Branched radio frequency multipole

Systems and methods of the invention include a branched radio frequency multipole configured to act, for example, as an ion guide. The branched radio frequency multipole comprises multiple ion channels through which ions can be alternatively directed. The branched radio frequency multipole is configured to control which of the multiple ion channels ions are directed, through the application of appropriate potentials.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is in the field of ion optics.

2. Description of Related Art

Ion guides comprising four electrodes are used to transport ions from one place to another. For example, in mass spectrometry ion guides may be used to transport ions from an ion source to an ion analyzer. Some types of ion guides operate using radio frequency potentials applied to the four electrodes. Neighboring electrodes (orthogonal to each other) in the ion guide are operated at potentials of opposite polarity, while opposing electrodes in the ion guide are operated at the same potentials. The use of appropriate potentials results in the generation of a quadrupole field and an ion channel through which ions will preferentially travel. In some instances, such ion guides also operate as a mass filter or collision cell.

SUMMARY OF THE INVENTION

Roughly described, a branched multipole structure constructed in accordance with an embodiment of the invention has a plurality of electrodes arranged in pairs opposed across an ion flow axis. The electrodes define first and second ion channels, which have a shared or common portion and a divergent portion. An RF voltage source applies RF voltages to at least a portion of the plurality of electrodes to establish RF fields that radially confine ions within the ion channels. By adjusting the phase and/or magnitude of the RF voltages applied to one or more electrodes, the ions are caused to preferentially travel along the first or second ion channel. In some implementations, a DC axial field may be established along at least a portion of the first and/or second ion channels to assist in transporting ions through the multipole structure and thereby improve transmission efficiency.

DETAILED DESCRIPTION

The invention comprises a branched radio frequency multipole for guiding ions from a source toward alternative ion destinations, or from a plurality of ion sources to an ion destination. The invention may comprise two ion destinations or two ion sources. The branched radio frequency multipole comprises electrodes divided into segments, and is configured to guide ions through different ion channels by applying different radio frequency (RF) voltages to these segments.

FIG. 1illustrates a perspective view of a branched radio frequency multipole system, according to various embodiments of the invention. Branched radio frequency multipole system100comprises branched electrodes110aand110b, disposed parallel to each other. Branched radio frequency multipole system also comprises orthogonal electrodes120A,120B,120C,120D,120E,120F,130A, and130B. The orthogonal electrodes120A-120F,130A, and130B are disposed orthogonally to the branched electrodes110A and110B such that the branched radio frequency multipole100comprises a first ion channel between ports140and150and a second ion channel between ports140and160of branched radio frequency multipole100. Port140is an opening defined by the branched electrodes110A and110B and the orthogonal electrodes120A and120D. Port150is an opening defined by the branched electrodes110A and110B and the orthogonal electrodes120C and130A. Port160is an opening defined by the branched electrodes110A and110B and the orthogonal electrodes120F and130B. The first ion channel and the second ion channel overlap in part of the branched radio frequency multipole100adjacent to port140and diverge at a branch point170before continuing to port150and port160, respectively.

The RF voltages applied to orthogonal electrodes120B,120C and130A may be controlled such that the first ion channel comprising a path between port140and port150is opened. Alternatively, the RF voltages applied to orthogonal electrodes120E,120F, and130B may be controlled such that the second ion channel comprising a path between port140and port160is opened. Thus, the paths by which ions traverse branched radio frequency multipole100can be controlled by the selection of appropriate voltages.

FIG. 2illustrates a top view of the branched radio frequency multipole system100ofFIG. 1, having orthogonal electrodes split into segments, according to various embodiments of the invention. The branched radio frequency multipole system100also comprises a radio frequency voltage source210. Radio frequency voltage source210may be coupled to the orthogonal electrodes120A,120B,120C,120D,120E,120F,130A, and130B. Several, but not all, of these connections are shown inFIG. 2. Radio frequency voltage source210may also be coupled to the branched electrodes, e.g.110A and110B.

The RF voltages applied to orthogonal electrodes120A-120F,130A,130B, and branched electrodes110A and110B may be controlled such that the first ion channel comprising a path between port140and port150is opened. For example, the RF voltages applied to orthogonal electrodes120A-120F,130A and130B may be controlled such that the RF voltage on orthogonal electrode120E-120F and130B is at least 1.1, 1.5, 2, or 3 times the RF voltage on orthogonal electrodes120A-120D and130A. Alternatively, the RF voltages applied to orthogonal electrodes120A-120F,130A,130B and branched electrodes110A and110B may be controlled such that the second ion channel comprising a path between port140and port160is opened. For example, the RF voltages on orthogonal electrodes120A-120F,130A and130B may be controlled such that the RF voltage on orthogonal electrode120B-120C and130A is at least 1.1, 1.5, 2, or 3 e times the RF voltage on orthogonal electrodes120A,120D-120F and130B.

The branched radio frequency multipole system100also comprises optional ion source/destinations220,230, and240. Ion source/destination220, ion source/destination230, and ion source/destination240may each be an ion source and/or an ion destination. As ion sources they may comprise, for example, an electron impact (EI) ion source, an electrospray (ESI) ion source, a matrix-assisted laser desorption (MALDI) ion source, a plasma source, an atmospheric pressure chemical ionization (APCI) ion source, a laser desorption ionization (LDI) ion source, an inductively coupled plasma (ICP) ion source, a chemical ionization (CI) ion source, a fast atom bombardment (FAB) ion source, an electron source, a liquid secondary ions mass spectrometry (LSMIS) source, or the like. As ion destinations they may comprise, for example, a mass filter, a chemical analyzer, material to be treated by the ion, a time of flight (TOF) mass analyzer, a quadrupole mass analyzer, a Fourier transform ion cyclotron resonance (FTICR) mass analyzer, a 2D (linear) quadrupole, a 3d quadrupole ion trap, a magnetic sector mass analyzer, a spectroscopic detector, a photomultiplier, a ion detector, an ion reaction chamber, or the like.

FIG. 3illustrates a top view of the branched radio frequency multipole system100, wherein branched electrodes110A and110B are each split into segments, according to various embodiments of the invention. In these embodiments, branched electrode110and branched electrode110B each include electrode segments310A,310B, and310C. The electrode segments310A,310B, and310C are disposed relative to each other such that a branched shape is formed. Branched radio frequency multipole system100also comprises orthogonal electrodes320A,320B,330A, and330B, disposed orthogonally to electrode segments310A,310B, and310C.

RF voltages applied to electrode segment310C and orthogonal electrodes320A,320B,330A, and330B may be controlled such that ions are directed through the first ion channel between port140and port150. When an ion channel is open, those members of electrode segments310A,310B, and310C that are adjacent to the open channel are normally operated at RF voltages having a polarity opposite of an RF voltage applied to the orthogonal electrodes320A,320B,330A and330B. When part of an ion channel is closed, this relationship between electrode segments of the branched electrodes and the orthogonal electrodes is not maintained, e.g. the same potentials may be applied to both a segment of the branched electrodes and the orthogonal electrodes.

For example, the RF voltage applied to electrode segment310C may be to the same as the RF voltages applied to orthogonal electrodes320A,320B,330A, and330B. Setting the same potential on all four electrodes forming a branch of an ion channel allows the ion guide to reproduce an electric potential distribution closely analogous to a theoretical electric potential distribution if electrode segment330A were continued following its curvature until it merged into electrode segment320B. This configuration would be effectively equivalent, in terms of electric field distribution and ion transfer, to a regular curved four-electrode set. In this case, ions will successfully be passed through the first ion channel between port140and port150, but will not traverse between port160and port140. Alternatively, the RF voltages applied to electrode segment310B and orthogonal electrodes320A,320B,330A, and330B may be the same. In this case, ions are directed through the second ion channel between port140and port160and will not successfully pass between port140and port150.

FIG. 4Aillustrates a top view of the branched radio frequency multipole system100, wherein the branched electrodes110A and110B are each split into segments, according to various embodiments of the invention. The branched electrode110A is split into segments410A,410B,410C, and410D, which are disposed relative to each other such that a branched shape is formed. Orthogonal electrodes420A,420B,430A, and430B are disposed orthogonally to the electrode segments410A,410B,410C, and410D.

In a manner similar to that described inFIG. 3, RF voltages may be applied to electrode segments410A,410B,410C,410D and orthogonal electrodes420A,420B,430A and430B in order to open the first ion channel between port140and port150, or alternatively, the second ion channel between port140and port160. Electrode segment410B is typically maintained at the same RF voltages as electrode segment410A.

FIG. 4Billustrates a side view of the branched radio frequency multipole system100ofFIG. 4A, according to various embodiments of the invention. This view shows that electrode segment410B is displaced relative to electrode segment410A. Specifically, an inter-electrode distance440between the two instances of electrode segment410B that make up part of branched electrode110A and110B (FIG. 1) is greater than an inter-electrode distance450between the two instances of electrode segment410A that make up part of branched electrode110A and110B. In various embodiments, the inter-electrode distance440differs from the inter-electrode distance450by greater than 4, 8, 12 or 15 percent of inter-electrode distance450. In some instances, the embodiments of branched radio frequency multipole100illustrated byFIGS. 4A and 4Bprovide a greater control of the opening and closing of ion channels than the embodiments illustrated byFIG. 3. For example, the embodiments illustrated byFIGS. 4A and 4Ballow for better shaping of the electric potential close to electrode410B where the most significant distortion of electric field occurs because of electrode branching. This may result in better ion transmission efficiency in the open channel. In alternative embodiments, electrode segments410A and410B are a single piece shaped to achieve the inter-electrode distances440and450.

FIG. 5is a diagram of a circuit configured to supply radio frequency voltages to a branched radio frequency multipole system, according to various embodiments of the invention. Circuit500is optionally included in radio frequency voltage source210. Circuit500comprises a phase switch510, inductors520,530,540,550,560, and570, and an RF source580. The phase of RF voltages on inductors530and560are dependent on the state of the phase switch510. When phase switch510is OFF, both of these inductors will have the same RF voltages. When phase switch510is ON, inductors530and560will have RF voltages of opposite polarity, e.g. be 180 degrees out of phase with each other. Inductors520and540respond to the inductance on inductor530. Inductors550and570respond to the inductance on inductor560. Thus, depending on whether the phase switch is on or off, one of410D (or310C) and410C (or310B) will have the same polarity as410A,410B, while the other will have the opposite polarity. Ion channels will be opened and closed accordingly. With this circuit500, turning on and off the phase switch510can be used to open and close ion channels in the branched radio frequency multipole100.

FIG. 6is a flowchart illustrating a method, according to various embodiments of the invention. In this method, electrode RF voltages are adjusted to alternatively pass ions to different destinations. A step610comprises setting electrode RF voltages such that the first ion channel between ports140and150of the branched radio frequency multipole100is opened to allow a first ion from an ion source, e.g. ion source/destination220, to pass through the first ion channel toward a first ion destination, e.g. ion source/destination230. A step620comprises introducing the first ion into the branched radio frequency multipole100and passing the first ion to the first ion destination. A step630comprises setting electrode RF voltages such that the second ion channel between ports140and160of the branched radio frequency multipole100is opened to allow a first ion from an ion source, e.g. ion source/destination220, to pass through the first ion channel toward a second ion destination, e.g. ion source/destination240. A step640comprises introducing the second ion into the branched radio frequency multipole100and passing the second ion to the second ion destination.

FIG. 7is a flowchart illustrating a method, according to various embodiments of the invention. In this method, electrode RF voltages are adjusted to alternatively pass ions to different destinations. A step710comprises setting electrode RF voltages such that the first ion channel between ports140and150of the branched radio frequency multipole100is opened to allow a first ion from a first ion source, e.g. ion source/destination230, to pass through the first ion channel toward an ion destination, e.g. ion source/destination220. A step720comprises introducing the first ion into the branched radio frequency multipole100and passing the first ion to the ion destination. A step730comprises setting electrode RF voltages such that the second ion channel between ports140and160of the branched radio frequency multipole100is opened to allow a first ion from a second ion source, e.g. ion source/destination240, to pass through the first ion channel toward the ion destination, e.g. ion source/destination220. A step740comprises introducing the second ion into the branched radio frequency multipole100and passing the second ion to the ion destination.

Several embodiments are specifically illustrated and/or described herein. However, it will be appreciated that modifications and variations are covered by the above teachings and within the scope of the appended claims without departing from the spirit and intended scope thereof. For example, while the embodiments described above and depicted in the figures utilize electrodes of generally planar shape, the invention should not be construed as being limited thereto. Other embodiments may utilize electrodes having a square cross-section, or electrodes having an inwardly directed curved (e.g., round or hyperbolic) surface. In each case, the electrodes are arranged into at least two pairs, with each electrode being opposed across an ion flow axis to a corresponding electrode.

In certain implementations, the branched multipole structure may function as a collision/reaction cell to produce controlled dissociation of the entering ions, for example via collision induced dissociation. For such an implementation, a collision or reaction gas is added through a collision/reaction gas source (which may include a gas supply, metering valve and conduit) to at least a portion of the interior volume of the multipole structure. A set of plates or similar structures having conductance limiting apertures may be utilized to create a pressurized region within the multipole's interior volume. The addition of a collision or damping gas may also be utilized to provide collisional focusing of ions and thereby improve ion transmission efficiences through the multipole.

It may be beneficial to establish an axial (longitudinal) DC field along at least a portion of the first/and or second ion channels to assist in urging ions to travel along the ion flow axes. This may be particularly advantageous where the multipole is operated at a relatively high pressure, and the ion undergo large number of collisions with atoms/molecules of collision or background gas, thereby reducing the ions' kinetic energy. Techniques for establishing axial DC fields in RF multipoles are well known in the art, and are disclosed, for example, in U.S. Pat. No. 6,111,250 by Thomson et al. (“Quadrupole with Axial DC Field”) and U.S. Pat. No. 7,067,802 by Kovtoun (“Generation of Combination of RF and Axial DC Electric Fields in an RF-Only Multipole”), the disclosures of which are incorporated herein by reference. Generally speaking, a DC voltage source is provided for applying DC voltages to DC axial field electrodes which extend or are spaced longitudinally along the first and/or second ion channels. The DC axial field electrodes may be external to or integrated with the multipole electrodes to which the RF voltages are applied. In certain implementations, the DC voltages applied to the axial field electrodes may be adjusted in accordance with the selection of the first or second ion channel as the preferred ion channel.