Patent Application
20230207302
The present invention relates to a time-of-flight, TOF, mass spectrometer, MS and a method of controlling a TOF MS.
Conventional time-of-flight, TOF, mass spectrometers, MS, coupled to laser desorption-ionization, LDI, sources typically employ two-stage acceleration configurations. A time delay is introduced between desorption-ionization and subsequent acceleration of ions towards a detector in what is also known as delayed pulsed extraction. This time-delay method is used to introduce a spatial spread and consequently, create a potential energy difference between ions having the same m/z ratio but having different initial velocities, therefore permitting isochronous arrival of these ions at the detector plane. This time-delay method may be considered an extension of an earlier method used in electron ionization TOF mass spectrometry for minimizing the adverse effects of turn-around time on mass resolving power. However, delayed pulsed extraction is strongly mass dependent and different time-delays or pulsed-extraction voltages are required to bring ions having different m/z ratios into focus at the detector. Hence, various time-dependent acceleration schemes for enhancing mass resolving power over extended m/z ranges and/or improving performance and/or utility of matrix-assisted LDI, MALDI, TOF MS, for example, have been described.
However, there remains a need to improve mass resolving power, for example over extended m/z ranges, for TOF MS, for example MALDI TOF MS.
It is one aim of the present invention, amongst others, to provide a time-of-flight, TOF, mass spectrometer, MS and a method of controlling a TOF MS which at least partially obviates or mitigates at least some of the disadvantages of the prior art, whether identified herein or elsewhere. For instance, it is an aim of embodiments of the invention to provide a TOF MS having an improved mass resolving power, for example over an extended m/z range. For instance, it is an aim of embodiments of the invention to provide a method of controlling a TOF MS that provides enhanced time-focusing of ions having the same m/z ratio but having different initial velocities over an extended m/z range.
A first aspect provides a time-of-flight, TOF, mass spectrometer, MS, comprising:
an ion source for supplying a group of ions, including a first ion having a first mass-to-charge ratio m1/z1, a second ion having a second mass-to-charge ratio m2/z2 and a third ion having a third mass-to-charge ratio m3/z3 wherein m3/z3>m2/z2>m1/z1, at a time t0;
a first set of electrodes, including a first electrode, and a second set of electrodes, including a first electrode and an Nth electrode, wherein the first set of electrodes and the second set of electrodes are mutually spaced apart by a gap therebetween;
an ion detector for detecting the ions;
a set of power supplies, including a first power supply, electrically coupled to the first set of electrodes and to the second set of electrodes; and
a controller configured to control the set of power supplies to apply respective potentials to the first set of electrodes and the second set of electrodes;
wherein the controller is configured to control the set of power supplies to:
provide a first substantially field-free region between the ion source and the first set of electrodes to allow the group of ions to expand theretowards and/or therein, at the time t0;
apply an extraction potential Vextraction to the first set of electrodes at a time textraction>t0, to extract the expanded group of ions, while maintaining a second substantially field-free region beyond the first set of electrodes, in the gap between the first set of electrodes and the second set of electrodes; and
optionally, change an acceleration potential Vacceleration applied to the second set of electrodes during a time period Δt=toff−ton, wherein ton>textraction, to vary acceleration of the extracted group of ions based, at least in part, on respective mass-to-charge ratios.
A second aspect provides a method of controlling a time-of-flight, TOF, mass spectrometer, MS, the method comprising:
supplying a group of ions, including a first ion having a first mass-to-charge ratio m1/z1, a second ion having a second mass-to-charge ratio m2/z2 and a third ion having a third mass-to-charge ratio m3/z3 wherein m3/z3>m2/z2>m1/z1, from an ion source at a time t0 and allowing the group of ions to expand towards and/or into a first substantially field-free region between the ion source and a first set of electrodes, including a first electrode;
applying an extraction potential Vextraction to the first set of electrodes at a time textraction>t0, to extract the expanded group of ions, while maintaining a second substantially field-free region beyond the first set of electrodes, in a gap between the first set of electrodes and a second set of electrodes, including a first electrode and an Nth electrode, wherein the first set of electrodes and the second set of electrodes are mutually spaced apart by the gap;
changing an acceleration potential Vacceleration applied to the second set of electrodes during a time period Δt=toff−ton, wherein ton>textraction, to vary acceleration of the extracted group of ions based, at least in part, on respective mass-to-charge ratios; and
detecting the ions.
A third aspect provides a computer comprising a processor and a memory configured to implement, at least in part, a method according to the second aspect.
A fourth aspect provides a computer program comprising instructions which, when executed by a computer comprising a processor and a memory, cause the computer to perform, at least in part, a method according to the second aspect.
A fifth aspect provides a non-transient computer-readable storage medium comprising instructions which, when executed by a computer comprising a processor and a memory, cause the computer to perform, at least in part, a method according to the second aspect.
According to the present invention there is provided a TOF MS, as set forth in the appended claims. Also provided is a method of controlling a TOF MS. Other features of the invention will be apparent from the dependent claims, and the description that follows.
TOF MS
The first aspect provides a time-of-flight, TOF, mass spectrometer, MS, comprising:
an ion source for supplying a group of ions, including a first ion having a first mass-to-charge ratio m1/z1, a second ion having a second mass-to-charge ratio m2/z2 and a third ion having a third mass-to-charge ratio m3/z3 wherein m3/z3>m2/z2>m1/z1, at a time t0;
a first set of electrodes, including a first electrode, and a second set of electrodes, including a first electrode and an Nth electrode, wherein the first set of electrodes and the second set of electrodes are mutually spaced apart by a gap therebetween;
an ion detector for detecting the ions;
a set of power supplies, including a first power supply, electrically coupled to the first set of electrodes and to the second set of electrodes; and
a controller configured to control the set of power supplies to apply respective potentials to the first set of electrodes and the second set of electrodes;
wherein the controller is configured to control the set of power supplies to:
provide a first substantially field-free region between the ion source and the first set of electrodes to allow the group of ions to expand theretowards and/or therein, at the time t0;
apply an extraction potential Vextraction to the first set of electrodes at a time textraction>t0, to extract the expanded group of ions, while maintaining a second substantially field-free region beyond the first set of electrodes, in the gap between the first set of electrodes and the second set of electrodes; and
optionally, change an acceleration potential Vacceleration applied to the second set of electrodes during a time period Δt=toff−ton, wherein ton>textraction, to vary acceleration of the extracted group of ions based, at least in part, on respective mass-to-charge ratios.
Hence, the ions initially (i.e. between the time t0 and the time textraction>t0) expand towards and/or into the first substantially field-free region, between the ion source and the first set of electrodes, during which equilibration of the ions takes place. At the time textraction>t0, the extraction potential Vextraction is applied to the first set of electrodes, thereby extracting the expanded group of ions from the first substantially field-free region. Thus, the first set of electrodes defines a first ion acceleration stage, for accelerating the ions from the ion source theretowards and/or therethrough. Importantly, the extraction potential Vextraction is applied to the first set of electrodes while maintaining the second substantially field-free region beyond the first set of electrodes, in the gap between the first set of electrodes and the second set of electrodes. By maintaining the second substantially field-free region beyond the first set of electrodes while applying the extraction potential Vextraction, penetration of the electric field in due to the second set of electrodes is attenuated, minimised or even eliminated, thereby reducing, avoiding or even preventing otherwise prompt acceleration of the ions theretowards and/or diminishing or eliminating distortion of phase space during the time period tdelay=textraction−t0 (i.e. a time delay), prior to the application of the extraction potential Vextraction. Additionally and/or alternatively, penetration of an electric field due to the ion source, for example a sample plate thereof, theretowards is also attenuated, minimised or even eliminated by the gap. Such prompt acceleration of the ions and/or distortion of the phase space otherwise further broadens a spatial distribution of the group of ions and/or a distribution of velocities of the group of ions. Hence, further broadening of the spatial distribution of the group of ions and/or the distribution of velocities of the group of ions is lessened, for example eliminated, by maintaining the second substantially field-free region beyond the first set of electrodes while applying the extraction potential Vextraction. Particularly, elimination of such prompt acceleration allows for a more precise correlation between initial ion position and initial velocity to be established at the onset of application of the extraction potential Vextraction, which has a strong effect on mass resolving power. Thus, by constraining the spatial distribution of the group of ions and/or the distribution of velocities of the group of ions, the mass resolution is thus improved. Subsequently, during the time period Δt=toff−ton, wherein ton>textraction, the acceleration potential Vacceleration applied to the second set of electrodes is optionally changed to vary acceleration of the extracted group of ions based, at least in part, on respective mass-to-charge ratios (i.e. of the ions). Thus, the second set of electrodes defines a second ion acceleration stage for accelerating the ions from the first set of electrodes theretowards and/or therethrough, for example towards the ion detector. By changing the acceleration potential Vacceleration applied to the second set of electrodes during the time period Δt=toff−ton to vary acceleration of the extracted group of ions based, at least in part, on respective mass-to-charge ratios, ions having relatively higher mass-to-charge ratios, for example, and traversing through the second set of electrodes at relatively later times are accelerated by a relatively changed, for example an increased, accelerating field due to the second set of electrodes compared with ions having relatively lower mass-to-charge ratios, for example, and traversing through the second set of electrodes at relatively earlier times. In this way, the relatively slower third ion, having the third mass-to-charge ratio m3/z3, is subject to an increased accelerating field, for example, compared with the relatively faster first ion having the first mass-to-charge ratio m1/z1. Particularly, ions having the same mass-to-charge ratio m/Z but different initial ion energies and hence velocities are similarly subject to different accelerating fields, thereby more effectively correcting for the initial ion energy spread and thus improving mass resolution. Particularly, in this way, time focusing of ions having the same mass-to-charge ratio m/Z but different initial ion energies is achieved.
In more detail, the inventors have identified a novel ion optical acceleration scheme for time-of-flight mass spectrometry of laser-produced ions from a solid target, for example, achieving enhanced time-focusing over an extended m/z range. The ion optical acceleration scheme involves a multiple stage acceleration configuration comprising a time-delay introduced between desorption-ionization, for example, and an extraction pulse applied across a first acceleration stage to transfer ions through a field-free gap into a second acceleration stage supplied with a time-dependent voltage ramp whereby heavier ions traversing the second acceleration stage at later times experience a quasi-linear, most preferably a linear, increase in the magnitude of the accelerating field, for example.
The ion optical acceleration scheme provides advantages over conventional acceleration configurations, as demonstrated using a new set of analytical equations, numerical analysis tools such as simulations and experimental measurements.
The prevention of electric field penetration, for example axial and/or radial field penetration, of the second accelerating stage into the first pulsed extraction stage, to eliminate prompt acceleration of ions and distortion of phase space during the time-delay and prior to the application of the extraction pulse, is accomplished by introducing a short intermediate field-free gap. Elimination of the prompt acceleration effect allows for a more precise correlation between initial ion position and initial velocity to be established at the onset of the extraction voltage pulse, which has a strong effect on mass resolving power. The short intermediate field-free gap also allows for using electrodes with increased size apertures, enhancing transmission of heavier ions with considerably wider initial kinetic energy spreads, while also minimizing the amount of material deposited on critical surfaces, especially those in the desorption-ionization region, extending the operational lifetime of the system.
Furthermore, the short intermediate field-free gap created between the two consecutive electrodes (i.e. between the first set of electrodes and the second set of electrodes) decouples the application of the extraction voltage pulse (i.e. the extraction potential Vextraction) to the first set of electrodes and the application of the high voltage ramp (i.e. the acceleration potential Vacceleration) to the second set of electrodes. By decoupling the application of these potentials such that their application is mutually independent, a complexity of analogue electronics design, for example, is considerably reduced. For example, the extraction pulse is applied to the entrance electrode of the field-free gap while the voltage ramp is applied independently to the electrode defining the exit end of the field-free gap while both the extraction pulse and the voltage ramp may be produced with high integrity and/or stability. In other words, respective reproducibilities of extraction pulses and voltage ramps may be improved, thereby reducing mass resolution aberrations otherwise due to pulse to pulse and/or ramp to ramp variations.
In addition and in further contrast to conventional ion optical schemes where a post acceleration stage is be coupled to a two-stage pulsed extraction scheme through an elongated field free region, for example, a preferred example comprises a single stage pulsed-extraction region closely coupled with a consecutive field-free gap, which is capable of reducing the time difference between ions over an extended m/z range being transferred into the second acceleration stage where the ramp potential can be applied more effectively to correct for the initial energy spread.
The inventors have produced a new set of analytical equations, as detailed below, to optimize the new acceleration scheme numerically and further validate results by ion optical simulations using linear and quasi-linear ramped potentials created experimentally.
Delayed Extraction
Conventionally, delayed extraction is used for TOF MS to improve mass resolution. A plume (also known as a packer or a group) of ions is generated, for example by pulsed laser desorption/ionization from a flat surface of target plate or pulsed electron ionization or resonance enhanced multiphoton ionization in a narrow space between two plates of the ion extraction system, during a short pulse of typically a few nanoseconds. Once generated, the plume is allowed to expand for about 50 ns to 100 ns before extraction is initiated. Otherwise, the ions are extracted through the ‘dense’ cloud of non-ionised material that is also generated, that will scatter the ions of interest and thus degrade resolution. Ion equilibration in the plasma plume occurs within about 100 nanoseconds, after which most ions (irrespective of their mass) initially move with an average velocity, having a distribution, in the direction of extraction. To compensate for this distribution in average velocity and thereby improve mass resolution, extraction of the ions towards the flight tube is delayed by typically a few hundred nanoseconds to a few microseconds, typically 200 ns to 500 ns. This is referred to as ‘time-lag focusing’ for ionization of atoms or molecules by resonance enhanced multiphoton ionization or by electron impact ionization in a rarefied gas and as ‘delayed extraction’ for ions produced generally by laser desorption/ionization, for example of molecules adsorbed on flat surfaces or microcrystals placed on conductive flat surface. The extraction delay can produce TOF compensation for ion energy spread and hence improve mass resolution.
Delayed extraction is conventionally used with MALDI or laser desorption/ionization (LDI) ion sources where the ions to be analyzed are produced in an expanding plume moving from the sample plate with a high speed (400-1000 m/s). Since the thickness of the ion packets arriving at the detector is important to mass resolution, on first inspection it can appear counter-intuitive to allow the ion plume to further expand before extraction. Delayed extraction is more of a compensation for the initial momentum of the ions: it provides the same arrival times at the detector for ions with the same mass-to-charge ratios but with different initial velocities.
In delayed extraction of ions produced in vacuum, such as in LDI or MALDI sources for example at a few milliTorr, the ions having relatively lower forward momentum (i.e. in the direction of extraction) are initially accelerated at a relatively higher potential since they are relatively further from the extraction plate when the accelerating extraction field is turned on. Conversely, those ions having relatively greater forward momentum are initially accelerated at a relatively lower potential since they are relatively closer to the extraction plate. Hence, at the exit from the acceleration region, those ions, having a specific m/z ratio, having initially relatively lower forward momentum at the back of the plume are accelerated to greater velocities than those ions, having the same specific m/z ratio, having initially relatively higher forward momentum at the front of the plume. Thus, after delayed extraction, the ions, having the same specific m/z ratio, that exit the ion source relatively earlier have relatively lower velocities in the direction of the acceleration compared with those ions, having the specific m/z ratio, that exit the ion source relatively later. If ion source parameters, particularly the time delay, are properly adjusted, these relatively faster ions catch up with these relatively slower ions at the ion detector, which thus detects relatively more simultaneous arrival of the ions having the same specific m/z ratio. That is, ions having the same m/z ratio effectively drift through the flight tube to the detector in the same time, despite having different initial forward momentum. In its way, the delayed application of the acceleration field acts as a one-dimensional time-of-flight focusing element.
Nevertheless, delayed extraction requires proper adjustment of the ion source parameters, particularly the time delay, to produce TOF compensation for ion energy spread and hence improve mass resolution. Thus, conventional implementations of delayed extraction are still combined with additional methods of improving mass resolution. For example, orthogonal acceleration, OA, TOF MS effectively reduces the average velocity distribution by collisional cooling and extracting the cooled ions orthogonally from the cooled ion beam. For example, reflectron TOF MS uses a constant electrostatic field to reflect the ions back towards the ion detector: more energetic ions, having a specific m/z ratio, penetrate relatively deeper into the reflectron and thus take a relatively longer path to the ion detector than less energetic ions such that the ion detector thus detects relatively more simultaneous arrival of the ions having the same specific m/z ratio. Thus, complexity, cost and size of the TOF MS is increased by these additional methods. Hence, there remains a need to improve delayed extraction that improves mass resolution, that is more robust to adjustment of the ion source parameters, particularly the time delay, and/or that does not require combination with additional methods of improving mass resolution. Particularly, there remains a need to improve mass resolution of linear TOF MS, for example for an extended m/z range of interest.
Theory
A set of analytical equations is developed below to describe ion motion in accelerating electric fields with linearly varying voltage ramps. A force exerted on a charged particle, expressed as a rate of change in momentum, dP/dt, is proportional to the product of charge q of the charged particle and an electric field intensity E of the accelerating electric field. A voltage V0 applied initially to an entrance electrode of the second stage of acceleration increases over time at a rate r, measured in units Vs−1, to a final value V. The thus ramped electric field is established across the second accelerating region having a length d, as defined by Equations (1)-(3):
Substituting and solving for the time dependent momentum gives Equation (4):
p(t)−p(0)=∫otq(V0+rt)dt
Integrating and rearranging Equation (4) gives Equation (5):
Equation (5) may be expressed in terms of potential energy U, where m is the mass of the charged particle, as shown by Equation (6):
The equation of ion motion of the charged particle may be obtained by integration of Equation (6), giving Equation (7):
Expressing distance as a function of time x(t) and substituting into Equation (7) gives Equation (8):
where U(0) is the ion potential energy (of the charged particle) at the entrance of the second accelerating field, d. The equation of ion motion is then used to optimize the three-stage acceleration configuration with the field-free gap in-between the first pulsed extraction region and the second acceleration region supplied with the voltage ramp.
TOF MS
The first aspect provides the TOF MS. In one example, the TOF MS comprises and/or is a linear TOF MS, for example having a linear flight tube arranged between the second set of electrodes and the detector. In one example, the TOF MS comprises and/or is a reflectron TOF MS, for example having a reflectron arranged between the second set of electrodes and the detector.
Ion Source
The TOF MS comprises the ion source. In one example, the ion source comprises and/or is a pulsed ion source, for example a pulsed laser ion source. In one example, the ion source comprises and/or is a LDI ion source, preferably a pulsed LDI ion source, for example a MALDI ion source or a surface assisted laser desorption/ionization SALDI, source. In one example, the ion source comprises and/or is laser ablation electrospray ionization, LAESI, source, a pulsed electron ionization and/or a resonance enhanced multiphoton ionization source. In one example, the pulsed ion source has a pulse duration in a range from 0.1 ns to 50 ns, preferably in a range from 0.5 ns to 20 ns, more preferably in a range from 1 ns to 5 ns. Generally, reducing the pulse duration is preferable since spread of start times is reduced, as understood by the skilled person, while increasing pulse homogeneity and/or reproducibility further improves mass resolution. In one example, the pulsed laser ion source has a wavelength in a range from 266 to 355 nm (i.e. ultraviolet).
In one example, the ion source comprises, in use, a sample plate, for example a LDI sample plate such as a MALDI sample plate or a laser ablation sample plate. It should be understood that generally, a sample plate is inserted into the TOF MS for mass spectrometry of a sample thereon. That is, sample plate is not only permanently installed in the TOF MS.
The ion source is for supplying (i.e. in use) the group of ions, including a first ion having a first mass-to-charge ratio m1/z1, a second ion having a second mass-to-charge ratio m2/z2 and a third ion having a third mass-to-charge ratio m3/z3 wherein m3/z3>m2/z2>m1/z1, at a time t0. In use, the group of ions forms a plume, for example upon MALDI of a sample. It should be understood that supplying the group of ions at the time t0 is not instantaneous but instead during a relatively short duration, for example during the pulse duration, as described above. However, this relatively short duration is short compared with durations for ion equilibration, a time period tdelay=textraction−t0, the time period Δt=toff−ton during which the acceleration potential Vacceleration applied to the second set of electrodes is optionally changed and/or a flight time of the ions.
First Set of Electrodes
The TOF MS comprises the first set of electrodes, including the first electrode. The first set of electrodes defines the first ion acceleration stage, as described above, for accelerating the ions from the ion source, for example from a sample plate, theretowards and/or therethrough. The first ion acceleration stage may be thus defined between the sample plate (i.e. of the ion source, the ion source) and the first set of electrodes. It should be understood that each electrode, for example the first electrode, comprises a respective ion aperture (also known as a passageway) therethrough for passage of ions therethrough. It should be understood that these respective ion apertures are linearly aligned, for example defining a first axis therethrough. In one example, the first electrode comprises a plate, having an ion aperture therethrough.
In one example, the first set of electrodes includes M electrodes, including the first electrode, wherein M is a natural number greater than or equal to 1, for example 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or more, wherein the M electrodes are mutually spaced apart, preferably mutually equispaced apart. By increasing M, a homogeneity and/or a linearity of the first ion acceleration stage may be improved. Conversely, by decreasing M, a complexity and/or a size of the first ion acceleration stage may be reduced. In one example, the first set of electrodes consists of the first electrode i.e. M is equal to 1. In this way, the first electrode thus provides one end of the first substantially field-free region, defines the first ion acceleration stage and provides one end of the second substantially field-free region therebeyond, noting that the first substantially field-free region, the first ion acceleration stage and the second substantially field-free region are provided at different times. That is, by controlling potentials applied to just the first electrode, the first electrode may be provide, at least in part, the first and second substantially field-free regions and the first ion acceleration stage. In this way, a complexity and/or a size of the first ion acceleration stage may be reduced. In one example, the first electrode comprises a plate or a ring, having an ion aperture therethrough, having a width in a range from 10 μm to 2 mm, preferably in a range from 100 μm to 1 mm. In one example, the first set of electrodes is provided, for example by metallization, in the bore of an electrically insulating tube or a pipe, having a ion aperture (i.e. the bore) therethrough, the first electrode having a width in a range from 10 μm to 2 mm, preferably in a range from 100 μm to 1 mm. The M electrodes may be as described with respect to the first electrode.
In one example, a diameter D of the first electrode and/or of the Mth electrode of the first set of electrodes is at least twice a length g of the gap, wherein D≥2 g and preferably D≥3 g. In this way, radial field penetration from the second set of electrodes into the second substantially field-free region is reduced.
Second Set of Electrodes
The TOF MS comprises the second set of electrodes, including the first electrode and the Nth electrode. The second set of electrodes defines the second ion acceleration stage for accelerating the ions from the first set of electrodes theretowards and/or therethrough. It should be understood that the first electrode and the Nth electrode of the second set of electrodes are mutually spaced apart and arranged at mutually opposed ends of the second set of electrodes. It should be understood that each electrode, for example the first electrode and the Nth electrode, comprises a respective ion aperture (also known as an aperture) therethrough for passage of ions therethrough. It should be understood that these respective ion apertures are linearly aligned, for example defining a second axis therethrough. In one example, the first axis and the second axis are coaxial.
In one example, the second set of electrodes includes N electrodes, including the first electrode and the Nth electrode, wherein N is a natural number greater than or equal to 2, for example 2, 3, 4, 5, 6, 7, 8, 9, 10 or more, wherein the N electrodes are mutually spaced apart, preferably mutually equispaced apart. By increasing N, a homogeneity and/or a linearity of the first ion acceleration stage may be improved. In one example, the first electrode comprises a plate or a ring, having a ion aperture therethrough, having a width in a range from 10 μm to 2 mm, preferably in a range from 100 μm to 1 mm. In one example, the second set of electrodes is provided, for example by metallization, in the bore of an electrically insulating tube or a pipe, having an ion aperture (i.e. the bore) therethrough, the first electrode having a width in a range from 10 μm to 2 mm, preferably in a range from 100 μm to 1 mm. The N electrodes may be as described with respect to the first electrode.
In one example, a diameter D of the first electrode of the second set of electrodes is at least twice a length g of the gap, wherein D≥2 g and preferably D≥3 g. In this way, radial field penetration from the second set of electrodes into the second substantially field-free region is reduced.
Gap
The first set of electrodes and the second set of electrodes are mutually spaced apart by the gap therebetween. It should be understood that the gap is thus a void comprising at most a gas preferably at a relatively low pressure, for example at a high vacuum for example at an operating pressure of at most 5×10−5 mbar, preferably of at most 5×10−6 mbar, through which the ions traverse from the first set of electrodes to the second set of electrodes i.e. from the first stage of acceleration to the second stage of acceleration. In one example, the first set of electrodes includes M electrodes and the Mth electrode of the first set of electrodes and the first electrode of the second set of electrodes are mutually spaced apart by the gap therebetween. That is, the Mth electrode of the first set of electrodes and the first electrode of the second set of electrodes may be adjacent. In one example, the first set of electrodes consists of the first electrode and the first electrode of the first set of electrodes and the first electrode of the second set of electrodes are mutually spaced apart by the gap therebetween. That is, the respective first electrodes of the first set of electrodes and the second set of electrodes may be adjacent. In one example, the gap is along an ion path defined from the ion source to the detector via the first set of electrodes and the second set of electrodes. In one example, the gap is a linear gap.
In one example, a length g (also known as axial extent) of the gap between the first set of electrodes and the second set of electrodes is at least a diameter d of an ion aperture in the first set of electrodes, for example in the first electrode or the Mth electrode thereof, or the second set of electrodes, for example in the first electrode thereof. That is, in one example, g≥d, preferably g≥3/2 d, more preferably g≥2 d. In this way, electric field penetration, for example axial field penetration, of the second stage of acceleration into the first stage of acceleration may be reduced. Increasing the length g to greater than, for example 5d does not further reduce electric field penetration significantly while increases path length. In one example, g≤20 d, preferably g≤10 d, more preferably g≤5 d.
Ion Detector
The TOF MS comprises the ion detector for detecting the ions. In one example, the ion detector comprises and/or is a microchannel plate, MCP, detector and/or a fast secondary emission multiplier, SEM, for example having a flat first converter plate (dynode) is flat. Other ion detectors are known. An electrical signal from the ion detector due, at least in part, to the detected ions is typically measured using a time-to-digital converter, TDC, or a fast analogue-to-digital converter, ADC.
Set of Power Supplies
The TOF MS comprises the set of power supplies, including the first power supply, electrically coupled to the first set of electrodes and to the second set of electrodes. In one example, the first power supply comprises and/or is a high-voltage, HV, power supply. Generally, each power supply of the set power supplies may be as described with respect first power supply. Suitable power supplies for applying the respective potentials to the first set of electrodes and to the second set of electrodes are known. Suitable power supplies are available from Spellman High Voltage Electronics Corporation (Hauppauge, N.Y., USA), Matsusada Precision Inc. (Shiga, Japan) and Applied Kilovolts Ltd. (Worthing, UK). In one example, the first (extraction pulse) power supply comprises and/or is a 1 kV to 5 kV, for example a 2.5 kV power supply unit (PSU), having a stability of <1000 ppm. In one example, the second (ramp pulse) power supply comprises and/or is a 5 kV to 20 kV, for example a 10 kV PSU having a stability of <100 ppm. In one example, the third (source) power supply is a 5 kV to 20 kV, for example a 10 kV PSU having a stability of <100 ppm. In one example, a fourth (ramp bias) power supply, electrically coupled, for example only electrically coupled, to the second set of electrodes, comprises and/or is a 5 kV to 20 kV, for example a 10 kV PSU having a stability of <100 ppm. Reversible versions of these power supplier may be employed to enable switching between the analysis of positive and negative ions.
In one example, the set of power supplies includes the first power supply electrically coupled, for example only electrically coupled, to the first set of electrodes and a second power supply electrically coupled, for example only electrically coupled, to the second set of electrodes. In this way, the respective potentials applied to the first set of electrodes and to the second set of electrodes may be independently supplied and/or controlled.
In one example, the ion source comprises, in use, a sample plate and the set of power supplies includes a third power supply electrically coupled, for example only electrically coupled, in use to the sample plate. In this way respective potentials applied to the sample plate and to the first set of electrodes and/or to the second set of electrodes may be independently supplied and/or controlled.
In one preferred example, the ion source comprises, in use, a sample plate and the set of power supplies includes the first power supply electrically coupled, for example only electrically coupled, to the first set of electrodes, a second power supply electrically coupled, for example only electrically coupled, to the second set of electrodes and a third power supply electrically coupled, for example only electrically coupled, in use to the sample plate.
Controller
The TOF MS comprises the controller configured to control the set of power supplies to apply the respective potentials to the first set of electrodes and the second set of electrodes. Typically, controllers for MS are implemented using a combination of electronics, firmware and/or software, for example using a computer comprising a processor and a memory, as understood by the skilled person.
In one example, the controller is configured to control the ion source, for example to supply the group of ions at the time t0. For example, for a MALDI ion source, the controller may be controlled to fire a laser pulse at the time t0.
First Substantially Field-Free Region
The controller is configured to control the set of power supplies to provide the first substantially field-free region between the ion source and the first set of electrodes to allow the group of ions to expand theretowards and/or therein, at the time t0. The time t0 is as described previously.
More generally, in one example, the controller is configured to control the set of power supplies to provide the first substantially field-free region between the ion source and the first set of electrodes for a time period tdelay=textraction−t0. That is, for the time period tdelay=textraction−t0 before the extraction potential Vextraction is applied to the first set of electrodes at the time textraction>t0, the first substantially field-free region between the ion source and the first set of electrodes is provided. In one example, the time period tdelay=textraction−t0 is in a range from 100 ns to 10 μs, preferably in a range from 500 ns to 2 μs.
It should be understood that the first substantially field-free region comprises at most a relatively low electric field, for example compared with that electric field due to the extraction potential Vextraction. In one example, the first substantially field-free region comprises an electric field in a range from 0 Vmm−1 to 50 Vmm−1, preferably in a range from 1 Vmm−1 to 25 Vmm−1, more preferably in a range from 2 Vmm−1 to 10 Vmm−1.
In one example, the controller is configured to control the set of power supplies to provide the first substantially field-free region between the ion source and the first set of electrodes by applying the same potential to the first set of electrodes as the potential applied to a sample plate of the ion source, for example.
In one example, the controller is configured to control the set of power supplies to provide the first substantially field-free region between the ion source and the first set of electrodes by applying a static voltage VPE=VB to the first set of electrodes, wherein the voltage VB is applied to the sample plate of the ion source.
The first ion and the third ion may define a mass-to-charge range of interest. For example, the first ion may define the lower bound and the third ion may define the upper bound of the mass-to-charge range of interest. In one example, the controller is configured to control the set of power supplies to change the acceleration potential Vacceleration applied to the second set of electrodes from the time ton, when the first ion, the second ion and the third ion are between the first electrode and the Nth electrode of the second set of electrodes, for example when the first ion is relatively proximal the Nth electrode and the third ion is relatively proximal the first electrode. That is, the time ton is no earlier than or coincides with the third ion having travelled through the gap between the first set of electrodes and the second set of electrodes. In other words, all ions in the mass-to-charge range of interest are in the second stage of acceleration before the acceleration potential Vacceleration is changed.
Extraction
The controller is configured to control the set of power supplies to apply the extraction potential Vextraction to the first set of electrodes at the time textraction>t0, to extract the expanded group of ions. That is, controller is configured to control the set of power supplies to apply the extraction potential Vextraction after providing the first substantially field-free region. In other words, expansion of the ions into the first substantially field-free region and extraction of the ions therefrom are successive, for example immediately successive, while not overlapping in time. It should be understood that the extraction potential Vextraction comprises and/or is a pulse i.e. an extraction potential pulse. In one example, the extraction potential Vextraction is applied for a pulse duration textraction_duration in a range from 0.1 μs to 50 μs, preferably in a range from 0.5 μs to 20 μs, more preferably in a range from 2 μs to 10 μs. Generally, the duration of the extraction potential pulse is long enough so that all the ions of interest have left the extraction region. Hence, the pulse duration depends, at least in part, on a given ion optical configuration and/or a mass-charge range of interest, as described below in more detail. In one example, ton≤(textraction+textraction_duration)≤toff. In one example, the extraction potential Vextraction is in a range from 0.1 kV to 10 kV, preferably in a range from 0.5 kV to 5 kV.
Second Substantially Field-Free Region
The controller is configured to control the set of power supplies to apply the extraction potential Vextraction to the first set of electrodes at the time textraction>t0, while maintaining the second substantially field-free region beyond the first set of electrodes, in the gap between the first set of electrodes and the second set of electrodes. That is, the second substantially field-free region is in the gap between the first set of electrodes and the second set of electrodes and the second substantially field-free region is maintained whilst the extraction potential Vextraction is applied to the first set of electrodes. More generally, in one example, the controller is configured to control the set of power supplies to provide the second substantially field-free region beyond the first set of electrodes, in the gap between the first set of electrodes and the second set of electrodes, during a duration from the time textraction to the time ton>textraction.
It should be understood that the second substantially field-free region comprises at most a relatively low electric field, for example compared with that electric field due to the extraction potential Vextraction and/or the acceleration potential Vacceleration. In one example, the second substantially field-free region comprises an electric field in a range from 0 Vmm−1 to 50 Vmm−1, preferably in a range from 1 Vmm−1 to 25 Vmm−1, more preferably in a range from 2 Vmm−1 to 10 Vmm−1.
In one example, the controller is configured to control the set of power supplies to maintain the second substantially field-free region beyond the first set of electrodes, in the gap between the first set of electrodes and the second set of electrodes, to at most 1% of the extraction potential Vextraction/mm.
The field free gap or region (i.e. the second substantially field-free region) primarily prevents electric field penetration of the second accelerating stage into the first pulsed extraction stage. This eliminates prompt acceleration of ions and distortion of phase space during the time-delay and prior to the application of the extraction pulse. Elimination of the prompt acceleration effect allows for a more precise correlation between initial ion position and initial velocity to be established at the onset of the extraction voltage pulse, which has a strong effect on mass resolving power.
The field free gap also decouples the application of the extraction voltage pulse across the first extraction stage and the application of the high voltage dynamic ramp across the second acceleration stage. The extraction pulse is applied to the electrode defining the entrance of the field-free gap while the voltage ramp is applied independently to the electrode defining the exit end of the field-free gap. Decoupling the application of the two signals allows them to be produced with high integrity and stability. For example, HV pulsing induces different DC offsets (depending on duty cycle and amplitude of the pulse) on the entrance (pulsed) electrode and exit (ramp) electrode that can only be effectively tuned out by adjusting the bias power supplies independently, which can only be done by decoupling the application of the extraction and ramp pulses as described herein.
Effective field free gaps can be formed by metal tubes or thick electrodes, but these approaches do not allow for the decoupling of the pulsed and ramped voltages applied at the entrance and exit of the field free gap respectively. The field free gap is preferably formed by two apertured planar electrodes that enable the application of different HV pulses to each electrode.
The field free gap could also be formed in a volume, typically bounded, for example, by a metal cylinder, designed to allow a gas pressure somewhat above the source high vacuum, e.g. a collision cell. However, the gas pressure within the cell should be low enough, typically <5×10−6 kPa, to not significantly scatter the ion beam as it passes through the cell and degrade the resolution improvement achieved by the implementation of the field-free gap. Again, forming the field free gap in this way would not allow for the decoupling of the pulsed and ramped voltages applied at either end of the field free region.
Due to the finite outer diameter of electrodes and apertures, radial and/or axial field penetration will give rise to residual electric fields within the field free gap. However, the dimensions of the field free gap can be defined to minimize the residual field within this region. The residual field that can be tolerated in the field-free gap is ultimately determined by the resulting distortion of phase space in the extraction region and its effect on resolving power. Electric fields of <10 Vmm−1 in the field free gap have been found not to significantly distort the phase space in the extraction region with an extraction potential of 1 kV, for example, applied across the extraction region.
The length of the field free gap required is defined, at least in part, by the extent of field penetration from second acceleration to first acceleration stages which itself depends on the potentials applied to the electrodes bounding the field free gap and the size of the apertures in the electrodes, but typically the axial extent g of the gap (i.e. the length g of the gap between the first set of electrodes and the second set of electrodes) should be greater than or equal to the aperture diameter d, i.e. g≥d and preferably g≥2 d to minimize the effects of axial field penetration.
The diameter D of the electrodes forming the field free region (i.e. the outer diameter or dimension of the Mth electrode of the first set of electrodes and/or the first electrode of the second set of electrodes) must be large enough, with respect to the length g of the gap, to prevent radial field penetration into the field free region. Typically, D≥2 g has been found to prevent radial field penetration while preferably D≥3 g to prevent significant radial field penetration.
In one example, the controller is configured to control the set of power supplies to provide a substantially linear field in the second set of electrodes while providing the first substantially field-free region between the ion source and the first set of electrodes.
Acceleration Potential
The controller is configured to control the set of power supplies to optionally change the acceleration potential Vacceleration applied to the second set of electrodes during the time period Δt=toff−ton, wherein ton>textraction, to vary acceleration of the extracted group of ions based, at least in part, on respective mass-to-charge ratios. It should be understood that the acceleration potential Vacceleration is changed during the time period Δt=toff−ton and thus the acceleration potential Vacceleration is time-dependent. It should be understood that the acceleration potential Vacceleration is applied from a time ton until a later time toff. In one example, the acceleration potential Vacceleration applied to the second set of electrodes during the time period Δt=toff−ton is a time-dependent acceleration potential Vacceleration.
As described above, by changing the acceleration potential Vacceleration applied to the second set of electrodes during the time period Δt=toff−ton to vary acceleration of the extracted group of ions based, at least in part, on respective mass-to-charge ratios, ions having relatively higher mass-to-charge ratios, for example, and traversing through the second set of electrodes at relatively later times are accelerated by a relatively changed, for example an increased, accelerating field due to the second set of electrodes compared with ions having relatively lower mass-to-charge ratios, for example, and traversing through the second set of electrodes at relatively earlier times. In this way, the relatively slower third ion, having the third mass-to-charge ratio m3/z3, is subject to an increased accelerating field, for example, compared with the relatively faster first ion having the first mass-to-charge ratio m1/z1. Particularly, ions having the same mass-to-charge ratio m/z but different initial ion energies and hence velocities are similarly subject to different accelerating fields, thereby more effectively correcting for the initial ion energy spread and thus improving mass resolution. Particularly, in this way, time focusing of ions having the same mass-to-charge ratio m/z but different initial ion energies is achieved.
It should be understood that a magnitude of the change ΔVacceleration of the acceleration potential Vacceleration during the time period Δt=toff−ton depends, at least in part, on a mass-to-charge range of interest and/or a duration of the time period Δt=toff−ton. Hence, the magnitude of the change ΔVacceleration of the acceleration potential Vacceleration during the time period Δt=toff−ton may be relatively smaller for a relatively smaller mass-to-charge range of interest and conversely, relatively larger for a relatively larger mass-to-charge range of interest. In one example, a magnitude of the change ΔVacceleration of the acceleration potential Vacceleration during the time period Δt=toff−ton is in a range from 10% to 100%, preferably in a range from 25% to 75% of the acceleration potential Vacceleration. In one example, a magnitude of the change ΔVacceleration of the acceleration potential Vacceleration during the time period Δt=toff−ton is in a range from 0.1 kV to 10 kV, preferably in a range from 0.5 kV to 5 kV.
It should be understood that a duration of the time period Δt=toff−ton depends, at least in part, on a mass-to-charge range of interest and/or a rate of change of the acceleration potential Vacceleration during the time period Δt=toff−ton. Preferably, all of the ions in the mass-to-charge range of interest, for example the first ion, the second ion and the third ion, are within the second acceleration stage (i.e. traversing the second set of electrodes and hence between the first electrode and the Nth electrode of the second set of electrodes) at the start ton of the change ΔVacceleration of the acceleration potential Vacceleration and the ion having the largest mass-to-charge ratio, for example the third ion, is at, or beyond, the exit of the second acceleration stage (i.e. at or beyond the second set of electrodes and hence at or beyond the Nth electrode of the second set of electrodes) at the end toff for time focusing to be achieved over this mass-to-charge range. In one example, a duration of the time period Δt=toff−ton is in a range from 1 μs to 100 μs, preferably in a range from 5 μs to 50 μs, more preferably in a range from 15 μs to 40 μs.
In one example, the controller is configured to control the set of power supplies to change a magnitude of the acceleration potential Vacceleration applied to the second set of electrodes monotonically during the time period Δt=toff−ton. It should be understood that the magnitude of the acceleration potential Vacceleration applied to the second set of electrodes is based, at least in part, on respective mass-to-charge ratios while the acceleration potential Vacceleration is time-dependent. Hence, by changing the magnitude of the acceleration potential Vacceleration monotonically during the time period, a voltage ramp is applied to the second set of electrodes. In one example, the controller is configured to control the set of power supplies to quasi-linearly or linearly change the acceleration potential Vacceleration applied to the second set of electrodes during the time period Δt=toff−ton. Preferably, the acceleration potential Vacceleration applied to the second set of electrodes during the time period Δt=toff−ton is changed linearly i.e. with time. In this way, the acceleration field in the second stage of acceleration changes directly proportional to the respective mass-to-charge ratios of the ions and thus provides the best possible improvement in mass resolution the second stage of acceleration. However, in generating sufficiently reproducible linear voltage ramps, over the relatively short time period Δt=toff−ton and/or relatively large change in acceleration potential, is not trivial and hence an approximation to a linear voltage ramp may be used. Such an approximation to a linear is thus termed quasi-linear in this context. A quasi-linear voltage ramp may be provided by and RC exponential ramp, for example. In one example, a root mean square, RMS, deviation of the change in the acceleration potential Vacceleration applied to the second set of electrodes during the time period Δt=toff−ton is at most 5%, preferably at most 2.5%, preferably at most 1% with respect to a linear change in the acceleration potential Vacceleration applied to the second set of electrodes during the time period Δt=toff−ton. In one example, a maximum deviation of the change in the acceleration potential Vacceleration applied to the second set of electrodes during the time period Δt=toff−ton is at most 10%, preferably at most 5%, preferably at most 2.5% with respect to a linear change in the acceleration potential Vacceleration applied to the second set of electrodes during the time period Δt=toff−ton.
Pressure
It should be understood that the TOF MS is maintained, in use, at vacuum, for example at an operating pressure of at most 5×10−5 mbar, preferably of at most 5×10−6 mbar. That is, the ion source, the first set of electrodes, the second set of electrodes and the detector are maintained at such a vacuum, such that the first substantially field-free region and the second substantially field-free region are maintained at such a vacuum. In one example, the TOF MS does not comprise a gas inlet for supplying a gas, for example a collision gas or a reaction gas, to the ion source, the first set of electrodes and/or the second set of electrodes.
Mass Range and Mass Resolution
In one example, the TOF MS has a mass range in a range from 50 Da to 50 kDa, preferably in a range from 0.5 kDa to 35 kDa, more preferably in a range from 1 kDa to 25 kDa, most preferably in a range from 2 kDa to 17 kDa. In one example, the TOF MS has a mass resolution in a range from 100 to 10,000, preferably in a range from 250 to 5,000, more preferably in a range from 500 to 2,750, wherein the mass resolution is according to the IUPAC definition, for example across the mass range.
In one preferred example, the TOF MS is a linear TOF MS and comprises:
the ion source, wherein the ion source is a LDI ion source, for supplying the group of ions, including the first ion having the first mass-to-charge ratio m1/z1, the second ion having the second mass-to-charge ratio m2/z2 and the third ion having the third mass-to-charge ratio m3/z3 wherein m3/z3>m2/z2>m1/z1, at the time t0;
the first set of electrodes, consisting of the first electrode, and the second set of electrodes, including the first electrode and the Nth electrode, wherein the first set of electrodes and the second set of electrodes are mutually spaced apart by the gap therebetween;
the ion detector for detecting the ions;
the set of power supplies, including the first power supply, electrically coupled to the first set of electrodes and to the second set of electrodes; and
the controller configured to control the set of power supplies to apply respective potentials to the first set of electrodes and the second set of electrodes;
wherein the controller is configured to control the set of power supplies to:
provide the first substantially field-free region between the ion source and the first set of electrodes to allow the group of ions to expand theretowards and/or therein, at the time t0 for a time period tdelay=textraction−t0;
apply the extraction potential Vextraction to the first set of electrodes at a time textraction>t0, to extract the expanded group of ions, while maintaining the second substantially field-free region beyond the first set of electrodes, in the gap between the first set of electrodes and the second set of electrodes, during a duration from the time textraction to the time ton>textraction; and
quasi-linearly or linearly change the acceleration potential Vacceleration applied to the second set of electrodes during the time period Δt=toff−ton, wherein ton>textraction, to vary acceleration of the extracted group of ions based, at least in part, on respective mass-to-charge ratios.
Method of Controlling a TOF MS
The second aspect provides a method of controlling a time-of-flight, TOF, mass spectrometer, MS, the method comprising:
supplying a group of ions, including a first ion having a first mass-to-charge ratio m1/z1, a second ion having a second mass-to-charge ratio m2/z2 and a third ion having a third mass-to-charge ratio m3/z3 wherein m3/z3>m2/z2>m1/z1, from an ion source at a time t0 and allowing the group of ions to expand towards and/or into a first substantially field-free region between the ion source and a first set of electrodes, including a first electrode;
applying an extraction potential Vextraction to the first set of electrodes at a time textraction>t0, to extract the expanded group of ions, while maintaining a second substantially field-free region beyond the first set of electrodes, in a gap between the first set of electrodes and a second set of electrodes, including a first electrode and an Nth electrode, wherein the first set of electrodes and the second set of electrodes are mutually spaced apart by the gap;
optionally, changing an acceleration potential Vacceleration applied to the second set of electrodes during a time period Δt=toff−ton, wherein ton>textraction, to vary acceleration of the extracted group of ions based, at least in part, on respective mass-to-charge ratios; and
detecting the ions.
The TOF MS, supplying the group of ions, including the first ion, the second ion and the third ion, the ion source, the time t0, the first set of electrodes including the first electrode, applying the extraction potential Vextraction, the time textraction>t0, the first substantially field-free region, the second substantially field-free region, the gap, the second set of electrodes, including the first electrode and the Nth electrode, applying the acceleration potential Vacceleration, the time period Δt=toff−ton and/or detecting the ions may be as described with respect to the first aspect, mutatis mutandis. The second aspect may include any step and/or feature described with respect to the first aspect, mutatis mutandis.
In one example, the method comprises providing the first substantially field-free region between the ion source and the first set of electrodes by applying a static voltage VB to the first set of electrodes.
In one example, the method comprises providing the first substantially field-free region between the ion source and the first set of electrodes during a time period tdelay=textraction−t0.
In one example, the method comprises providing a substantially linear field in the second acceleration stage while providing the first substantially field-free region between the ion source and the first set of electrodes.
In one example, maintaining the second substantially field-free region beyond the first set of electrodes, in the gap between the first set of electrodes and the second set of electrodes, comprises maintaining the second substantially field-free region beyond the first set of electrodes, in the gap between the first set of electrodes and the second set of electrodes, to at most 1% of the extraction potential Vextraction mm.
In one example, a length of the gap between the first set of electrodes and the second set of electrodes is at least a diameter of an ion aperture in the first set of electrodes or the second set of electrodes.
In one example, changing the acceleration potential Vacceleration applied to the second set of electrodes during the time period Δt=toff−ton comprises changing a magnitude of the acceleration potential Vacceleration applied to the second set of electrodes monotonically during the time period Δt=toff−ton.
In one example, changing the acceleration potential Vacceleration applied to the second set of electrodes during the time period Δt=toff−ton comprises quasi-linearly or linearly changing the acceleration potential Vacceleration applied to the second set of electrodes during the time period Δt=toff−ton.
In one example, the first set of electrodes consists of the first electrode.
In one example, the method comprises independently applying respective voltages to the first set of electrodes and to the second set of electrodes.
Computer, Computer Program, Non-Transient Computer-Readable Storage Medium
The third aspect provides a computer comprising a processor and a memory configured to implement, at least in part, a method according to the second aspect.
The fourth aspect provides a computer program comprising instructions which, when executed by a computer comprising a processor and a memory, cause the computer to perform, at least in part, a method according to the second aspect.
The fifth aspect provides a non-transient computer-readable storage medium comprising instructions which, when executed by a computer comprising a processor and a memory, cause the computer to perform, at least in part, a method according to the second aspect.
Throughout this specification, the term “comprising” or “comprises” means including the component(s) specified but not to the exclusion of the presence of other components. The term “consisting essentially of” or “consists essentially of” means including the components specified but excluding other components except for materials present as impurities, unavoidable materials present as a result of processes used to provide the components, and components added for a purpose other than achieving the technical effect of the invention, such as colourants, and the like.
The term “consisting of” or “consists of” means including the components specified but excluding other components.
Whenever appropriate, depending upon the context, the use of the term “comprises” or “comprising” may also be taken to include the meaning “consists essentially of” or “consisting essentially of”, and also may also be taken to include the meaning “consists of” or “consisting of”.
The optional features set out herein may be used either individually or in combination with each other where appropriate and particularly in the combinations as set out in the accompanying claims. The optional features for each aspect or exemplary embodiment of the invention, as set out herein are also applicable to all other aspects or exemplary embodiments of the invention, where appropriate. In other words, the skilled person reading this specification should consider the optional features for each aspect or exemplary embodiment of the invention as interchangeable and combinable between different aspects and exemplary embodiments.
For a better understanding of the invention, and to show how exemplary embodiments of the same may be brought into effect, reference will be made, by way of example only, to the accompanying diagrammatic Figures, in which:
In this example, the TOF MS comprises:
an ion source 109, 110 for supplying a group of ions, including a first ion m1 having a first mass-to-charge ratio m1/z1, a second ion m2 having a second mass-to-charge ratio m2/z2 and a third ion m3 having a third mass-to-charge ratio m3/z3 wherein m3/z3>m2/z2>m1/z1, at a time t0 (time=0);
a first set of electrodes SE1, including a first electrode 103, and a second set of electrodes SE2, including a first electrode 105 and an Nth electrode 107, wherein the first set of electrodes SE1 and the second set of electrodes SE2 are mutually spaced apart by a gap g therebetween;
an ion detector 111 for detecting the ions;
a set of power supplies (not shown), including a first power supply (not shown), electrically coupled to the first set of electrodes SE1 and to the second set of electrodes SE2; and
a controller (not shown) configured to control the set of power supplies to apply respective potentials to the first set of electrodes SE1 and the second set of electrodes SE2;
wherein the controller is configured to control the set of power supplies to:
provide a first substantially field-free region between the ion source and the first set of electrodes to allow the group of ions to expand theretowards and/or therein, at the time t0;
apply an extraction potential Vextraction (VPE−VB) to the first set of electrodes SE1 at a time textraction>t0 (time=t_ext), to extract the expanded group of ions, while maintaining a second substantially field-free region 104 beyond the first set of electrodes SE1, in the gap g between the first set of electrodes SE1 and the second set of electrodes SE2; and
change an acceleration potential Vacceleration (VR) applied to the second set of electrodes during a time period Δt=toff−ton=toff−t_on, wherein ton>textraction, to vary acceleration of the extracted group of ions based, at least in part, on respective mass-to-charge ratios.
In this example, the TOF MS comprises and/or is a linear TOF MS, for example having a linear flight tube arranged between the second set of electrodes and the detector.
In this example, the ion source is a MALDI ion source (pulsed laser energy 110 shown, passing through apertures in electrodes 103 and 105). In this example, the ion source comprises, in use, a MALDI sample plate 101, having a sample 109 thereon. In this example, the first electrode 103 of the first set of electrodes SE1 comprises a plate, having an ion aperture therethrough. In this example, the first set of electrodes consists of the first electrode 103. In this example, a diameter D of the first electrode and/or of the Mth electrode of the first set of electrodes is at least twice a length g of the gap. In this example, the second set of electrodes SE2 includes N electrodes 105, 108, 108 and 107, including the first electrode 105 and the Nth electrode 107, wherein N is a equal to 4, wherein the N electrodes are mutually spaced apart, preferably mutually equispaced apart. In this example, a diameter D of the first electrode 105 of the second set of electrodes SE2 is at least twice a length g of the gap. In this example, a length g (also known as axial extent) of the gap between the first set of electrodes SE1 and the second set of electrodes SE2 is at least a diameter d of an ion aperture 100 in the first set of electrodes SE1, for example in the first electrode 103, and the second set of electrodes, for example in the first electrode 105 thereof. In this example, the ion detector 111 is a microchannel plate, MCP, detector.
In more detail,
The first substantially field-free region, of length ‘s’, is provided between the sample plate 101 and the first electrode 103 of the first set of electrodes SE1 of the first set of electrodes SE1, during ablation and ionisation of the sample 109. This first substantially field-free region eliminates the prompt acceleration of ions and distortion of phase space during the time-delay prior to the application of the extraction pulse, due, for example, to the electrical fields therebeyond in the gap 104 of length ‘g’. Elimination of the prompt acceleration effect allows for a more precise correlation between initial ion position and initial velocity to be established at the onset of the extraction voltage pulse, which has a strong effect on mass resolving power. Subsequently, the first acceleration stage 102, of length ‘s’, is formed between the sample plate 101 and the first electrode 103 of the first set of electrodes SE1 of the first set of electrodes SE1 (i.e. the first substantially field-free region becomes the first acceleration stage 102) and simultaneously, the field free gap 104 (i.e. the second substantially field-free region), of length ‘g’, is formed between the first electrode 103 of the first set of electrodes SE1 and the first electrode 105 of the second set of electrodes SE2. This second substantially field-free region eliminates distortion phase space in the first pulsed extraction stage 102, due, for example, to the second acceleration stage 106 of the second set of electrodes SE2, which would otherwise adversely affect mass resolving power. The second acceleration stage 106, of length ‘d’, is formed between the first electrode 105 of the second set of electrodes SE2 and the Nth electrode 107 of the second set of electrodes SE2, with several intermediate electrodes 108 (two shown) distributed evenly through the second acceleration stage 106.
The voltages applied to the electrodes that form the multiple-stage acceleration configuration are shown in
During the period ton to toff the potential distribution across the second acceleration stage 106 is modified by a time-dependent voltage ramp ΔV(t) of duration Δt=toff−ton, applied to the first electrode 105 of the second set of electrodes SE2, whereby heavier ions traversing this stage 106 at later times experience a linear, most preferably a quasi-linear, increase in the magnitude of the accelerating field thus enhancing mass resolving power over the extended m/z range of interest.
All the ion over the m/z range of interest (m1 to m3) must be within this second acceleration stage 106 at the onset (ton) of the dynamic ramp (
In this example, the set of power supplies SPS includes the first power supply 202 electrically coupled, for example only electrically coupled, to the first set of electrodes SE1, a second power supply 204 electrically coupled, for example only electrically coupled, to the second set of electrodes SE2 and a third power supply 201 electrically coupled, for example only electrically coupled, in use to the sample plate 101. In this example, the set of power supplies SPS includes a fourth power supply 207 electrically coupled, for example only electrically coupled, to the second set of electrodes SE2.
In this example, the first (extraction pulse) power supply 202 is a 2.5 kV power supply unit (PSU), having a stability of <1000 ppm, for example an Applied Kilovolts HP2.5×AA025. In this example, the second (ramp pulse) power supply 204 is a 10 kV PSU having a stability of <100 ppm, for example an Applied Kilovolts HPO10×AA025. In this example, the third (source) power supply 201 is a 10 kV PSU having a stability of <100 ppm, for example an Applied Kilovolts HPO10×AA025. In this example, the fourth (ramp bias) power supply 207 is a 10 kV PSU having a stability of <100 ppm, for example an Applied Kilovolts HPO10×AA025. Reversible versions of these power supplier may be employed to enable switching between the analysis of positive and negative ions.
Ion Optics Simulation
The multiple-stage configuration 120 is shown with ion groups over a m/z range from 2 kDa 611 to 17 kDa 612 at time ton=6.3 μs 610, the start of the application of the ‘dynamic ramp’ across the second acceleration stage 106. Only the m/z range of ions, 2 kDa to 17 kDa, within the second acceleration stage 106 at this time (ton=6.3 μs) will be time focused at the detector. In this example, the length of first acceleration stage 102 s=6.4 mm, field free gap 104 g=3 mm, second acceleration stage 106 d=70 mm and field free distance 112, from exit of second acceleration stage 107 to detector 111, I=500 mm. Sample plate 101 has static potential of 9.5 kV applied, first electrode 103 is pulsed from initial potential of 9.5 kV 604 to 8 kV 605 after time delay textraction=800 ns 606, and the second electrode ramped from static ‘ramp bias’ potential of 8 kV 607 to 13 kV 608 over 10 μs window 609 after initial time-delay of 6.3 μs 610. Four intermediate electrodes 108 ensure a linear field is maintained across second acceleration stage 106 with no significant radial field penetration.
Plot (1) 620 shows result of simulation with amplitude of ‘dynamic ramp’, across the second acceleration stage 106, set to zero (ΔV=0 kV), that is, a static potential across the second acceleration stage 106, which is equivalent to traditional two-stage acceleration configuration. Results in sharp peak in resolution of 1800 at m/z of 2 kDa 623 (actual m/z position of peak resolution determined by delayed-extraction pulse time, here textraction=800 ns). Resolution rapidly fall away from peak value 623 with increasing m/z, as would be expected in the absence of any time-dependent acceleration scheme.
Plot (2) 621 shows the resolution obtained with application of an ‘ideal’ linear 402 ‘dynamic ramp’ (ΔV=5 kV, Δt=10 μs) applied across the second acceleration stage 106. Enhanced resolution is now obtained over the entire m/z range of interest, resolution of 2000 at 2 kDa 624 to 2200 at 17 kDa 625, demonstrating a significant improvement in resolution over this extended m/z range with respect to resolution achieved 620 in the absence of any time-dependent acceleration scheme.
Plot (3) 622 shows resolution obtained with application of practical quasi-linear ‘dynamic ramp’ 603, equivalent to 8 kV pulsed across and RC network (R=132 kΩ, C=65 pF) in 10 μs window, creating exponential ramp of amplitude ΔV=5 kV 626 over Δt=10 μs 609. Resolution 622 is reduced slightly, compared to that achieved with ‘ideal’ linear ‘dynamic ramp’ 621 at the higher end of the m/z range of interest, but resolution is still significantly enhanced with respect to the resolution obtained 620 with the ‘static ramp’ configuration, over the whole extended mass range.
In more detail,
At S901, the method comprises supplying a group of ions, including a first ion having a first mass-to-charge ratio m1/z1, a second ion having a second mass-to-charge ratio m2/z2 and a third ion having a third mass-to-charge ratio m3/z3 wherein m3/z3>m2/z2>m1/z1, from an ion source at a time t0 and allowing the group of ions to expand towards and/or into a first substantially field-free region between the ion source and a first set of electrodes, including a first electrode.
At S902, the method comprises applying an extraction potential Vextraction to the first set of electrodes at a time textraction>t0, to extract the expanded group of ions, while maintaining a second substantially field-free region beyond the first set of electrodes, in a gap between the first set of electrodes and a second set of electrodes, including a first electrode and an Nth electrode, wherein the first set of electrodes and the second set of electrodes are mutually spaced apart by the gap.
At S903, the method comprises optionally, changing an acceleration potential Vacceleration applied to the second set of electrodes during a time period Δt=toff−ton, wherein ton>textraction, to vary acceleration of the extracted group of ions based, at least in part, on respective mass-to-charge ratios.
At S904, the method comprises detecting the ions.
The method may include any of the steps described herein.
Field free gap: region between first and second acceleration regions (i.e. between extraction plate 103 and first ramp electrode 105)
Second acceleration region: ‘dynamic ramp’ acceleration region from first ramp electrode 105 (exit of field free gap) to ground electrode 107 (entrance to TOF analyser).
In more detail,
In more detail,
In more detail,
In this example, this is probably the earliest time when the extraction potential Vextraction would be switched off since the largest m/z ions have passed beyond its influence. Essentially, the extraction pulse is preferably ‘on’ to maintain the field free gap between the first two acceleration stages.
In more detail,
In more detail,
In more detail,
In more detail,
In this example, the extraction delay (textraction) is a variable for tuning purposes, but typically ˜800 ns.
In this example, the ‘Dynamic ramp’ ton and toff times, 6.3 μs and 16.3 μs respectively, are fixed values determined by design of the MS.
At time t0<t<textraction, a field free region (
At t=textraction, the extraction pulse is applied to the extraction electrode, dropping the potential on this electrode from 9000 V to 8000 V; thus creating a potential difference of 1000V across the first acceleration region (
The extraction pulse should remain ‘on’ for the whole time any ions of interest are still in the first acceleration region. If the extraction pulse is switched ‘off’ then the first acceleration region will revert to field free (
So, when can the extraction pulse be switched ‘off’? The quick answer is when the highest m/z ions of interest have passed from the first acceleration region, through the field free gap and into the second acceleration region. However, we also need to be aware that a modification to the field in the first acceleration region may affect the field in the second acceleration region, so we might want to wait a little longer.
Whilst the extraction pulse is ‘on’, we have a field free region between the two acceleration stages that minimises any influence the field in the first acceleration might have on the field in the second acceleration region.
The best way to determine the earliest time the extraction can be switched ‘off’ is by plotting the resolution of the highest m/z ions (17 kDa) against extraction duration (
The resolution plot below shows the minimum extraction duration to be ˜6 μs. A duration of 5 μs, for example, would be too short and the mass resolution would be degraded.
For illustration, two potential plots are shown, both at time t=6 μs, but with different extraction durations of 5 μs and 6 μs. The location of the highest m/z ions (17 kDa) at time t=6 μs is marked in each case. Clearly the fields at this location are different, for the two extraction durations, giving rise to the difference in resolution shown in the plot.
An extraction duration of 6 μs is a minimum value in this example, extraction durations longer than this (e.g. 10 μs or 100 μs) will not degrade the resolution since all the ions of interest will have passed into the second acceleration stage, or beyond, by the time the extraction is switched ‘off’.
Pulsed extraction duration is not generally a tuning variable. However, it needs to be set such that all ions of interest experience the required acceleration as per the design.
Turning ‘off’ the extraction potential too soon will compromise the velocity focusing and thus the resolution of the higher m/z ions.
Thus, there is a minimum extraction potential, for the instrument discussed here −6 μs, but there is no specific maximum value, for example 10 μs and 100 μs would be equally effective as 6 μs in this case.
Essentially, in this case, extraction duration=>6 μs. 6 μs could be used, as could 10 μs and 100 μs. Sometimes a larger value, such as 100 μs, might be chosen to move any electrical noise, associated with switching ‘off’ the extraction potential, outside the time-of-flight range of the analyser. However, the extraction duration must be <<repetition period for the instrument e.g. for an instrument running at 1 kHz the extraction duration must be <<1 ms to ensure the pulser electronics re-stabilise before next pulse triggered.
At S1201, the controller provides a laser trigger pulse.
At S1202, a laser light pulse is emitted, ablating and ionising the sample, in response to the laser trigger pulse, as described with respect to S901. The laser light pulse typically has a peak width of about 1 ns (FWHM). The sample plate 101 is maintained at a constant potential of 9 kV.
At S1203, a laser pulse synchronisation signal is provided by a photodiode illuminated by a fraction of the laser light pulse, that defines the time t0, which occurs at a fixed time after the laser light pulse.
At S1204, the extraction potential Vextraction is applied to the extraction plate 103 at a time textraction>t0 i.e. after the time delay tdelay=textraction−t0, which is about 800 ns in this example, as described with respect to S902. The extraction potential Vextraction is a square wave of amplitude −1 kV and a duration textraction_duration of 10 μs, superimposed on the otherwise constant potential of 9 kV applied to the extraction plate 103.
At S1205, the acceleration potential Vacceleration applied to the first electrode 105 during the time period Δt=toff−ton is varied, as described with respect to S903. The time period Δt is 10 μs and ton−t0 is 6.3 μs. The maximum amplitude of the acceleration potential Vacceleration is +5 kV, superimposed on the otherwise constant potential of 8 kV applied to the first electrode 105. At S1206 (not shown), the ions are detected, as described with respect to S904.
Steps S1201 to S1206 are repeated, for example at a frequency of 1 kHz.
Although a preferred embodiment has been shown and described, it will be appreciated by those skilled in the art that various changes and modifications might be made without departing from the scope of the invention, as defined in the appended claims and as described above.
In summary, the invention provides a novel ion optical acceleration scheme to enhance time-focusing over an extended m/z range. The inventors have arrived at several advantages of the proposed ion optical scheme over prior art acceleration configurations largely by decoupling the first ‘pulsed extraction’ acceleration stage 102 from the second time-dependent acceleration stage 106:
Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.
All of the features disclosed in this specification (including any accompanying claims and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at most some of such features and/or steps are mutually exclusive.
Each feature disclosed in this specification (including any accompanying claims, and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.
The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2007309.4 | May 2020 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/GB2021/051184 | 5/18/2021 | WO |
