Abstract:
An improved ion optical lens designed to increase the amount of ion current delivered into a multi-pole ion detector or transfer device, such as quadrupole mass analyzer, an ion guide, collision cell, etc. A device and method is disclosed that utilizes a tubular entrance lens to introduce ions into or sample ions at a field-free or near field-free region disposed at the junction of two sets of multi-pole assemblies operating with radio frequency potentials shifted 180 degrees out of phase with respect to each other. The method is useful for increasing the transport of ions into as they enter into or exit out of a multi-pole mass analyzer, such as a quadrupole mass analyzer, an ion guide, collision cell, etc.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of PPA Ser. 61/201,781, filed 2008 Dec. 15 by the present inventors. 
    
    
     GOVERNMENT SUPPORT 
     Not applicable. 
     SEQUENCE LISTING OF PROGRAM 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     This invention relates to lenses for mass spectrometers, specifically to such lenses which are used at the entrances and exits of multi-pole assemblies with oscillatory and direct current potentials, such as quadruple mass spectrometers, multi-pole ion guides and collision cells, etc. 
     2. Prior Art 
     Quadrupole mass spectrometers commonly use an entrance lens in front of the quadrupole with a can or chamber encasing the quadrupole assembly to contain the oscillating fields of the quadrupole assembly inside of a metal can to prevent these fields from reaching out into the ion source region; detrimentally influencing the trajectories of ions. These fields at the entrance (and exit) of quadrupoles are commonly referred to as “fringe fields” and are composed of axially projected fields. 
     Originally entrance lenses where a flat plate with an aperture, referred to as a “shime or a diaphragm” (Steffen, 1965; Wollnick et al. 1965). However, this just prevented the fringe fields from entering the ion source. But as ions passed through the aperture, and were directed towards the quadrupole assembly, they were subjected to these fringe fields which lead to dispersing or defocusing the ion beam. Thereby, causing some ions to be lost (or rejected) and not enter the quadrupole assembly. 
     Thereafter, inventors disclosed several types of entrance lenses to introduce ions into a quadrupole assembly in such a way as to reduce these disperseive fields. U.S. Pat. Nos. 3,129,327 (1964), 3,371,204 (1968), 3,555,271 (1971), 3,783,279 (1974) all to Brubaker or Brubaker et al. disclosed a quadrupole assembly commonly referred to as a “pre-quad” disposed between the quadrupole mass spectrometer and the entrance lens. This pre-quad was relatively short compared to the quadrupole mass spectrometer and only powered with the RF electrical component or a derivative potential of the quadrupole mass spectrometer. Thereby, controlling the electrodes of the pre-quad to produce a decrease in the ratio of the static (DC, direct current) to the peak alternating potential (AC, alternating current)—delaying the onset of the DC component, with the ratio of DC to AC potential substantially zero at the inlet end of the quadrupole mass spectrometer. 
     Several types of entrance lens that are conical or tubular (snouts) shaped have been disclosed—for example U.S. Pat. No. 3,560,734 to Barnett et al. (1971), U.S. Pat. Nos. 3,867,632 (1975), 3,936,634 (1976), 3,937,954 (1976), and 4,013,887 (1977) all to Fite, and U.S. Pat. No. 6,153,880 to Russ IV et al. (2000). Barnett et al. disclosed an entrance lens comprised of two flat plates with conical (or tubular) snouts whose apexes are positioned inside the entrance of the quadrupole assembly an equal distance between each rod penetrating the fringe fields present at the entrance. When ions are accelerated through the lens into the central axis of the quadrupole mass analyzer they are shielded from these fringe fields. As the ions near the exit of the snout inside of or at the entrance to the quadrupole assembly they experience the defocusing effect of the fringe fields that are substantially reduced but still present. 
     Fite disclosed in a series of patents an entrance lens comprised of a flat metal plate with a dielectric tube (or snout) whose exit also is positioned inside the entrance region of the quadrupole assembly. The dielectric tube permitted the oscillatory fields from the quadrupole mass spectrometer to penetrate the tube thereby focusing the ions, as shown with the pre-quad by Brubaker, but block the defocusing direct current fields. The ions exit the tube in a similar fashion to the lens described above by Barentt et al. where the defocusing effect of the fringe fields are substantially reduced but still present 
     Russ IV et al. disclosed an entrance lens that is conical in shape that penetrates slightly into the central axis of the quadrupole assembly where the voltage supply for the lens is phase coherent with the voltage applied to the mass filter allowing more ions to be transmitted through the lens and into the mass filter. But nevertheless all the lenses at the entrances of multi-pole assemblies heretofore known suffer from a number of disadvantages: 
     (a) Entrance lens such as “shims or diaphragms” do completely block these defocusing fringe fields upstream of the entrance lens but ions upon entering and passing through the aperture of the lens, and before entering the multi-pole assembly, experience defocusing fringe fields (both axially alternating and direct current fields) which lead to the lost of ions as they traversed the distance from the lens to the multi-pole assembly—before they enter the multi-pole assembly. 
     (b) The use of pre-quads eliminates the defocusing DC fringe fields at the entrance of the pre-quad (delays the DC fields) but the axially RF defocusing fields remain. 
     (c) Lens with a metal snout or tube only shield the ions while the ions are inside the snout, but as they near the exit of the tube they experience these fringe fields in an increasing manner and some of the ions are lost, impacting into the inside walls of the tube and onto the rods of the quadrupole analyzer after they exit. 
     (d) Lens with a metal plate and a dielectric snout offer some improvement, but as the ions pass from the flat metal plate into the dielectric tube they experience these axial oscillatory fields and are potentially lost to the inside walls of the dielectric tube. In addition, as charged and neutral material accumulate on the inside of the tube the dielectric nature of the tube changes requiring constant adjustment of the potential of the tube; and cleaning. 
     (e) Combining pre-quads with either a lens with a metal or dielectric snout doses not eliminate the axial oscillating defocusing fields as the ions exit the tubes. 
     (f) If the radio frequency phase of the quadrupole assembly is applied to the entrance lens, ions experience defocusing fields upstream of the entrance lens and are possibly lost before passing through the entrance lens and into the multi-pole assembly. 
     3. Objects and Advantages 
     Accordingly several objects and advantages of the present invention are: 
     (a) to provide entrance and exit lenses for a quadrupole mass analyzer which can cancel or neutralize the defocusing fringe fields present at the inlet and outlet of quadrupole mass analyzers; 
     (b) to provide an entrance lens which will allow a larger percentage of ions from an ion source to pass through the entrance lens and into the central axis of the quadrupole mass analyzer uninhibited; 
     (c) to provide an entrance and exit lenses for quadrupole mass analyzers which can replace existing lens; 
     (d) to provide lens whose production allows for the individual parts to easily removed, disassembled, cleaned, and reassembled; 
     (e) to provide an entrance and exit lens for a multi-pole collision cell which can restrict the flow of gas out of the collision cell into the surrounding vacuum chamber; and 
     (f) to provide an exit lens from an high pressure multi-pole ion guide which can restrict the flow of gas out of the ion guide into the surrounding vacuum chamber. 
     Further objects and advantages are to provide a lens which can be easily installed and inexpensive to manufacture; which can be mass produced; can be comprised of metal, such as, stainless steel and insulating material, such as, Teflon or Vespel; which can be used with multi-pole assemblies such as a quadrupole mass analyzers, hexapole, octopole or quadrupole ion guides or collision cells; use with multi-plate ion guides or collision cells; as an exit lens for a multi-pole assembly; can replace entrance lens in electron ionization ion sources to quadrupole mass analyzers; and can be easily retrofitted to existing assemblies or instrumentation. Still further objects and advantages will become apparent from a consideration of the ensuing description and drawings. 
     SUMMARY 
     In accordance with the present invention a lens comprises a flat plate with an aperture and a snout, and a set of multi-poles with a radio frequency potential 180 degrees out of phase with an adjacent multi-pole assembly thereby creating a field-free or near field-free region where the two sets of multi-pole assemblies meet. 
    
    
     
       DRAWINGS 
       Figures 
       In the drawings, closely related figures have the same number but different alphabetic suffixes. 
         FIG. 1  shows a cross-sectional view of the inlet. 
         FIG. 2A  show a cross-sectional view of the inlet with the shout of the lens with a cylindrical shape and an exit tapering-down into a conical shape, with the exit opening of the shout smaller than the entrance opening. 
         FIG. 2B  show a similar view of the inlet with the shout of the lens with a cylindrical shape and restriction at the exit of the lens. 
         FIG. 2C  shows a similar view of the inlet with aperture plate with a tubular lens on the outer edges of the plate. 
         FIG. 2D  shows a cross-sectional view of the inlet with a ring electrode. 
         FIG. 3  show a cross-sectional view of the lens adjacent to a quadrupole mass analyzer comprised a set of pre-quads and an RF/DC qaudrupole mass filter. 
         FIG. 4  shows a cross-sectional view of an ion guide assembly, comprised of a hexa-pole assembly, with the tens as an exit lens. 
         FIG. 5A  shows a similar view of a multi-pole assembly with the lens as an entrance and exit lens of a high-pressure RF collision cell. 
         FIG. 5B  shows a similar view with the lens as an entrance and exit lens of quadrupole mass analyzer. 
     
    
    
     DRAWINGS 
     Reference Numbers 
     
         
           10  electrode or lens 
           11  tubular extension 
           12  snout 
           14  conical shape aperture 
           16  restriction or aperture 
           20  ion source region 
           22  incident in beam 
           30  poles 
           32  tubular shaped electrode 
           36  quadrupole mass analyzer 
           38  poles of the quadrupole mass analyzer 
           40  RF-only set of pre-quads 
           50  electric leads 
           52  controller 
           60  tubular lens 
           62  detector 
           100  region 
           102  exit lens 
           104  second lens assembly 
           200  entrance lens 
           201  entrance lens 
           202  exit lens 
           204  entrance lens 
           206  exit lens 
           210  hexa-pole assembly 
           220  RF collision cell 
       
    
     DETAILED DESCRIPTION 
     FIG.  1 —Preferred Embodiment 
     A preferred embodiment of the present invention is an inlet or entrance lens assembly to a quadrupole RF/DC (radio frequency/direct current) mass analyzer, see  FIG. 1 , with an incident ion beam  22  directed from an ion source region  20  through the inlet into a quadrupole analyzer  36 . This device is intended for use in collection, focusing, and introducing ions from low pressure ion sources, such as but not limited to, electron and chemical ionization sources, photo-ionization sources, etc.; ion optic assemblies that make up high pressure direct current (DC) and radio frequency (RF) collision cells; and ion optic assemblies (comprised of elements utilizing direct current (DC) and radio frequency (RF) potentials) that makeup the low pressure components of atmospheric or near-atmospheric pressure sources, such as but not limited to electrospray, atmospheric pressure chemical ionization, photo-ionization, laser desorption (including matrix desorption), inductively coupled plasma, and discharge ionization. 
     The inlet is comprised of an electrode or lens  10  and a set of four poles  30   a ,  30   b ,  30   c  (not shown),  30   d  (not shown), where the exits of the individual poles are adjacent to and symmetrically aligned with the corresponding poles  38   a ,  38   b ,  38   c  (not shown),  38   d  (not shown) of the quadrupole mass analyzer  36 . The lens  10  may be formed from an aperture plate by adding a snout  12  for extending between the four poles  30   a ,  30   b ,  30   c  (not shown),  30   d  (not shown) of the lens along the central axis. The snout  12  is tubular in nature but may be conical or have an irregular cylindrical shape. The length of the snout depends on the length of the multi-pole assembly and the spacing between the individual poles  30  of the multi-pole assembly and the poles  38  of the mass analyzer  36 . Typically, the individual poles  30   a ,  30   b ,  30   c  (not shown),  30   d  (not shown) of the lens are separated from the poles  38   a ,  38   b ,  38   c  (not shown),  38   d  (not shown) of the quadrupole analyzer  36  by an insulator or dielectric disk or rod (not shown). The snout  12  extends past the end of the multi-pole assembly plus ½ the distance separating the individual rods  30   a ,  30   b ,  30   c  (not shown),  30   d  (not shown) of the inlet from the rods  38   a ,  38   b ,  38   c  (not shown),  38   d  (not shown) that make up the quadrupole analyzer forming region  100 . Typical distances separating the rods are 1-2 millimeters and are determined by the peak-to-peak potentials of the RF potentials and the DC potentials of the abutting/adjacent poles. This corresponds to region  100  being approximately 0.5 to 1 millimeter from the ends of the individual poles, respectively. 
     Electric leads  50   a ,  50   b ,  50   c  schematically depict the connections required to supply the lens with DC and RF potentials, along with leads supplying RF and DC potentials to the quadrupole mass filter. Both are controlled by and may output results to a controller  52 . The RF potentials supplied the inlet are 180 degrees out-of-phase with the RF potentials supplied the quadrupole mass spectrometer. 
     FIGS.  2 A,  2 E, and  3   
     Additional Embodiments 
     Additional embodiments are shown in  FIGS. 2  thru  3 . In  FIG. 2A  the snout  12  of the lens is cylindrical shaped with a conical shaped aperture  14 ; in  FIG. 2B  the snout  12  is shown cylindrical shaped with a restriction or aperture  16  at the exit; in  FIG. 2C  a tubular extension  11  is added to the outer edge of the aperture plate  10 ; in  FIG. 2D  the multi-pole assembly is replaced with a ring or tubular shaped electrode  32 .  FIG. 3  illustrated the lens adjacent to a mass analyzer comprised of a RF-only set of pre-quads  40  and an RF/DC quadrupole mass analyzer  36  with corresponding electrical leads  50   a ,  50   b ,  50   c , and  50   d.    
     FIGS.  4  and  6   
     Alternate Embodiments 
     There are various possibilities with regard to the relative disposition of the lens as illustrated in  FIGS. 4-5 .  FIG. 4  illustrates an embodiment where the lens function as an exit lens  102  of an ion guide comprised of a hexa-pole assembly  210 , utilizing direct current (DC) and radio frequency (RF) potentials. The multi-pole assembly of the lens are comprised of 6 poles  30   e ,  30   f ,  30   g  (not shown),  30   h  (not shown),  301  (not shown),  30   j  (not shown), with the individual poles axially aligned with their corresponding poles of the hexa-pole assembly. The lens is upstream of a tubular lens  60  leading into a second lens  104  assembly which leads into a quadrupole mass analyzer (not shown). The second lens is comprised of 4 poles  30   a ,  30   b ,  30   c  (not shown),  30   d  (not shown) where the individual poles are in turn axially aligned with the corresponding 4 poles of the mass analyzer. Corresponding electrical for the DC and RF controllers are shown  60   a ,  60   b ,  50   e ,  60   f ,  60   g , and  50   h .  FIG. 5A  illustrates the use of a set of lenses used as an entrance  200  and exit  202  lens to a high-pressure RF collision cell  220  comprised of a quadrupole assembly from a MS-MS analyzer, such as a triple quadrupole analyzer, comprised of upstream analyzer (Q 1 ) and a downstream analyzer (Q 3 ); a quadrupole-time-of-flight analyzer, etc. Alternatively, the inlet illustrated in  FIG. 2D  may be configured as an entrance and an exit lens of a collision cell or ion-guide assembly comprised of alternating plates.  FIG. 5B  illustrates the lens used both as an entrance  201  and exit  206  lens for a quadrupole mass analyzer with an electron ionization source  20   a  upstream of the entrance lens  204  and a detector  62  comprised of a dynode and electron multiplier downstream of the exit lens  206 . 
     Operation 
     FIGS.  1  thru  5   
     The manner of using the inlet to introduce ions into a quadrupole mass spectrometer is similar to that for inlets in present use. Namely a potential difference is established between the ion source  20  and inlet. Ions are attracted to the inlet and enter the aperture  10  and are directed into and through the conduit. As the ions exit the conduit they are introduced into region  100 . Region  100  is the region which is field-free or near field-free and is formed by the positioning the multi-poles adjacent to the corresponding poles of the quadrupole mass spectrometer and by supplying the multi-poles of the inlet with a RF potential 180 degrees out of phase with the corresponding pole of the mass spectrometer. 
     The inlet can be used to restrict the flow of gas from the ion source into the quadrupole assembly by placing a restriction at the exit of the conduit, as shown in  FIGS. 2A and 2B . This restriction can be formed tapering the ends of the conduit to form a conical shaped aperture  14  or alternatively a restriction  16  may be placed on the end of the conduit. 
     As shown in  FIGS. 4  thru  5 , when the inlet is used as an entrance  104 ,  200 ,  201  and exit lens  102 ,  202 ,  206 , at the junction of the multi-poles, is field-free or near field-free, allowing the sampling of ions into the conduit in a field-free or near field-free regions  100 . 
     Advantages 
     From the description above, a number of advantages of our lens assembly become evident: 
     (a) By placing the lens adjacent to the entrance to a quadrupole mass analyzer the defocusing fringe fields will be neutralized, and will permit the uninhibited passage of ions from an ion source through the lens and into the central axis of the quadrupole mass analyzer. 
     (b) By neutralizing the fringe fields present at the inlet the inside diameter of the snout can be larger, occupying more of the central axis of the quadrupole assembly and permit more ions to enter the quadrupole assembly. 
     (c) By having a similar footprint as existing entrance and exit lenses for quadrupole mass analyzers, the lenses can easily incorporated into existing instruments. 
     (d) The limited number of components and the nature of materials used to produce the individual parts, allows the lens to be easily removed, disassembled, cleaned, and reinserted back onto the can surrounding the multi-pole assembly or into the vacuum chamber wall adjacent the multi-pole assembly. 
     (e) By using the lens as an entrance and exit lens for a high pressure multi-pole collision cell the gas load imposed on the vacuum system can be reduced. 
       FIGS. 2A and 2B  show the exit of the snout with a tapered conical shape and a smaller opening than the entrance, respectively thereby restricting the flow of gas through the lens.  FIG. 2D  shows a similar lens with the multi-pole assembly replaced with a single ring for neutralizing the fields of an adjacent assembly comprised axially aligned ring or plate electrodes. 
     CONCLUSION, RAMIFICATION AND SCOPE 
     Accordingly, the reader will see that the lens of this invention once placed adjacent to the entrance or exit to a multi-pole assembly can be use to create a field-free or near field-free region at the junction of the lens and the multi-pole assembly; and a set of lens can be used between adjacent multi-pole assemblies—thereby neutralizing the defocusing fringe fields present at the entrance and exit of RF/DC and RF multi-pole devices. In addition, when a lens is placed adjacent to the entrance to a quadrupole mass analyzer with an electron ionization source, ions from the ion sources can be transferred from the ion source region into the central axis of the quadrupole mass analyzer without exposing the ions to the defocusing fringe fields. Furthermore, the lens has the advantages in that:
         it permits the introduction of a wider beam of ions into the central axis of the quadrupole mass analyzer;   it provides entrance and exit lenses that can easily replace existing lens assemblies;   it provides entrance and exit lenses which are easily and inexpensive to produce, clean, disassembled and reassembled;   it provides entrance and exit lenses for a high pressure collision cell that determine the rate of gas flow from the cell into the vacuum chamber; and   it provides an exit lens for a multi-pole ion guide to restrict the flow of gas out of the ion guide into the surrounding vacuum chamber.       

     Although the description above contains many specifications, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently embodiments of this invention. For example, the lens can be incorporated into existing mass analyzers without the need to change the quadrupole assembly; the snout and aperture of the lens can have other symmetrical shapes, such as, a square shaped, oval shaped, etc.; the length of the individual rods of the lens&#39; multi-pole assembly can be variable depending on the application; the multi-pole assembly can be composed of six or eight rods; the RF potential applied to the rods can be the same as or a derivative of the potential applied to the adjacent multi-pole assembly; the potentials, both direct and oscillatory, applied to the lens can be variable and either changed manually or by computer control; the potentials applied to the lens can track or mirror the potentials applied to the adjacent multi-pole assembly; the rods of the multi-pole assembly can have other shapes, such as, oval, square, rectangular, etc.; the rods can be solid or hollow; the rods can be oriented, such as, flat face to flat face for square or rectangular shaped rods, corner to corner for square shaped rods, etc. 
     Thus the scope of the invention should be determined by the appended claims and their legal equivalents, rather than by the examples given.