Abstract:
A virtual ion trap that uses electric focusing fields instead of machined metal electrodes that normally surround the trapping volume, wherein two opposing surfaces include a plurality of uniquely designed and coated electrodes, and wherein the electrodes can be disposed on the two opposing surfaces using plating techniques that enable much higher tolerances to be met than existing machining techniques.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
     This document is a Continuation of and claims priority to U.S. patent application Ser. No. 10/878,989, filed Jun. 28, 2004, now U.S. Pat. No. 7,227,138 which claims priority to U.S. Provisional Patent Application Ser. No. 60/482,915, filed Jun. 27, 2003. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to storage, separation and analysis of ions according to mass-to-charge ratios of charged particles and charged particles derived from atoms, molecules, particles, sub-atomic particles and ions. More specifically, the present invention is a device for performing mass spectrometry using a virtual ion trap, wherein the aspect of being virtual is in reference to the elimination of electrodes to thereby remove physical obstructions that result in more open access to a trapping volume. 
     2. Description of Related Art 
     Mass spectrometry (MS) is one of the most important techniques used by analytical chemists for identifying and quantifying trace levels of chemical elements and compounds in environmental and biological samples. Accordingly, MS can be performed as an independent process. However, MS becomes more powerful when coupled to separation techniques such as gas chromatography, liquid chromatography, capillary electrophoresis, and ion mobility spectrometry. 
     In MS, ions are separated according to their mass-to-charge ratios in various fields, including magnetic, electric, and quadrupole. One type of quadrupole mass spectrometer is an ion trap. Several variations of ion trap mass spectrometers have been developed for analyzing ions. These devices include hyperbolic configurations, as well as Paul, dynamic Penning, and dynamic Kingdon traps. In all of these devices, ions are collected and held in a trap by an oscillating electric field. Changes in the properties of the oscillating electric field, such as amplitude, frequency, superposition of an AC or DC field and other methods can be used to cause the ions to be selectively ejected from the trap to a detector according to the mass-to-charge ratios of the ions. 
     Mass spectrometers are mainly classified by reference to a mass analyzer that is used. These mass analyzers included magnetic and electric sector, ion cyclotron resonance (ICR), quadrupole, time-of-flight (TOF), and radio frequency (RF) ion trap. 
     Each of these mass analyzers has its own advantages and disadvantages. For example, sector and ICR instruments are known for their high mass resolution, TOF for its speed, and quadrupoles and ion traps for their simplicity and small size. ICR and sector instruments are typically large and complex to operate, and as with TOF, require high vacuum, while quadrupoles and ion traps operate at higher pressures but deliver lower mass resolution. Most analytical problems can be solved using lower performance instruments. Therefore, quadrupole and ion trap mass spectrometers, that are significantly less expensive, are used ubiquitously in the industry. 
     A mass spectrometer is comprised of an ion source that prepares ions for analysis, an analyzer that separates the ions according to their mass-to-charge ratios, and a detector that amplifies the ion signals for recording and storage by a data system. 
     It was noted above that one particular advantage of ion trap mass spectrometers is that these devices typically do not require as high a vacuum within which to operate as other types of mass spectrometers. In fact, the performance of the ion trap mass spectrometer can be improved due to collisional dampening effects due to the background gas that is present. Ion trap mass spectrometers typically operate best at pressures in the mTorr range. 
     It is also observed that the smaller the ion trap, the higher the possible operating pressure. This is an important advantage for portable and handheld instruments, not only because of the reduced size of the ion trap, the electronics and power requirements, but also because of the reduced size of the vacuum pump that must be used. 
     It is important to also note that there has been considerable interest in reducing the size of ion trap mass spectrometers for portable and handheld use. Disadvantageously, a major problem with reducing the size of the ion trap is that machining tolerances become more critical at small sizes while trying to retain good ion trap resolution. One example of a small ion trap was reported by a research group at Oak Ridge. The device is basically a miniaturized version of a cylindrical ion trap with no real changes in the structure, but just the size. 
     It is also noted that the capacity for trapping ions is another issue when dealing with a small ion trap because of the issue of space-charge repulsion of particles within the trap. 
     Accordingly, what is needed is an ion trap that can be easily miniaturized without compromising resolution of the MS, provide easier access to the trapping volume, maximize space within a trapping volume, and meet manufacturing tolerances more easily than prior art machining techniques. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a virtual ion trap that provides easier access to the trapping volume. 
     It is another object to provide a virtual ion trap that can be manufactured more easily than existing machining techniques. 
     It is another object to provide a virtual ion trap that can be miniaturized without sacrificing resolution of the MS. 
     In a preferred embodiment, the present invention is a virtual ion trap that uses electric focusing fields instead of machined metal electrodes that normally surround the trapping volume, wherein two opposing plates include a plurality of uniquely designed and coated electrodes, and wherein the electrodes can be disposed on the two opposing plates using photolithography techniques that enable much higher tolerances to be met than existing machining techniques. 
     In a first aspect of the invention, a plurality of electrodes generating electrical fields are disposed on two opposing plates to thereby create a trapping volume. 
     In a second aspect of the invention, a trapping field can be modified by changing the applied voltages to the plurality of electrodes, changing the number of electrodes, changing the orientation of the electrodes, and changing the shape of the electrodes. 
     In a third aspect of the invention, a plurality of trapping volumes can be created within a single ion trap using the plurality of electrodes described above. 
     In a fourth aspect of the invention, virtual ion trap arrays can be created that are massively parallel or in series. 
     In a fifth aspect of the invention, the virtual ion trap can electronically correct imperfections in the electric potential field lines that are generated to create the trapping volumes. 
     These and other objects, features, advantages and alternative aspects of the present invention will become apparent to those skilled in the art from a consideration of the following detailed description taken in combination with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a perspective view of a prior art ion trap that is known to those skilled in the art. 
         FIG. 2  is an edge view of a first embodiment that is made in accordance with the principles of the present invention. 
         FIG. 3  is a profile view of an inside face of one of the two parallel and opposing surfaces of the first embodiment. 
         FIG. 4  is a profile view of an outside face of one of the two parallel and opposing surfaces of the first embodiment. 
         FIG. 5  is a perspective view of another embodiment of the present invention where the circular opposing faces of the virtual ion trap of  FIG. 2  are now shaped as rectangles. 
         FIG. 6  is an edge-on profile view of virtual ion trap of  FIG. 5 . 
         FIG. 7  is an example of a more complete illustration of the electrical potential field lines that are present in a first embodiment. 
         FIG. 8  is an identical illustration of electrical potential field lines that can be generated within a state of the art ion trap. 
         FIG. 9  is a perspective view of a planar open storage ring ion trap. 
         FIG. 10  is a perspective cross-sectional view of the planar open storage ring ion trap of  FIG. 9 . 
         FIG. 11  is an illustration of a cross-sectional view of the planar open storage ring ion trap of  FIGS. 9 and 10  that at least partially illustrates electrical potential field lines. 
         FIG. 12  is a perspective cross-sectional view of a cylindrical ion trap. 
         FIG. 13  is a cross-sectional and elevational view of the cylindrical ion trap of  FIG. 12  that at least partially illustrates electrical potential field lines. 
         FIG. 14  is a perspective view of a plate  82  and cylinder  84  virtual ion trap. 
         FIG. 15  is a perspective cross-sectional view of the plate and cylinder virtual ion trap shown in  FIG. 14 . 
         FIG. 16  is provided to illustrate the electric potential field lines that are present within the plate and cylinder virtual ion trap of  FIG. 15 . 
         FIG. 17  is a perspective and see-through view of a cylindrical virtual ion trap. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made to the drawings in which the various elements of the present invention will be given numerical designations and in which the invention will be discussed so as to enable one skilled in the art to make and use the invention. It is to be understood that the following description is only exemplary of the principles of the present invention, and should not be viewed as narrowing the claims which follow. 
     It is important to understand several important issues from the outset of the description of the present invention. First, it should be understood that there is no single preferred embodiment, but rather various embodiments having different advantages. No assumptions should be implied as to the best embodiment from the order in which they are described. 
     Second, the present invention is a virtual ion trap that is typically used in conjunction with a mass spectrometer that is typically used to perform trapping, separation, and analysis of various particles including charged particles and charged particles derived from atoms, molecules, particles, sub-atomic particles and ions. For brevity, all of these particles are referred to throughout this document as ions. 
     The present invention can first be described in terms of its functions. Specifically, the present invention is an ion trap for use in a mass spectrometer, but instead of using machined metal electrodes that surround trapped ions, electric focusing fields are generated from electrodes disposed on generally planar, parallel and opposing surfaces. The term “virtual” thus applies to the fact that the confining walls of electrodes are replaced with the “virtual” walls created by the electric focusing fields. 
     The detailed descriptions thus briefly begins by describing some of the better known ion traps as known to those skilled in the art. Consider  FIG. 1  which is a perspective view of a typical ion trap of the prior art. The prior art ion trap  10  is comprised of a metal ring electrode  12  and two metal end caps  14 . The metal ring electrode  12  is equatorially centered. More simplified geometries for ion traps can be found in the prior art such as a simple cylinder ring electrode with solid flat or grid end caps, thereby forming a cylindrical ion trap. Another form of a trap is a linear ion trap. The trapping field is formed using four or more solid metal rods arranged around a central axis, with electrostatic ends caps disposed at each end of the rods. A toroidal ion trap and the cyclical linear trap are similar to a linear quadrupole, but with the electrode rods bent into a circle. This configuration eliminates the need for endcaps. Ions are trapped within the annular space between the four circular rods. Additional ion traps that are known to those skilled in the art include RF and DC Kingdon, DC orbitron, and DC linear, among others. It is noted that traps based only on DC fields require that the ions have significant kinetic energies and defined trajectories. The DC-only traps do not operate in the presence of a buffer gas (i.e., a low vacuum) because buffer gas dampens the trajectories of the ions. 
     What is important to understand from the prior art is that the electrodes used to create the trapping volume are creating substantial barriers, by themselves, to the flow of ions, photons, electrons, particles, and atomic or molecular gases into and emissions out of the ion traps. 
       FIG. 2  is provided as a typical but by no means simplest form of a virtual ion trap  20  that is made in accordance with the principles of the present invention. However, this edge view of the first embodiment demonstrates several important principles of the invention that are common to all embodiments of the invention to be described hereinafter. 
     First, some solid physical electrode surfaces of linear RF quadrupoles and other prior art ion traps are eliminated in favor of virtual electrodes. The virtual electrodes are formed by arranging a series of one or more electrodes on these opposing faces  22  that generate constant potential surfaces similar to the solid physical surfaces that the electrodes replace. 
     Second, the opposing faces  22  are aligned so as to be mirror images of each other. 
     Third, the opposing faces  22  are substantially parallel to each other. 
     Fourth, the opposing faces  22  are substantially planar. However, it is mentioned that the opposing faces  22  may be modified to include some arcuate features. However, optimum results will be maintained by making the opposing faces  22  generally symmetrical with respect to any arcuate features that they may have to thereby make it easier to create a desired trapping volume. 
     The specific features of the first embodiment of  FIG. 1  are now described as follows. The inside and opposing faces  22  have an oscillating electrical field applied thereto. The application of an oscillating field is common to all ion traps described above. The outside faces  24  have a common potential applied thereto that is a common ground in this case. However,  FIGS. 3 and 4  demonstrate some other important features. 
       FIG. 3  shows that both inside faces  22  are coated with an electrically conductive material in a unique pattern so that the lattice of circular patterns  26  remains uncoated. The center of each of the circular patterns  26  has an aperture  28  disposed therethrough to the outside faces  24 . The outside faces  24  and the apertures disposed through the centers of the uncoated circular patterns  26  are also coated with an electrically conductive material that is electrically isolated from the electrically conductive material on the inside faces  22 . 
     It is also noted that the lattice of circular patterns  26  on each of the opposing faces  23  not only are disposed to face each other, but the circular patterns are also concentrically aligned. 
     Another observation needs to be made with respect to coatings. The term “coatings” as used in the present invention refers to conductive materials, non-conductive or insulating materials, and semi-conductive materials that can be disposed on a substrate to give selected portions of electrodes or substrates very specific electrical properties. For example, the coatings can actually function as the electrodes that are disposed on substrates to create the electrical potential field lines to generate trapping volumes. 
     It is also noted that although the lattice of circular patterns  26  is being used in this embodiment, alternatively the patterns can be other shapes as desired, such as squares. 
     When an alternating or oscillating electric field is applied to the two inside faces  22  of the virtual ion trap  20 , and a constant electrical potential is applied to the outside faces  24  and apertures  28 , each of the circular patterns  26  and its opposing circular pattern  26  create a trapping electrical field that can retain ions therein. 
     In the embodiment shown in  FIGS. 2 ,  3  and  4 , the trapped ions are focused toward the center of each of the circular patterns  26  between the opposing faces  22 . A slowly increasing potential difference between the opposing faces  22  can be applied to create a dynamically changing electric field that selectively ejects ions out of the traps at one side or the other according to their mass-to-charge ratios. 
     The virtual ion trap of the present invention has several distinct and important advantages over the state of the art in ion traps. One of the most important aspects of the present invention is the high precision that can be used to construct the electrodes that are disposed on opposing faces. The state of the art relies on machined metal electrodes. The tolerances that can be achieved using machined metal parts are substantially less than the tolerances that can be achieved using photolithography. 
     Photolithography or any other plating technology can be used to dispose electrically conductive traces, or electrodes, on the opposing faces of a virtual ion trap. Obviously, plating techniques such as photolithography are capable of very high precision compared to machined metal parts. For example, the opposing faces  22  of  FIGS. 2 ,  3 , and  4  can be constructed on silicon wafers such as those used in the chip manufacturing industry. Obviously, very high precision is possible because of the advances in precision and reduction in size of traces as known to those skilled in the art of chip manufacturing. 
     Other distinct advantages of the present invention include, but are not limited to, simple fabrication, low cost, miniaturization, and mass reproducibility. 
       FIG. 5  is a perspective view of another embodiment of the present invention.  FIG. 5  shows that the circular opposing faces  22  of the virtual ion trap  20  are now shaped as rectangles  32  in virtual ion trap  30 . The electrodes  34  are now disposed adjacent to opposite edges  36  and  38  of the rectangular opposing faces  32 . The space  40  between the electrodes  34  on the rectangular opposing faces  32  is a resistive material. The oscillating electric field is thus applied to the electrodes  34 , while a constant or common mode potential voltage is applied to outside rectangular faces  42 . 
     Alternatively, the oscillating electric field can be applied to the outside rectangular faces  42 , which the common mode potential is applied to the electrodes  34 . 
       FIG. 6  is an edge-on profile view of virtual ion trap  30 . Note the position of electrodes  34 . Electrical potential field lines  44  are shown at the center of the virtual ion trap  30 . These electrical potential field lines  44  are only partially shown, and illustrate the orientation of the electric potential field lines with respect to each other and the rectangular opposing faces  32 . 
     Another important advantage of the present invention is due to the ability to further shape electric potential field lines that are being generated by the present invention. Shimming is the process whereby additional electrodes are strategically disposed at ends of surfaces, plates, cylinders and other structures that are forming the virtual ion trap of the present invention. The additional electrodes are added in order to modify electrical potential field lines. By applying electrical potentials to these additional electrodes, it is possible to substantially straighten them or make them substantially parallel to each other. This action results in improved performance of the present invention because of the affect of the electrical potential field lines on the ions. 
     However, the affect of shimming is not confined to straightening field lines. It may be that the “idealized” field profile may have lines that are not straight or parallel. Accordingly, shimming can be performed to create a field profile that is “idealized” for any particular application, even if that application requires arcuate field lines. 
     In the embodiment of  FIGS. 5 and 6 , it is observed that shimming electrodes can be added in more than one location. For example, the shimming electrodes can be added as a vertical electrode extending between the opposite edges  36  and  38 . Alternatively, the shimming electrodes can be disposed adjacent to the electrodes  34  that generate the desired electrical potential field lines that create the trapping volume. In another alternative embodiment, the electrodes  34  can even be cut so as to electrically isolated from a portion of the electrodes near the ends of the rectangular opposing faces  32 . 
       FIG. 7  is provided as only an example of a more complete illustration of the electrical potential field lines  44 . Note that a gap  46  is completely open. This gap  46  enables the virtual ion trap  30  to be completely transparent to ejected ions, thereby leading to higher detection efficiency. In addition, the virtual ion trap  30  enables optical beams to penetrate the virtual ion trap to a trapping volume, to thereby enable excitation, ionization, fragmentation, or other photochemical or spectroscopic processes. 
     In contrast to  FIG. 7 ,  FIG. 8  illustrates an identical illustration of electrical potential field lines  52  that can be generated within a state of the art ion trap  50 . However, access to a trapping volume is completely blocked by electrode or wall structure  54 . Thus, the only possible access would be through some small apertures through the wall structure  54 , or through perforations in an endcap (not shown). 
       FIG. 9  is a perspective view of a planar open storage ring ion trap  60 . In an alternative embodiment, the storage ring configuration can be replaced with solid disks that have no aperture through a center axis. The electrodes are disposed in the same locations. 
       FIG. 10  is a perspective cross-sectional view of the planar open storage ring ion trap  60  of  FIG. 9 . Note the electrodes  62  that are disposed adjacent to a center aperture  64  disposed coaxially around a center axis  68 , and adjacent to an outer edge  66 . 
       FIG. 11  is an illustration of a cross-sectional view of the planar open storage ring ion trap  60  of  FIGS. 9 and 10  that at least partially illustrates electrical potential field lines  69 . 
       FIG. 12  is a perspective cross-sectional view of a cylindrical ion trap  70 . Note that electrodes  72  are disposed adjacent to the edges  76 , and disposed coaxially around a center axis  74 . 
       FIG. 13  is a cross-sectional elevational view of the cylindrical ion trap  70  that at least partially illustrates electrical potential field lines  78 . 
       FIG. 14  is a perspective view of a plate  82  and cylinder  84  virtual ion trap  80 . 
       FIG. 15  is a perspective cross-sectional view of the plate and cylinder virtual ion trap  80  shown in  FIG. 14 . Note that there is an electrode  86  disposed inside the cylinders  84  and adjacent to a connection with the plates  82 . Note also the electrode  88  disposed inside and on the plates  82  and adjacent to the connection with the cylinders  84 . 
       FIG. 16  is provided to illustrate the electric potential field lines  90  that are present within the plate and cylinder virtual ion trap  80 . It is noted that an alternative embodiment of the present invention, the view of  FIG. 16  can be extended outwards from the page. In other words, the ion trap can be a linear extension of the walls  82  and  84  that are shown. 
       FIG. 17  is a perspective and see-through view of a cylindrical virtual ion trap  100  wherein an outer cylinder  102  and an inner cylinder  104  have a plurality of electrodes  106  spaced apart and arranged around a circumference thereof. 
     Some other materials that can be used for the construction of a virtual ion trap include a leaded glass semiconductor. The leaded glass semiconductor can be polished or treated to thereby create conductive areas, and not polished or treated to leave resistive areas. 
     Consider also a circuit board as commonly used generally in the art of electronics. On a face side, a plurality of electrodes can be disposed as electrical traces thereon. Apertures can be used to electrically connect the electrodes via resistors on a backside of the circuit board. 
     It is to be understood that the above-described arrangements are only illustrative of the application of the principles of the present invention. Numerous modifications and alternative arrangements may be devised by those skilled in the art without departing from the spirit and scope of the present invention. The appended claims are intended to cover such modifications and arrangements.