region to convert a first attenuated ion beam to a substantially continuous attenuated ion beam; passing the substantially continuous ion beam to a second ion beam attenuator, and repeatedly switching the second ion beam attenuator between a high ion transport mode and a low ion transport mode to produce a second discontinuous ion beam at an output of the second ion beam attenuator.
In an embodiment, the second non-continuous ion beam passes through the second gas-filled region and is converted to a substantially continuous attenuated ion beam. That is, the method may include passing a second attenuated ion beam through a second gas-filled region to produce a substantially continuous attenuated ion beam.
The first ion beam attenuator and/or the second ion beam attenuator may comprise one or more electrostatic lenses. The one or more electrostatic lenses may comprise one or more electrodes, wherein the state of the ion beam attenuator may be alternated by varying one or more voltages applied to the electrodes. However, other arrangements are of course possible. For example, the one or more ion beam attenuators may include a mechanical shutter or a mechanical ion beam attenuator. Alternatively, the one or more ion beam attenuators may comprise magnetic ion gates or magnetic ion beam attenuators.
The output from each ion beam attenuator may pass through the gas filled region. Typically, the gas-filled region comprises an ion guide or a gas chamber. Thus, differential pumping holes may be provided at the inlet and/or outlet of the gas-filled region.
The gas pressure within the gas fill region and the length of the gas fill region may be selected to substantially completely convert the attenuated ion beam into a continuous ion beam between each of the ion beam attenuators.
The first ion beam attenuator and the second ion beam attenuator may have the same attenuation factor (and may alternate at the same frequency). Alternatively, the first ion beam attenuator and the second ion beam attenuator may provide different attenuation factors.
When more than one ion beam attenuator is used in this manner, there may be more than one way to achieve the desired level of attenuation. For example, if it is desired to use two lenses to attenuate the intensity to 1%, the first attenuator may be set to 1% and the second attenuator to 100%, or vice versa. Alternatively, both devices may operate at an intermediate value to give a combined transmission rate of 1%. For example, both the first and second beam attenuators may be operated at 10%, or one of the beam attenuators may be operated at 20%, the other of the beam attenuators may be operated at 5%, and so on. Since the attenuation apparatus may become contaminated during prolonged use, it may be desirable to balance the attenuation evenly between the first and second ion beam attenuators, or to periodically replace the attenuator most often used for attenuation, to extend the period between maintenance, cleaning and/or replacement. Thus, in an embodiment, when it is desired to provide a target total attenuation, the method may include adjusting the relative attenuations provided by the first and second ion beam attenuators in a manner that maintains the target total attenuation.
According to a further aspect, there is provided a method of single ion charge detection mass spectrometry in which a signal is analysed in real time and used to prematurely terminate a capture event that would not produce useful data. For example, trapping events that do not accommodate ions or that have more than a maximum number of ions present may be terminated prematurely.
It will be understood that the present invention in any one of these further aspects may incorporate any or all of the features described in relation to the first and second aspects of the invention, and vice versa, at least to the extent that they are not mutually inconsistent. Those skilled in the art will also appreciate that all of the described embodiments of the invention described herein may suitably comprise any one or more or all of the features described herein.
Drawings
Various embodiments will now be described, by way of example only, and with reference to the accompanying drawings, in which:
FIG. 1 schematically illustrates a single Charge Detection Mass Spectrometry (CDMS) apparatus that may be used in embodiments;
FIG. 2 illustrates how the detector signal may vary when more than one ion is present within the ion trap of a CDMS apparatus (such as the CDMS apparatus shown in FIG. 1);
FIG. 3 shows how the rate at which good transients are obtained varies with time after which unwanted transients may be terminated;
fig. 4A and 4B illustrate how an ion beam may be attenuated;
fig. 5 schematically illustrates an ion beam attenuating device that may be used in an embodiment;
figure 6 illustrates the use of ion optics for selectively or sequentially delivering respective ions to a plurality of CDMS devices;
figure 7 shows an apparatus comprising a plurality of CDMS devices arranged within an ion guide;
figure 8 shows an example of a CDMS apparatus with multiple charge detectors within a single ion trap; and is
Fig. 9A, 9B, 9C, and 10 illustrate the operation of a SpiroTOF apparatus that can be used as an ion trap for a CDMS apparatus according to embodiments.
Detailed Description
Various embodiments relate to methods of Charge Detection Mass Spectrometry (CDMS). It will be appreciated that CDMS typically involves simultaneous measurement of the mass-to-charge ratio (m/z) and charge (z) of an ion. In this way, the mass (m) of the ions can then be (indirectly) determined. The charge of the ions can generally be measured directly using charge detection electrodes. For example, as ions are passed through (or past) the charge detection electrode, they will induce a charge on the charge detection electrode which can then be detected by, for example, suitable detection (signal processing) circuitry connected to the charge detection electrode. The mass-to-charge ratio of the ions can generally be determined in various suitable ways. For example, the mass to charge ratio can be determined from the time of flight or ion velocity (as long as the energy of each charge is known) of the ions within the CDMS apparatus. Thus, various examples of CDMS experiments are known, and it will be understood that the embodiments described herein may generally be applied to any suitable CDMS experiment as desired.
In general, however, the mass-to-charge ratio can be determined from, for example, the frequency of oscillation of ions located within the trapping field. Accordingly, a CDMS apparatus may generally comprise an ion trap within which ions to be analysed are contained. Thus, the ions are analyzed in discrete "ion trapping events". Thus, in each ion trapping event, the ion trap is opened to allow ions to enter the ion trap for analysis. At the end of an ion capture event, those ions may then be ejected and a new ion capture event initiated.
For example, in some CDMS experiments such as that described in Keifer et al, "Charge detection Mass Spectrometry with nearly perfect Charge accuracy", analytical chemistry 2015,87, 10330-. In the CDMS experiment described by Keifer et al, ions were repeatedly passed through a metal cylinder located at the centre of the ion trap, which was connected to an amplifier and a digitizer. When the ion is located at the center of the cylinder, the magnitude of the charge induced on the cylinder is equal to the charge on the ion.
Fig. 1 schematically illustrates a single CDMS apparatus according to one embodiment. As shown in fig. 1, the apparatus comprises an electrostatic ion trap in the form of a
However, other arrangements will of course be possible. Thus, although figure 1 shows a
Thus, in a well calibrated system, the amplitude of the recorded signal can be used to measure the charge on the ions. However, because of the low signal-to-noise ratio, many ion passes may typically be required for accurate charge measurements. For example, current prior art instruments are capable of producing a resolution better than the resolution of a unit charge, such that the charge on nearly all trapped ions can be accurately determined. The frequency of oscillation of ions in the trap is related to their mass-to-charge ratio. Although the signal is typically significantly non-sinusoidal, the fourier transform of the recorded transient allows the mass-to-charge ratio to be measured (although at a lower resolution). Taken together, the measurement of mass-to-charge ratio and charge allows the mass of the ions to be determined.
It will be appreciated that this method may be particularly useful for generating mass spectra of high molecular weight species (e.g. in the megadalton and above range) because conventional electrospray mass spectrometry, for example, may be difficult to interpret in such cases, as the different charge states tend to be difficult to resolve from each other. However, CDMS techniques can be relatively slow. For example, typically thousands of ion capture events may be required to establish a useful mass spectrum. Therefore, a method of shortening the time required for generating a map is particularly significant.
Various examples of the present disclosure will now be described.
Single ion selection
In some embodiments, it may be desirable to select a single ion (N ═ 1) for analysis to efficiently operate the CDMS device. According to the technique described by Kiefer et al, the average of the arrival of ions in the poisson distribution is set to one ion (over a fill period of-0.5 milliseconds). However, this means that in most cases (-63%) the filling will result in no ions (N ═ 0) or more than one ion (N > 1). When N is 0, (long) acquisition time (up to-three seconds) is wasted. Furthermore, when more than one (N >1) ion is accommodated in the ion trap, the signal may be severely contaminated due to space charge effects.
Thus, in an embodiment, the detector signal may be monitored in real time and if after a period of time (e.g., 10 milliseconds or 50 milliseconds or 100 milliseconds) the signal processing indicates that N-0 or N >1, the current acquisition may be terminated early and a new fill event started, thereby improving throughput. For example, the collection may be terminated by applying a suitable electric field to (rapidly) remove all ions from the CDMS apparatus. For example, by removing the trapping field and/or applying one or more ejection fields, ions can then be "ejected" (or otherwise removed) from the trap and lost into the system or into collisions with electrodes.
Alternatively, in other embodiments, when it is determined that N >1, the ions in the trap may be excited to eject N-1 ions (such that these ions are lost, as described above), leaving only a single ion for analysis. This may be done deterministically or additional monitoring may be performed to check that only one ion remains. It will be appreciated that ejecting ions from the trap may be advantageous compared to starting a new fill event, as in that case the success rate may be close to 100% (whereas a new fill will typically only succeed 37% — i.e. a new fill will have a-63% probability of resulting in no ions or more than one ion).
Similarly, in this way, if ions are lost during the trapping period (so that N ═ 0), for example due to scattering or unstable trajectories of residual gas, the acquisition can be terminated early, allowing a new fill event.
Thus, in contrast to more conventional methods where a fixed ion capture period is used for CDMS measurements (even if no ions are measured or there are multiple ions that compromise the signal), in embodiments, the ion capture event may be terminated early if the signal processing indicates that N ═ 0 or N > 1. Alternatively, if the signal processing indicates that N >1, the operation of the CDMS apparatus may be adjusted until N ═ 1. Thus, the CDMS apparatus can be dynamically controlled based on a determination of how many ions are present in the apparatus.
Any suitable technique may be used to monitor the detector signal. For example, in some embodiments, the real-time signal processing may consist of a series of overlapping apodized fast fourier transforms. The estimate of the number of ions present in the trap may be based, for example, on the number of masses present in the profile above a noise threshold, or the total charge detected, or a combination of these.
Embodiments for fine tuning the ion arrival rate to maximize the probability of N-1 are also contemplated. For example, in some instances, one or more Dynamic Range Enhancement (DRE) lenses may be used to control the flux of an ion beam over a wide dynamic range. For example, configurations involving multiple DRE lenses separated by a gas-filled chamber at collision chamber pressure for beam recombination can help control the flux of the ion beam over a wide dynamic range to help maximize the probability of N ═ 1 ions reaching the CDMS device.
In some embodiments, instead of exciting ions from the ion trap when it is determined that more than one ion is present, the ion trap itself may be designed such that when more than one ion is present, the ion trajectory becomes unstable, causing ejection of all but one ion. In other words, the ion trap may be designed as a so-called "leaky" single ion trap. This may be achieved, for example, using appropriately designed geometries and/or by applying one or more appropriate electric fields to the ion trap. In an embodiment, one or more of the ion traps may be of the type described in us patent No. 8,835,836 (MICROMASS), in which, once the charge capacity of the ion trap is reached, excess ions will leak or otherwise be ejected from the trap due to the forces exerted on the ions by coulomb repulsion.
Ion trap-space charge effect
Figure 2 shows a series of spectra obtained by simulating the motion and detecting two identical ions with 100eV energy in a conical trap configured for CDMS after 0.05, 0.08, 0.2 and 1 seconds, respectively. The transient is sampled at a rate of 1.25 MHz. A Fast Fourier Transform (FFT) is used to obtain a map from the original transient. The mass of the ion is 100kDa and the charge is 100, so its mass to charge ratio is 1000 Th.
Specifically, fig. 2 compares ideal data obtained when ions do not interact with each other with data obtained when actual space charge effects are considered. The ideal data is essentially the same as that obtained for a single ion and shows, as expected, a steady increase in resolution with increasing time, with the peak centered at the correct mass-to-charge ratio. On the other hand, in the case where two ions are able to interact, it can be seen that even after 0.05 seconds there is still a deviation from the correct mass to charge ratio and by 0.08 seconds the signal has split into two distinct peaks. By 0.2 seconds, the two peaks have collapsed and by the end of the transient (at 1 second), the data is completely corrupted.
By providing and analyzing these data while the transient is still in progress, it can be determined whether there is more than one ion in the trap in 0.08 seconds or even earlier. This determination may be made using statistical or bayesian model comparisons (comparing the probability of one peak being present with the probability of two peaks or more) or hypothesis testing, or by simply counting peaks in a smoothed version of the map, or by measuring the full width of the map at the maximum intensity score compared to the expected width of a single peak, or by various other possible methods.
In this case, terminating the capture after 0.2 seconds (allowing 120 milliseconds for data processing) saves 0.8 seconds of wasted acquisition time, since the entire transient is 1 second in length.
Thus, figure 2 shows that it is possible to identify very quickly when the ion trap contains more than one ion, to allow for early termination of transients, or to allow for control of the ion trap to eject one or more ions. It is clear that this situation can also be recognized very quickly when no signal is present, in which case the transient can also be terminated early.
More generally, if the total transient time is TLAnd the transient is at time T in the event that it contains no ions or more than one ionSEnding later, the rate at which good transients are obtained is:
where λ is the average number of ions that enter the trap during the trap fill period. Regardless of TLAnd TSHow value of (a) is, when λ ═ 1, RGood effectIs maximized and therefore the intensity of the ion beam supplying the trap should be optimized to achieve this rate as much as possible. For a value of 1, for a lambda,
FIG. 3 shows for TLA fixed value of 1 andTSvariation of RGood effectHow to vary. For TS0.2, with RGood effectA rate of 0.74, which is the inability to terminate undesirable transients prematurely (i.e., T), yields good single ion transientsS＝TL1) or more than twice the rate obtained.
High dynamic range ion beam attenuation
As described above, embodiments are contemplated for controlling the flux of an ion beam in real time over a wide dynamic range to help maximize the probability of N-1 ions reaching a CDMS apparatus. It should be appreciated, however, that there are many situations where it is desirable to reduce the intensity of an ion beam in a controlled, quantitative, unbiased manner. That is, the degree of attenuation should not depend on m/z, ion mobility, propensity for fragment or charge reduction, or any other ion characteristic within the relevant range of each property.
For example, it may be desirable to avoid undesirable problems caused by high ion fluxes, including overfilling of traps (including those used in ion mobility experiments) (leading to loss of run and bias of ions or undesirable fragmentation), space charge effects, detector saturation (leading to loss of quantitative accuracy, mass accuracy and artificial peaks), and charging of instrument internal surfaces, which leads to further loss of ions or distortion of the forward-transported ion beam in a range of applications including, but not limited to, generating controlled low ion fluxes for use in experiments involving single ions or small numbers of ions (such as CDMS).
After the beam has been attenuated in a quantitative and unbiased manner, many of the properties of the ideal signal obtained from the original unattenuated beam can generally be recovered by simply rescaling or otherwise adjusting the data produced by the instrument in question (e.g., the intensity of the mass spectral peak produced by the mass spectrometer).
The degree of attenuation may be constant over the duration of the experiment, or may vary in a predetermined manner, or in response to information obtained from data already acquired during the experiment (in a data-dependent manner).
Beam attenuation can also result in the loss of small signals that fall below the detection threshold after attenuation. For this reason, the instrument may alternate between two or more modes of operation with different degrees of attenuation. The final combined data set may then be reconstructed from the two or more data sets by taking the small signal from the less attenuated data and the larger signal from the more attenuated data.
U.S. patent No. 7,683,314 (MICROMASS) discloses a method of attenuating an ion beam by transmitting a transmission rate therein of substantially 100% (for time Δ Τ)2) And a mode in which the transmission rate is substantially 0% (for a time deltat)1) Operate alternately between modes. This may be achieved, for example, by alternating the deceleration voltage to repeatedly switch the ion beam between the two states.
Fig. 4A shows the ideal beam intensity versus time after this attenuation step. Since the resulting beam is discontinuous or cut off, this device, located upstream of the ion guide or gas collision cell, can be operated so as to convert it into a fraction Δ Τ that has been reduced to its original intensity2/ΔT1As shown in fig. 4B.
However, since a finite time is inevitably required for the ion beam to respond fully to the voltage change expected to switch between the on-state and the off-state, the duration Δ T when the on-state occurs2When it becomes too short, there is not enough time to recover 100% of the transmission before the next voltage change and the decay is no longer linear or quantitative. On the other hand, at the time interval Δ T1It is no longer possible to restore the beam to a substantially continuous beam when it becomes comparable in time to the passage through the downstream gas cell or ion guide.
This means that there is a practical limit to the quantitative degree of attenuation that can be achieved with this device (e.g. to 1% of the original intensity in a typical device).
According to an embodiment of the present disclosure, there is provided an attenuation method using two attenuation devices of the above-mentioned type, separated by a gas cell or ion guide designed to convert an ion beam into a substantially continuous beam.
Fig. 5 shows an example of an attenuation device according to an embodiment. As shown, the device comprises a
The
If each of the first and
It is obvious thatThe concept can be extended to include more than two devices separated by an ion guide or gas collision cell designed to produce a substantially continuous beam. For example, when N devices each individually capable of reducing an ion beam to a fraction p of its original intensity are combined in this manner, the fraction p of the original beam intensity can be quantitatively achievedN. This power-law behavior means that extremely high attenuation factors can be achieved quantitatively with relatively few devices. This may be required, for example, to achieve the low ion arrival rates required to obtain a high probability of filling the trap with a single ion.
In practice, the attenuating device or the associated gas cell does not have to be arranged continuously in the apparatus. The attenuation device or associated gas chamber may be separated by other means such as a reaction chamber, mass filter, ion transfer device, etc. Each of these additional devices may be used for several purposes or operate in several different modes, and may be configured to react, fragment, or filter ions, or (possibly simultaneously) convert a pulsed ion beam into a substantially continuous ion beam.
In addition, one or the other or both of the attenuation means may be operated continuously in a full transmission mode, wherein attenuation is only activated as required.
Space charge tolerance of wells
In an embodiment, it may be desirable for a CDMS apparatus to be able to analyze multiple ions simultaneously to increase throughput. However, as noted above, in the case of conventional CDMS apparatus such as the CDMS apparatus described by Kiefer et al, space charge effects can significantly affect performance when more than one ion is present in the ion trap.
Accordingly, in some embodiments, it is contemplated that the CDMS apparatus may include multiple ion traps. For example, a CDMS apparatus may comprise a plurality of parallel ion traps arranged to receive a plurality of ions from an upstream apparatus, each ion trap having an associated one or more charge detection electrodes. In this example, a plurality of ions from an upstream device may be shared between a plurality of ion traps using suitable ion optics (e.g., ion lenses or beam splitting devices). Thus, the system may be arranged such that (single) ions are sequentially or selectively transferred to one of a plurality of different ion traps.
An example of such an arrangement is shown in fig. 6, in which two CDMS devices of the general type shown in fig. 1 are arranged in parallel, and in which a plasma
As another example, a CDMS apparatus may comprise a series of "leaky" ion traps, wherein each ion trap has a geometry configured such that the trajectory becomes unstable when more than one ion is present. In this case, if the ions are properly confined within the CDMS apparatus, the ions will naturally distribute themselves along a series of traps due to space charge effects. Thus, a series of ion traps may be housed within a plasma guide such as a stacked ring ion guide.
An example of such an arrangement is shown in figure 7, in which two CDMS apparatus 72, 74 of the general type shown in figure 1 are formed within a single ion guide 70, with the electrodes of the ion guide thus providing the CDMS apparatus with an ion trap and charge detector. For example, suitable RF and/or DC potentials may then be applied to the electrodes of the ion guide 70 in order to confine ions (radially) within the ion guide 70, and also to define one or more axial trapping regions along the length of the ion guide, with the electrodes being located in the centre of the one or more trapping regions, so that a charge detector is then provided to perform CDMS measurements. Accordingly, ions may be injected into the ion guide 70 and may be allowed to naturally distribute between the ion trapping regions defining the CDMS arrangements 72, 74. CDMS measurements may then be performed in each CDMS apparatus 72, 74 in parallel, prior to ejection of ions from each of the ion traps (and ion guide 70). Although fig. 7 shows only two CDMS devices 72, 74, it should be understood that any number of CDMS devices may be used in this arrangement.
In these embodiments, each of the ion traps within the CDMS apparatus may be arranged to analyse only a single ion. For example, N ion traps (where N >1) may be provided to analyze N ions.
However, embodiments are also envisaged in which multiple ions (N >1) are analysed within a single ion trap. For example, if it can be arranged such that the trajectories diverge (fan out) outside the region of the charge detector electrodes, the capacity of the ion trap can be increased beyond a single ion (while still providing sufficient signal quality). For example, in three dimensions, the trajectories may occupy a "dumbbell" (or rotating "H") shape. In this case, when the ions are moving slowly, the ions will tend to separate furthest and, therefore, the space charge effect will be reduced. Thus, in an embodiment, a plurality of ions (N >1) may be analysed simultaneously, with their ion trajectories arranged to diverge outside the region of the charge detector electrodes.
Alternatively or additionally, the ion trap may be extended to accommodate more than one charge detection electrode. For example, the ions may be caused to take a folded flight path, such as a trajectory within an ion trap, in which the ions are caused to pass back and forth repeatedly between two reflective electrodes, for example in a multi-pass operation, so as to follow a substantially zig-zag or "W" shaped path. Charge detection electrodes may then be periodically placed along the folded flight path (e.g., instead of periodic focusing elements that may be found within the folded flight path instrument). Thus, each ion may pass through each of the plurality of charge detection electrodes (so that multiple measurements may be made for each ion, potentially improving signal quality). As another example, instead of using a folded flight path type geometry, a multi-detector configuration can be circled in a circle to give a cyclic CDMS apparatus with multiple charge detection electrodes. The signals from each charge detection electrode may be analysed separately or, if more convenient, some of the signals may be coupled electronically and the combined signal deconvolved in post-processing.
As yet another example, the device may be linear or circular, with no orthogonal trapping and a number of charge detection electrodes arranged along the flight path (e.g., in a manner similar to ion velocity fourier transform mass spectrometry techniques).
For example, fig. 8 shows an example of a CDMS apparatus in which a plurality of independent charge detection electrodes are disposed within a single
Improved trajectories for higher resolution or faster operation
The applicants have further recognised that the use of near-quadratic potentials within an ion trap can improve the energy tolerance of the device, for example, because ions of the same mass to charge ratio but of different energies will produce signals having more similar (or substantially the same) shapes. More harmonic (sinusoidal) signals can produce a sharper pattern (with reduced harmonics). Thus, in an embodiment, substantially second order potentials are used to confine ions within the ion trap such that the ions experience substantially harmonic motion within the ion trap (and through the one or more charge detector electrodes). In this case, the charge detector electrode may be positioned at substantially the center of the secondary potential. However, other arrangements will of course be possible.
Various existing geometries with appropriate substantially secondary potentials may be utilized. For example, it is contemplated that an Orbitrap-type device or a SpiroTOF device may be used (e.g., as described in U.S. patent No. 9,721,779 (MICROMASS) or U.S. patent application publication No. 2017/0032951 (MICROMASS)). Devices with a central electrode, particularly Orbitrap, have relatively high space charge tolerance.
Fig. 9A, 9B, 9C, and 10 illustrate the operation of a SpiroTOF apparatus that can be used as an ion trap for a CDMS apparatus according to embodiments. As shown in fig. 9A, ions are injected into an annular region defined between an
A substantially second order axial potential may then be established along the device to cause the ions to begin to oscillate axially with substantially simple harmonic motion, as shown in figure 10. The conditions may be chosen such that the trajectory remains circular (as shown in fig. 10), or the ions may be allowed to oscillate radially (by applying some radial excitation during initial acceleration). The charge detector 1100 may then be positioned within the device, for example in the center of the device, such that ions repeatedly pass near the detector electrodes to generate a signal. The charge detector 1100 may include one or more of the segments selected from existing electrodes used for substantially quadrupolar-logarithmic potentials (quadruporo-logarithmic potentials) in a stationary device, or they may be additional electrodes having geometries and voltages designed to minimize perturbations to the potentials.
An advantage of this arrangement is that even for small numbers of ions, the average initial separation between ions can be increased by beam expansion during initial implantation, thereby reducing space charge effects. In addition, the
However, other arrangements will of course be possible. For example, an Orbitrap type geometry using a substantially quadrupole-logarithmic potential may also provide similar advantages. This may also be the case, for example, for cassini orbits such as those described in us patent No. 8,735,812 (BRUKER dalton GMBH), depending on the trajectory chosen.
Signal processing
It is well known that the use of fourier transform processing on non-harmonic signals produces artifact "harmonics". However, in embodiments, forward fitting/bayesian signal processing using one or more model peak shapes may be used. This can significantly reduce the intensity of harmonics and improve the signal-to-noise ratio of the inferred spectra. This, in turn, can provide a higher quality resolution in a fixed time (or similarly achieve the same resolution in a shorter time). For example, the applicants have recognized that similar techniques, such as those described in U.S. patent application publication No. 2016/0282305 (MICROMASS), for processing ion mobility data may also be advantageously used to process CDMS signals obtained in accordance with the various embodiments described herein. For example, by using similar such techniques, charge values may be extracted from the fitted magnitudes in embodiments. Such signal processing methods may therefore be able to extract high mass spectra from trapping events containing more than one ion, particularly if the space charge limitations are reduced.
While the present invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the scope of the invention as set forth in the following claims.