Patent Application
20130187044
This invention relates to an ion guide device, especially being capable of introducing ions from higher pressure (or low vacuum) environment into a low pressure environment for mass spectrometry analysis.
In the field of mass spectrometry in order to introduce ions from ion source into mass analyzer at higher pressure (1˜104 Pa or 0.0075˜75 torrs), a high-frequency (or RF) guiding device is normally used. The effective potential barrier formed with the high frequency voltage applied on the electrodes in this device would accelerate ions towards the central axis for focusing. The ions will lose a large portion of their kinetic energy due to collisions with the neutral gas molecules, and hence ions would be confined in the vicinity of the central axis before passing through the aperture for differential pumping and entering into the lower pressure region of a mass spectrometer. This kind of RF focusing device has different variations including D. J. Douglas' initial invention of the multi-pole guide system (U.S. Pat. No. 5,179,278), R. D. Smith's ion funnel (U.S. Pat. No. 6,107,628), N. Inatsugu, H. Waki's Q-array device (U.S. Pat. No. 6,462,338 B1), and Bateman's (U.S. Pat. No. 7,095,013) travelling wave ion guide. However, as the first ion guide right after the ion source, it would experience very strong gas flow induced by the pressure difference. Sometimes the effect of the gas flow on the ions is even stronger than that of the electric field. In such case, the electrodes themselves, or their mounting brackets often inevitably interfere with the gas flow. In addition, the effect of the position of the pipes on pumping may also cause turbulence or flow jitter on the ion path, and further affect the transmission of the ions.
In the U.S. Pat. No. 5,572,035, the inventor Franzen has proposed to use wire electrodes to form ion reflector in order to confine ions. This reflector design can theoretically have good transmission for gas molecules. But generally the meshes are very soft, and the inventor did not give an example of how to securely install them at a specific location without effects from the air flow. If mounted with additional brackets, the additional brackets will also affect the direction of the gas flow. In U.S. Pat. No. 7,391,021, they raise a structure to confine ions in a serial set of stacked RF diaphragm, but the slimsy diaphragms are still with high risk to be shape-changed under high flow rate toward its axis.
In addition, in the ion guiding devices developed in the past, the opposite phases of the high frequency voltages applied on adjacent electrodes (either parallel to each other (between lines or between surfaces), or being concentric rings or arcs) would create very large capacitance between electrodes. For example, U.S. Pat. No. 6,107,628 described an ion funnel design using sheet electrodes for applying voltages with opposite polarity. Similar structure was also introduced in U.S. Pat. No. 7,595,486's RF multipole design. Two groups of adjacent wires with opposite phase RF superimposed are placed all parallel to shape a pole rod surface for confining the ions inside.
In these design, the power consumption of the power supply is very large due to the large capacitance caused by the multiple parallel layers (this is equivalent to many parallel capacitors for the high frequency power supply).
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.
One of the purposes of this invention is to design an ion guide device which can efficiently focus ions at high pressure and high gas flow conditions in a low-vacuum region. This device should be able to minimize the adverse impact of ion guide structure on the neutral gas flow and to minimize the capacitance between electrodes.
In order to reduce the impact of electrode structure on the gas flow, this invention proposes to use wire electrodes to generate the required electric field. However, due to the lack of rigidity of the wire electrodes, it is difficult to precisely fix them in space. Therefore, one need find a good way of fixing the electrodes in space to form appropriate electrode geometry. In addition, the wire electrodes formed in space will have to not only form the required electric fields, but also meet the conditions of minimized inter-electrode capacitance.
This invention presents a kind of ion guide device consisting of multiple layers of stretched wire electrodes distributed along a defined ion guide axis in the ion guide device. Each layer of the wire electrodes contains at least two wire electrodes with some distance away from the ion guide axis, and rotates with an angle relative to their adjacent layers of wire electrodes. The ion guide contains multiple layers of wire electrodes to form a cage-like ion guide channel. A power supply provides voltage to each layer of the wire electrodes, and creates an electric field which focuses the ions towards the ion guide axis.
As a preferred embodiment of the invention, the plane for each layer of the wire electrodes is roughly orthogonal to the said ion guide axis, and the angle between the two ranges from 85° to 95°. In other embodiments of the invention, the angle between the said plane and the said ion guiding axis can be expanded to between 70° to 110°.
As an embodiment of the present invention, it also proposes that each layer of the wire electrodes contains a pair of stretched thin wires equally spaced from the ion guide device. Each layer of the wire electrodes is substantial perpendicular (90 degrees relative) to the next layer of electrodes, and the phases of the high frequency voltages applied on the adjacent pairs are opposite.
As a preferred embodiment of the invention, it also involves reducing the distance between the wire electrodes and the ion guide axis to form a funnel type ion guide device for the purpose of improving ion focusing effect and reducing the adverse effect of the gas flow.
In the embodiment of the present invention, the angle between each layer of wire electrodes and its adjacent layer around the ion guide axis can have multiple variations. For example, the angle can be 360/N degrees, for which N=4, 5, 6, 7, 8, 9, 10, 11 or 12. Thus, we can construct the quadrupole field, hexapole field, octapole field and so on. In some embodiments, the shape of each layer can be formed with wire electrodes with different geometry such as triangle, pentagon and other polygons.
In the embodiment of the present invention, there are several ways of forming the electric field to focus ions towards the ion guide axis. For example, one can provide high frequency voltages to the adjacent electrodes with different phases which could be opposite phases or phase difference being 360/M degrees (M is an integer greater than 1) The amplitude of the high-frequency voltages can be changed. In another example, DC voltages can be provided to wire electrodes on each layer in order to form a gradually changing DC gradient along the ion guide device and its components contain electric field which can focus ions towards the ion guide axis.
In the embodiment of the present invention, the high frequency voltage source includes a number of high-frequency high-voltage switches in order to generate high-frequency square wave voltage.
In the embodiment of the present invention, one can also apply different DC potential to at least some of the wire electrodes so that a potential gradient can be formed along the direction of the ion guide axis.
In the embodiment of the present invention, the ion guide axis cannot only be straight line, but also be curved lines. In this or other embodiments one can further include a DC voltage source to provide a DC potential difference between the opposite wire electrodes of the same pair for at least some of the layers in order to bend the ion beam along the ion guiding axis.
In the embodiment of the invention, gas flow exists in at least part of the ion guide device, and the ion drift direction caused by the potential gradient is opposite to the direction of the gas flow so that only ion with specific mobility can be transmitted effectively.
In one embodiment of the invention, the space settings between wire electrodes and high frequency voltage setting for at least part of the layers will make the ions entering the ion guide device pass, being blocked, or splattered mass selectively near the wire electrodes.
In another embodiment of the invention, the high voltage settings and the potential gradient settings along the ion guide axis will make the ions collide with the neutral gas molecules effectively, and efficiently transmit the product ions, fragment ions, or desolvated ions.
In the embodiment of the invention, gas flow exists in at least part of the ion guide device, and the pressure of the said gas flow is between 10-10000 Pascal (0.075-75 Torrs). In this gas flow environment, the diameter of the wire electrodes is kept less than 0.5 mm in order to reduce their impact on the gas flow. In order to effectively fix the wire electrodes without the interference of the gas flow, one can mount the wire electrodes of the different layers outside of the cage-like ion guide device for which the fixed support or frame have wire electrodes wound around, soldered, or clamped. In the embodiment for which a relatively enclosed frame was used, one can set exhaust holes on the outer wall of the frame in order to reduce the effect of the gas flow.
The embodiment of this invention also proposes a combination of ion guide device structure which includes multiple said ion guide devices, and at least some of the ion guide devices are aligned in parallel in a first direction in order to achieve convergence and/or divergence of ion guide axes.
Further, in some of the embodiments, at least some of the ion guide devices are aligned in series in order to connect ion guide axes in tandem.
These and other aspects of the present invention will become apparent from the following description of the preferred embodiment taken in conjunction with the following drawings, although variations and modifications therein may be affected without departing from the spirit and scope of the novel concepts of the disclosure.
Further features and benefits of the present invention will be apparent from a detailed description of preferred embodiments thereof taken in conjunction with the following drawings, wherein similar elements are referred to with similar reference numbers.
The present invention is more particularly described in the following examples that are intended as illustrative only since numerous modifications and variations therein will be apparent to those skilled in the art. Various embodiments of the invention are now described in detail. Referring to the drawings, like numbers indicate like components throughout the views. As used in the description herein and throughout the claims that follow, the meaning of “a”, “an”, and “the” includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein and throughout the claims that follow, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise. Additionally, some terms used in this specification are more specifically defined below.
The terms used in this specification generally have their ordinary meanings in the art, within the context of the disclosure, and in the specific context where each term is used. Certain terms that are used to describe the disclosure are discussed below, or elsewhere in the specification, to provide additional guidance to the practitioner regarding the description of the disclosure. The use of examples anywhere in this specification, including examples of any terms discussed herein, is illustrative only, and in no way limits the scope and meaning of the disclosure or of any exemplified term. Likewise, the disclosure is not limited to various embodiments given in this specification.
As used herein, “around”, “about” or “approximately” shall generally mean within 20 percent, preferably within 10 percent, and more preferably within 5 percent of a given value or range. Numerical quantities given herein are approximate, meaning that the term “around”, “about” or “approximately” can be inferred if not expressly stated.
As used herein, the terms “comprising,” “including,” “having,” “containing,” “involving,” and the like are to be understood to be open-ended, i.e., to mean including but not limited to.
Embodiments of the present invention are described below with reference to the accompanying drawings, and in the accompanying drawings like reference numerals represent like elements.
In an ion guide structure which uses alternating electric field for confining those charged particles, the process in converging and guiding charged particles is often interfered by the disturbance of gas flow that trapping electrode structure caused. One method to avoid the interference is to miniaturize scale of these electrodes. Thin sheet or wire electrode structure can avoid unwanted disturbance of gas flow. But these thin structures are usually too slimsy to form a stable geometry under strong flow condition.
In accordance with the present invention, an ion guide device was presented comprising multiple layers of stretched wire electrodes crossing in space. These wire electrodes surround a volume space and form a spatial structure similar as the famous Chinese Pavilion building established in 2010 Shanghai World Expo. This structure of wire electrode system provides high transparence and low interference to the gas flow passing through or across the device. A cage-like ion guide tunnel can be formed around the central axis of the device structure with high transmission for the charged particle streams injected. In order to stabilize the whole structure and the position of each wire electrode, a mounting frame is introduced to fix up all the wires with enough tension, and insulate these separated wire electrode parts. As an optimal parameter, the cross angle of adjacent wire electrodes between a pair of neighboring wire layers are substantially perpendicular to each other with the tolerance of +/−5 degrees. Under such conditions, the parasite capacitance load between neighboring layers can be obviously decreased, thus reducing the power consumption of the AC voltage power supply, especially in high frequency (>100 KHz) or radio frequency (RF, commonly >1 MHz) region. The parasite capacitance load between adjacent layers slightly increases if a larger angle tolerance exists for this kind of perpendicular wire position. But even if the cross angles of wires are reduced to 45 degrees, the inter-parasite capacitance load of this kind of wire ion guide device is still much smaller compared to previous ion guide geometry with substantially parallel neighboring electrode such as the conventional ion tunnel and ion funnel presented in the prior arts.
The schematic view
In the ion guide device shown in
Generally, in the ion device described in
In order to reduce the power consumption of the RF power supply for the ion guide, the capacitance load between different layers was reduced by introducing none-zero cross angle between different adjacent layers. Relative thin wire electrodes also help to reduce the power consumption for RF driving especially when high amplitude and frequency RF voltage is applied, which strongly focuses the ion beam through the ion guide device towards its central guiding axis. To avoid the risk of RF discharging between wire electrodes, and suppress the escape of confined ions due to the diffusion of neutral background molecular, commonly wire electrodes and the axis of ion guide should be within enough distance. Considering the adverse effect cause by the fringe field around the cross position of near wires, the proper value of this distance should be no less than 1 millimeter.
The described ion guide apparatus works properly particularly when a gas or molecular flow existed in at least a part of the ion guide structure. The open structure of the ion guide can transmit ions along the ion guide axis without the disturbance on or from the flow in a proper pressure. Typically, the proper pressure of the flow region should be in the range between 0.075 Torr and 75 Torrs.
As an alternative solution for fewer disturbances to gas flow, the diameter of the wires should be equal or less than 0.5 mm. Typically, metal wires such as copper, nickel and stainless steel wire can be used to fabricate the wire electrode. For increasing the surface conductivity of the electrodes, the wire can be plated with a thin layer of high electric conductive materials, such as gold or silver. Under this treating method, the inner body of the wire electrode can be insulator. Both high elastic material, rubber for example, and high strength material, such as quartz glass fiber or capillary, can be used as the inner insulator, and support several segmented wire electrodes on a single wire structure.
In order to form an axial electric field in the guiding tunnel 21, different DC bias potential can be superimposed to each wire layer by a DC power supply 19 through a multiple-node voltage divider 18. Each layer of wire electrodes was connected by one component of voltage divider 18 to produce an axial DC potential gradient along the guiding tunnel 21, which helps to promote the transmission efficiency and lower speed of ions in the ion guide device, especially under a relatively higher working pressure over 0.1 Torr.
In the embodiment of present invention illustrated by
The electric field distribution in this funnel shape ion guide device is also different from that in another kind of conventional ion guide device which is quadrupole rod sets.
Computer simulation is used to prove and estimate this converging process of ion beam in the wire ion guide device. Opposite RF levels were applied to neighboring wire layers and hence the wire layers were defined as two groups. In a specific simulation, the high-frequency voltage applied on each wire group is ±150V (0-peak) sine wave with 1 MHz frequency. The background pressure of ion guide device is 20 torrs (2660 Pa). The distance between wire and guiding axis at the entrance layer is 5.25 mm and gradually reduced to 1.25 mm at the exit layer. The diameter of all wire electrodes is 0.2 mm, and the axial spacing between two layers of wire electrodes is 1 mm. Referring to
In the above simulation, the wire electrodes were driven by a high-frequency sine wave RF power supply. Actually, the high-frequency voltage power supply can also be replaced by a group of high-frequency high-voltage switch, which switches the RF potential of different wire electrodes groups between a high DC level and a low DC level with high frequency (>10 KHz). By this method, a high-frequency square wave voltage signal can be induced between wire electrode groups in order to substitute the conventional sine wave radial trapping voltages.
In the embodiment of the invention, a variety of shapes in the frame can be used to fix the wire electrodes. The simplest shape of the frame can be rectangle (
Another solution for the supporting structure of wire electrode is a kind of column-shaped bracket shown in
In other embodiments of the invention, the wire electrode ion guide device can not only generate quadrupole based focusing field for ion beam focusing, but also be made to generate other forms of multipole radial focusing field such as substantial hexpole or octapole field. As shown in
Similar electrode geometry is shown in
In the next layer of the wire square pattern (constitutes by wire electrodes 921,922,923,924) is rotated by 45 degrees around the guide axis relative to the direction of wire pattern of previous layer, and repeated periodically, thus forming an octapole trapping field ion guide device. Compared to quadrupole field, the hexapole and octapole fields are weaker for ion focusing in the central part of the ion guide, but stronger near the periphery region.
An advantage of this invention is that the beam convergence characteristics in the wire ion guide device can be different in different region as we select. By selecting the pole number of major multipole field with wire patterns, the convergence performance of ion beam in the center or periphery region can be adjusted. With the innate strong rebound performance of ions around the wire electrode region, the combination of different wire patterns in axial projection view can meet the different characteristics of the ion source and gas flow characteristics under the specific local ion guiding requirements, such as expanding the ion beam radius for lower space charge effect or desolvation process, or focusing the ion beam for adapting small aperture to the next vacuum stage.
In addition to the above perspective, the rotation angle also can be 360/N degrees (N=5,7,9,10,11 or 12) to achieve other polygon wire patterns with different multipole fields other than quadrupole, hexpole or octapole.
In the above described embodiment, in guide device all the rotation angles around the guide axis between neighboring wire layer were maintained as a constant, for example 90°, 60°, 45°, etc. However, in further embodiments, these rotation angles are not necessarily fixed. For example,
As a further extended embodiment, the phase difference of the high-frequency potential applied between neighboring layers of wire electrode may not be exactly 180 degrees. If the high-frequency potentials applied between neighboring layers of wire electrodes are 120 or 90 degrees, within 3 or 4 wire layers, the phase shift on wire layers go through one cycle. This embodiment can be further extended to make the phase difference of high frequency potential between neighboring layers as 360/M degree, where M is an integer greater than one. M different phases of high frequency potential waveform are provided to the wire electrode layer 1st to Mth according to the layer sequence along the axis. Such phase-relation pattern repeats within M layers a cycle periodically. In this device, periodic multiphase wire layers can induce a high frequency axial electric field and produce axial travelling wave along the guiding axis to facilitate the guiding to ions towards the direction of transmission. Advantageously, in this case, it is not necessary to introduce an axial DC potential gradient for driving ions, so that the upstream and downstream ion optics of the travelling wave wire ion guide can be substantially equipotential. This can avoid using complex high voltage bias circuit for the serial ion optics linked with the wire ion guide device.
The above descriptions show the example of ion guide example using periodic phase high-frequency voltage to transmit ion along the guide axis. As an extension of this travelling wave ion driving method, the pattern of phase shifts applied on the wire layers can be non-periodic. In this situation, a series of multiphase high frequency potential are applied to these neighboring layers of wire electrodes, which induce an alternating high-frequency electric field between neighboring layers, even in this situation, the alternating electric field inside ion guide device also contains a converge component toward the ion guide axis. The local speed of traveling wave can be adjusted with those phase shifts of high-frequency voltages applied on the neighboring wire layers.
Considering the effect of gas flow, when a gas flow with the same direction and enough strength as the electric field exists inside the wire ion guide device, even without the help of additional axial DC potential gradient (e.g., for positive ions, along the axis of the guide potential gradient is relatively positive to negative from inlet to outlet) or travelling waves, ions in the wire ion guide can be effectively transmitted with the help of high-frequency radial electric field in the wire ion guide. A backward DC gradient (e.g., for positive ions, along the axis of the guide potential gradient is relative negative to positive from inlet to outlet) can also be applied at least on a segment of the ion guide, for blocking high mobility charged particle species. Either the gas flow rate or backward axial potential gradient can be changed in order to separate the ions according to their mobility.
Viewed from the installation process, the entire wire electrodes should be straightened with high tension and the external mounting bracket should have a solid structure and accurate scale. In order to achieve these goals,
In preferred embodiment of present invention, the plane in which wire electrodes of each layer are located is roughly vertical to the ion guiding axis. For example, angle between the layer plane and guiding axis is set to between 85° to 95°. It is understandable that there is no severe impact on ion focusing efficiency if the wire electrode is not vertical to the guiding axis strictly. Typically, the angle between the layer plane and guiding axis can be defined in the range of 90+/−20 degrees, i.e. 70°-110°. In addition, the guiding axis defined in embodiment of the invention is not necessarily a straight line. A curved ion guiding axis can also exist in the wire ion guide device.
The purpose of the embodiments mentioned above is just to indicate the possibility in fabricating this multi-layer wire ion guiding device. It also gives rough approaches and some technical details for fabrication of the ion guide device. However, the wire distribution and electric field structure are not limited to the forms we have described above in present invention. For example, wire electrodes can be in the form of pentagon, pentagram, rectangle, or even hexagon or octagon, etc. Understandably, in these embodiments, it is better to distribute wire electrodes in the similar form between layers. But it is still allowed to have difference in geometry size while with similar geometry form between layers. It is also allowed to have a slight difference in geometry form between layers.
Moreover, voltages applied to wire electrodes can be in the form of square wave, sawtooth wave, pulse sequence or the combination of these forms. As for amplitude of RF voltage which is used to confine ions radially, it is not necessarily the same between layers. For instance, one can change this amplitude applied on wire electrodes of at least some of layers according to distance between two parallel wires on the same layer with the purpose to select ions of different mass-to-charge ratio. Taking that shown in
Furthermore, for DC voltage which is used to produce axial potential, it is not a must in form of arithmetic distribution between layers. As an alternative, this distribution can be changed by setting values in resistor network 18 as needed. For example, a rather negative DC voltage can be applied on certain layers firstly with the purpose to capture ions in this region. Then the DC voltage distribution returns to normal and ions can be released. Another example is to accelerate ions with the axial electric field, or to oscillate ions with a high frequency RF voltage in radial/axial direction, which can increase the number of collisions between ions and neutral gas molecules at high gas pressure. The collision induced reaction product ions, ion fragments or desolvated products can then be guided into analyzer.
There are actually many combinations of variations of embodiments we have described above and we do not necessary to elaborate here. People skilled in the art should be capable of forming practical embodiments by combining the variations of embodiments we described.
For example, the part or whole of straight wire electrodes can be made of thermal resistance materials. A current supply which supplies heating current can be applied between the ends of resistance wire. The heating effect and accompanied infrared rays (IR) can be used to help desolvation, thermal dissociation, IR dissociation of the target ions, etc. In addition, as a variant of the resistance wire, the high permeability material can also be used to make the wire electrodes. A high frequency AC voltage supply, rather than a current supply, is in need to supply the heating by the magnetic inductive eddy current which is similar to that in an induction cooker.
Another example is to combine multiple said guide devices (as shown in 141.1 and 141.2 of
Combination of ion guiding devices shown in
Finally, it should be pointed that in this ion guiding device, the focusing effect to ions can also be achieved by applying particular DC voltages only on wire electrodes. When different DC voltages are applied on these wires, a variable DC electric field strength along guiding axis is produced inside the guiding tunnel. By setting these DC voltages, a DC electric field component can be formed to focus ions in the radial direction.
The foregoing description of the exemplary embodiments of the invention has been presented only for the purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations are possible in light of the above teaching.
The embodiments were chosen and described in order to explain the principles of the invention and their practical application so as to activate others skilled in the art to utilize the invention and various embodiments and with various modifications as are suited to the particular use contemplated. Alternative embodiments will become apparent to those skilled in the art to which the present invention pertains without departing from its spirit and scope. Accordingly, the scope of the present invention is defined by the appended claims rather than the foregoing description and the exemplary embodiments described therein.
