Patent Application
20090140135
This application claims priority to United Kingdom Application GB0722038.7, filed Nov. 9, 2007, which is hereby incorporated by reference.
Not Applicable.
The invention relates to electrode structures for use with charged particle beams. In particular, the invention provides for electrode structures that in various configurations act as an ion trap, ion guide, ion lens, collision cell or mass analyser to trap, transfer, collide, collimate, focus, analyse or filter a beam of ions. The electrode structure may be used to trap, guide or filter ions of interest, generated from a molecular beam, for analysis by their mass to charge ratio in an analytical instrument such as a mass spectrometer detector.
Mass spectrometry (MS) is a powerful analytical technique that is used for the qualitative and quantitative identification of organic molecules, peptides, proteins and nucleic acids. MS offers speed, accuracy and high sensitivity. Key components of a mass spectrometer are the ion source, ion coupling optics, mass analyser and detector. The ion source transforms analyte molecules into a stream of charged particles, or ions, through a process of electron addition or subtraction. The ions can be ‘steered’ using electric or magnetic fields. Ion coupling optics or lenses collimate the ion flux from the ion source into the mass analyser. The analyser separates ions by their mass to charge ratio. Several different kinds of mass analyser are known in the art, including, but not limited to; magnetic sector, quadrupole, ion trap, time of flight and cycloidal. The ions exit the analyser in order of mass to charge ratio and in so doing produces a mass spectrum which is a unique signature or ‘fingerprint’ for the analyte. Ions are directed to a detector where they impact and discharge an ion current which may be counted and amplified by signal electronics before being displayed on a computer screen as a mass spectrum. The detector is normally an electron multiplier. These components together form the analytical sub-system of the mass spectrometer system.
Other mass spectrometer system components include vacuum pumps, a vacuum chamber, drive electronics, data acquisition electronics, power supplies and enclosures.
The mass analyser, or mass filter, as the name implies allows an ion of a chosen mass to charge ratio (m/z) to pass through while rejecting all the others. The possibility of using an electrodynamic quadrupole field was first proposed by Wolfgang Paul at the University of Bonn in the 1950s. This research culminated in a seminal paper by Paul in 1958, and in a basic patent U.S. Pat. No. 2,939,952. The advantages of the quadrupole as a mass analyser include its compactness, mechanical simplicity and sensitivity.
Several types of mass analysers have been developed which utilise electrodynamic quadrupole fields; the quadrupole mass filter, the monopole and the quadrupole ion trap (i.e. the “Paul Trap”). Various other mass analyser geometries that approximate to the quadrupole field have been proposed, including mass filters making use of square rather than cylindrical rods, quadrupole mass analysers driven with square waves rather than sinusoidal waves, cylindrical ion traps, linear ion traps based on the classic quadrupole geometry, so-called rectilinear ion traps using flat plate electrodes, linear ion traps based on segmented quadrupole rods, ‘toroidal’ ion traps wherein a Paul trap is turned into a torus, and so on. However, all of these geometries have it in common that they are used to generate quadrupole electrodynamic fields to trap or filter ions. A major contribution in 1960s was made by Brubaker by providing a pre-filter to provide a delay in the DC ramp, and thereby effecting enhanced sensitivity and resolution in the quadrupole mass filter. A number of more exotic quadrupole-like geometries have been proposed including the monopole, the quadrupole monopole, the use of a spherical retarding-field electrode at the exit of the quadrupole filter to enhance resolution at high mass, a four-fold monopole with a central round rod inside a housing with a square cross section, a ‘solenoid’ mass filter and a static twisted quadrupole.
Until recently, the mass analyser components that are used to generate an electrodynamic quadrupole field have been manufactured from materials like steel and ceramic using conventional “machine shop” processes such as milling, turning, grinding, lapping and polishing. These manufacturing processes and materials are still the mainstay of the mass spectrometer industry and are the basis of almost all products on the market that make use of the quadrupole electrodynamic field principle such as; the hyperbolic ion trap or ‘Quisitor’ (i.e. ‘quadrupole ion store’), the cylindrical ion trap, the orbitrap and ion guides like hexapoles and octopoles. Conventional mass spectrometer components like these are manufactured and assembled using machine tools and other workshop practices. Because mechanical precision is critical to the final performance of the mass spectrometer, these parts are fixed in place and assembled by a trained technician using precise, proprietary tooling.
In U.S. Pat. No. 6,683,301 B2 [Whitehouse et al.], an electrostatic potential is applied to counter electrode positioned above or across from a surface or array of RF electrodes. The counter electrode has an electrostatic potential applied to it which drives ions between the counter electrode and the RF surface towards or away from the RF surface. Ions approaching the RF surface are prevented from hitting the RF electrodes by the repelling pseudopotential field generated by the RF voltage applied the RF electrodes. In US2005/0258364A1 [Whitehouse et al.] a RF surface electrode array is disclosed that has, addition to the counter electrode of U.S. Pat. No. 6,683,301, DC electrodes positioned behind (the “back electrodes”) and on the sides (the “side electrodes”) of the RF surface electrode array. In accordance with US2005/0258364A1, these DC back and side electrodes may be used to control ions in the pseudopotential field between the RF surface electrode array and the counter-electrode. The RF surface electrode array may be made up of an array of spherical electrodes. The RF voltage applied to the RF surface electrodes can be combined with the electrostatic potentials applied the back and side electrodes to control the movement of ions in the pseudopotential field region above the RF electrode array. The main objective of US2005/0258364A1 is to provide a ‘pusher’ electrode in a Time of Flight (TOF) mass spectrometer. This pusher electrode array is intended to be used to generate a ‘pulsed’ packet of ions down the flight tube of the TOF.
It should be noted that U.S. Pat. No. 6,683,301 and US2005/0258364A1 both disclose a RF surface, which can be an array of electrodes, that has a counter electrode positioned opposite, and also behind and around, the RF surface. By applying an electrostatic potential to the counter electrode and a RF voltage to the RF surface, a pseudopotential field may be generated between the counter electrode and the RF surface that traps ions. However, both U.S. Pat. No. 6,683,301 and US2005/0258364A1 rely on DC counter electrodes (i.e. across, behind or to the sides) in cooperation with a RF surface (or RF electrode array) to trap or guide ions.
Several attempts have been made to miniaturise and integrate quadrupole mass analysers using micromachining techniques, or using semiconductor microfabrication processes and micro-electromechanical systems (MEMS) technology, some of which are described in our previously filed British applications, GB 0202665.6 and GB 0217815.0. An example of a miniature quadrupole mass filter is described in our application, GB 0403122.5.
The principal advantages of miniaturised mass analysers are the significantly reduced system requirements, in particular smaller power supplies, electronics and vacuum systems. This dividend is a consequence of the scaling laws associated with geometrically reduced electrical fields, and the shorter mean free path between collisions of molecules. In this way, a miniaturised mass analyser permits the development of mass spectrometer detector systems that are highly deployable, and may be configured for applications and markets that heretofore were not addressed. Examples of these applications include the use of hand-portable mass spectrometer detectors for the detection of explosives, hazardous chemicals and pollutants in the field, or on-line monitoring of reaction processes in the petrochemical industry.
For a miniaturised, or portable, mass spectrometer to be commercially viable, it must achieve the performance required of it by the application. Unless the mass spectrometer detector has acceptable resolution, mass range and sensitivity, it will fail to detect the chemical species of interest with any degree of accuracy. For example, in some applications (in particular explosives detection) sensitivity is particularly valued. The success or failure of a mass spectrometer instrument in these markets will be determined in great part by its performance, and not just by ergonomic factors such as detector size, weight and power consumption. The performance of the mass spectrometer system is determined by the characteristics of the mass analyser used. Therefore, efficient mass analysers are required which can at once combine the benefits of the scaling laws associated with miniaturisation of the analyser, such as smaller power supplies and vacuum pumps, with the raw performance of large, conventionally manufactured mass analysers.
To date, a number of miniaturised mass spectrometer systems have been demonstrated, and a subset of these have been marketed and sold. However, none of the commercially available portable mass spectrometer systems approaches the resolution, sensitivity or mass range of large, conventional ‘benchtop’ mass spectrometer systems. Typical resolution of the commercially available, portable mass spectrometers is limited to a peak width of approximately 1.0 amu across a mass range of 1-450 m/z, and sensitivity (without the aid of a pre-concentrator) of mid to low parts per billion. The goal of this invention is to provide a mass analyser that substantially enhances the performance of a miniaturised mass spectrometer, in particular its resolution and sensitivity, while maintaining the system advantages arising from physical scaling laws such as the relationship between the mean free path and the operating pressure of the mass analyser.
There is therefore a need to provide an improved mass analyser that overcomes these and other disadvantages associated with the prior art.
These and other problems are addressed by the present invention in providing an electrode cell formed from a plurality of individual electrodes arranged relative to one another to define a three dimensional geometric structure having individual ones of the plurality of electrodes located at each of the vertices of the geometric structure and wherein each of the electrodes of the cell present a curved surface to each other electrode of the cell. One or more of these electrode cells may be configured to act as a mass filters, ion guides or ion traps.
The electrodes may be formed having only curved surfaces and may be fabricated in geometries such as those defined by spheroids, hyperboloids and/or super ellipsoids. Within the context of the present invention the term spheroid means is a quadric surface in three dimensions obtained by rotating an ellipse about one of its principal axes.
Electrodes formed in accordance with the teaching of the present invention will have a conducting surface but could be a fabricated in a solid piece or for example could be fabricated having two or more constituents, the outer constituent or layer being formed from a conducting material such as a metal. Examples of such a latter arrangement may include metal coated insulated substrates such as glass or ceramic. A further modification to an electrode structure could be provided by fabricating the electrode from a structure having a conducting surface encapsulating a hollow core. Electrodes formed in accordance with the teaching of the present invention could also be formed from a conductive composite material.
It will be understood that by defining a three dimensional geometric structure that the individual electrodes of the cell are separated from other electrodes of the cell in three dimensions, i.e. the X, Y and Z direction.
In a simplest configuration the cell may be fabricated in a cube geometry having eight corners, each of the corners equidistant from a mid point of the cell, the distance between neighbouring corners also being equal. In such an arrangement the cell includes 8 electrodes, one provided at each corner of the cubic structure.
It will be understood that in a cubic structure the distances between each of the neighbouring electrodes is equal. It is not intended to limit the teaching of the present invention to such a specific geometrical structure in that the distances between adjacent or neighbouring electrodes may be different. In such latter examples it will be understood that a plurality of different geometrical configurations may be defined by the relative orientation of the individual ones of the plurality of electrodes relative to one another.
The invention also provides an electrode matrix formed by combining two or more individual electrode cells. In this context the cell may be considered as a constituent building block of the matrix. It will be understood that certain matrix structures may benefit from using the same building block cells whereas other matrix structure may utilise cells of different geometries. When multiple building blocks are combined, it will be understood that adjacent cells may share the same side walls, in that the electrodes forming the vertices or corners of a first cell may also be considered as forming the vertices or corners of a second neighbouring cell. As the curved surfaces of the electrodes are desirably symmetrical in a three dimensions it will be understood that field lines generated in a first direction from an individual electrode will be equivalent to field lines generated in a second opposing direction.
By providing the individual electrodes in a spheroid geometry located at each of the corners of the cells it will be understood that they present curved surfaces to each of the neighbouring electrodes. If the electrodes are substantially identical, then the field lines that are generated by each of the electrodes will be equal allowing the generation of a pseudopotential well which may be used to filter, trap or guide ions or other charged particles. In an exemplary arrangement the spheroid shape is generated through a circular ellipse such that the resultant electrode is spherical in shape. In this way the electrode cell will be formed from a plurality of spherical electrodes each being centred on the corners of the cells. By applying suitable potential and frequency to the individual ones of the electrodes defining the corners of the cell or indeed by switching the phase of the RF potential applied to each electrode, it is possible to change the velocity of a charged particle and control its trajectory. It will be understood that in this usage, that the term velocity is considered a vector physical quantity, having both speed and direction components.
A cell or matrix arrangement provided in accordance with the teaching of the invention may be considered useful in the fabrication of a number of different structures. As was mentioned above, useful applications include filters, traps or ion guides. It will be understood that quadrupole mass filters are commonly an arrangement of four rods in parallel. In such an arrangement, the rods are typically cylindrical electrodes, or four rod electrodes having a hyperbolic surface facing each other, or more unusually four square rods or four flat plates. By applying equal RF voltages out of phase to diagonally opposed pairs of rods, equipotential field lines are generated between the rods which can be represented by sets of hyperbolae in the x-y plane with a geometrically four-fold symmetry about the z axis. A charged particle, or more conventionally an ion, will have a stable trajectory down the centre of the quadrupole, the trajectory describing a spiral around the z axis. In the context of the present invention the x-y plane the quadrupole may be represented by four circles of radius r, i.e. spheroids where the generating ellipse is a circle, organised symmetrically at the corners of square, such that all four circles are equidistant. A circle of radius r0 touching all four circles can be drawn in the centre of the square.
This x-y plane is a cross section of the quadrupole's four rods which are parallel to the z axis. Each of the diagonally opposite pairs of rods is connected to a RF supply so that the two pairs are out of phase. The rods form an electric multipole, so that ions travelling down the centre of the quadrupole are simultaneously attracted and repelled by oppositely charged rod pairs. The polarity of the rod pairs changes with each cycle of the RF power, so that the ion is attracted and repelled between alternate pairs of rods. In this way the ion describes a stable trajectory between the rods.
A second cross section of the quadrupole rods in the x-y plane may be taken at a point further along the z axis. This point may a distance along the z axis that is equal to the length of one side of a square drawn orthogonally to the z axis between the centrelines of the rods, the centrelines of the rods orthogonally intersecting each corner of the square. These two squares may be joined by lines, parallel to the centrelines of the rods. These connecting lines are therefore equal in length to the sides of the two squares, and together the squares and lines form a cube, which may be considered an electrode cell. Alternatively, if the second cross section is taken at distance along the z axis which is not equal to the length of one side of the square connecting the centrelines of the rods, then together the squares and lines connecting the squares form a cuboid, which again could be considered an electrode cell as formed in accordance with the teaching of the present invention.
At each corner of this cube the rods' cross sectional area may be represented by discs or elliptical (or in the case where the rods have hyperbolic surfaces facing each other, the cross sections are super-elliptical) surfaces drawn in the x-y plane. The quadrupole rods may now be abstractly represented by a cube with a disc at each of its vertices. Four axes may be parallel to they axis, and down four sides of this cube. Each axis intersects the centres of two discs symmetrically. By rotating two of the discs around their common, intersecting axis, two spherical volumes of rotation may be generated. Repeating this for each of the four axes, eight spherical volumes of rotation may be generated, the centre points of which intersect each of the vertices of the cube. In this way an electrode structure cell of eight sphere electrodes of equal volume at each of the vertices of a cube may be constructed. Alternatively, if as a starting point cross sections are taken from quadrupole rods with hyperbolic surfaces, then volumes of rotation of these cross sections around intersection axes of rotation will generate superellipsoids, or superellipsoidal electrodes at each of the vertices of the cube, or cuboid. Similarly, if the quadrupole is constructed from rods with elliptical cross sections then ellipsoids, or ellipsoidal electrodes, may be generated at each of the vertices of the cube, or cuboid. Likewise if the rod surfaces are parabolic then paraboloids, or paraboloidal electrodes, may be generated at the vertices, and so on.
Together, the spheroids (a category which includes spheres, superellipsoids, ellipsoids and paraboloids) subtend an internal spherical volume which intersects the surfaces of each spheroid at a tangent. This internal spherical volume has a radius rs, which although not the same as r0, can be considered to be analogous. This region of the invention is enclosed by the electrode cell. By applying DC and RF voltages in various permutations to the electrodes at the vertices of the cube, the electrode cell, of this invention, various electrostatic and pseudopotential fields may be generated inside this spherical volume that will have the effect of filtering, guiding or trapping ions.
In the context of a cubic electrode cell, if each of the electrodes of the cell is connected to a RF voltage supply, such that each electrode is out of phase with the three electrodes immediately adjacent to it (i.e. the electrodes at each of three connected vertices), then six electrodynamic quadrupole fields may generated across each of the cube's faces. As the polarity of these multipoles is alternated by the AC or RF voltage applied to the electrodes, they form electrodynamic quadrupole fields. An ion approaching along an axis orthogonal to any of these six faces will have a stable trajectory as it nears the cube, and between these spheroids as it passes inside the cube or cuboid. In this configuration, each face of the cube forms an electric multipole. The electrode structure is in effect an ‘all-axis’ (or six-axis) ion guide. If the spheroid electrodes are wired up in this way, then each face of the cube forms a plane with an electrodynamic quadrupole field between the four spheroids at each corner of the face.
It should be noted that when operated in this mode, the electrode structure cell could function as a collision cell for use in a tandem mass spectrometer. ‘Parent’ ions exiting the first quadrupole mass filter enter the electrode structure collision cell along the x axis, and can be collided with reagent ions, or chemical reagents, entering the collision cell along the y and/or z axis. The ‘parent’ ions react with the reagent, and ‘product’ ions are produced which exit the collision cell and are further analysed in the second stage quadrupole mass filter of the tandem mass spectrometer.
Clearly, the electrode structure configured in this way can be multiplied in arrays along the x, y and z axes. Very large arrays of six-axis ion guides may be constructed by replicating the electrode cell structure in this way.
In one embodiment of arrays of the electrode structure, arrays may be configured as N×N, N×M or N×M×O ion guides (where N, M and O are the number of ion channels desired along the x, y and z axis respectively). A lattice or matrix of ion guides, which is analogous to ‘cross-connect’ switches used as components linking and routing fibreoptical and optoelectronic networks, may be constructed from an array of N, M and O electrode structures. This may have applications in mass spectrometry, particle physics and quantum computing.
In another embodiment, the electrode structure may be configured as an RF ion guide or mass filter. In this embodiment, two diagonally opposed pairs of electrodes are connected to an RF voltage supply in phase, with the other two diagonally opposed pairs of electrodes connected to the same RF voltage out of phase. In this way, the electrode structure can be operated as a pseudo quadrupole mass filter in all-pass, or RF only, mode.
By applying a DC voltage ramp to the RF voltage supplies to the four electrode pairs, the electrode structure can be operated as a quadrupole mass filter and will scan ions in order of mass to charge ratio as the DC voltage is ramped. However, a major difference is that in a quadrupole, the DC voltage may be applied only to all four rods, whereas the electrode cell structure of the present invention has the advantage that it may be applied to some subset of all the electrodes. In this way, the electrode structure may be configured to approximate to the performance of a quadrupole with segmented rods—in other words a pseudo-quadrupole mass filter. For example, successive grids of electrodes, or successive electrode structure cells of eight electrodes, could be set up alternately with RF-only, RF and DC and so on, thereby alternately passing, filtering, passing, filtering ions and so on. Another option is to operated the first ‘grid’ of four electrodes in RF-only mode, and apply RF and DC to the second set of electrodes, thereby effecting a delayed DC ramp of the kind described by Brubaker. This configuration would emulate the behaviour of a quadrupole mass filter with pre and post filters.
Therefore it can be see that a unique advantage of the invention is the inherent flexibility of the electrode cell structure, which may be repeated in along all axes serially and/or in parallel, permits many more modes of operation than a traditional quadrupole, segmented quadrupole or tandem quadrupole mass spectrometer.
In common with the ‘all-axis’ RF ion guide described above, or like the ‘ion lattice’ or ion ‘cross-connect’ also described above, in another embodiment the electrode structure cell may be repeated serially to create longer ion guides, quadrupole mass filters or pseudo-quadrupole mass filters. Similarly, the electrode structure cell may be repeated in parallel to create N parallel ion channels, N quadrupole mass filters or N pseudo-quadrupole mass filters. In this way the electrode structures may be configured to act as arrays of multiple, parallel quadrupole-like mass filters, or arrays of multiple parallel RF ion guides. By switching the electrical connectivity of the electrode structure, the direction of the stable ion trajectory through the electrode structure may be ‘switched’ by 90 degrees, so that the trajectory is now parallel to any of the x, y or z axes. A three dimensional array of electrode structure cells may be reconfigured in this way to ‘switch’ the direction of the stable ion trajectory, or to switch the direction of mass analysis through the array, so that it effectively operates as an N×N, N×M or N×M×O ion switch.
In a further embodiment, the electrode structure cell may be operated as an ion trap. If we return to the concept of the quadrupole mass filter operated as an RF ion guide (i.e. in RF-only mode with no ramp of DC voltage), let us imagine an ion with a stable trajectory along the z axis of the quadrupole mass filter. The ion passes through the quadrupole until it reaches a cube (or cuboid) region subtended at each the cube's vertices by eight circular rod cross-sections. Normally, the ion will pass through this cube region and exit at the ends of the rods to a detector, typically an electron multiplier. Let us now imagine this quadrupole mass filter is mounted on its side (i.e. the rods are parallel to the z-x plane) on a stationary turntable. After the ion has passed through the quadrupole aperture and as it is nearing the half way point of the quadrupole we switch on the turntable so that the quadrupole mass filter now rotates at some frequency ω. The quadrupole field is now rotating around the ion. This rotating field should have the effect of trapping the ion within a region intersected by the axis of rotation of the quadrupole mass filter.
If we again consider the cubical (or cuboidal) volume that is subtended between the electrodes (as described above), and which forms an exemplary electrode structure cell of the invention, by careful configuration of the electrical contacts to the electrodes, and the sequence with which the RF voltage phase is applied to the individual spheroid electrodes, we will see that the electrode structure can made to simulate the ‘spinning’ of a quadrupole mass filter around an axis of rotation intersecting the centre of the cubical volume. In once case, this ‘virtual’ axis of rotation may be orthogonal to the top face of the cube (i.e. along the y axis). This axis of rotation may also be parallel to the x or z axes.
By applying RF voltage in phase to a set of two diagonally opposed pairs of electrodes parallel to the x axis, and by applying RF voltage out of phase to the other two diagonally opposed pairs, also parallel to the x axis, we can generate electrodynamic quadrupole fields between these pairs so that together these fields form a pseudopotential well through the electrode structure. Therefore, the electrode structure mimics the behaviour of a quadrupole operated in RF-only mode; in other words, when the quadrupole is functioning as an RF ion guide.
To ‘rotate’ this quadrupole RF ion guide counter-clockwise (for example) around they axis, we next disconnect the same RF voltage supply in phase from the first set of two pairs, and instead connect it to a second set of two diagonally opposed pairs of electrodes, this time parallel to the z axis. Out of phase RF voltage is connected to the other set of diagonally opposed pair of electrodes, also parallel to the z axis. The quadrupole operated as a RF ion guide has now ‘rotated’ 90 degrees.
If this sequence is repeated for the sets of electrode pairs parallel to the x axis, and thereafter to the electrode pairs parallel to the z axis and so on, this should has the effect of creating a rotating pseudopotential well within the electrode structure. This rotating pseudopotential well will act as an electrodynamic saddle point, trapping an ion within the electrode structure cell.
This pseudopotential well, or saddle point, may be visualised as the point of intersection of each of the electrodynamic quadrupole planes. These planes are formed at any instant between a set of four spheroid electrodes, made up of two pairs of diagonally opposed electrodes, connected up to reproduce the field across the cross section of a quadrupole mass filter operated in RF-only mode. The first diagonally opposed pair is electrically connected in phase to the RF voltage supply, and the second pair is connected out of phase, thereby forming an electrodynamic quadrupole field between the four electrodes. This electrodynamic quadrupole field can be thought of as a plane. By sequencing the connection of pairs parallel to the x axis and z axis, these field planes can be made to rotate around they axis (or the x or z axis), thereby generating a stable, saddle point at the point of intersection of these planes.
These and other features and benefit will be understood with reference to the following exemplary embodiments.
A detailed description of preferred exemplary embodiments of the invention is provided with reference to
It will be understood that the presentation of the conventional rod structure as provided in
In this way, if the quadrupole mass filter rods have hyperbolic, elliptical or parabolic surfaces, then the cross-sections will be hyperbolae, super-ellipses, ellipses or parabolas, and hyperboloid, superellipsoid, ellipsoid or paraboloid solids may be generated at each of the vertices of the cube (or cuboid) by similar rotational operations around axes 601, 602, 603 and 604.
It can be seen in
In
Depending on electrical connectivity, individual electrode cells in
The ‘all-axis’ ion guide may also function as a collision cell of the kind used in tandem mass spectrometers. The ‘parent’ ions enter along the x axis (for example) from the first-stage quadrupole (or other mass analyser such as time of flight, or trap etc) mass spectrometer stage and are collided with reagent ions entering along the z or y axis, and react with the reagent ions to generate chemically specific ‘product’ ions. The product ions are now transmitted out of the collision cell along the x axis to a second-stage mass analyser for further analysis.
A delayed DC ramp (which has the effect of increasing transmission and resolution) may be obtained by only applying the ramp in DC voltage to the middle and third electrode cells. By applying RF voltage to the first cell, and not to the later cells, this has the effect of delaying the DC ramp until the second and later cells, thereby operating the first cell as a ‘pre-filter’. Similarly, by only applying the DC to the middle cell, one can operate the first and last cells as ‘pre’ and ‘post’ filters respectively.
As can be seen, this cellular electrode structure geometry is highly scaleable and versatile, and may be reconfigured to operated in a variety of different modes such as all axis ion guide, single axis ion guide, pseudo quadrupole mass filter and as large, multidimensional arrays of the all the above modes.
We now turn our attention to operation of the electrode cell structure as an ion trap.
The invention disclosed here is a means of virtually reproducing the rotation of the quadrupole's electrodynamic field around the axes of symmetry of cube (or cuboid) so that an ion (or ions) may be trapped at the point of intersection of these axes. If we now take
However, to ‘rotate’ the quadrupole field between the electrodes in the anti-clockwise direction, the electrodes are connected as shown in
The quadrupole fields may be visualised as planes subtended between two diagonally opposed electrode pairs, where each diagonally opposed electrode pair has opposite polarity to the other pair. In
Likewise, to aid visualisation,
Heretofore the cells or matrices provided in accordance with the teaching of the invention have been described with reference to the geometrical shapes defining the cell and not with reference to physical devices incorporating such geometries.
It will be appreciated by those skilled in the art that the function of an ion guide is to transmit ions along stable trajectories from one point at the entrance of the guide to another point at the exit. The arrangement of electrode structures described for example in
For example, an RF ion guide may be used as a ‘collision cell’ if placed between two mass analysers inside a tandem mass spectrometer. In the collision cell, ions moving along stable trajectories may be transmitted from the first mass analyser to the second analyser in the tandem mass spectrometer. While inside this ion guide, these ions may be reacted with other ions or neutral species or indeed photons by collision with these particles within the cell.
This collision chamber may be linear or curved or describe an arc and connect two mass analysers. In
A further use of an ion guide is to transmit ions from a first vacuum chamber through a second chamber to a third vacuum chamber. If the ion guide is carefully designed, it may be used to focus ions through orifices of progressively smaller diameters through chambers held at progressively lower pressures and in so doing reduce the pumping load on the vacuum system of the mass spectrometer system. If the ion guide is used to transmit ions from a first vacuum chamber through a second chamber to third vacuum chamber, and if the ion guide is operated within a region which is held at a substantially higher pressure than the third chamber, than a phenomenon known as ‘collisional focussing’ may take place. Such focussing permits the formation of a narrow beam of ions, and this beam may be transmitted by the ion guide through a small orifice between the second chamber and the third chamber, and this small orifice may reduce pumping load on the mass spectrometer system's vacuum system.
It will be appreciated that what has been described herein are exemplary arrangements of one or more electrode structures formed from a plurality of individual electrodes, the plurality of electrodes being arranged relative to one another to define a three dimensional geometric structure with individual ones of the plurality of electrodes located at each of the vertices of the geometric structure and wherein each electrode of the cell presents a curved surface to each other electrode of the cell. Such a structure may be configured as a RF ion guide, mass filter or ion trap. Pseudopotential wells formed between the electrodes of the invention may be used to transfer, guide, manipulate, collimate, focus, filter, analyse or trap ions or other charged particles. The electrode structures may be used to efficiently transfers ions from one location to another along pseudopotential field lines generated between the electrodes by applying a RF voltage supply to the electrodes. The ions may exit and enter the cell through spaces defined between the electrodes. The electrode structure of the invention is inherently scaleable and very large arrays of ion guides, ion traps and mass filters may be constructed. In some embodiments an array of electrode structures may be operated as an ion or charged particle ‘cross-connect’ switch, to guide and redirect ions along orthogonal axes. Applications of the invention may include particle physics, quantum computing and mass spectrometry. While the teaching of the present specification has been explained with reference to exemplary arrangements herein it will be understood that modifications can be made without departing from the spirit and or scope of the present invention. Integers or components that are described with reference to any one Figure could be interchanged or replaced with those of another Figure without departing from the present teaching.
It will be understood that while the cells or matrix structures described herein have not been delimited by dimension that such cells or matrix structures particularly lend themselves to being fabricated as a microengineered or microfabricated structure. Within the context of the present invention the term microengineered or microengineering or microfabricated or microfabrication is intended to define the fabrication of three dimensional structures and devices with dimensions in the order of microns. It combines the technologies of microelectronics and micromachining. Microelectronics allows the fabrication of integrated circuits from silicon wafers whereas micromachining is the production of three-dimensional structures, primarily from silicon wafers. This may be achieved by removal of material from the wafer or addition of material on or in the wafer. The attractions of microengineering may be summarised as batch fabrication of devices leading to reduced production costs, miniaturisation resulting in materials savings, miniaturisation resulting in faster response times and reduced device invasiveness. Wide varieties of techniques exist for the microengineering of wafers, and will be well known to the person skilled in the art. The techniques may be divided into those related to the removal of material and those pertaining to the deposition or addition of material to the wafer. Examples of the former include:
Whereas examples of the latter include:
These techniques can be combined with wafer bonding to produce complex three-dimensional, examples of which are the cells provided by the present invention.
Where the words “upper”, “lower”, “top”, bottom, “interior”, “exterior” and the like have been used, it will be understood that these are used to convey the mutual arrangement of the layers relative to one another and are not to be interpreted as limiting the invention to such a configuration where for example a surface designated a top surface is not above a surface designated a lower surface.
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0722038.7 | Nov 2007 | GB | national |
