Abstract:
An electrode for influencing ion motion in mass spectrometers, having a dielectric substrate and a conducting layer on portions of the substrate, wherein peripheral borders, edges or convex shapes of the conducting layer adjoin free regions of the substrate. According to the invention, a dielectric layer is provided on transitions from the conducting layer to the adjoining free regions of the substrate such that at least some of the peripheral borders, edges or convex shapes of the conducting layer are covered.

Description:
STATEMENT OF RELATED APPLICATIONS 
     This patent application claims priority on and the benefit of German Application No. 20 2009 002 192.0 having a filing date of 16 Feb. 2009, which is incorporated herein in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The invention relates to an electrode for influencing ion motion in mass spectrometers, having a dielectric substrate and a conducting layer on portions of the substrate, wherein peripheral borders, edges or convex shapes of the conducting layer adjoin free regions of the substrate. “Free regions” are such regions which are not covered by the conducting layer. The layers are relatively thin, of the order of micrometers. Borders, edges or convex shapes are typically hardly visible or invisible elevations relative to the surface of the substrate. 
     The invention particularly relates to electrodes for ion-optical elements in mass spectrometers, preferably combined with a high DC voltage or a radiofrequency high voltage. A preferred application of the invention relates to electrodes with a metallic coating on an insulating substrate. 
     2. Related Art 
     Termination electrodes which are formed from a ceramic plate as a substrate in combination with a metallic coating may be provided in curved radiofrequency ion traps. The metallic coating is only applied to the substrate in regions, specifically in a region which is “visible” to the ions. Electrical discharges can be observed in the region of the peripheral edges of the electrically conducting, metallic coating. It is even possible for whiskers to be formed over time in the vicinity of the peripheral edges. This increases the risk of an electrical flashover or breakdown still further. 
     Electrical discharges can occur under all vacuum conditions, even in RF ion traps for ion storage or cooling. The distances of the voltage-carrying parts to each other are important for avoiding electrical charges. Although said distances can be altered, this is always associated with undesired changes in the electric field geometry. 
     U.S. Pat. No. 6,316,768 B1 describes the use of printed circuit boards (PCBs) in mass spectrometers. Here, the PCBs can also have electrodes suitable for ion transport. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the present invention to avoid electrical flashovers between electrodes in a mass spectrometer. Further objects of the invention are the avoidance of whisker formation and leakage currents. 
     The electrode according to the invention is characterized by a dielectric layer on transitions from the conducting layer to the adjoining free regions of the substrate such that at least some of the peripheral borders, edges or convex shapes of the conducting layer are covered. Preferably, the adjoining free regions on the one hand and the adjoining regions of the conducting layer on the other hand are also covered by the dielectric layer. Electrical flashovers emanating from the edges or convex shapes are effectively prevented in otherwise unchanged operating conditions. It is possible to work with higher voltages, depending on the application and individual part in question in the mass spectrometer. 
     Advantageously, the dielectric substrate, which is typically a plate-shaped substrate, is composed of a ceramic or glass-ceramic material. The conducting layer on the substrate is in particular a metallic layer. 
     According to a further idea of the invention, it is possible for a plurality of conducting layers to be arranged on the substrate, in particular next to one another or with at least one conducting layer on both sides of the substrate. The conducting layers can also merge into one another or be connected to one another, for example in the region of outer edges or recesses in the substrate. 
     Advantageously, the dielectric layer is composed of glass or a glass-ceramic material. The use of epoxide or polycarbonate layers is also possible. 
     According to a further idea of the invention, it is possible for the substrate to have an opening, bore or recess, for example for ions to pass through in a lens arrangement. An ion beam is then preferably aligned perpendicularly to the plane of the substrate. 
     According to a further idea of the invention, it is possible for the dielectric layer to cover all free edges or convex shapes of the conducting layer and regions of the substrate adjacent thereto. Alternatively, the dielectric layer covers only part of the free edges or convex shapes of the conducting layer. 
     According to a further idea of the invention, a slit which extends through the substrate is provided in a portion of a transition from the conducting layer to the substrate—next to a border region of the conducting layer. 
     The invention also relates to a mass spectrometer with at least one electrode according to the invention. In particular, the mass spectrometer has a mass analyzer designed like an Orbitrap ion trap. In the Orbitrap, the trapped ions move in orbits about a central electrode due to electrostatic attraction and, in the process, oscillate along the axis of the central electrode. The frequency of the oscillation generates signals which are converted into mass/charge ratios by a Fourier transform. “Orbitrap” is a registered trademark of Thermo Fisher Scientific (Bremen) GmbH, Germany. 
     According to a further idea of the invention, the mass spectrometer can have an ion trap which is combined with a mass analyzer. Here, the mass analyzer itself can also be built in the design of an ion trap. It is a curved ion trap, in particular in the case of the ion trap arranged outside of the mass analyzer. 
     Advantageously, an API ion source is assigned to the mass spectrometer. API is an abbreviation for “atmospheric pressure ionization”. However, it is also possible to use other ion sources. 
     According to a further idea of the invention, the mass spectrometer has a mass analyzer designed like a TOF analyzer. TOF is an abbreviation for “time of flight”. However, it is also possible to use other mass analyzers. 
     According to a further idea of the invention, the mass spectrometer can have a collision cell or a reaction cell. Said cell is preferably coupled to an ion trap. 
     The invention also relates to a method for determining the mass of ions which has the following features:
         the ions are generated in an ion source, led through an electric field and analyzed thereafter to determine their weight,   the electric field is generated by electrodes,   at least one of the electrodes has an electrically conducting layer on a dielectric substrate,   free regions of the substrate, to be precise regions without an electrically conducting layer, are provided,   a dielectric layer is provided at least in part on transitions from the conducting layer to the adjoining free regions of the substrate.       

     In the method, an API ion source can be provided as an ion source. 
     According to a further idea of the invention, the dielectric substrate is composed of at least one of the following materials: glass, ceramics, glass ceramics, silica glass, silicate glass, organic glass and polycarbonate. One or more of these materials can also be provided as material for the dielectric layer. 
     Advantageously, the electrically conducting layer on the substrate is a metallic layer. 
     Peripheral borders, edges or convex shapes of the conducting layer can adjoin the free regions of the substrate. Here, the borders, edges or convex shapes can at least in part be covered by the dielectric layer. In the process, the free regions of the substrate can at least in part also be covered by the dielectric layer. 
     Incidentally, further features of the invention are disclosed in the description and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       Advantageous embodiments of the invention will be explained in more detail below on the basis of the drawings, in which: 
         FIG. 1  shows a schematic illustration of the invention in a plan view, 
         FIG. 2  shows a modification with respect to  FIG. 1 , 
         FIG. 3  shows a further modification with respect to  FIG. 1 , 
         FIG. 4  shows a cross section of part of an ion trap, 
         FIG. 5  shows a modification with respect to  FIG. 4 , 
         FIG. 6  shows a cross section of an arrangement of a number of electrodes, for example for a TOF mass spectrometer, 
         FIG. 7  shows a schematic design of a mass spectrometer with an Orbitrap analyzer, 
         FIG. 8  shows a modification with respect to  FIG. 7 , and 
         FIG. 9  shows a schematic illustration of a TOF mass spectrometer. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     According to  FIG. 1 , an electrode  11  to be installed in a mass spectrometer consists of at least one electrically insulating substrate  12 , a conducting layer  13  applied at least in part thereon and a dielectric layer  14 . The substrate  12  is preferably designed like a plate, rectangular in this case, and is in particular composed of a ceramic, chemically inactive material. 
     The substrate  12  has, slightly eccentrically, an in this case circular opening  15  for ion beams to pass through. The substrate  12  is covered by the conducting layer  13  around the opening  15  and in further regions of said substrate. 
     The conducting layer  13  is preferably metallic and comprises a number of functional regions: an electrode surface  16  extends around the opening  15 . There is a contact surface  18  in the vicinity of an outer border  17  of the substrate  12 , which contact surface  18  is connected to the electrode surface  16  via a conductor region  19 . 
     The electrode surface  16  preferably extends rectangularly or even squarely around the opening  15 . By contrast, the contact surface  18  is designed to be slightly smaller, but is preferably also rectangular. The conductor region  19  represents the shortest, in this case strip-like, connection between the electrode surface  16  and the contact surface  18 . 
     The dielectric layer  14  covers the border regions of the conducting layer  13 , in particular a peripheral edge  20 . In  FIGS. 1 to 3 , the conducting layer  13  is shaded from top left to bottom right, and the dielectric layer  14  from bottom left to top right. There is a cross hatched region  21  wherever there is covering. 
     The dielectric layer  14  is preferably composed of glass, a glass-like or glass-ceramic material which advantageously is as chemically inactive as possible. The layer has a thickness of, for example, 50 μm, with an insulation of approximately 1.5 kV per 25 μm. Other layer thicknesses can be obtained by multiple applications. 7401 Glass Encapsulant by DuPont is a preferred material. 
     The conducting layer  13  is formed, for example, by a metallic layer with a thickness of 3 μm to 4 μm. The conductor region  19  of said conducting layer is completely covered by the dielectric layer  14 , see the cross hatching in  FIGS. 1 to 3 . 
     The contact surface  18  is covered on the edge face by the dielectric layer  14  with a central region remaining uncovered for electrical connections (not shown). Analogously, this is true for the electrode surface  16 . In this case too, a central region, not covered by the dielectric layer  14 , is provided which has the opening  15  in the middle. Hence, the dielectric layer  14  extends over portions of the conducting layer  13  and free regions of the substrate  12 . In this sense, free regions are those regions which are not covered by the conducting layer  13 . 
     As a result of the dielectric layer  14 , electrical flashovers and the formation of whiskers, particularly in the region of the peripheral edge  20 , are avoided or higher voltages can be applied using the same geometrical conditions. This is of great importance, particularly in the context of applying the invention in a mass spectrometer and the vacuum conditions prevailing there. 
     In the embodiment in accordance with  FIG. 2 , a slit  22  extends around part of the electrode surface  16 , preferably about approximately half the electrode surface  16  and adjacent to a border  23  of the substrate  12 , the border  23  lying opposite the border  17 . 
     Like the opening  15 , the slit  22  also extends through the substrate  12 , but, alternatively, it is also possible for said slit to be worked into the surface, in the manner corresponding to a groove. In  FIG. 2 , the dielectric layer  14  is only provided outside of the slit  22 . In particular, the slit  22  prevents voltage drains or discharges along the surface of the substrate  12 . Leakage paths are preferably approximately 4 mm or more. It is also possible for the leakage paths to be between 1 and 10 mm, depending on the application. 
     The width of the slit  22  is preferably approximately 0.3 mm or more. In particular, provision is made for the width of the slit to be between 0.5 mm to 1 mm. 
     Whereas an ion beam can pass through the electrodes illustrated in  FIGS. 1 and 2 ,  FIG. 3  illustrates an exemplary embodiment for a closed termination electrode which however can be designed precisely as illustrated in  FIG. 1 , with the exception of the lack of the opening  15 . 
       FIG. 4  shows an application of the electrode  11  according to the invention, specifically the arrangement in an ion trap. It can be seen from the cross-sectional view that the substrate  12  is also coated with the conducting layer  13  in the region of the opening  15 , so that the electrically conducting layer  13  on a side A of the electrode  11  is electrically conductively connected to the conducting layer on side B of the electrode  11 . 
     In this case, the dielectric layer  14  is also provided on both sides A, B of the electrode  11 . The peripheral edge  20  and adjoining regions of the substrate  12  on the one hand, and the conducting layer  13  on the other hand are in particular covered on both sides A, B. Rod-like radiofrequency electrodes  24  are arranged upstream (or downstream) of the electrode  11  in the direction of an ion trajectory. 
     For reasons of improved clarity, no shadings are shown for the parts  12 ,  13 ,  14  illustrated in cross-section in  FIGS. 4 to 6 . 
     In order to complete the illustration in  FIG. 4 ,  FIG. 5  shows rod-like electrodes  24 ,  25  on both sides of the electrode  11 . Said rod-like electrodes can also be designed and arranged like a multipole. As long as the electrode  11  in  FIGS. 4 and 5  is provided as a termination electrode of an ion trap, the electrode can also be designed like the exemplary embodiment in accordance with  FIG. 3 , that is to say without an opening  15 . 
     The electrodes  11  according to the invention can also be arranged one behind the other repeatedly, for example to form an ion lens or an acceleration or reflector element (see  FIG. 6 ). In said case, three electrodes  11  are arranged one behind the other, for example for use in a TOF mass spectrometer. Similar arrangements can also be provided in so-called stacked plate ion guides and ion mobility spectrometers, see GB 2 389 704 A, for example in combination with a radiofrequency DC voltage. Stacked plate ion guides and other possible applications are also known from Gerlich, D., Ng, C. &amp; Baer, M. (ed.), State-Selected and State-to-State Ion-Molecule Reaction Dynamics, Part 1: Experiment, Inhomogeneous RF fields: A Versatile Tool for the Study of Processes with Slow Ions, John Wiley and Sons, Inc., 1992, LXXXII, 1-176. 
     A number of electrodes according to the invention are arranged in a mass spectrometer  26  in accordance with  FIG. 7 . Starting from an atmospheric pressure ion source (API ion source)  27 , ions reach a vacuum chamber  29  with a multipole ion guide  30  through an ion interface  28  (comprising, for example, skimmers and ion lenses). This is adjoined by an ion lens  31  and a further vacuum chamber  32  with a further multipole ion guide  33  and a further ion lens  34 . 
     A curved radiofrequency ion trap  35  in a vacuum chamber  36  is arranged downstream of the ion lens  34 . The ion trap  35  terminates with a termination lens  37 , analogously to  FIG. 3 . 
     The design of the electrodes according to the invention makes optimization of the ion guide, particularly in the region of the vacuum chambers  29 ,  32 ,  36 , possible. Thus, the distances between the ion lenses and the multipole ion guides can be designed to be relatively small. 
     The curved ion trap  35  has ion optics  38  radially on the inside, by means of which ions can be transferred from the ion trap into a mass analyzer  39 . In this case, the latter is designed like an Orbitrap analyzer. To this end, the ions are ejected from the ion trap  35  by high voltage pulses. 
       FIG. 8  shows a mass spectrometer  40 , analogous to  FIG. 7 , but additionally with a collision cell  41  arranged behind the ion trap  35 . Accordingly, an ion-permeable lens  42  is provided instead of the termination lens  37 . By way of example, ions can be fragmented or ions can react with other ions or molecules in the collision cell  41 , and subsequently be returned to the ion trap for storage and/or cooling. From there, the ions are ejected in the direction of the mass analyzer  39 . 
     In the exemplary embodiment of  FIG. 9 , a TOF mass analyzer is provided arranged behind the ion lens  34  instead of the ion trap  35 . Said TOF mass analyzer  43  has on its input side an orthogonal accelerator  44  by means of which the ions reach a receiver element  47  through an acceleration lens  45  and via a reflector-lens arrangement  46 . The mass spectrometer designed in this fashion is labeled with the number  48  in  FIG. 9 . 
     Electrodes according to the invention can in particular be provided in the region of parts  28 ,  31 ,  34 ,  37 ,  42 ,  44 ,  45 ,  46  and  47 . Naturally, other types of ion sources can also be used instead of API ion sources. The invention is preferably used in conjunction with any type of electrode for influencing ion motion in mass spectrometers. Accordingly, it is also possible to use different mass analyzers than the ones presented here. 
     Typically, there is a pressure of 5×10 −5  to 2×10 −4  mbar in the collision cell  41 . Typically, the pressure in the ion trap  35  is similar. However, it is also possible for the collision cell  41  to have a significantly higher pressure of up to 10 −2  mbar. 
     The distances between the electrodes according to the invention and other ion-optical elements are typically 1 mm to 1.2 mm, preferably 0.2 mm to 5 mm. A voltage of up to 2500 V at approximately 3.1 MHz is typically applied in the ion trap  35 . Frequencies between 200 kHz and 10 MHz, in particular 1 to 5 MHz, are preferred. The ion lenses or electrodes have a voltage of typically up to 250 V. The ejection voltage is typically 3500 V, but can also be up to 5000 V. 
     LIST OF REFERENCE SYMBOLS 
     
         
           11  Electrode 
           12  Substrate 
           13  Conducting layer 
           14  Dielectric layer 
           15  Opening 
           16  Electrode surface 
           17  Border 
           18  Contact surface 
           19  Conductor region 
           20  Peripheral edge 
           21  Cover (cross-hatched) 
           22  Slit 
           23  Border 
           24  Radiofrequency electrodes 
           25  Radiofrequency electrodes 
           26  Mass spectrometer 
           27  API ion source 
           28  Ion interface 
           29  Vacuum chamber 
           30  Multipole ion guide 
           31  Ion lens 
           32  Vacuum chamber 
           33  Multipole ion guide 
           34  Ion lens 
           35  Radiofrequency ion trap 
           36  Vacuum chamber 
           37  Termination lens 
           38  Ion optics 
           39  Mass analyzer 
           40  Mass spectrometer 
           41  Collision cell 
           42  Lens 
           43  TOF Mass analyzer 
           44  Orthogonal accelerator 
           45  Acceleration lens 
           46  Reflector-lens arrangement 
           47  Receiver element 
           48  Mass spectrometer