Abstract:
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. Thus, ions can alternatively be directed down different ion channels without the use of a mechanical valve.

Description:
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 transportions from one place to another. For example, in mass spectrometry ion guides may be used to transportions 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 
   Systems and methods of the invention include a branched radio frequency multipole configured to act 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. Thus, ions can alternatively be directed down different ion channels without the use of a mechanical valve. 
   In some embodiments, the branched radio frequency multipole is used to alternatively direct ions from one ion source to more than one alternative ion destination. For example, the branched radio frequency multipole can be configured to direct an ion from an ion source to one of two alternative mass spectrometers. In some embodiments, the branched radio frequency multipole is used to direct ions from alternative ion sources to a single ion destination. For example, the branched radio frequency multipole can be configured to direct ions alternatively from an electron impact ion source and an atmospheric pressure ion source to a single mass spectrometer. 
   In some embodiments, the branched radio frequency multipole is used as a collision cell. In some embodiments, the branched radio frequency multipole is configured to act as a mass filter. 
   In some embodiments, the branched radio frequency multipole comprises at least a first branched electrode and a second branched electrode disposed parallel to each other, and a plurality of orthogonal electrodes disposed orthogonally to the first branched electrode and the second branched electrode. The branched electrodes and the orthogonal electrodes are configured to form an ion guide comprising at least a first ion channel and a second ion channel that diverge at a branch point. The first ion channel and the second ion channel overlap in part of the branched radio frequency multipole and diverge at the branch point. 
   The system also comprises a radio frequency voltage source for applying radio frequency voltages to the first branched electrode, the second branched electrode, and the plurality of orthogonal electrodes. The amplitude and/or phase of the radio frequency voltages are selected for establishing a radio frequency potentials configured to form regions of ion stability in alternatively the first ion channel or the second ion channel and, thus, direct ions alternatively through the first ion channel or the second ion channel, respectively. 
   In some embodiments, the invention comprises a method of using a branched radio frequency multipole, the method comprising setting voltages on segments of the branched electrodes and/or the orthogonal electrodes such that ions are directed down alternatively the first ion channel or the second ion channel. 
   In some embodiments, the invention includes a method of using a branched radio frequency multipole, the method comprising setting radio frequency voltages such that the radio frequency voltages opposite a first ion channel are different from the radio frequency voltages in a second ion channel. The method also comprises applying radio frequency voltages to orthogonal electrodes and branched electrodes in an opposite polarity alternating in time. The method also comprises introducing an ion from an ion source into the ion guide through an ion inlet and passing the ion to a first ion destination through the first ion channel. The method also comprises introducing a second ion from the ion source into the ion guide through an ion inlet and passing the second ion to a second ion destination through the second ion channel. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a perspective view of a branched radio frequency multipole system, according to various embodiments of the invention. 
       FIG. 2  illustrates a top view of the branched radio frequency multipole system of  FIG. 1 , having orthogonal electrodes split into segments, according to various embodiments of the invention. 
       FIG. 3  illustrates a top view of a branched radio frequency multipole system, having branched electrodes split into segments, according to various embodiments of the invention. 
       FIG. 4A  illustrates a top view of a branched radio frequency multipole system, having a branched electrode split into segments, according to various embodiments of the invention. 
       FIG. 4B  illustrates a side view of the branched radio frequency multipole system of  FIG. 4A , according to various embodiments of the invention. 
       FIG. 5  is a diagram of a circuit configured to supply radio frequency potentials to a branched radio frequency multipole system, according to various embodiments of the invention. 
       FIG. 6  is a flowchart illustrating a method, according to various embodiments of the invention. 
       FIG. 7  is a flowchart illustrating an alternative method, according to various embodiments of the invention. 
   

   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. 1  illustrates a perspective view of a branched radio frequency multipole system, according to various embodiments of the invention. Branched radio frequency multipole system  100  comprises branched electrodes  110   a  and  110   b , disposed parallel to each other. Branched radio frequency multipole system also comprises orthogonal electrodes  120 A,  120 B,  120 C,  120 D,  120 E,  120 F,  130 A, and  130 B. The orthogonal electrodes  120 A- 120 F,  130 A, and  130 B are disposed orthogonally to the branched electrodes  110 A and  110 B such that the branched radio frequency multipole  100  comprises a first ion channel between ports  140  and  150  and a second ion channel between ports  140  and  160  of branched radio frequency multipole  100 . Port  140  is an opening defined by the branched electrodes  110 A and  110 B and the orthogonal electrodes  120 A and  120 D. Port  150  is an opening defined by the branched electrodes  110 A and  110 B and the orthogonal electrodes  120 C and  130 A. Port  160  is an opening defined by the branched electrodes  110 A and  110 B and the orthogonal electrodes  120 F and  130 B. The first ion channel and the second ion channel overlap in part of the branched radio frequency multipole  100  adjacent to port  140  and diverge at a branch point  170  before continuing to port  150  and port  160 , respectively. 
   The RF voltages applied to orthogonal electrodes  120 B,  120 C and  130 A may be controlled such that the first ion channel comprising a path between port  140  and port  150  is opened. Alternatively, the RF voltages applied to orthogonal electrodes  120 E,  120 F, and  130 B may be controlled such that the second ion channel comprising a path between port  140  and port  160  is opened. Thus, the paths by which ions traverse branched radio frequency multipole  100  can be controlled by the selection of appropriate voltages. 
     FIG. 2  illustrates a top view of the branched radio frequency multipole system  100  of  FIG. 1 , having orthogonal electrodes split into segments, according to various embodiments of the invention. The branched radio frequency multipole system  100  also comprises a radio frequency voltage source  210 . Radio frequency voltage source  210  may be coupled to the orthogonal electrodes  120 A,  120 B,  120 C,  120 D,  120 E,  120 F,  130 A, and  130 B. Several, but not all, of these connections are shown in  FIG. 2 . Radio frequency voltage source  210  may also be coupled to the branched electrodes, e.g.  110 A and  110 B. 
   The RF voltages applied to orthogonal electrodes  120 A- 120 F,  130 A,  130 B, and branched electrodes  110 A and  110 B may be controlled such that the first ion channel comprising a path between port  140  and port  150  is opened. For example, the RF voltages applied to orthogonal electrodes  120 A- 120 F,  130 A and  130 B may be controlled such that the RF voltage on orthogonal electrode  120 E- 120 F and  130 B is at least 1.1, 1.5, 2, or 3 times the RF voltage on orthogonal electrodes  120 A- 120 D and  130 A. Alternatively, the RF voltages applied to orthogonal electrodes  120 A- 120 F,  130 A,  130 B and branched electrodes  110 A and  110 B may be controlled such that the second ion channel comprising a path between port  140  and port  160  is opened. For example, the RF voltages on orthogonal electrodes  120 A- 120 F,  130 A and  130 B may be controlled such that the RF voltage on orthogonal electrode  120 B- 120 C and  130 A is at least 1.1, 1.5, 2, or 3 e times the RF voltage on orthogonal electrodes  120 A,  120 D- 120 F and  130 B. 
   The branched radio frequency multipole system  100  also comprises optional ion source/destinations  220 ,  230 , and  240 . Ion source/destination  220 , ion source/destination  230 , and ion source/destination  240  may 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. 3  illustrates a top view of the branched radio frequency multipole system  100 , wherein branched electrodes  110 A and  110 B are each split into segments, according to various embodiments of the invention. In these embodiments, branched electrode  110  and branched electrode  110 B each include electrode segments  310 A,  310 B, and  310 C. The electrode segments  310 A,  310 B, and  310 C are disposed relative to each other such that a branched shape is formed. Branched radio frequency multipole system  100  also comprises orthogonal electrodes  320 A,  320 B,  330 A, and  330 B, disposed orthogonally to electrode segments  310 A,  310 B, and  310 C. 
   RF voltages applied to electrode segment  310 C and orthogonal electrodes  320 A,  320 B,  330 A, and  330 B may be controlled such that ions are directed through the first ion channel between port  140  and port  150 . When an ion channel is open, those members of electrode segments  310 A,  310 B, and  310 C 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 electrodes  320 A,  320 B,  330 A and  330 B. 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 segment  310 C may be to the same as the RF voltages applied to orthogonal electrodes  320 A,  320 B,  330 A, and  330 B. 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 segment  330 A were continued following its curvature until it merged into electrode segment  320 B. 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 port  140  and port  150 , but will not traverse between port  160  and port  140 . Alternatively, the RF voltages applied to electrode segment  310 B and orthogonal electrodes  320 A,  320 B,  330 A, and  330 B may be the same. In this case, ions are directed through the second ion channel between port  140  and port  160  and will not successfully pass between port  140  and port  150 . 
     FIG. 4A  illustrates a top view of the branched radio frequency multipole system  100 , wherein the branched electrodes  110 A and  110 B are each split into segments, according to various embodiments of the invention. The branched electrode  110 A is split into segments  410 A,  410 B,  410 C, and  410 D, which are disposed relative to each other such that a branched shape is formed. Orthogonal electrodes  420 A,  420 B,  430 A, and  430 B are disposed orthogonally to the electrode segments  410 A,  410 B,  410 C, and  410 D. 
   In a manner similar to that described in  FIG. 3 , RF voltages may be applied to electrode segments  410 A,  410 B,  410 C,  410 D and orthogonal electrodes  420 A,  420 B,  430 A and  430 B in order to open the first ion channel between port  140  and port  150 , or alternatively, the second ion channel between port  140  and port  160 . Electrode segment  410 B is typically maintained at the same RF voltages as electrode segment  410 A. 
     FIG. 4B  illustrates a side view of the branched radio frequency multipole system  100  of  FIG. 4A , according to various embodiments of the invention. This view shows that electrode segment  410 B is displaced relative to electrode segment  410 A. Specifically, an inter-electrode distance  440  between the two instances of electrode segment  410 B that make up part of branched electrode  110 A and  110 B ( FIG. 1 ) is greater than an inter-electrode distance  450  between the two instances of electrode segment  410 A that make up part of branched electrode  110 A and  110 B. In various embodiments, the inter-electrode distance  440  differs from the inter-electrode distance  450  by greater than 4, 8, 12 or 15 percent of inter-electrode distance  450 . In some instances, the embodiments of branched radio frequency multipole  100  illustrated by  FIGS. 4A and 4B  provide a greater control of the opening and closing of ion channels than the embodiments illustrated by  FIG. 3 . For example, the embodiments illustrated by  FIGS. 4A and 4B  allow for better shaping of the electric potential close to electrode  410 B 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 segments  410 A and  410 B are a single piece shaped to achieve the inter-electrode distances  440  and  450 . 
     FIG. 5  is 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. Circuit  500  is optionally included in radio frequency voltage source  210 . Circuit  500  comprises a phase switch  510 , inductors  520 ,  530 ,  540 ,  550 ,  560 , and  570 , and an RF source  580 . The phase of RF voltages on inductors  530  and  560  are dependent on the state of the phase switch  510 . When phase switch  510  is OFF, both of these inductors will have the same RF voltages. When phase switch  510  is ON, inductors  530  and  560  will have RF voltages of opposite polarity, e.g. be 180 degrees out of phase with each other. Inductors  520  and  540  respond to the inductance on inductor  530 . Inductors  550  and  570  respond to the inductance on inductor  560 . Thus, depending on whether the phase switch is on or off, one of  410 D (or  310 C) and  410 C (or  310 B) will have the same polarity as  410 A,  410 B, while the other will have the opposite polarity. Ion channels will be opened and closed accordingly. With this circuit  500 , turning on and off the phase switch  510  can be used to open and close ion channels in the branched radio frequency multipole  100 . 
     FIG. 6  is 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 step  610  comprises setting electrode RF voltages such that the first ion channel between ports  140  and  150  of the branched radio frequency multipole  100  is opened to allow a first ion from an ion source, e.g. ion source/destination  220 , to pass through the first ion channel toward a first ion destination, e.g. ion source/destination  230 . A step  620  comprises introducing the first ion into the branched radio frequency multipole  100  and passing the first ion to the first ion destination. A step  630  comprises setting electrode RF voltages such that the second ion channel between ports  140  and  160  of the branched radio frequency multipole  100  is opened to allow a first ion from an ion source, e.g. ion source/destination  220 , to pass through the first ion channel toward a second ion destination, e.g. ion source/destination  240 . A step  640  comprises introducing the second ion into the branched radio frequency multipole  100  and passing the second ion to the second ion destination. 
     FIG. 7  is 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 step  710  comprises setting electrode RF voltages such that the first ion channel between ports  140  and  150  of the branched radio frequency multipole  100  is opened to allow a first ion from a first ion source, e.g. ion source/destination  230 , to pass through the first ion channel toward an ion destination, e.g. ion source/destination  220 . A step  720  comprises introducing the first ion into the branched radio frequency multipole  100  and passing the first ion to the ion destination. A step  730  comprises setting electrode RF voltages such that the second ion channel between ports  140  and  160  of the branched radio frequency multipole  100  is opened to allow a first ion from a second ion source, e.g. ion source/destination  240 , to pass through the first ion channel toward the ion destination, e.g. ion source/destination  220 . A step  740  comprises introducing the second ion into the branched radio frequency multipole  100  and 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, the branched electrodes discussed herein may be curved on sides facing toward the first ion channel and the second ion channel. E.g., the branched electrodes may be parabolic or round. For example, in some embodiments, branched radio frequency multipole  100  may be used as a collision cell or as a mass filter. For example, the segmentation of the orthogonal electrodes illustrated in  FIG. 2  can be used in combination with segmentation of the branched electrodes illustrated in  FIGS. 3 ,  4 A, and  4 B. 
   Collision gas can be used to reduce significant excursion of ion trajectories from a center line of the ion guide because of collisional damping. This may simplify forming appropriate electric fields using a combination of electrode segments and associated voltages. For example, with collisional dampening, a spatial region that preferably approximates a standard curved four-electrode ion guide may be reduced to a narrow spatial region around the center line of ion trajectories, relative to a system without collisional damping. 
   The embodiments discussed herein are illustrative of the present invention. As these embodiments of the present invention are described with reference to illustrations, various modifications or adaptations of the methods and/or specific structures described may become apparent to those skilled in the art. All such modifications, adaptations, or variations that rely upon the teachings of the present invention, and through which those teachings have advanced the art, are considered to be within the spirit and scope of the present invention. Hence, these descriptions and drawings should not be considered in a limiting sense, as it is understood that the present invention is in no way limited to only the embodiments illustrated.