CN113871286A

CN113871286A - Ion guide with different multipoles

Info

Publication number: CN113871286A
Application number: CN202110722780.0A
Authority: CN
Inventors: 陈通; G·佩雷尔曼; C·A·弗洛里
Original assignee: Agilent Technologies Inc
Current assignee: Agilent Technologies Inc
Priority date: 2020-06-30
Filing date: 2021-06-28
Publication date: 2021-12-31

Abstract

An ion guide includes electrodes elongated along an axis from an entrance end to an exit end and spaced about the axis to surround an interior. The electrode has a polygonal shape with an inner surface disposed at a radius from the axis and having an electrode width tangent to a circle inscribed by the electrode. An aspect ratio of the electrode width to the radius varies along the axis. The electrodes are configured to generate a two-dimensional RF electric field in the interior, the two-dimensional RF electric field having a multipole component that includes one or more low-order multipole components and one or more high-order multipole components and that varies along the axis according to the varying aspect ratio, and the two-dimensional RF electric field having an RF voltage amplitude that varies along the axis.

Description

Ion guide with different multipoles

Technical Field

The present invention relates to ion guides, in particular linear (two-dimensional) multipole ion guides, as may be used in mass spectrometry systems to guide or transport ions.

Background

Mass Spectrometry (MS) systems generally comprise an ion source for ionising components (particularly molecules) of a sample under investigation, followed by one or more ion processing devices providing various functions, followed by a mass analyser for separating ions based on their different mass-to-charge ratios (or m/z ratios, or more simply "mass"), followed by an ion detector to which the mass-sorted ions arrive and are thereby detected (e.g. counted). The MS system further comprises electronics for processing the output signals from the ion detector as needed to produce user-interpretable data in a format such as a chromatogram or mass spectrum, which typically exhibits a series of peaks indicative of the relative abundance of detected ions (e.g., ion signal intensity, such as the number of ion counts per ion detected) as a function of their m/z ratio. The mass spectra (and/or MS/MS fragment spectra) can be used to determine the molecular structure of the components of the sample, thereby enabling qualitative and quantitative characterization of the sample, including the identification and abundance of the chemical compounds of the sample (and possibly also isotopologues and/or isotopes of each compound found in the analysis).

The mass spectrometry technique can be enhanced by combining it with another analytical separation technique prior to the MS analysis stage, thus acting as a first stage of analytical separation. Examples include chromatographic techniques such as Liquid Chromatography (LC) or Gas Chromatography (GC) and electrophoresis-based techniques such as Capillary Electrophoresis (CE). In a hybrid LC/MS, GC/MS, or CE/MS system, the separated compounds eluted from the chromatography column or electrophoresis instrument (e.g., CE capillary) are introduced into the ion source of the MS system, and the MS system processes the separated compounds as outlined above. A hybrid MS system may combine the advantages of a first stage analytical separation technique (e.g., LC, GC, or CE) and a second stage analytical separation technique (MS). For example, a hybrid MS system can acquire three-dimensional (3D) LC/MS, GC/MS, or CE/MS data from a sample, characterized by retention time (or elution time or acquisition time), ion abundance, and m/z as sorted by the MS system. Multidimensional MS data is used to measure and distinguish different compounds of complex samples. For example, two different compounds may co-elute from the chromatography column at about the same time, but because they have different masses, they can be subsequently separated by the MS system to avoid overlapping peaks in the data (assuming the MS system is operating with sufficient resolution).

MS systems include one or more ion guides, which are typically configured as linear (two-dimensional) multipole ion guides. Generally, the ion guide has an electrode arrangement surrounding an interior space between the ion inlet and the ion outlet. The ion guide transports ions from a preceding device to a succeeding device of the MS system through an inner space thereof. To this end, the ion guide is configured to generate a Radio Frequency (RF) field in its interior space that is effective to focus ions as an ion beam on a central longitudinal axis of the ion guide. Conventionally, linear multipole ion guides have pairs of cylindrical electrodes (or "rods") arranged parallel to each other and circumferentially about a common longitudinal axis. Each pair of electrodes, which are diametrically opposed to each other on either side of the longitudinal axis, are electrically interconnected and supplied with an RF voltage potential. The RF voltage potential applied to one or more electrode pairs is 180 degrees out of phase with the RF voltage potential applied to the other adjacent electrode pair(s). Due to the corresponding pseudo-potential traps caused by the RF electric field, the RF multipole is able to confine the ions in a plane orthogonal to the longitudinal axis, which limits the radial trajectories of the ions and thus focuses them as a beam on the central axis. For this purpose, the parameters of the RF electric field are suitably set such that ions having a desired mass range will be stabilized in the ion guide. In particular, the RF value (i.e., the rapid rate of change of the RF electric field) and RF amplitude (i.e., the strength of the RF electric field pushing or pulling the ions) are set such that the ions will remain focused on the ion guide axis and will not collide with the ion guide electrodes as they travel down the length of the ion guide. At any given time, ions accelerated by the RF electric field towards a certain electrode will then be rapidly accelerated towards a different electrode operating in the opposite phase to the first electrode, whereby the time-averaging effect is an on-axis beam focusing due to the (effective) constant two-dimensional (radial) restoring force imparted by the RF electric field directed towards the axis.

The ion guide may be part of a collision cell. In the collision cell, the electrode structure of the ion guide is enclosed in a housing filled with a "collision" gas (also known as a damping gas, buffer gas, or bath gas, typically an inert gas such as nitrogen, argon, etc.). The collision cell may function as an ion cooler that helps focus ions on the longitudinal axis by reducing (damping) their kinetic energy (or "thermalizing" the ions) via collisions with neutral collision gas molecules (i.e., "collision cooling" or "collision focusing"). The collisions cause the ions to lose their kinetic energy and move toward the central longitudinal axis where the effective potential is at a minimum. Thus, the collision cell reduces the cross-section of the ion beam and the radial kinetic energy spread of the ions. Alternatively, the collision cell may function as an ion fragmentation device, where the pressure is high enough (typically a few millitorr to a few tens of millitorr) to ensure efficient ion fragmentation via collisions with neutrals. That is, in addition to thermalizing ("precursor") ions, collision cells can yield "fragment" ions (or "product" ions) by means of collision induced dissociation (CID, also known as collision activated dissociation or CAD). In either case, in addition to the RF potential, a Direct Current (DC) potential gradient in the axial direction is imposed over the length of the collision cell to counteract the axial kinetic energy loss of ions due to collisions and thereby address issues related to kinetic energy loss, such as ion stagnation in the collision cell. Conventionally, the ion guide electrodes are coated with a resistive material so that a DC potential can be established along the longitudinal axis. Alternatively, the ion guide electrodes are segmented in the axial direction and different magnitudes of DC potentials are applied to the individual segments to form an axial DC gradient. In some cases, a DC potential barrier may be temporarily applied at or additionally to the exit end of the ion guide to operate the ion guide as an ion accumulator or ion trap.

It is often desirable to have an ion guide that effectively converges an ion beam passing through its interior space from its ion entrance to its ion exit, particularly when part of a collision cell. That is, the ion guide should provide a large ion acceptance rate at the ion inlet in order to maximize the amount of ions captured from the preceding device, and a small ion emittance at the outlet in order to minimize beam phase space, for example, to effectively transfer to the succeeding device when it is desired to transmit ions through a small gas-conducting confinement aperture in front of the succeeding device. One example is an MS system in which the ion source is followed by a quadrupole mass filter or mass analyser (with four electrodes extending in the axial direction), then a collision cell with a multipole ion guide, and then a (final) mass analyser (such as a time of flight (TOF) analyser). The quadrupole mass filter transfers precursor ions of a selected mass into a collision cell that fragments the precursor ions into product ions via CID. The product ions are then transferred to a final mass analyser from which fragment ions of different masses are successively transferred to an ion detector. In such systems, the ion beam entering the final mass analyzer from the collision cell should have a significantly smaller cross-sectional diameter than the ion beam exiting the quadrupole mass filter and entering the collision cell.

There is a continuing need for further development in the field of ion guides, including those utilized in collision cells.

Disclosure of Invention

To address the foregoing problems, in whole or in part, and/or other problems that may have been observed by those skilled in the art, the present disclosure provides methods, processes, systems, devices, apparatuses, and/or devices, as described by way of example in the implementations set forth below.

According to one embodiment, an ion guide comprises: an ion inlet end; an ion outlet end; and a plurality of electrodes elongated along an ion guide axis from the ion entrance end to the ion exit end and spaced apart from each other about the ion guide axis to surround an ion guide interior, the electrodes comprising a polygonal shape with respective inner surfaces disposed at a radius from the ion guide axis, wherein: the inner surface is inscribed on a circle on the ion guide axis having the radius; the inner surfaces have respective electrode widths tangent to the circle; the aspect ratio of the electrode width to the radius varies along the ion guide axis; and the plurality of electrodes are configured to generate a two-dimensional RF electric field in a transverse plane orthogonal to the axis within the ion guide interior, the RF electric field comprising a superposition of a lower order multipole component and a higher order multipole component, wherein an amplitude ratio of the lower order component to the higher order component varies along the ion guide axis according to the varying aspect ratio, and the RF electric field has an RF voltage amplitude that varies along the ion guide axis.

According to another embodiment, a collision cell includes: a housing; and an ion guide according to any one of the embodiments disclosed herein disposed in the housing.

According to another embodiment, a Mass Spectrometry (MS) system includes: the ion guide according to any one of the embodiments disclosed herein; and a mass analyzer in communication with the ion guide.

According to another embodiment, a Mass Spectrometry (MS) system includes: the ion guide according to any one of the embodiments disclosed herein; and a controller comprising an electronic processor and a memory and configured to control the steps of the method according to any one of the embodiments disclosed herein, in particular to control the operation of the ion guide.

According to another embodiment, a method for transporting ions comprises: applying RF voltage potentials to the plurality of electrodes of an ion guide configured according to any of the embodiments disclosed herein to generate the two-dimensional RF electric field in the ion guide interior; and admitting the ions into the ion guide interior so as to subject the ions to the two-dimensional RF electric field and confine the ions radially to an ion beam along the ion guide axis.

According to another embodiment, a method for analyzing a sample comprises: generating analyte ions from the sample; transporting the analyte ions into an ion guide according to any of the embodiments disclosed herein; and operating the ion guide according to any one of the embodiments disclosed herein.

Other apparatuses, devices, systems, methods, features, and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims.

Drawings

The invention may be better understood by reference to the following drawings. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. In the drawings, like reference numerals designate corresponding parts throughout the different views.

Fig. 1 is a graph of the magnitude of pseudo-potential (normalized in arbitrary units) as a function of radial distance (normalized in arbitrary units) from the ion guide axis (or "device axis") of a linear multipole ion guide.

Fig. 2 is a schematic cross-sectional view of an example of a linear (e.g., rectangular) geometry ion guide according to an embodiment of the present disclosure.

Fig. 3 is a schematic cross-sectional view of another example of a linear geometry ion guide according to the present disclosure.

FIG. 4 is a graph of seven different multipole coefficients Φ as a function of aspect ratio of a linear multipole ion guide_N(N-0, 1, 2, 3, 4, 5, 6).

Fig. 5A is a schematic perspective view of an example of a linear geometry ion guide according to an embodiment of the present disclosure.

Fig. 5B is a schematic cross-sectional side (length-wise) view of the ion guide illustrated in fig. 5A, showing one electrode pair for clarity.

Fig. 5C is a schematic end view of the ion guide shown in fig. 5A at its ion inlet.

Fig. 5D is a schematic end view of the ion guide shown in fig. 5A at its ion outlet.

Fig. 6A is a schematic perspective view of an example of a linear geometry ion guide according to another embodiment of the present disclosure.

Fig. 6B is a schematic side (length-wise) view of the ion guide shown in fig. 6A, wherein one of the electrodes is not shown for clarity.

Fig. 6C is a schematic end view of the ion guide shown in fig. 6A at its ion inlet.

Fig. 6D is a schematic end view of the ion guide shown in fig. 6A at its ion outlet.

Fig. 7A is a schematic perspective view of an example of a linear geometry ion guide according to another embodiment of the present disclosure.

Fig. 7B is another schematic perspective view of the ion guide shown in fig. 7A.

Figure 7C is an end view of the ion guide shown in figure 7A at its ion inlet.

Fig. 7D is an end view of the ion guide shown in fig. 7A at its ion exit.

Fig. 8 is a schematic cross-sectional view of an example of a linear geometry ion guide according to another embodiment of the present disclosure.

Fig. 9A is a schematic perspective view of an example of an ion guide electrode according to an embodiment of the present disclosure.

Fig. 9B is a schematic perspective view of a section of the ion guide electrode shown in fig. 9A.

FIG. 10 is a schematic diagram of an example of a Mass Spectrometry (MS) system according to the present disclosure.

Detailed Description

As described above, the RF electric field generated in the ion guide creates a pseudo-potential well. The pseudo-potential (or effective mechanical potential) describes the time-averaged effect of the RF electric field in the ion guide. The RF electric field is a composite or linear superposition of multipole components of different orders N that contribute to the overall RF electric field to varying degrees (i.e., some multipole components are stronger than others) and thus affect ion motion to varying degrees. Some common examples of multipole components include quadrupole (N-2), hexapole (N-3), octupole (N-4), decapole (N-5), and decapole (N-6) components. The shape of the pseudo-potential well corresponding to the multipoles of different orders is different. This is illustrated in figure 1 for quadrupole, hexapole and octapole components, which is a graph of the magnitude of the pseudo-potential (normalized in arbitrary units) as a function of the radial distance r (normalized in arbitrary units) from the ion guide axis (or "device axis") of the sub-guide. The pseudo-potential is related to the radial distance r and multipole order N as follows:

U_eff(r)∝r^(2N-2)

this relationship results in the pseudo-potential well shape shown. The pseudo-potential increases with the radial position (distance r from the sub-guide axis equal to 0). Thus, ions further from the axis will push them back to the axis due to the stronger effect they experience from the pseudo-potential. Also, ions close to the axis will experience very little radial restoring force from the pseudo-potential, i.e. such ions are in the depth of the trap. As also shown in fig. 1, the higher order multipole pseudo-potential trap has a wider base, i.e., the effective potential, which is relatively flat near the ion guide axis, increases more rapidly as the electrodes approach (i.e., steeper near the electrodes). In a potential well with a wide bottom, ions can occupy a larger portion of the cross-sectional area inside the ion guide before undergoing significant restoring forces from the pseudo-potential back toward the axis. Thus, a broad-bottomed potential well is advantageous in capturing ions having a greater emittance (such as from a widely diverging ion beam), reducing space-charge forces in the ion guide, and transferring ions having a wider range of m/z ratios. At the same time, however, a broad-bottomed potential well is not advantageous for transferring the ion beam through a narrow gas-conducting confinement aperture at the interface between the ion guide and the following device or pumping stage receiving the ion beam from the ion guide (to optimize multi-stage pumping operation), since many ions of the large beam space will strike the walls around the aperture and thus be lost from the ion workflow. On the other hand, the lower order multipole pseudo potential trap has a narrower base, which is advantageous for compressing the ion beam diameter (focused ion beam) in combination with collisional cooling.

Due to the functional requirements of the collision cell, three main features should be realized. First, the effective bottom of the pseudo-potential well should be tapered to compress the ion beam diameter while reducing the radial kinetic energy spread of the ions, as further facilitated by collisional cooling. Second, the two-dimensional RF electric field used to confine ions in a plane orthogonal to the longitudinal ion guide axis (referred to herein as the transverse plane) needs to vary with changes in ion beam characteristics (e.g., beam radius, radial energy, etc.) in the axial (longitudinal) dimension to meet adiabatic requirements, i.e., so that the RF electric field does not transfer too much heat to the ions. For example, RF heating of ions may not properly compete with desired collisional cooling of ions. The effective pseudo-potential well depth of the multipole ion guide has the following analytical solution:

where n is the order of the multipole, q is the ion charge in coulombs, m is the mass of the ion, ω is the angular frequency of the applied RF voltage in radians (ω -2 f where frequency f is in hertz), r₀Is the radius of a circle in a transverse plane inscribed by the ion guide electrode, in meters, and V₀Is the magnitude (zero to peak) of the applied RF voltage in volts.

Adiabatic requirements impose a low mass cut-off (the minimum mass of ions that will be stable in the ion guide and can therefore be transferred by the ion guide) in the following equation:

where K is the ion radial kinetic energy in joules (J), and η_maxIs a characteristic parameter of the insulation requirement.

The RF voltage should be adapted to the changes in other parameters as the ion beam is radially compressed by tapering the bottom of the pseudo-potential well, thereby meeting adiabatic requirements to maintain a range of m/z ratios to be transferred by the ion guide. The considerations of thermal insulation are further discussed in Gerlich, D., Inhomogenes RF Fields: A Versatile Tool for the Study of Processes with Slow Ions, State-Selected and State-to-State Ion-molecular Reaction Dynamics, Part 1: Experimental, John Wiley & Sons company (1992), pages 10-26; the entire contents of which are incorporated herein by reference.

Third, in order to move ions forward in the axial (longitudinal) dimension under relatively high pressure environments (e.g., in a collision cell), a longitudinal potential difference (or axial DC potential gradient) needs to be established to compensate for the energy lost in the collisions of the ions with neutral gas molecules.

While RF multipole electrodes typically have a cylindrical geometry to better approximate the ideal multipole shape, RF multipole electrodes having alternative cross-sectional shapes (e.g., linear or other polygonal shapes) possess some other features of interest.

Fig. 2 is a schematic cross-sectional view of an example of a linear (e.g., rectangular) geometry ion guide 200 taken at some arbitrary point along an ion guide axis (or device axis) 204 of the ion guide 200 between an ion entrance end and an ion exit end, in accordance with embodiments of the present disclosure. For reference purposes, FIG. 2 includes an arbitrarily positioned Cartesian (x-y-z) reference frame. In this example, ion guide axis 204 corresponds to the z-axis, and a transverse plane orthogonal to ion guide axis 204 corresponds to the x-y plane. In the context of the present disclosure, the term "axial" refers to the direction of the ion guide axis 204 or generally parallel to the ion guide axis 204.

The ion guide 200 comprises a plurality of electrically conductive

ion guide electrodes

208A, 208B, 208C and 208D which are elongate along the ion guide axis 204 from the ion entrance end to the ion exit end and are circumferentially spaced from one another in a transverse plane about the ion guide axis 204 to surround an ion guide interior 212 which is therefore also elongate along the ion guide axis 204.

Electric powerThe poles

208A, 208B, 208C, and 208D are typically circumferentially spaced apart from one another by equal distances at a given axial position. However, in some embodiments, the circumferential spacing between the

electrodes

208A, 208B, 208C, and 208D may vary as one moves along the ion guide axis 204. The

electrodes

208A, 208B, 208C, and 208D may be precisely positioned relative to one another and electrically isolated from one another in any known manner, such as by utilizing electrically insulating structures and fastening elements, as will be appreciated by those skilled in the art. The

electrodes

208A, 208B, 208C, and 208D have a polygonal shape with corresponding inner surfaces 216 disposed at a radius r from the sub-guide axis 204₀And faces the ion guide interior 212. In the present example, the

electrodes

208A, 208B, 208C, and 208D are planar or plate-shaped with a rectangular cross section, but may have any polygonal or prismatic shape as a whole. In the present example, the electrode structure or arrangement of ion guide 200 is a quadrupole. That is, ion guide 200 includes four

longitudinal electrodes

208A, 208B, 208C, and 208D. Specifically, the ion guide 200 comprises two electrode pairs, wherein the electrodes of each pair are diametrically opposed to each other with respect to the ion guide axis 204. Thus, in the illustrated example, ion guide 200 includes a pair of

X electrodes

208A and 208B and a pair of

Y electrodes

208C and 208D. In other embodiments, ion guide 200 may include a greater number of electrodes, such as in the case of a hexapole (six electrodes), an octapole (eight electrodes), or higher multipole, for example. As shown in FIG. 2, each pair of opposing electrodes 208A/208B and 208C/208D is at 2r₀Or the diameters of the L are spaced apart from each other. More generally, in an exemplary embodiment, the number of longitudinal ion guide electrodes of ion guide 200 is 2N, where N is an integer equal to or greater than 2. As shown in further examples described below, the plurality of electrodes of ion guide 200 may have 2N times rotational symmetry about ion guide axis 204 from the ion entrance end to the ion exit end. For example, the quadrupole structure shown in fig. 2 can have 4 times rotational symmetry along the entire axial length of the ion guide 200.

In some embodiments,

electrodes

208A, 208B, 208C, and 208D are in electrical communication with an ion conductorThe deflector axis 204 is parallel such that the radius r₀Is constant along the ion guide axis 204. In other embodiments,

electrodes

208A, 208B, 208C, and 208D are oriented at an angle to ion guide axis 204 such that radius r₀Is variable along the ion guide axis 204. That is,

electrodes

208A, 208B, 208C, and 208D may converge toward one another (or diverge away from one another) in a given direction along ion guide axis 204. In some embodiments, the dimensions (shape and size) of the

electrodes

208A, 208B, 208C, and 208D are constant along the ion guide axis 204. In other embodiments, one or more of the dimensions (shapes and/or sizes) of the

electrodes

208A, 208B, 208C, and 208D are varied along the ion guide axis 204. As shown in further examples described below, the plurality of electrodes of ion guide 200 may have 2N times rotational symmetry about ion guide axis 204 from the ion entrance end to the ion exit end, where again N is an integer equal to or greater than 2. For example, the quadrupole structure shown in fig. 2 can have 4 times rotational symmetry along the entire axial length of the ion guide 200.

The plurality of

electrodes

208A, 208B, 208C, and 208D are configured to generate a two-dimensional, time-varying (radio frequency or RF) ion guide electric field in the ion guide interior 212 (i.e., between opposing pairs of

electrodes

208A, 208B, 208C, and 208D). The RF electric field has a multipole component with low and high order components that vary along the ion guide axis 204, and an RF voltage amplitude that varies along the ion guide axis 204 in a manner described below.

Typically, each pair of opposing electrodes 208A/208B and 208C/208D is electrically interconnected (as illustrated) to facilitate application of appropriate RF voltage potentials that drive the RF ion guide field. An RF power supply 220 (generally representing suitable known components (e.g., one or more waveform generators, one or more amplifiers, RF circuitry, etc.)) is schematically depicted as including a first RF voltage source + V in communication with the first electrode pair 208A/208B_RFAnd a second voltage source-V in communication with the second electrode pair 208C/208D_RF. For generating one or more two-dimensional RF electric fields, an RF voltage potential V of general form is applied_RFcos (ω t) is applied to the opposing pairs of interconnected electrodes 208A/208B and 208C/208D, where the potential applied to one electrode pair 208A/208B is 180 degrees out of phase with the potential applied to the other electrode pair 208C/208D. In general, from the perspective of the transverse plane of fig. 2, regardless of how many electrode pairs are provided, each electrode will typically be driven by an RF potential 180 degrees out of phase with the two electrodes adjacent to the electrode. Typically, the amplitude V of the RF potential is for all

electrodes

208A, 208B, 208C, and 208D of ion guide 200_RFAnd the absolute value of the frequency co will be the same. The basic theory and application of the generation of multipole RF fields for ion focusing, cooling and other processes is generally known to those skilled in the art and, therefore, need not be described in detail herein.

Additionally, a DC power supply 224, generally representing suitable known components (e.g., one or more amplifiers, DC circuits, etc.), is schematically depicted as including two DC voltage sources in communication with the

electrodes

208A, 208B, 208C, and 208D. DC power supply 224 is configured via

electrodes

208A, 208B, 208C, and 208D to generate an axial DC potential gradient along the length of ion guide 200 (ion guide axis 204), an example of which is described below.

The two-dimensional time-varying potential Φ (x, y) of a rectangular quadrupole (such as the ion guide 200 illustrated in fig. 2) has an analytical solution described below:

where n is the order of the multipole, x and y are spatial coordinates in a transverse plane orthogonal to the ion guide axis 204, and L-2 r₀Is the lateral distance between the diametrically opposed pairs of electrodes 208A/208B and 208C/208D.

The potential solution can be extended by a series of multipole components as described below:

wherein,

is an nth order multipole term or coefficient (also known as a spatial harmonic), A_NIs an nth order multipole term

Amplitude or intensity of Re [ (x + iy)^N]Is a complex variable function (x + iy)^NA real part of (a), and i²Is-1. Thus, in an expanded form including the first few multipole terms (for quadrupole, hexapole, octupole, decapole and twelve), the potential solution can be expressed as:

for symmetrical spacing from the ion guide axis by radius r, as a few examples₀The four-pole, six-pole, eight-pole, ten-pole and twelve-pole potentials are respectively as follows:

see further Douglas et al, Linear Ion traces in Mass Spectrometry, Mass Spectrometry Reviews, Vol.24, Wiley Periodicals, Inc. (2005), pages 1-19; and Konenkov et al, Spatial Harmonics of linear multipoles with round electrodes, International Journal of Mass Spectrometry, Vol.289, Elsevier B.V. (2010), p.144-149; the entire contents of which are incorporated herein by reference.

Fig. 3 is a schematic cross-sectional view of another example of a linear geometry ion guide 300 according to the present disclosure. As in the previous example, the ion guide 300 includes two pairs of

ion guide electrodes

308A, 308B, 308C and 308D surrounding an ion guide interior 312, with respective inner surfaces 316 facing the ion guide interior 312. Each

electrode

308A, 308B, 308C, and 308D, or at least each inner surface 316 thereof, has an electrode width W in a lateral plane. The

electrodes

308A, 308B, 308C, and 308D, or more specifically the inner surfaces 316 thereof, are collectively inscribed with an inscribed diameter D (corresponding to the transverse distance L-2 r in fig. 2) in a transverse plane of the ion guide interior 312 (i.e., the transverse distance L-2 r in fig. 2)₀) The circle of (c). (rectangle) electrode width W and inscribed radius r₀The ratio of (i.e., referred to herein as the "aspect ratio" of the rectangular ion guide 300) determines the composition of the multipole electric field (i.e., the coefficient of the multipole component in the potential expansion). If the

electrodes

308A, 308B, 308C, and 308D each have the same width at any axial point along the length of the ion guide 300, the electrode width W for the aspect ratio may be the width of a single one of the

electrodes

308A, 308B, 308C, and 308D. Alternatively, the total width of the

electrodes

308A, 308B, 308C, and 308D (four in this example) may be used as the electrode width W in the aspect ratio.

This fact is illustrated in FIG. 4, which is a graph of seven different multipole coefficients Φ as a function of aspect ratio_N(N-0, 1, 2, 3, 4, 5, 6). It should be noted that the quadrupole component Φ compares to other higher order multipole components₂Is the dominant or dominant component, and the twelve-pole component Φ₆Is the dominant high-order term. Further, only data points for the quadrupole, the decapole, and the dodecapole components are clearly visible, while data points for the other multipoles are blurred. As is evident from fig. 4, as the aspect ratio increases, the quadrupole component of the RF electric field becomes more dominant (or stronger) and the higher order multipole component becomes less dominant (or weaker). Accordingly, ion guide 300 may be configured (i.e., with respect to the geometry and/or relative positions/orientations of

electrodes

308A, 308B, 308C, and 308D) such that the multipole component of the generated RF electric field varies in the axial direction from the ion entrance to the ion exit of ion guide 300. As one example, the aspect ratio may be increased in axial dimension such that the resulting ion beam has a relatively large acceptance rate at the ion entrance to maximize ion capture from the preceding device and converges downward to a relatively small emittance at the ion exit to maximize ion transfer to the succeeding device. Alternatively and conversely, depending on the function of the ion guide, the aspect ratio may decrease in the axial direction from the ion entrance to the ion exit, causing the ion beam to diverge. It can be seen that the electrode width W and/or inscribed radius r can be varied along the ion guide axis as desired for a given embodiment₀To change the aspect ratio.

Thus, according to one aspect of the present disclosure, the ion guide, and in particular its plurality of ion guide electrodes (typically 2N electrodes), is configured to generate a two-dimensional RF ion confining electric field in a transverse plane orthogonal to the axis in the ion guide interior. The RF electric field is or includes a superposition of a lower order multipole component (i.e., at least one lower order multipole component, or one or more lower order multipole components) and a higher order multipole component (i.e., at least one higher order multipole component, or one or more higher order multipole components). Different multipole component phi_NOf corresponding amplitude a_NIn the axial direction (i.e. along the ion guide axis) as a function of the varying aspect ratio of the electrode structure. In other words, the relative multipole amplitude of the different multipole components varies in the axial direction according to the varying aspect ratio. In other words, the RF electric field can be characterizedTo have a multipole amplitude ratio (i.e., a ratio of the amplitude of the one or more lower order components to the amplitude of the one or more higher order components) and the multipole amplitude ratio varies along the ion guide axis according to the varying aspect ratio.

In the present context, the terms "low order multipole component" and "high order multipole component" are generally interpreted relative to each other. As one non-limiting example, quadrupole, hexapole, and octopole components may be considered low-order multipole components, while higher-order multipole components than octupole may be considered high-order multipole components, such as decapole, dodecapole, and the like.

As part of an example of this aspect of the present disclosure (where only two multipoles are considered for simplicity), fig. 4 shows that the ratio of quadrupole (low order) amplitude to decapole (high order) amplitude varies in the axial direction as the aspect ratio varies.

According to other aspects of the present disclosure, the magnitude of the applied RF voltage potential and/or the magnitude of the applied DC voltage potential may also vary (particularly gradually) along the ion guide axis, as described further below. Several non-exclusive examples of more specific embodiments are described below.

Fig. 5A is a schematic perspective view of an example of a linear geometry ion guide 500 according to an embodiment of the present disclosure. As in the previous example, the ion guide 500 includes a plurality of

ion guide electrodes

508A, 508B, 508C and 508D that are elongated along the ion guide axis 504 and are circumferentially spaced apart from each other in a transverse plane about the ion guide axis 504 to surround an axially elongated ion guide interior 512 that extends from an ion entrance (end) 520 to an ion exit (end) 524.

Electrodes

508A, 508B, 508C, and 508D have respective inner surfaces 516 facing ion guide interior 512. In fig. 5A, one of the electrodes (508B) is not shown for clarity. Fig. 5B is a schematic cross-sectional view of ion guide 500, showing one electrode pair for clarity. Fig. 5C is an end view of the ion guide 500 at the ion entrance 520 and fig. 5D is an end view of the ion guide 500 at the ion exit 524.

In this embodiment, the

electrodes

508A, 508B, 508C, and 508D (or at least their inner surfaces 516) are oriented at an angle to each other (i.e., tilted or tapered toward each other) relative to the ion guide axis 504 such that they converge toward each other in the axial direction from the ion inlet 520 to the ion outlet 524. Thus, the inscribed radius r of the

electrodes

508A, 508B, 508C, and 508D₀Tapering along the ion guide axis 504. Also in this embodiment, the electrode width W (at least the width of the inner surface 516) remains constant along the ion guide axis 504, while the circumferential gaps between

adjacent electrodes

508A, 508B, 508C, and 508D in the transverse plane gradually decrease along the ion guide axis 504. In fig. 5A, the electrode width W appears tapered, but this is due only to the three-dimensional perspective. Thus, the aspect ratio increases in axial dimension according to a predetermined function or pattern, which in this example is linear, but in other embodiments may be non-linear. With this configuration, the high order (e.g., N) of the generated RF electric field>2) The multipole component is larger at the ion entrance 520 and gradually decreases towards the ion exit 524, resulting in an increasingly dominant quadrupole field when moving in the axial direction from the ion entrance 520 to the ion exit 524 (see fig. 4 and the accompanying description above). Such RF electric field distribution provides a wide-bottomed pseudo-potential trap for trapping ions with greater emittance at the ion entrance 520, and a narrow-bottomed pseudo-potential well for better compressing the ion beam diameter (and associated beam phase space) at the ion exit 524 (see fig. 1 and accompanying description above). Thus, the RF electric fields generated by the electrode structure of the ion guide 500 focus the ions into a converging ion beam 528, as schematically depicted in fig. 5B.

Fig. 6A is a schematic perspective view of an example of a linear geometry ion guide 600 according to another embodiment of the present disclosure. As in the previous example, the ion guide 600 includes a plurality of

ion guide electrodes

608A, 608B, 608C and 608D that are elongated along an ion guide axis 604 and are circumferentially spaced apart from each other in a transverse plane about the ion guide axis 604 to surround an axially elongated ion guide interior 612 that extends from an ion inlet (end) 620 to an ion outlet (end) 624.

Electrodes

608A, 608B, 608C, and 608D have respective inner surfaces 616 that face ion guide interior 612. Fig. 6B is a schematic side (length-wise) view of ion guide 600, where one of the electrodes (608B) is not shown for clarity. Fig. 6C is an end view of ion guide 600 at ion entrance 620, and fig. 6D is an end view of ion guide 600 at ion exit 624. In this embodiment, the

electrodes

608A, 608B, 608C and 608D are shaped such that their inner surfaces 616 are angled toward each other relative to the ion guide axis 604 and thus converge toward each other in an axial direction from the ion inlet 620 to the ion outlet 624, while other surfaces or edges of the

electrodes

608A, 608B, 608C and 608D are parallel or orthogonal to the ion guide axis 604. In contrast, in the implementation shown in fig. 5A-5D, the entire structure of

electrodes

508A, 508B, 508C, and 508D are tilted toward one another.

Another difference between the ion guide 500 shown in fig. 5A-5D and the ion guide 600 shown in fig. 6A-6D relates to the electrode width W of the

inner surfaces

516 and 616 of the sets of electrodes 508A/508B/508C/508D and 608A/608B/608C/608D. The electrode width W in ion guide 500 is relatively wide and the electrode width in ion guide 600 is relatively narrow. As a further option and as illustrated, the electrode width W in the ion guide 500 can be greater than the radial height of each

electrode

508A, 508B, 508C, and 508D. By comparison, the electrode width W in the ion guide 600 can be less than the radial height of each

electrode

608A, 608B, 608C, and 608D. The foregoing features may be reversed between the two embodiments. That is, the electrodes 508A/508B/508C/508D of the ion guide 500 may have a narrow electrode width W, or the electrodes 608A/608B/608C/608D may have a wide electrode width W.

In addition to the foregoing, the configuration illustrated in fig. 6A-6D may generally be the same as or similar to the configuration described above and illustrated in fig. 5A-5D. That is, the inscribed radius of

electrodes

608A, 608B, 608C, and 608Dr₀The electrode width W (of at least the inner surface 616) remains constant while the circumferential gap between

adjacent electrodes

608A, 608B, 608C and 608D gradually decreases along the ion guide axis 604. Thus, as described above, the aspect ratio increases in the axial dimension, whereby in operation, the higher order (e.g., N) of the generated RF electric field>2) The multipole component is larger at the ion entrance 620 and tapers towards the ion exit 624, resulting in a compressed (and in this example converging) ion beam.

Fig. 7A is a schematic perspective view of an example of a linear geometry ion guide 700 according to another embodiment of the present disclosure. Fig. 7B is another schematic perspective view of ion guide 700. As in the previous example, ion guide 700 includes a plurality of

ion guide electrodes

708A, 708B, 708C and 708D that are elongated along and circumferentially spaced from each other in a transverse plane about an ion guide axis to surround an axially elongated ion guide interior 712 that extends from an ion entrance (end) 720 to an ion exit (end) 724.

Electrodes

708A, 708B, 708C, and 708D have respective inner surfaces 716 that face ion guide interior 712. Fig. 7C is an end view of ion guide 700 at ion entrance 720, and fig. 7D is an end view of ion guide 700 at ion exit 724. In this embodiment, the

electrodes

708A, 708B, 708C, and 708D are parallel to the ion guide axis such that the inscribed radius r of the

electrodes

708A, 708B, 708C, and 708D is₀Along the axial length of ion guide 700 remains constant. However, the electrode width W (of at least the inner surface 716) varies in the axial dimension. Specifically, in the illustrated embodiment, the electrode width W (particularly the width of the inner surface 716) increases in the direction from the ion entrance 720 to the ion exit 724. Thus, the aspect ratio increases in axial dimension, thereby altering the multipole component of the generated RF electric field (as described above) in operation, resulting in a compressed (and in this example converging) ion beam.

FIG. 8 is a linear geometry according to another embodiment of the present disclosureA schematic cross-sectional view of an example of an ion guide 800. As in the previous example, the ion guide 800 includes a plurality of

ion guide electrodes

808A, 808B, 808C and 808D that are elongated along and circumferentially spaced from each other in a transverse plane about the ion guide axis to surround an axially elongated ion guide interior 812 that extends from an ion entrance (end) to an ion exit (end).

Electrodes

808A, 808B, 808C, and 808D have respective inner surfaces 816 that face the ion guide interior 812. Electrode width W of inner surface 816 and radius r inscribed by them₀May be constant or may vary along the axial dimension (into the drawing page) as desired to vary the aspect ratio according to a predetermined function or pattern as described herein. In addition to the foregoing, the cross-sectional areas of the

electrodes

808A, 808B, 808C, and 808D in the transverse plane can generally have any shape desired, such as a complex or irregular polygon or a combination of polygons and circular features. Such cross-sectional shapes may be used for a desired function or purpose in addition to achieving an axially varying aspect ratio. In this embodiment, for example, the cross-sectional shape of each

electrode

808A, 808B, 808C, and 808D is a combination of a straight segment and a trapezoidal segment, wherein the inner edge of the trapezoidal segment corresponds to the inner surface 816 having the predetermined electrode width W as described herein. Such cross-sectional shapes may provide advantages, such as, for example, increased rigidity and/or simplified fabrication of the

electrodes

808A, 808B, 808C, and 808D.

In any of the ion guides described herein, as the geometry (particularly the aspect ratio) of the ion guide electrodes varies in axial dimension, the amount of heat deposited into the ion beam by the RF electric field (i.e., RF heating) will also vary in axial dimension. For example, if the aspect ratio increases in the direction from the ion entrance to the ion exit to converge the ion beam, the amount of heat deposited will correspondingly increase and the above adiabatic condition may be violated. In accordance with one aspect of the disclosure, in any embodiments of the ion guide disclosed herein, the magnitude of the RF potential applied to the ion guide electrode can be in the axial dimension (substantially) of the ion guide electrodeAbove) effectively match changes in aspect ratio and thereby prevent changes in the manner in which adiabatic conditions are violated. In other words, the RF voltage amplitude may vary according to a predetermined function that maintains (substantially) an approximate adiabatic condition along the device axis. For example, where the aspect ratio increases in the direction from the ion entrance to the ion exit, the magnitude of the applied RF potential may correspondingly taper in the same direction to counteract the effect of the changing aspect ratio on the RF heating. In an embodiment, the function according to which the RF voltage amplitude varies may be configured to satisfy at least one (one or both) of the following conditions: low mass cutoff m exhibited by the ion guide as ions are transported through the ion guide_LMaintained constant over a range of +/-1 amu; and/or the standard deviation σ of the kinetic energy K of ions travelling in the ion guide (e.g., in a radial direction) is maintained below 0.1 electron volts (eV) for at least a second half of the axial length of the ion guide toward the ion exit end (i.e., the half section of the ion guide that terminates at the ion exit end).

Additionally, the DC potential may be gradually decreased from the inlet to the outlet to establish an axial DC potential difference or gradient to keep the ions moving forward, particularly when the ion guide is part of a pressurized device (such as a collision cell). The axial DC electric field generated in the interior of the ion guide adds an amount of energy to the ions that is effective to increase or at least maintain the kinetic energy of the ions in the forward direction from the ion entrance end to the ion exit end.

Thus, according to one aspect of the present disclosure, an ion guide (in particular an ion guide electrode) is configured to generate an RF ion confining electric field with an axially varying (in particular gradually varying) RF amplitude. According to another aspect of the disclosure, the ion guide (in particular the ion guide electrode) is configured to generate a DC electric field having an axially varying (in particular gradually varying) DC magnitude.

Fig. 9A is a schematic perspective view of an example of an ion guide electrode 908 according to another embodiment of the present disclosure. Fig. 9B is a schematic perspective view of a section of ion guide electrode 908. A plurality of such electrodes 908 may be provided in any of the ion guides disclosed herein, with the inner surface 916 facing the interior of the ion guide. The electrode 908 comprises a plurality of conductive electrode sections 932 axially spaced from one another and configured to apply an RF voltage of a two-dimensional RF electric field at a continuously varying value of RF voltage amplitude (e.g., a value of RF voltage amplitude that gradually decreases in a direction from an ion entrance to an ion exit of the ion guide). Further, the plurality of conductive electrode sections 932 may be configured to apply the DC voltage at a continuously varying value of DC voltage magnitude (e.g., a value of DC voltage magnitude that gradually decreases in a direction from the ion entrance to the ion exit). The axially spaced electrode sections 932 may be implemented in any suitable manner. As one non-limiting example, the electrode 908 may be plated with a thin metal layer that is cut into a plurality of strips that are axially spaced from each other and oriented orthogonal to the ion guide axis and serve as the electrode section 932. Electrode segments 932 (here, the illustrated strips) may be electrically isolated from each other, except for being connected to receive predetermined RF and DC voltage potentials through resistor 936 and capacitor 940.

The electrode 908 of this embodiment has advantages over known electrodes having resistive coatings. Electrodes with resistive coatings are susceptible to structural deformation caused by heating when an AC/DC current is passed through the resistive material. Structural deformation can distort the electric field and cause performance degradation of the ion guide and any devices of which it is a part, such as the collision cell. The electrode 908 of this embodiment avoids the use or need for a resistive coating, and is therefore expected to improve the robustness and performance of the ion guide.

For simplicity, the various embodiments of the ion guides described thus far have a straight axial geometry. However, it will be understood that any of the embodiments described herein may be modified to have a curved or bent geometry, e.g., may be U-shaped, C-shaped, S-shaped, etc. Such embodiments are also considered herein as linear multipole ion guides because their axial length (whether curved or straight) is typically much greater than their inscribedField radius r₀And they provide a two-dimensional RF electric field that confines ions between electrodes elongated along the axis of the ion guide.

Fig. 10 is a schematic diagram of an example of a Mass Spectrometry (MS) system 1000 according to the present disclosure. The MS system 1000 may include one or more ion guides according to any embodiment described herein. The operation and design of the various components of a mass spectrometry system are generally known to those skilled in the art and therefore need not be described in detail herein. Rather, certain components are briefly described to facilitate an understanding of the presently disclosed subject matter.

The MS system 1000 may generally include an ion source 1004, one or more

ion transfer devices

1008, 1012, and 1016 (or ion processing devices), and a (final) mass analyzer 1020. Three

ion transfer devices

1008, 1012, and 1016 are shown by way of example only, as other embodiments may include more than three, less than three, or no ion transfer devices. The MS system 1000 includes a plurality of chambers defined by one or more housings (shells) and arranged in series such that each chamber is in communication with at least one adjacent (upstream or downstream) chamber. Each of the ion source 1004, the

ion transfer devices

1008, 1012, and 1016, and the mass analyzer 1020 includes at least one of these chambers. Thus, the MS system 1000 defines a flow path for ions and gas molecules that generally passes from the chamber of the ion source 1004, through the chambers of the

ion transfer devices

1008, 1012, and 1016, and into the chamber of the mass analyzer 1020. From the perspective of fig. 10, the flow path is generally from left to right. Each chamber is physically separated from adjacent chambers by at least one structural boundary (e.g., wall). The wall includes at least one opening to accommodate the flow path. The wall openings can be quite small relative to the overall size of the chambers, thus acting as gas conduction barriers that limit the transfer of gas from a preceding chamber to a succeeding chamber and facilitate independent control of the respective vacuum levels in adjacent chambers. The walls may serve as electrodes or ion optics. Alternatively or additionally, the electrodes and/or ion optics may be mounted to or positioned proximate to the wall. Any of the chambers can include one or more ion guides, such as linear multipole ion guides (e.g., quadrupole, hexapole, octopole, etc.) or ion funnels. One or more of the chambers may include an ion guide configured as disclosed herein.

At least some of the chambers may be considered to be reduced pressure chambers or vacuum stages that operate under controlled sub-atmospheric internal gas pressures. To this end, the MS system 1000 includes a vacuum system in communication with the vacuum ports of the chambers. In the illustrated embodiment, each of the ion source 1004, the

ion transfer devices

1008, 1012, and 1016, and the mass analyzer 1020 includes at least one chamber having a

respective vacuum port

1024, 1026, 1028, 1030, and 1032 in communication with a vacuum system. Typically, as the MS system 1000 is operated to analyze a sample, each chamber continuously reduces the gas pressure below the level of the previous chamber, ultimately to the very low vacuum level (e.g., from 10) required to operate the mass analyzer 1020^-4Bracket to 10^-9The range of torr). In fig. 10,

vacuum ports

1024, 1026, 1028, 1030, and 1032 are schematically represented by broad arrows. The vacuum system as a whole is schematically represented by these broad arrows, it being understood that the vacuum system includes vacuum lines leading from the

vacuum ports

1024, 1026, 1028, 1030 and 1032 to one or more vacuum generating pumps and associated plumbing and other components as understood by those skilled in the art. In operation, one or more of

vacuum ports

1024, 1026, 1028, 1030, and 1032 may remove non-analyte neutral molecules from the ion path through MS system 1000.

The ion source 1004 may be any type of continuous beam or pulsed ion source suitable for generating analyte ions for mass spectrometry analysis. Examples of ion source 1004 include, but are not limited to, electrospray ionization (ESI) sources, Photoionization (PI) sources, Electron Ionization (EI) sources, Chemical Ionization (CI) sources, Field Ionization (FI) sources, plasma or corona discharge sources, Laser Desorption Ionization (LDI) sources, and matrix-assisted laser desorption ionization (MALDI) sources. Some of the examples just presented are or may optionally be Atmospheric Pressure Ionization (API) sources, as they operate only at or near atmospheric pressure (such as ESI sources) or may be configured as such (such as Atmospheric Pressure Photoionization (APPI) sources and Atmospheric Pressure Chemical Ionization (APCI) sources). Nonetheless, the API source includes a vacuum port 1024 (exhaust port) through which gases, contaminants, etc. can be removed from the chamber. The chamber of the ion source 1004 is an ionization chamber in which sample molecules are broken down into analyte ions by an ionization device (not shown). The sample to be ionized may be introduced into the ion source 1004 by any suitable means, including coupled techniques, where the sample is the output 136 of an analytical separation instrument, such as, for example, a Gas Chromatography (GC) or Liquid Chromatography (LC) instrument (not shown). The ion source 1004 may include a skimmer 1040 (or two or more skimmers axially spaced from one another), also referred to as an intercept plate, a skimmer cone, or a sampling cone. The skimmer 1040 has a central aperture. The skimmer 1040 is configured to preferentially allow ions to pass through to the next chamber while blocking non-analyte components. The ion source 1004 may also include other components (electrodes, ion optics, etc., not shown) for organizing the generated ions into a beam that can be efficiently transferred into the next chamber.

In some embodiments, first ion transfer arrangement 1008 may be primarily configured as a pressure reduction stage. To this end, the ion transfer arrangement 1008 may include ion transfer optics 1044 configured to maintain the ion beam in focus along a primary optical axis of the MS system 1000. The ion transfer optics 1044 can have various configurations known to those skilled in the art, such as, for example, a multipole arrangement of electrodes elongated along an axis (e.g., a multipole ion guide), a series arrangement of ring electrodes, an ion funnel, split cylindrical electrodes, and the like. In some embodiments, the ion transfer optics 1044 may be configured as an ion trap. One or more lenses 1046 may be positioned between the ion transfer device 1008 and an adjacent ion transfer device 1012.

In some embodiments, the second ion transfer device 1012 may be configured as a mass filter or ion trap configured to select ions having a particular m/z ratio or range of m/z ratios. To this end, the ion transfer arrangement 1008 may include ion transfer optics 1048, such as a multipole arrangement of electrodes (e.g., a quadrupole mass filter). One or more lenses 1050 may be positioned between the ion transfer device 1012 and the adjacent ion transfer device 1016. In other embodiments, the ion transfer device 1012 may be primarily configured as a pressure reduction stage.

In some embodiments, the third ion transfer device 1016 may be configured as a cooling cell or collision cell. To this end, the ion transfer device 1016 may include ion transfer optics 1052 configured as a non-mass resolving RF-only device, such as a multipole arrangement of electrodes. A cooling gas (or damping gas), such as, for example, argon, nitrogen, helium, etc., may be flowed into the chamber of the ion transfer device 1016 to cool (or "thermalize," i.e., reduce the kinetic energy of) the ions by virtue of collisions between the ions and gas molecules during operation in the analysis mode. In other embodiments, the ion transfer device 1016 may be configured as an ion fragmentation device, such as a collision cell. In one example, ion fragmentation is accomplished by means of Collision Induced Dissociation (CID), in which case the gas added to the cell ("collision gas") results in a gas pressure sufficient to enable fragmentation by CID. In an embodiment, the ion transfer device 1016 includes an ion guide as disclosed herein. In an embodiment, the ion guide or other ion transfer optics 1052 is enclosed in a housing having a cell inlet, a cell outlet spaced from the cell inlet along the longitudinal axis of the cooling or collision cell (of the third ion transfer arrangement 1016), and a gas supply port in communication with the interior of the housing for admitting collision gas. Ion beam shaping optics 1054 may be positioned between the ion transfer device 1016 and the MS 1020. In other embodiments, the ion transfer device 1016 may be configured primarily as a pressure reduction stage.

Thus, in some embodiments, an ion guide as disclosed herein is disposed in an enclosure or housing (such as a collision cell) configured to maintain the ion guide interior at a pressure effective to thermalize the ion guide interiorIons in the sections or at least some ions in the interior of the further fragmentation ion guide, particularly in preparation for obtaining fragment ion spectra, as will be appreciated by those skilled in the art. In some embodiments, the ion guide interior is maintained or maintained at from 5x10^-2To 1x10^-8Torr, pressure within the range of torr.

The mass analyzer 1020 may be any type of mass analyzer and includes an ion detector 1062. In the illustrated embodiment, by way of example only, mass analyzer 1020 is depicted as a time-of-flight mass spectrometer (TOF) analyzer. In this case, mass analyzer 1020 includes an evacuated field-free flight tube 1058 into which ions are implanted by an ion pulser 1066 (or ion thruster, ion tractor, ion extractor, etc.). As understood by those skilled in the art, the beam shaping optics 1054 direct the ion beam into an ion pulser 1066, which pulses ions into a flight tube 1058 as ion packets. The ions drift through flight tube 1058 toward ion detector 1062. Ions of different masses travel through flight tube 1058 at different velocities and therefore have different total flight times, i.e., ions of smaller mass travel faster than ions of larger mass. Each ion packet is spread out (dispersed) in space according to the time-of-flight distribution. The ion detector 1062 detects and records the time at which each ion arrives (impacts) at the ion detector 1062. The data acquisition device then correlates the recorded time of flight to the m/z ratio. The ion detector 1062 may be any device configured to collect and measure the flux (or current) of mass-differentiated ions output from the mass analyzer 1058. Examples of ion detectors include, but are not limited to, multi-channel plates, electron multipliers, photomultipliers, and faraday cups. In some implementations, as shown, the ion pulser 1066 accelerates ion packets into the flight tube 1058 in a direction orthogonal to the direction along which the beam shaping optics 1054 transmit ions into the ion pulser 1066, known as orthogonal acceleration TOF (oa-TOF). In this case, flight tube 1058 often includes an ion mirror (or reflectron) 1071 to provide approximately 180 ° of reflection or steering in the ion flight path for extending the flight path and correcting for the kinetic energy distribution of the ions.

In other embodiments, mass analyzer 1020 may be another type of mass analyzer, such as, for example, a mass filter, an ion trap, an Ion Cyclotron Resonance (ICR) cell, an electrostatic ion trap, or an electrostatic and/or magnetic sector analyzer.

In operation, a sample is introduced into the ion source 1004. The ion source 1004 generates sample ions (analyte ions and background ions) from the sample and transfers the ions to one or more

ion transfer devices

1008, 1012, and 1016. The one or more

ion transfer devices

1008, 1012, and 1016 transfer ions through one or more pressure reduction stages and into the mass analyzer 1020. Depending on the type or types of

ion transfer devices

1008, 1012, and 1016 included, the one or more

ion transfer devices

1008, 1012, and 1016 may perform additional ion processing operations, such as mass filtering, ion fragmentation, beam shaping, and the like, as described above. Further, one or more of the

ion transfer devices

1008, 1012, and 1016 may include an ion guide configured and operated in accordance with any of the embodiments described herein. The mass analyzer 1020 mass resolves ions as described above. The measurement signals output from the ion detector 1062 are processed by the electronics of the MS system 1000 to generate a mass spectrum.

Exemplary embodiments

Exemplary embodiments provided in accordance with the presently disclosed subject matter include, but are not limited to, the following:

1. an ion guide, comprising: an ion inlet end; an ion outlet end; and a plurality of electrodes elongated along an ion guide axis from the ion entrance end to the ion exit end and spaced apart from each other about the ion guide axis to surround an ion guide interior, the electrodes comprising a polygonal shape with respective inner surfaces disposed at a radius from the ion guide axis, wherein: the inner surface is inscribed on a circle on the ion guide axis having the radius; the inner surfaces have respective electrode widths tangent to the circle; the aspect ratio of the electrode width to the radius varies along the ion guide axis; and the plurality of electrodes are configured to generate a two-dimensional RF electric field in a transverse plane orthogonal to the axis within the ion guide interior, the RF electric field comprising a superposition of a lower order multipole component and a higher order multipole component, wherein an amplitude ratio of the lower order component to the higher order component varies along the ion guide axis according to the varying aspect ratio, and the RF electric field has an RF voltage amplitude that varies along the ion guide axis.

2. The ion guide of embodiment 1, wherein the aspect ratio increases along the ion guide axis in a forward direction from the ion entrance end to the ion exit end for converging an ion beam in the forward direction.

3. The ion guide according to any one of the preceding embodiments, wherein the electrodes are inclined towards the ion guide axis such that the radius varies along the ion guide axis.

4. The ion guide according to any one of the preceding embodiments, wherein the inner surface tapers towards the ion guide axis such that the radius varies along the ion guide axis.

5. The ion guide according to any one of the preceding embodiments, wherein the radius decreases along the ion guide axis.

6. The ion guide according to any one of the preceding embodiments, wherein the width of each electrode is constant along the ion guide axis.

7. The ion guide according to any one of embodiments 1 to 5, wherein the electrodes are tapered such that the width of each electrode varies along the ion guide axis.

8. The ion guide according to any one of embodiments 1-5, wherein the width of each electrode increases along the ion guide axis.

9. The ion guide according to any one of embodiments 1-4, 6 and 7, wherein the radius is constant along the ion guide axis.

10. The ion guide according to any one of the preceding embodiments, wherein the inner surface is flat.

11. The ion guide according to any preceding embodiment, wherein the amplitude ratio varies according to at least one of: the amplitude ratio increases in a direction from the ion entrance end to the ion exit end; the amplitude ratio decreases in a direction from the ion entrance end to the ion exit end.

12. The ion guide according to any one of the preceding embodiments, wherein the low order multipole component comprises at least one of: a quadrupole component; a hexapole component; an octupole component.

13. The ion guide according to any one of the preceding embodiments, wherein the RF voltage amplitude decreases in a forward direction from the ion entrance end to the ion exit end along the ion guide axis.

14. The ion guide according to any of the preceding embodiments, wherein the RF voltage amplitude varies as a function of maintaining an approximately adiabatic condition along the device axis, the approximately adiabatic condition defined by at least one of: the low mass cutoff is maintained constant within a range of +/-1 amu; the standard deviation of the kinetic energy of the ions is maintained below 0.1eV at least in a second half axial length of the ion guide towards the ion exit end.

15. The ion guide according to any one of the preceding embodiments, wherein the aspect ratio increases in a forward direction from the ion entrance end to the ion exit end along the ion guide axis and the RF voltage amplitude decreases in the forward direction along the ion guide axis.

16. The ion guide according to any one of the preceding embodiments, wherein the plurality of electrodes have 2N times rotational symmetry about the ion guide axis from the ion entrance end to the ion exit end, where N is an integer equal to or greater than 2.

17. The ion guide according to any one of the preceding embodiments, wherein the plurality of electrodes is 2N, where N is an integer equal to or greater than 2.

18. The ion guide according to any one of the preceding embodiments, wherein the plurality of electrodes is four.

19. The ion guide according to any one of embodiments 1-17, wherein the plurality of electrodes is greater than four.

20. The ion guide according to any one of the preceding embodiments, wherein the plurality of electrodes are configured to generate an axial DC electric field in the ion guide interior effective to increase or maintain kinetic energy of ions in a forward direction from the ion entrance end to the ion exit end.

21. The ion guide according to any one of the preceding embodiments, wherein each of the electrodes comprises a plurality of conductive electrode segments axially spaced from each other and configured to apply an RF voltage of the two-dimensional RF electric field at a continuously varying value of RF voltage amplitude.

22. The ion guide of embodiment 21, wherein the plurality of conductive electrode segments are configured to apply a DC voltage at a value of continuously varying DC voltage magnitude.

23. The ion guide according to any one of the preceding embodiments, comprising an RF voltage source in communication with the plurality of electrodes and configured to apply an RF voltage potential to the plurality of electrodes.

24. The ion guide of any preceding embodiment, comprising a DC voltage source in communication with the plurality of electrodes and configured to apply a DC voltage potential to the plurality of electrodes.

25. A method for transporting ions, the method comprising: applying RF voltage potentials to the plurality of electrodes of an ion guide configured according to any of the embodiments disclosed herein to generate the two-dimensional RF electric field in the ion guide interior; and admitting the ions into the ion guide interior so as to subject the ions to the two-dimensional RF electric field and confine the ions radially to an ion beam along the ion guide axis.

26. The ion guide of embodiment 25, wherein said two-dimensional RF electric field is effective to converge said ion beam in a forward direction from said ion entrance end to said ion exit end.

27. The ion guide according to any one of the preceding embodiments, comprising applying a DC voltage potential to the plurality of electrodes to generate an axial DC electric field in the ion guide interior effective to increase or maintain kinetic energy of the ions in a forward direction from the ion entrance end to the ion exit end.

28. The ion guide according to any one of the preceding embodiments, comprising maintaining the ion guide interior at from 5x10^-2To 1x10^-8Torr, pressure within the range of torr.

29. The ion guide according to any one of the preceding embodiments, comprising maintaining the ion guide interior at a pressure effective to thermalize the ions in the ion guide interior.

30. The ion guide according to any one of the preceding embodiments, comprising maintaining the ion guide interior at a pressure effective to fragment at least some of the ions in the ion guide interior.

31. A collision cell, comprising: a housing; and an ion guide according to any one of the preceding embodiments disposed in the housing.

32. A Mass Spectrometry (MS) system, comprising: the ion guide according to any one of the preceding embodiments; and a mass analyzer in communication with the ion guide.

33. A Mass Spectrometry (MS) system, comprising: the ion guide according to any one of the preceding embodiments; and a controller comprising an electronic processor and a memory and configured to control the steps of the method according to any one of the preceding embodiments, in particular to control the operation of the ion guide.

34. A method for analyzing a sample, the method comprising: generating analyte ions from the sample; transporting the analyte ions into an ion guide according to any one of the preceding embodiments; and operating the ion guide according to any one of the preceding embodiments.

35. An ion guide, comprising: an ion inlet end; an ion outlet end; and a plurality of electrodes elongated along an ion guide axis from the ion entrance end to the ion exit end and spaced apart from each other about the ion guide axis to surround an ion guide interior, the electrodes comprising a polygonal shape with respective inner surfaces disposed at a radius from the ion guide axis, wherein: the inner surface is inscribed on a circle on the ion guide axis having the radius; the inner surfaces have respective electrode widths tangent to the circle; and the aspect ratio of the electrode width to the radius varies along the ion guide axis.

36. The ion guide of embodiment 35, comprising one or more features of any one of embodiments 1-34.

It will be understood that terms such as "communicate" and "communicating" (e.g., a first component is in "communication" or "communicating") with a second component are used herein to indicate a structural, functional, mechanical, electrical, signaling, optical, magnetic, electromagnetic, ionic, or fluid relationship between two or more components or elements. Thus, the fact that one component is said to communicate with a second component is not intended to exclude the possibility that additional components may be present between and/or operatively associated or engaged with the first and second components.

It will be understood that various aspects or details of the invention may be changed without departing from the scope of the invention. Furthermore, the foregoing description is for the purpose of illustration only, and not for the purpose of limitation, the invention being defined by the claims.

Claims

1. An ion guide, comprising:

an ion inlet end;

an ion outlet end; and

a plurality of electrodes elongated along an ion guide axis from the ion entrance end to the ion exit end and spaced apart from each other about the ion guide axis to surround an ion guide interior, the electrodes comprising a polygonal shape with respective inner surfaces disposed at a radius from the ion guide axis, wherein:

the inner surface is inscribed on a circle on the ion guide axis having the radius;

the inner surfaces have respective electrode widths tangent to the circle;

the aspect ratio of the electrode width to the radius varies along the ion guide axis; and is

The plurality of electrodes are configured to generate a two-dimensional RF electric field in a transverse plane orthogonal to the axis within the ion guide interior, the RF electric field comprising a superposition of a lower order multipole component and a higher order multipole component, wherein an amplitude ratio of the lower order component to the higher order component varies along the ion guide axis according to the varying aspect ratio, and the RF electric field has an RF voltage amplitude that varies along the ion guide axis.

2. The ion guide according to claim 1, comprising at least one of:

wherein the aspect ratio increases along the ion guide axis in a forward direction from the ion entrance end to the ion exit end for converging an ion beam in the forward direction; wherein the RF voltage amplitude decreases in the forward direction along the ion guide axis.

3. The ion guide according to claim 1, comprising at least one of:

wherein the electrode is tilted towards the ion guide axis such that the radius varies along the ion guide axis;

wherein the inner surface tapers towards the ion guide axis such that the radius varies along the ion guide axis.

4. The ion guide according to claim 1, comprising at least one of:

wherein the radius decreases along the ion guide axis;

wherein the width of each electrode is constant along the ion guide axis.

5. The ion guide according to claim 1, comprising at least one of:

wherein the electrodes are tapered such that the width of each electrode varies along the ion guide axis;

wherein the width of each electrode increases along the ion guide axis;

wherein the radius is constant along the ion guide axis.

6. The ion guide according to claim 1, wherein the inner surface is flat.

7. The ion guide according to claim 1, wherein the amplitude ratio varies according to at least one of: the amplitude ratio increases in a direction from the ion entrance end to the ion exit end; the amplitude ratio decreases in a direction from the ion entrance end to the ion exit end.

8. The ion guide of claim 1, wherein the low order multipole component comprises at least one of: a quadrupole component; a hexapole component; an octupole component.

9. The ion guide of claim 1, wherein the RF voltage amplitude decreases in a forward direction from the ion entrance end to the ion exit end along the ion guide axis.

10. The ion guide of claim 1, wherein the RF voltage amplitude varies as a function of maintaining an approximately adiabatic condition along an axis of the device, the approximately adiabatic condition defined by at least one of: the low mass cutoff is maintained constant within a range of +/-1 amu; the standard deviation of the kinetic energy of the ions is maintained below 0.1eV at least in a second half axial length of the ion guide towards the ion exit end.

11. The ion guide according to claim 1, wherein the plurality of electrodes have 2N times rotational symmetry about the ion guide axis from the ion entrance end to the ion exit end, where N is an integer equal to or greater than 2.

12. The ion guide according to claim 1, wherein the plurality of electrodes is 2N, where N is an integer equal to or greater than 2.

13. The ion guide according to claim 1, comprising at least one of:

wherein the plurality of electrodes is four;

wherein the plurality of electrodes is greater than four.

14. The ion guide of claim 1, wherein the plurality of electrodes are configured to generate an axial DC electric field in the ion guide interior effective to increase or maintain kinetic energy of ions in a forward direction from the ion entrance end to the ion exit end.

15. The ion guide according to claim 1, wherein each of the electrodes comprises a plurality of electrically conductive electrode segments axially spaced from one another and configured according to at least one of:

the plurality of conductive electrode segments are configured to apply RF voltages of the two-dimensional RF electric field at continuously varying values of RF voltage amplitude;

the plurality of conductive electrode segments are configured to apply a DC voltage at a value of continuously varying DC voltage magnitude.

16. The ion guide according to claim 1, comprising at least one of:

an RF voltage source in communication with the plurality of electrodes and configured to apply an RF voltage potential to the plurality of electrodes;

a DC voltage source in communication with the plurality of electrodes and configured to apply a DC voltage potential to the plurality of electrodes.

17. A method for transporting ions, the method comprising:

applying RF voltage potentials to the plurality of electrodes of the ion guide of claim 1 to generate the two-dimensional RF electric field in the ion guide interior; and

the ions are admitted into the ion guide interior so as to be subjected to the two-dimensional RF electric field and are radially confined to an ion beam along the ion guide axis.

18. The method of claim 17, wherein the two-dimensional RF electric field is effective to converge the ion beam in a forward direction from the ion entrance end to the ion exit end.

19. The method of claim 17, comprising applying DC voltage potentials to the plurality of electrodes to generate an axial DC electric field in the ion guide interior effective to increase or maintain kinetic energy of the ions in a forward direction from the ion entrance end to the ion exit end.

20. The method of claim 17, comprising at least one of:

maintaining the ion guide interior at from 5x10^-2To 1x10^-8Pressure in the range of torr;

maintaining the ion guide interior at a pressure effective to thermalize the ions in the ion guide interior;

maintaining the ion guide interior at a pressure effective to fragment at least some of the ions in the ion guide interior.