GB2545485A

GB2545485A - Method and apparatus for ion mobility separations

Info

Publication number: GB2545485A
Application number: GB1522361.3A
Authority: GB
Inventors: Sydoruk Oleksiy
Original assignee: Imperial College of Science Technology and Medicine
Current assignee: Imperial College of Science Technology and Medicine
Priority date: 2015-12-18
Filing date: 2015-12-18
Publication date: 2017-06-21
Also published as: GB201522361D0

Abstract

An ion mobility separator is configured such that an electric field that propels ions in a drift direction A inside the drift region is spatially limited or localised to one or more activated regions 108a within the drift region. The activated region has an electrical field (201, 202) that reduces or decays preferably linearly in the drift direction A, and the activated region is moved or translated from a first end 101a to a second end 101b of the drift region. Preferably the electric field travels with a velocity that matches a drift velocity of an ion species. The activated region may be provided by selectively activating a subset of electrodes. The invention provides simultaneous bunching of ions of the same mobility and separation of ions of different mobilities without requiring any preliminary ion separation (figure 3).

Description

Method and apparatus for ion mobility separations

Field

The present invention relates to analytical separations, in particular, to drift-tube and travelling-wave ion mobility spectrometry.

Background

Analytical separation methods exist that rely on differences in mobilities of ionized analytes in a background gas. These methods are known collectively under the name ion mobility spectrometry (IMS).

The most common of these known techniques is drift-tube (DT) IMS. This technique is based on injecting a packet of ions into a region with a homogenous static electric field. Ions with different mobilities drift in the field with different velocities and arrive at the end of the drift region at different times. The ion charges can then be received by a detector, with the resulting ion spectrum giving information about the species present in the originally injected packet. Alternatively, the ions may undergo subsequent separation steps, or ions with only a specific mobility can be selected and then directed into a mass spectrometer.

Another separation method, called travelling-wave (TW) IMS, uses, instead of a constant electric field, a field in the form of a wave propagating along the drift region. Depending on the field strength and the wave velocity, ions with different mobilities interact with the electric field for different intervals of time during a period of the wave. As a result, the time that ions reside in the drift region depends in a complicated way on the ion mobility and the parameters of the traveling wave. Methods and instruments of TW IMS are disclosed, for example, in US patent 6,914,241 to Giles et al..

Ion separation combining both static and traveling-wave electric fields is disclosed in US patent 8,841,608 to Shvartsburg et al. Its essential feature is the use of two fields that pull ions in opposite directions. The fields can be a wave and a static field or two counter-propagating waves. In one operational regime, the actions of the wave and static fields are compensated for a particular ion species. Other regimes include low- and high-pass ion mobility filtering.

An important characteristic of how efficiently an ion mobility spectrometer separates different ion species is its resolving power. For the instruments in which ions are injected and separated in the form of packets, the resolving power is inversely proportional to the width of the packets at the instrument exit. The width at the exit depends, first, on the packet width at the instrument entrance and, second, on diffusion broadening of the packet in the drift direction.

One widely used technique to control the width of ion packets entering a drift region is by ion shutters. Two common configurations are Bradbury-Nielsen and Tyndall-Power shutters, both operating under similar principles. A shutter is formed by two sets of conducting wire grids placed in front of a drift region. An electric field is created between the wires so that ions approaching the shutter are captured and discharged on the wires, precluding them from entering the drift region. For a short period of time, typically of the order of 0.2 ms, the field is switched off, and an ion packet enters the drift region. The cycle is typically repeated after 20 ms, allowing the ions to traverse the drift region. The resulting duty cycle of 1% is low and compromises the instrument performance.

Methods to improve this basic scheme are known. US patent 5,200,614 to Jenkins discloses ion mobility spectrometers where ions first enter a field-free region allowing an ion population to build up. A high electric field is then applied across this region for a short time flushing the ions into the drift region.

In the ion mobility spectrometer disclosed in US patent 7,538,320 to Sperline, ion shutters consist of two spatially separated elements. At predetermined intervals, the electric field between the elements is made lower or higher than the field in the surrounding regions, modulating the ion velocity and compressing ion packets. US patent 6,924,479 to Blanchard teaches a method of ion injection without wire grids. Instead, the ions are first dynamically accumulated in ion wells created by suitable field gradients and then released into the drift region. The method avoids ion loss on the grids. A method to dynamically packetise ions is disclosed in US patents 6,812,453 and 7,095,013 to Bateman et al. In these patents it is described how ions enter a drift region as a continuous beam and are subjected to a travelling-wave electric field. The ions exit the drift region as packets, which are then directed into a time-of-flight mass spectrometer. US patent application 2014/0061457 to Berdnikov et al. discloses a similar packetising method but relying on a moving pseudopotential instead of a travelling wave. US patent 9,063,086 teaches a method to compress ion packets of different mass-to-charge ratios while preserving the spatial separation between them. Pre-separated ion packets are subjected to an electric-field gradient causing ions in a packet to drift with different velocities. A limitation of the above methods is that they create and manipulate ion packets either before or after separating the ions based on their mobility or charge-to-mass ratio.

Summary

These and other problems are addressed in accordance with the present teaching by separating and bunching ions according to their mobility so as to provide simultaneous bunching of ions of the same mobility and separation of ions of different mobilities while requiring no preliminary ion separation.

In accordance with the present teaching a method and apparatus are provided that provide an increased resolving power and signal strength by bunching ions of the same mobility values simultaneously with providing a separation of ions of different mobility values.

Accordingly, a first aspect of the present teaching provides an apparatus for separating and bunching ions according to their mobility, the apparatus comprising: a chamber defining a drift region of the apparatus; and an electrical module configured to operably create inside the drift region an electric field that propels ions in a drift direction, wherein the apparatus is configured such that the electric field is operably spatially limited to at least one activated region defined within the drift region, the activated region having an electrical field that decays in the drift direction, the electrical module being arranged to effect a movement of the at least one activated region in the drift direction from a first end of the drift region to a second end of the drift region such that the ions separate and bunch according to their respective mobilities within the activated region.

In a further development of the invention, the electrical module comprise a plurality of electrodes arranged along the drift region to operably create inside the drift region the electric field that propels ions in the drift direction.

In a further development of the invention, the apparatus is arranged to selectively activate a subset of electrodes of the plurality of electrodes to provide the activated region of the drift region and to vary activation of individual ones of the plurality of electrodes to effect the movement of the activated region in the drift region.

In a further development of the invention, is arranged, concurrently with providing the activated region, to provide a non-activated region using a second subset of electrodes of the plurality of electrodes.

In a further development of the invention, the apparatus is arranged selectively activate a third subset of electrodes of the plurality of electrodes to provide a second activated region separated from the activated region by the non-activated region.

In a further development of the invention, the electric field travels in the drift direction with a constant predetermined velocity.

In a further development of the invention, the predetermined velocity matches a drift velocity of a predetermined ion species.

In a further development of the invention, the electric field decays linearly in the drift direction.

In a further development of the invention, the apparatus further comprises an introduction device configured to introduce packets of non-separated ions with different mobility values into the drift region.

In a further development of the invention, the drift region is operably maintained at a pressure between 0.1 Pascal and atmospheric pressure.

In a further development of the invention, the electrical module is further configured to create in the drift region a pseudo-potential to prevent ions from spreading in the directions perpendicular to the drift direction.

According to a second aspect of the present invention, there is provided a method for separating and bunching ions according to their mobility, the method comprising subjecting ions to an electric field that propels the ions in a drift direction in a drift region, wherein the electric field is operably spatially limited to at least one activated region defined within the drift region that travels in the drift region in the drift direction from a first end of the drift region to a second end of the drift region and the electric field decays in the drift direction in the activated region such that the ions separate and bunch according to their respective mobilities within the activated region.

In a further development of the invention, the method further comprises introducing packets of non-separated ions with different mobility values into the drift region.

In a further development of the invention, the method further comprises creating in the drift region a pseudo-potential to prevent ions from spreading in the directions perpendicular to the drift direction.

There is therefore provided in accordance with the present teaching a method and an apparatus for ion mobility separation with increased resolving power and signal levels as compared to that achievable using conventional drift-tube ion mobility spectrometry. In accordance with the present teaching, ions are subjected to an inhomogeneous travelling electric field whose velocity is selected so as to match the velocity of a particular targeted ion species. Ions of the said species and of species with close but different mobility values will, in accordance with the present teaching, separate and bunch in different positions within a drift region of the spectrometer thereby achieving increased resolving power and signal strength.

Brief Description Of The Drawings

The present teaching will now be described, by way of example only, and with reference to the accompanying drawings in which:

Figure 1 illustrates an apparatus according to the present invention.

Figures 2a-2c show an examplary electric field strength along the drift region of the apparatus of Fig. 1 as a function of coordinate for three different given times.

Figures 3a-3b demonstrate the processes of simultaneous ion bunching and separation according to the present invention.

Figure 4 shows the ion spatial distributions for an exemplary calculation assuming a homogenous static electric field according to the conventional DT IMS.

Figure 5 shows the ion spatial distributions for an exemplary calculation where the ion parameters are the same as in Fig. 4 but with an inhomogeneous travelling electric field according to the present invention.

Detailed description of preferred embodiment of the invention

Figure 1 illustrates an apparatus 100 according to an exemplary aspect of the invention. The apparatus 100 comprises a chamber 120 defining a drift region 101 where ions may drift under an electric field in a drift direction A between an introduction device 102 located at a first end 101a of the drift region 101 and a collecting element 105 located at the second end 101b of the drift region 101 opposite the first end 101a. The drift region 101 may be filled with a background gas, for example air or nitrogen, which depending on the intended analysis can be held at one or more of a range of pressures, from for example 0.1 Pa to atmospheric pressure.

The introduction device 102 may be a shutter arranged to selectively introduce ions into the drift region 101. This introduction device 102 can adopt any physical form that may allow allowing ions to enter the drift region 101 in the form of ion packets.

The apparatus 100 further comprises an electrical module 110 arranged along the drift region 101. The electrical module 110 is configured to create inside the drift region 101, at least along a portion of the drift region 101, an electric field that will operably propel ions that are introduced into the chamber 120 in the drift direction A. In accordance with the present teaching the electrical module 110 is preferably configured to create an electric field that is spatially limited to define at least one activated region 108a within the drift region 101. The activated region 108a is moveable within the drift region 101 so as to travel in the drift region 101 in the drift direction A from the first end 101a of the drift region 101 to the second end 101b of the drift region 101. The electrical module 110 is also configured to operably provide an electrical field that will decay in the drift direction A of the activated region 108a. By providing an activated region 108a that both has a decay parameter and moves within the drift region 101, it is possible, in accordance with the present teaching, to provide for a simultaneous separation and bunching of ions within the activated region 108a according to their respective mobilities. It will be appreciated that advantageously the length of the activated region 108a and the level of decay for that activated region 108a will be constant for any particular analysis. In this way a moveable activated region 108a that has a constant decay parameter along its length will travel within the drift region 101.

The electrical module 110 preferably comprises a plurality of electrodes 103 located on at least one side of the drift region 101. The electrodes can, for example, be rings surrounding the drift region 101 for cylindrical designs of the drift region 101. Other geometrical forms such as planar electrodes located on at least one side of the drift region 101 for planar designs of the drift region 101 could also be advantageously employed. The electrical module 110 is electrically connected to a combination of voltage sources 109 and controlling circuits 104 to create the travelling decaying electric field in the drift region 101. Specifically the controlling circuits 104 are configured to selectively activate the electrodes of the plurality of electrodes 103, as will be described in more detail below.

The collecting element 105 is configured to collect the ions that have drifted in the drift region 101. It preferably comprises a protective grid 106 followed by a detector 107 that operably serves to detect the ions that pass through the grid. The detector 107 is configured to measure the time the ions spend in the drift region 101 and may provide an overview of the ion packet that was introduced into the drift region through a series of corresponding currents versus time readings that will provide information of the properties of the individual ions that formed the ion packet.

In operation ions enter the drift region 101 through the introduction device 102 at the first end 101a. In the drift region 101 the ions drift under the action or influence of the applied electric field. After passing through the drift region 101, the ions reach the collecting element 105 where they are detected.

According to the present invention, the controlling circuits 104 of the apparatus 100 are configured to selectively activate a first subset 103a of the plurality of electrodes 103, comprising at least one electrode of the plurality of electrodes 103, so as to generate the activated region 108a of the drift region 101. It will be appreciated that the activated electrodes create an electric field within the drift region, an example of which is illustrated in Figures 2a-2c. Advantageously according to the present invention, the electric field created by the first subset of electrodes 103a decays in the drift direction A along at least a portion of the drift region 101 that defines the activated region 108a. By varying activation of individual ones of the electrodes 103a, that is selectively activating other ones of the plurality of electrodes and deactivating previously activated electrodes, it is possible to provide for a movement of the activated region 108a such that the activated region 108a travels in the drift direction A in the drift region 101 from the first end 101a of the drift region 101 to the second end 101b of the drift region 101. This movement of the activated region 108a, and the fact that it has a decay parameter such that the electrical field within the activated region 108a is not a fixed value allows ions that are introduced in the drift region 101 to separate and bunch according to their respective mobilities within the activated region 108a.

It will be appreciated that the activated region 108a travels by selectively activating individual sets of electrodes of the plurality of electrodes 103. Preferably the activated region 108a travels by successively selectively activating neighbouring electrodes of the plurality of electrodes 103. Thus, an apparatus 100 according to the invention provides, along at least a portion of the drift region 101, a decaying electric field that travels in the drift region 101 in the direction A as illustrated in Figures 2a-2c and 3a-3b.

The controlling circuits 104 are also configured concurrently with providing the activated region 108a to provide at least one non-activated region 108b of the drift region 101 wherein a second subset 103b of the plurality of electrodes 103 are not activated, as shown in Figures 2a-2c.

The controlling circuits 104 of the apparatus 100 are further arranged to selectively activate a third subset of electrodes 103d of the plurality of electrodes 103 to provide a second activated region 108d separated from the activated region 108a by the non-activated region 108b as shown in Figure 2c. In this way two or more packets of ions that are introduced separately into the drift region 101 may each travel within their dedicated activated region 108a, 108d along the drift region 101 of the apparatus 100.

The controlling circuits 104 of the apparatus 100 may be further arranged to selectively activate a fourth subset of electrodes 103c of the plurality of electrodes 103 to provide a second non-activated region 108c separated from the non-activated region 108b by the activated region 108a as shown in Figure 2b.

Preferably the electrodes of the first subset 103a are neighbouring electrodes of the plurality of electrodes 103, as shown on Figures 2a-2c. Similarly the electrodes of the second subset 103b, respectively of the third subset 103d, respectively of the fourth subset 103c, are also neighbouring electrodes of the plurality of electrodes 103. Thus the regions 108a, 108b, 108c and 108d each form a continuous segment of the drift region 101.

More precisely the electric field created by the electrodes of the activated region 108a provide an inhomogeneous electric field in the drift region 101 that has a constant component along the drift region 101, that is a spatially homogeneous DC electric field component, and a travelling component along the drift region 101 in the direction A of the ion drift, that is a spatially inhomogeneous travelling AC or DC electric field.

Preferably the activated region 108a which is provided by a decaying electric field, travels in the drift direction A with a constant predetermined velocity that matches a drift velocity of a predetermined ion species. The choice of the predetermined ion species can be based either on a previous measurement in a homogeneous DC field or simply on an expectation from a user as to the intended ions of interest. For example, the user might be interested in whether ions with a specific mobility are present in a sample, and thus may tune the instrument to this mobility value.

In the drift region 101, the ions are therefore subjected to an inhomogeneous travelling electric field whose velocity substantially matches the velocity of the particular predetermined ion species having a specific mobility value. The ions of the predetermined mobility thus travel at a position along the decaying electric field in the activated region 108a of the drift region 101, while ion species with lower and higher mobilities travel respectively downstream and upstream of this position. The larger the difference in mobility, the further apart will be the ion species. Thus under the influence of the electric field in the activated region 108a ions of a same mobility value group together and ions of different mobility value separate. Hence ions of the predetermined species and ions of species with close but different mobility values separate from the ions of the predetermined species and from each other. Similarly, ions of each species bunch together at different positions inside the drift region 101. These ions then travel, at their respective position within the decaying electric field defined by the activated region 108a, along the drift region 101.

As the decaying electric field extends along a portion only of the drift region 101 that is the activated region 108a, ions species having mobilities much different from the velocity of the activated region will escape the activated region 108a. Once removed from the effect of the electric field defined by the activated region 108a, such ion species are not subjected to the decaying electric field and do not travel with the activated region 108a. Therefore, in the context of the present teaching, the expression ‘ion species with close mobility values’ means that those ion species that remain within the activated region 108a during a movement of the activated region 108a through the drift region 101 of the apparatus 100. In this way detected ion species at the end 101b of the drift region 101 will be dependent on predetermined mobility values and on the form of the electric field gradient. The skilled person therefore understands that ions that are too slow or too fast escape the activated region 108a and the travelling electric field and are not therefore bunched, although they may still be separated and detected.

Advantageously, packets of non-separated ions, preferably ions of different species having different mobility values to be sorted, are introduced in the drift region 101 and simultaneously grouped into bunches of same mobility values and separated according to their mobility values by the apparatus 100 as will be described below.

Figures 2a, 2b, 2c show schematically the electric field strength 201, 202, 203a, 203b created by the plurality of electrodes 103 along the drift region 101 at three different given times T1, T2, T3. As shown, the created electric field 201, 202, 203a, 203b decays in the direction A of ion drift along at least a portion of the drift region 101.

For illustrative purposes only, the decay of the electric field is shown herein as being a linear decay with the “x” coordinate along the drift direction A in the drift region 101. It will however be appreciated that other functional dependencies may be used. The created electric field propagates or travels in the drift direction A towards the second end 101b of the drift region 101, that is to the right hand side of Figures 2a-2c. In Figure 2a the electric field has therefore the form 201 at some given time while the electric field has the form 202 at a later given time in Figure 2b and the form 203a-20b at a still later given time in Figure 2c.

More precisely, at a first given time T1, as shown in Figure 2a, the decaying electric field 201 is created by the first subset of electrodes 103a in the activated region 108a, while the remaining electrodes of the plurality of electrodes 103 form the second subset of electrodes 103b of the non-activated region 108b. In the example of the Figure 2a the activated region 108a is located near the first end 101a of the drift region 101, the non-activated region 108b extending aside the activated region 108a until the second end 101b of the drift region 101.

At later given time T2, as shown in Figure 2b, other electrodes are activated to form the first subset of electrodes 103a in the activated region 108a, while the remaining electrodes of the plurality of electrodes 103 form the second and third subsets of electrodes 103b, 103c of the non-activated region 108b and second non-activated region 108c. The decaying electric field 202 created by the first subset of electrodes 103a in the activated region 108a has therefore moved toward the second end 101b of the drift region 101. In the example of the Figure 2b the activated region 108a is located in the middle of the drift region 101, the non-activated region 108b extends aside the activated region 108a until the second end 101b of the drift region 101 and the second non-activated region 108c extends aside the activated region 108a from the first end 101 a of the drift region 101.

At another later given time T3, as shown in Figure 2c, still other electrodes are activated to form the first subset of electrodes 103a in the activated region 108a and the decaying electric field 203a created by the first subset of electrodes 103a in the activated region 108a has again moved toward the second end 101b of the drift region 101. Moreover, the third subset of electrodes 103d is activated to provide the second activated region 108d and create the decaying electric field 203b. In the example of the Figure 2c the activated region 108a is located near the second end 101b of the drift region 101, the non-activated region 108b is located in the middle of the drift region 101 and the second activated region 108d extends aside the non-activated region 108b from the first end 101 a of the drift region 101.

Figures 3a and 3b show schematically the electric field strength 301, 302 created by the first subset of electrodes 103a along a segment of the drift region 101 at two different moments of time U in Figure 3a and h in Figure 3b and illustrate simultaneous bunching and separation of packets of ions 303 to 311 with three different mobility values according to the invention.

Expanding the electric field strength in a Taylor series and considering only the first two expansion terms, the electric field E strength of the inhomogeneous travelling electric field provided by the apparatus 100 according to the invention along a segment of the drift region 101 can be described by the equation E = Edc + G {x-ct), (1) where Edc is the DC electric field; G is the electric field gradient; t is the time and x is the coordinate along the drift region 101 in the drift direction A; c is the velocity of the electric field pattern. Preferably the velocity c of the electric field is constant with time and in the drift region 101. Preferably the velocity of the electric field pattern c is chosen to be equal to the velocity v of a particular ion species that is defined by v = K Edc for an ion species of mobility value K.

The electric field strength as a function of the coordinate x has the form 301 at a moment of time ft in Figure 3a and the form 302 at a later moment fe in Figure 3b.

In the example of Figure 3 ions 303, 304, and 305 have the same mobility value Ko. At the moment of time ft, they have different positions along the drift region 101, therefore different coordinates x in the drift direction A as shown in Figure 3a, and therefore experience each a different strength of electric fields. Ion 304 experiences an electric field equal to Edc; ion 305 experiences an electric field Ei smaller than Edc; ion 303 experiences an electric field Ei larger than Edc.

As a result, ion 304 drifts towards the second end 101b of the drift region 101, that is to the right hand side of Figures 3, with a velocity v = Ko Edc at the moment U. The velocity of the electric field pattern c is chosen to be equal to v. As a result, ion 304 experiences an electric field equal to Edc and has a velocity equal to v also at any moment of time other than h.

At the moment of time fi, ion 305 drifts with a velocity smaller than v, allowing ion 304 to catch up with ion 305 at the later moment tz, as illustrated in Figure 3b Ion 303, on the other hand, drifts at the moment fi with a velocity exceeding the value v. It will at the later moment tz catch up with the slower ion 304. As a result, according to the invention, ions 303, 304, and 305 will bunch together while drifting under the action of the electric field. The spatial width of the packet of ions consisting of ions 303, 304, and 305 in the drift direction A is smaller at the moment of time tz than at the preceding moment fi.

Ions 306, 307, and 308 have a mobility value Κι larger than Ko. At the moment of time h, ion 308 experiences the electric field Ei smaller than Edc. The velocity of the ion drift equals to E1K1 and coincides in the example with the field velocity c. As a result, ion 308 experiences the same electric field and drifts with the same velocity also at any moment of time other than U. Ions 306 and 307 both experience a field stronger than Ei, and at the moment ft they have velocities exceeding the value c. As a result, ions 306 and 307 catch up at the moment of time tz with ion 308. The spatial width of the ion packet consisting of ions 306, 307, and 308 is smaller at the moment of time tz than at the preceding moment f-ι, and the position of the packet of ions 306, 307, 308 differs from the position of the packet of ions 303, 304, and 305.

Ions 309, 310, and 311 have a mobility value Kz smaller than Ko. At the moment of time f-ι, ion 309 experiences the electric field Ez smaller than Edc. The velocity of the ion 309 drift equals to EzKz and coincides in the example with field velocity c. As a result, ion 309 experiences the same field and drifts with the same velocity also at any moment of time other than h. Ions 310 and 311 both experience a field weaker than Ez, and at the moment U they have velocities smaller than c. As a result, ion 309 catches at the moment of time tz with ions 310 and 311. The spatial width of the ion packet consisting of ions 309, 310, and 311 is smaller at the moment of time tz than at the preceding moment f-ι, and the position of the packet of ions 309, 310, and 311 differs from the position of the packet of ions 303, 304, and 305 and the position of the packet of ions 306, 307, and 308.

Ignoring diffusion, the movement of an ion with a mobility value K in an electric field E along the coordinate x is governed by an equation of motion of the form dx/dt = KE. The solution for the electrical field of the form of Equation (1) is of the form x(t) = ct - {KEdc-c)I(KG) + B exp(-KGt), (2) where B is a constant determined by the initial conditions, that is the moment of time at which the ion is introduced into the drift region 101. However, the magnitude of the third term on the right-hand side of the equation (2) will decrease exponentially with time, so that the relative importance of this term will also decrease with time. This term characterizes, therefore, the bunching of ions of the same mobility values. The values of the gradient G, of the mobility K and of the constant B will determine how quickly ion bunches will form.

If the third term in equation (2) is much smaller than the other two and can be ignored, all ions regardless of their mobility will move with the velocity c, as described by the first term in equation (2). The second term is this equation will therefore determine the spatial separation between the ions with different mobility values. Ions with mobility values Ki and K2 will be separated by a distance L = (c/G)|(1/Ki - VK2)\, which is independent of time.

It is therefore apparent from the present description that the ions introduced in the drift region 101 are sorted into distinct bunches of ions of the same mobility without requiring any preliminary ion separation. The apparatus 100 according to the invention provides simultaneously a bunching of ions of the same mobility while providing a separation of ions with different mobilities.

It is also apparent from the present description that the separation between ions of different species and the bunching of the ions of the same species will depend on the values of the parameters c, G, and Edc, as well as the total length of the drift region 101, which determines the ion residence time in the drift region 101. An optimum performance can be achieved for a chosen mobility value by adjusting the parameters of the electric field (c, G, and Edc), which is done by varying the voltages applied by the controlling circuits 104 to the plurality of electrodes 103.

It will be appreciated by those skilled in the art that, depending on the extent of the portion of the drift region 101 where the field gradient exists, a limit can exist to the mobility values offering themselves to the simultaneous bunching and separation by the process described above. Advantageously the parameters of the electric field may be adjusted to increase the resolving power and/or signal levels over a desired range of mobility values. Preferably these mobility ranges are chosen based on a previously measured data. Preferably the mobility ranges are adjacent covering a wide mobility range. Preferably the mobility ranges are partially overlapping.

It will further be appreciated by those skilled in the art that the existence of electric field gradients in the drift direction A will create a component of the electric field in the perpendicular direction to the direction A. This effect can be counteracted, at sub-atmospheric pressures, by a pseudopotential created in the drift region 101 to prevent ions from spreading in the directions perpendicular to the drift direction A. The pseudo-potential may be created by energizing the module 110 or electrodes of the plurality of electrodes 103 with high-frequency voltage sources, in addition to the voltage sources 109 creating the drift electric field.

Numerical example

The parameters chosen for the following numerical example are intended to be illustrative rather than exclusive. Three species of ions are chosen having the mobility values of Ko = 1.5 V/(cm2s), Ki = 1.6V/(cm2s), and K2 = 1.4 V/(cm2s). Ten ions of each species are introduced at the moment of time t = 0 and then every 0.001 ms until t = 1 ms.

In a first calculation, the ions are subjected to a conventional constant homogeneous electric field of the strength E = 200 V/cm.

In a second calculation, the ions are subjected to a travelling inhomogeneous electric field in the form of a inhomogeneous travelling electric field according to the invention of the form E = Edc + A(x-c(t-to)), with Edc = 200 V/cm, A = 200 V/cm2, c = 300 cm/s, and to = 0.5 ms. The coordinate x = 0 corresponds to the entrance of the drift region 101 at the first end 101a.

In both calculations, diffusion is simulated as a random walk in the drift direction A; the diffusion constants are found from the Einstein-Smoluchowski relation for a temperature of 300 K.

Figure 4 illustrates the spatial distributions of the ions of the three species after 10 ms according to the first calculation when the ions drift in the homogeneous static electric field Edc as is provided by a conventional DTIMS. Trace 401 shows the spatial distribution for the ions with the mobility K2; trace 402, for the ions with the mobility Ko; trace 403, for the ions with the mobility Κι. The separation is insufficient to resolve the ions with the three different mobilities Ko, Κι, K2.

Figure 5 illustrates the spatial distributions of ions of the three species after 10 ms according to the second calculation when the ions drift in an inhomogeneous travelling electric field E = Edc + A(x-c(t-to)) provided by the apparatus according to the invention. Trace 501 shows the spatial distribution for the ions with the mobility K2; trace 502, for the ions with the mobility Ko; trace 503, for the ions with the mobility K1. The ion peaks are clearly separated and the resolution is increased significantly compared to the spatial distributions of Figure 4. An additional improvement compared to Figure 4 is the increased amplitude of all peaks.

It is not intended to limit the present teaching to any one set of advantages or features of the preferred embodiment as modifications can be made without departing from the present teaching.

For example other suitable electrical modules 110 adapted to create the electric field to propel the ions may be used, for example grids and resistive glass.

Therefore, while exemplary arrangements have been described herein to assist in an understanding of the present teaching it will be understood that modifications can be made without departing from the spirit and or scope of the present teaching. To that end it will be understood that the present teaching should be construed as limited only insofar as is deemed necessary in the light of the claims that follow.

Furthermore, the words comprises/comprising when used in this specification are to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers , steps, components or groups thereof.

Claims

1. An apparatus (100) for separating and bunching ions according to their mobility, the apparatus comprising: a chamber (120) defining a drift region (101) of the apparatus; and an electrical module (110) configured to operably create inside the drift region (101) an electric field that propels ions in a drift direction (A), wherein the apparatus (100) is configured such that the electric field is operably spatially limited to at least one activated region (108a) defined within the drift region (101), the activated region (108a) having an electrical field that decays in the drift direction (A), the electrical module (110) being arranged to effect a movement of the at least one activated region (108a) in the drift direction (A) from a first end (101a) of the drift region (101) to a second end (101b) of the drift region (101) such that the ions separate and bunch according to their respective mobilities within the activated region (108a).

2. The apparatus of claim 1, wherein the electrical module (110) comprise a plurality of electrodes (103) arranged along the drift region (101) to operably create inside the drift region (101) the electric field that propels ions in the drift direction (A).

3. The apparatus of claim 2 arranged to selectively activate a subset of electrodes (103a) of the plurality of electrodes (103) to provide the activated region (108a) of the drift region (101) and to vary activation of individual ones of the plurality of electrodes to effect the movement of the activated region (108a) in the drift region (101).

4. The apparatus of claim 3, wherein the apparatus (100) is arranged, concurrently with providing the activated region (108a), to provide a non-activated region (108b) using a second subset of electrodes (103b) of the plurality of electrodes.

5. The apparatus of claim 3 or 4, wherein the apparatus (100) is arranged selectively activate a third subset of electrodes (103c) of the plurality of electrodes to provide a second activated region (108c) separated from the activated region (108a) by the non-activated region (108b).

6. The apparatus of any one of the preceding claims, wherein the electric field travels in the drift direction (A) with a constant predetermined velocity.

7. The apparatus of claim 5, wherein the predetermined velocity matches a drift velocity of a predetermined ion species.

8. The apparatus of any one of the preceding claims, wherein the electric field decays linearly in the drift direction (A).

9. The apparatus of any one of the preceding claims, further comprising an introduction device (102) configured to introduce packets of non-separated ions with different mobility values into the drift region (101).

10. The apparatus of any one of the preceding claims, wherein the drift region (101) is operably maintained at a pressure between 0.1 Pascal and atmospheric pressure.

11. The apparatus of any one of the preceding claims, wherein the electrical module (110) is further configured to create in the drift region (101) a pseudo-potential to prevent ions from spreading in the directions perpendicular to the drift direction (A).

12. A method for separating and bunching ions according to their mobility, the method comprising subjecting ions to an electric field that propels the ions in a drift direction (A) in a drift region (101), wherein the electric field is operably spatially limited to at least one activated region (108a) defined within the drift region (101) that travels in the drift region (101) in the drift direction (A) from a first end (101a) of the drift region (101) to a second end (101b) of the drift region (101) and the electric field decays in the drift direction (A) in the activated region (108a) such that the ions separate and bunch according to their respective mobilities within the activated region (108a).

13. The method of claim 12, wherein the electric field travels in the drift direction (A) with a constant predetermined velocity.

14. The method of claim 13, wherein the predetermined velocity matches a drift velocity of a predetermined ion species.

15. The method of any one of the claims 12 to 14, wherein the electric field decays linearly in the drift direction (A).

16. The method of any one of the claims 12 to 15, further comprising introducing packets of non-separated ions with different mobility values into the drift region (101).

17. The method of any one of the claims 12 to 16, further comprising creating in the drift region (101) a pseudo-potential to prevent ions from spreading in the directions perpendicular to the drift direction (A).