Abstract:
A self-aligned ion guide assembly comprises a plurality of 2N rods radially contained by at least one insulating collar having two grooves axially displaced on the outer periphery of the collar. A wire in each groove electrically contacts and mechanically bonds to alternate rods through respective radially directed holes. Alternatively, the collar is cast about the assembly of rods and wires.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 60/364,507 filed Mar. 12, 2002. 
    
    
     FIELD OF THE INVENTION 
     The invention relates in general to an assemblage of rods for generation of multipole fields to be employed for ion guide and mass analysis purposes, and more particularly to economical and precise construction arrangements thereof. 
     BACKGROUND OF THE INVENTION 
     The transport of ions over some spatial interval is a functional description of an ion guide. Such an ion guide is an electro-optical device for confining the ion trajectories to a generally axial locus and that confinement is achieved through the influence of an appropriate electric multipole field distribution that returns a non-axially directed ion trajectory back toward the axis. The most common structural form for such a guide consists of a number, 2N, of metal rods arranged equidistant from a central axis. Opposite and/or 180° phase shifted AC potentials are applied in common to alternate rods. The efficacy of the ion guide depends upon precise geometry of the rod assembly as well as the congruence of virtual source and exit apertures of the guide with the real apertures of devices between which the ion guide operates. In one system (a mass spectrometer), an ion source is disposed spaced apart from a mass analyzer with the ion guide therebetween. Separation of the ionization and mass analysis procedures and devices permits optimization of these procedures and hardware subject to the efficiency of the ion guide. 
     The prior art has approached ion guide construction through support of the array of rods with at least a pair of axially spaced support assemblies having holes for retaining the relative positions of the rods and also for providing the desired common electrical contact of alternate rods. These prior art support assemblies typically include a ceramic insulating ring having holes through which the rods pass, to define the relative disposition of the rods. Metal rings are secured to the opposite faces of the ceramic insulator and each metal ring forms a common electrical contact with one corresponding sub-set of alternate rods while maintaining electrical isolation from the other sub-set of rods. Such arrangements are described in U.S. Pat. No. 6,329,654 B 1 and in U.S. Pat. No. 5,852,294. 
     SUMMARY OF THE INVENTION 
     It is desired to achieve a precise geometry for an ion guide with a simplified assembly. This is obtained with a support collar construction employing an insulating ring having axial extent sufficient to accommodate two axially spaced peripheral grooves on the outer azimuthal surface. An inner azimuthal surface has rod conformal arcuate surface portions formed therein to locate each rod. Each groove is characterized by a set of radially directed holes azimuthally spaced 2π/N radians for an assembly of 2N rods. The angular positions for the hole set for one groove is staggered π/N with respect to the other groove. Common electrical contact for one sub-set of N rods is realized by a conductor disposed within the groove, which contacts a rod through the respective radially directed hole. 
     The arcuate surface receives and constrains the outward radial locus of a rod. Electrical contact is established with a strong conducting wire captured in the groove and stressed such that when bonded through the holes to respective rods, there is an outward force on the rods balancing the inward constraining force of the arcuate surface portion against the rod, e.g., preloading the rod against the collar. As a result the ion guide assembly is a robust self aligned structure and is characterized by an aperture limited only by the rods themselves. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 shows one context for the invention. 
     FIG. 2 a  is a perspective view of an ion guide assembly according to the invention. 
     FIG. 2 b  is an elevation of the ion guide of FIG. 2 a.    
     FIG. 2 c  is a section through one support collar of the invention. 
     FIG. 3 shows a detail of a support collar of the invention. 
     FIG. 4 is a perspective representation of an alternative form of constructing an ion guide similar to FIG. 2 a.   
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The context of the invention is schematically shown in FIG. 1 for one representative example. For this example, an ionization chamber  12  operates on a sample to produce an ionized portion of the sample that is extracted from the ionization chamber  12  for transport through ion guide  16  to analyzer  14 . Schematically shown herein is an ion trap mass analyzer comprising the ion trap  18 , detector/display, recorder  20  and trap support electronics  22 . The separation of the ionization process from the analysis is often advantageous for a variety of reasons outside the scope of this work. 
     The ion guide  16  shown in FIGS. 2 a  and  2   b  incorporates features of the present invention, as specialized for the example, a hexapole ion guide. An assembly  30  of 6 cylindrical rods  32  are disposed axially equidistant on a circular locus orthogonal to the rod axes. The assembly  30  further includes one or more (preferably, two) insulating collars  34  and  36  of generally cylindrical geometry FIG. 3 is a detailed illustration of the salient features of one such collar. Two grooves,  38  and  39 , are inscribed on the outer azimuthal periphery of collars  34  and  36  and mutually axially displaced. The inner azimuthal periphery is characterized by rod-conformal arcuate (scalloped) portions  40  disposed at equal angular increments (here 60°). Each scalloped portion  40  defines the radial location of the rod in contact therewith and determines the axis of that individual rod, which is most usually parallel to the axes the other rods. Each groove  38  and  39  has radially directed holes formed to coincide with alternate scalloped portions  40 . Conductors, preferably wires  42  and  44 , are disposed within respective grooves  38  and  39  and are suitably bonded through the radially directed holes to the corresponding rods to form electrical contact as well as a durable mechanical bond therewith. As an example, FIG. 3 shows a wire  44  bonded by spot weld  48  through hole  45  to one underlying rod  32 . The wires  42  and  44  are captured in the grooves  38  and  39  under sufficient stress that, when bonded through corresponding holes to respective rods, such rods are pre-loaded in respect to the collars at the scalloped portions. 
     Assembly of the ion guide  16  is practiced by arranging the rods about a mandrel centrally disposed within the set of rods to urge the rods outwardly against the scalloped portions  40 . A wire  42 , for example, is inwardly urged against the bottom of the groove and through each radially directed hole and preferably spot welded to the respective rod  11 . The mandrel is then withdrawn. For a hexapole ion guide  16  comprising  6  rods (denominated A, B, C, D, E and F) each urged against scalloped portions A′, B′, C′, D′, E′ and F′, respectively, a wire in groove  38  for example, communicates with rods A, C and E while a wire in groove  39  would contact rods B, D and F through the corresponding radially directed holes. Electrical contact from each sub-set of rods may be effected from the wire or by a radially outward directed lead from a selected rod of each subset of rods. 
     In one ion guide assembly of this invention, the insulating collars are formed from a plastic such as poly-ether-ether-ketone (PEEK) to produce a robust but flexible structure. Another choice is polyphnylene sulfide (PPS), preferably glass filled for temperature stability. Ceramic, or other brittle insulator would also suffice for this purpose. Rods may be constructed of any suitable conductor, although it is generally desired that these be relatively chemically inert to an ion flux of varying character. Stainless steel wire has been used for the wire conductor  42  and spot welding to rods  32  has proved a simple and effective bond. The inventive arrangement is inherently self-aligning and easily assembled. The resulting ion guide exhibits no limiting inner diameter due to the support collars as would be the case where rods are led through holes in such supporting member. It is useful for one collar to further include a radial extension forming a flange  46  to mate with a terminal device as represented by ionization chamber  12  or analyzer  18 . 
     A hexapole ion guide in accord with the above description has been constructed having gross dimensions of 6 cm. in length with outer collar diameter (excluding flange) of 0.50 inch. The rods were 2.4 mm. stainless steel disposed at equal 60° increments on a circle of 0.290 inch diameter. The connecting wires  42  were 0.020 in. stainless steel. The construction as described herein has been used in a mass spectrometer system as indicated in FIG.  1  and is particularly robust and tolerant of disassembly and re-assembly for cleaning, maintenance and the like. 
     In another embodiment illustrated in FIG. 4, the ion guide structure described above is constructed through a casting process by placing the rods  32  in a fixture  52  that establishes the desired spatial relationship of such rods. Conductor wires  42  are then bonded to the appropriate rods as described, via solder or tack welding. A mold  50  forms a slip fit about the assembly of the fixture  52  and rods  32  sufficient to contain a (temporary) fluid phase insulating medium, such as an epoxy. Any of a wide choice of epoxy materials may be found to be useful and a particular choice will depend upon electric field strength to be applied, outgassing characteristics, possible contaminating effects (upon the analysis instrument) realized from low energy ion induced erosion, mechanical and thermal properties and the like. These aspects are outside the scope of this work. By way of example, a commercially available epoxy, Epoxi-patch (1C-white), available from Dexter Corporation, Seabrook, NH, has been employed with satisfactory results. Molds  50  and fixture  52  were constructed of tetrafluoroethylene (Teflon) which will easily slide off the resulting casting. 
     It will be clear to one skilled in the art that the above embodiments may be altered in many ways without departing from the scope of the invention. Suitable applications of the conveyer may include applications other than mass spectroscopy applications. The ion guide need not be straight, but can take on a desired non-linear trajectory. Lengthy guides may be achieved with more collars spaced appropriately. Accordingly, the scope of the invention should be determined by the following claims and their legal equivalents.