US2691107A - Mass spectrometry - Google Patents

Description

Get. 5, 1954 c. E. BERRY 2,691,107

MASS SPECTROMETRY Filed Aug. 7, 1950 2 Sheets-Sheet 1 ER & RECORDER 1P0 TEN TM].

*5 F! G TRANS VERSE v FIELD /0/v ENE/PG) I POINT AT WHICH EFFECTIVE WIDTH OF VIRTUAL SLI T EQUALS WIDTH OF ION BEAM 0 POINT AT WHICH EFFECTUAL WIDTH 0F VIRTUAL SLIT lg EOUALS RESOLVING SLIT a /0/v BEAM INTENSITY INVENTOR CLIFFORD E. BERRY nEsoL w/va POWER y Z v a IX IM TRANS VERSE P0 TEN 77/41. A T TORNE Y Oct. 5, 1954 c. E. BERRY 2,691,107:

MASS SPECTRQMETRY Filed Aug. 7. 1950 2 Sheets-Sheet 2 F/G. Z x8 /0/vS B IONS g8, IONS ANAL YZER I8 .8 CHAMBER l l l RESOL V/NG 51/7 20 EQU/PO TENT/AL LINES 48 RESOLV/NG ELECTRODE 7 K29 SH/E L D ZCOLLECTOR EL ECZTRODE 60 6 /1 (MASS NUMBER 44) SPECTRUM EFFECTIVE RESOL VING WIDTH EFFECTIVE RESOL VING WIDTH GREA TER THAN SPECTRUM WIDTH LESS THAN SPECTRUM WIDTH 5 MERCURY I50 TOPE SPEC TRUM EFFEC T/ 1 5 RESOL w/va W/D Tl-I EFFECT/ VE RESoL w/vs WIDTH GREATER THAN SPACINGBETWEEN LESS THAN SPAC/NG BETWEEN ADJACENT MASSES ADJACENT MASSES F IG. 5A.

JNVENTOR. CLIFFORD E. BERRY A T TORNEV 'charged at a collector electrode.

Patented Oct. 5, I954 PATENT OFFICE MIA'SS SPECTROlwETR-Y Clifiord .E. EerryzxAltadena, Califl, assignorto Consolidated Engineering Corporation, 7 Pasa- .dena, Calif a corporation (if-California Application August '7, 1950, Serial-No. 178,149

5 Claims. ((31. 250-413) This invention relates to mass spectrometry and more specifically to methods and. apparatus for varying the resolving power ofa massspectrometer by electrical'means.

In mass spectrometry, a mixture to be analyzed is ionized, at least in part, by electron bombardment. The ions are then segregated in accordso that the heterogeneous beam is separated into a plurality of essentially homogeneous beams. A single. ion beam is focussed on and discharged at a collector electrodethe discharge current being a function of the partial pressure in the original sample of the particles from which the ions of the beam are derived. Frequently'the mass spectrum is scanned by successively focusing the several spatially separated beams on the collector electrode.

In the conventional instrument, the resolution .l

of the several beams is accomplished'by a barrier placed in front of the collector electrode and having a narrow slit through which ions'inust pass to reach the collector. This slit is commonly referred to as the resolving slit and is oricnted'with its majoraxis parallel to the magnetic field. Ions of differing mass-to-charge ratio arefocussed' on the resolving slit by varying the magnitude of the ion propelling field or by varying the magnetic field strength. The resolving power of such an instrument is a function of the width of the resolving slit, i. e. the length of its rni-nonaxis transverse to the direction of the magnetic'field. By varying this dimension, as by substitution of a barrier having as different size slit or by mechanical' adjustment of the slit size, the resolving "power or" the mass'spectrometer may be varied.

I have now discovered a method for varying the resolving power electrically together with apparatus designed for carrying out the method in the most effective manner. As willbecome more apparent in the description of the accompanying drawings, there is needfor some simple-andeiiective-method of varying the resolving'power of a massspectrometer in order that the instru- .12 .ment may. be operable over a wide range of mass numbers. 1 Thus the resolving power-imost-:efiec- .tive at loweremasses is inadequate forresolving her masses, whereas .1 the higher I resolving power necessary for-ithe higher masses: interferes with analysis of the lower masses. :In. one aspect, the invention contemplates a methodS-of varying the resolving power of. amass spectrometerin which anion beam :is focussed: throughaa resolving slit-andontoa collector: electrode which comprises .developingiin .therregion betweeni'the resolving slit and 'thecollector electrodaanelectrostatic. field havingv a component parallelttothe 1 direction of :.:ion travel and ninxzopposition to-ion travel to the collector electroderand a cornpo- "nent transverse to the dire'ction'of ion travel-and parallel to the minor axist-oi itheiresolving1slit, with the transversercomponent being of:.zero..intensity at the .axis of 'symmetry' andcontrolling the magnitude of' the electrostaticrfield so that an ion beamuof a :given'energy can traversei'lthe distance between the resolving? slit-mend: thetcollector electrode only in a path ofea'fiwidthnnot exceeding. the minor axis :ofitheresolvingi slit and centered on the axis of symmetry.

By shaping the electrostatiofield so aslto form 'a saddle, ice. aregion where the pot'entialtwill be at a maximumin the longitudinal?direction and aflminim-um in the. transverse:directionland son: the axis of symmetry, the field functions as a virtualresoluing slit when xthe potential .at' the saddlepoint approaches Ethe potentialof the point from which thez ionsoriginatedinithe ion source. *If thesadd'le is .wideriithan the resolving' slit; that is, ifthetregionover which. the 1 opposing potential is less than the starting potential of the ions'is widei than thet:minor .-axis "of the resolving slit, resolvingpowerof thel mass spectrometer will be determined by-the resolving slit. If the electrostaticefield strength is'increased,..the.saddlesissnarrowed, i.-e. the transverse area thereof having. an. opposing potential loss than the .potentialenergy of theions isrestricted to a width less than the resolving slit and the electrostatic field then determines the resolving power of the instrument.

The energy of an ion'is determined by'the potential existing at'thej point of ionformation. In the accelerating field, ions gainkinetic' energy at the expense or potential energy, the total energy remaining constant. Hence w-hen the ions enter the opposing"electrostaticfield between the resolving: slit and the" collector,- they :give 'upiki- .netic energy, ii'.' necessary, to ithe :.point where restored. if,

however, a particular ion seeks to traverse the opposing field in a region where the bucking potential exceeds the potential energy of the ion, it will be blocked. As the opposing potential approaches the potential energy of a particular ion, the saddle point for that ion narrows about the axis of symmetry. This in elfect narrows the available path between the resolving slit and the collector for that ion. As this path becomes narrower than the resolving slit, the resolving power of the instrument is increased.

The use of an opposing electrostatic fiield in the region of the collector electrode is not new in mass spectrometry. Such a field has been established by means of an electrode located between the resolving slit and the collector electrode and maintained at a potential opposing ion flow towards the collector electrode. However, the purpose of this prior use has been entirely different from the principles of the present invention and has related solely to the blocking of ghosts, e. g. ionized fragments resulting from metastable transitions, or ions that have been slowed through collision with gas molecules or the walls of the analyzer. These ghosts have been found to have an energy less than the energy of the ions of a given mass-to-charge ratio with which they are associated. The development of an electrostatic bucking potential of sufficient magnitude to block such lower energy ghost ions has been found to improve the operation of the mass spectrometry. However, the use of such a potential to vary the resolving power of a mass spectrometer is novel there having been no effort made to establish a field to act as a virtual resolving slit so as to restrict the path of the ions under investigation.

For the purpose of this invention, I have found that a single electrode disposed between the resolving slit and collector electrode is not entirely satisfactory for developing the type of electrostatic field necessary to vary the instruments resolving power.

Accordingly, the invention also contemplates in a mass spectrometer having an ionization chamber, means for ionizing molecules in the chamber, an analyzer, means for propelling ions from the ionization chamber as a beam through the analyzer, means for sorting the ions of the beam in the analyzer according to their specific mass,

and a target electrode for collecting ions impinging thereon, the combination comprising a conductive barrier disposed between the analyzer and the target electrode and having a resolving slit therein through which ions must pass in travelling from the analyzer to the collector, a plurality of interconnected parallel electrodes disposed between the barrier and the target and having slits therein aligned with and wider than the resolving slit, and means for impressing on the said plurality of electrodes a potential opposing ion travel from the barrier to the target.

I have found that a plurality of parallel and interconnected electrodes permits development of a bucking field having optimum shape for the purposes of this invention.

The invention will be more clearly understood from the following detailed description thereof taken in conjunction with the accompanying drawing wherein:

Fig. 1 is a schematic diagram of a mass spectrometer adapted to the practice of the invention;

Fig. 2 is an enlarged view of the collection end of the mass spectrometer of Fig. 1 showing the a nature of the field established between the collector electrode and resolving slit;

Fig. 3 is a diagram showing a plot of the transverse component of the electrostatic field in the region of the saddle point and illustrating its efiect as a virtual resolving slit;

Fig. 4 is a graph showing the variation in resolving power and discharge current intensity as the transverse potential component of the electrostatic field is varied; and

Figs. 5 and 5A illustrate sections of actual recordings of ion discharge currents showing the effect of variation in the width of the elfective resolving slit in the case of a low mass number spectrum and in the case of a high mass number spectrum respectively.

Referring to Fig. 1, the spectrometer there shown diagrammatically comprises a cylindrical head [0 which connects with a sample inlet tube H made of insulating material. Within the head, in the path of the gas entering from the inlet tube, there is a pusher or repeller electrode I2 in the form of a conductive plate insulated from the rest of the apparatus.

An electron beam I3 is produced within the head in the space immediately following the electrode l2 by means of an electron gun (not shown). The region of the electron beam is referred to as the ionization chamber [4, since gas molecules entering the head from the inlet tube H are ionized by the electron beam in this region. The front of the ionization chamber is formed by an intermediate propelling of collimating electrode l5 which is electrically connected to the head and is provided with a slit SI that is substantially in line with the pusher electrode and with the path of the electron beam. A second propelling or collimating electrode 16 is spaced in front of the first electrode by a ring I! of insulating material and is provided with a slit S2 aligned with the slit SI and the electron beam. The head I0 opens into an analyzer tube 18 through the slit S2. The analyzer tube in the illustrated type of spectrometer is semi-circular in shape and encloses an analyzer chamber ill.

The opposite end of the analyzer tube I8 is provided with an exit or resolving slit 2B which gives access to an ion collector or target 2!.

The head ID of the spectrometer, analyzer tube l8 and the collector electrode 2| are enclosed within an enevelope 23 through a wall of which the sample inlet tube H projects. A high degree of vacuum is maintained within the envelope by means of vacuum pumps (not shown) con nected to a gas outlet 24. Within the envelope and substantially surrounding the target is a metallic shield 21 in the form of a cylinder with a disk at its end interposed between the end of the analyzer tube and the target and having an aperture or slit 28 in line with the target and the resolving slit at the end of the analyzer tube.

Intermediate the resolving slit 20 and the shield 21 there is mounted, in accordance with the present invention, a plurality of interconnected parallel electrodes 29 which function in the manner hereinafter explained. The entire instrument is maintained in a magnetic field transverse to the direction of travel of the ion beam by means of a magnet (not shown). A battery or other direct current supply 30 is connected in an energizing network across a potentiometer 31. The positive end of this potentiom eter is connected through a slider BIA and a switch 32 to a capacitor 33. A potential dividing network is connected across this capacitor and ageenror:

takes: the: formof. I a. potentiometer :34: connected in series. with axsecond I'eSiStOI "35J A. lead '36" connected intermediatev the potentiometer 34- andresistorr35 is connected to the terminal propelling electrode 16 and to ground. Since the terminal. propellingelectrode [6: is electrically connected to. the analyzer-tube, .the latter is also grounded; Potentionieter 34 has afirst slider 34A connected to the intermediate: propelling electrode l5. Pusher electrode 12 isconnected to the potential dividingznetwork by means of'a slider 38 for reasons hereinafter explained.

'Asecond; adjustable tap 34B of the potentiometer-M is connected-to -theelectrodesZB in the collection endof themass= spectrometer. By connectingthe pusher electrodeto the potentiometer through slider 38?; rather than to the positive end of thepotentiometer, as is the conventional practice, theelectrodes 29 maybe operated at a somewhat higher positive potential than the pusher electrode. Such relationship constitutes-preferred practice for the reasonthat the potential on the axis of symmetry of the electrode 294s inherently somewhat lower than the potential impressed on the electrode and for optimum results it is desirable to be able to raise this potential (at the axis of symmetry) near'to that'of the pusher electrode. This can only-be done if electrode'29 is at a higher potentialthan the pusher electrode.

The target'or collector electrode 2| is connected to ground through a high resistance 42. This resistance is shunted by an amplifying and indicating or recording apparatus 44 which is preferably ofthe negative feed-backtypeso as to minimize the eiiect of the high resistance. The shield 21 is likewise connected to-ground'and hence the barrier in'which the'resolvingslit'20' is located and the shield 27 are at the same potential as the terminal accelerating electrode 16, which is likewise grounded. A variable capacitor 46 may be connected from the target to the circuit supplying the propelling potentials. The capacitor is connected between the target and the resistance 42 so that the path to ground is from the target through the capacitor 46" and resistor 35. Although the incorporation of capacitor -66 in the output circuit forms no part of the invention which is inno way limited to the'particular output" circuit shown in'the drawing, the capacitor is desirable since-itprovides meansfor' balancing out the effects of capacitance coupling between electrodes 2| and'29.

In the operation of the spectrometer, a sample of'gas to be analyzed is admitted to the-ionization'chamber through the gas inlet tube. Molecules of the gas sample are bombarded and ionized by the electron beam. The resulting ions are propelled'as-an unsorted ion beam B:

intothe analyzer tube IS by means ofielectrical potentials established between" the pusher and; the two collimating electrodes bythe electrical.

circuit as described. Thus, slits SI, S2 in;the propelling electrodes l5, l6 respectively,- through Whichthe ions pass, cause the-ions to. imm'erge.

into the analyzer as a thin ribbon, a cross section of .the slits.

travelof the. beams.

One of the ion beams. is -focussed on the are-- solving slit .20 and passes throughthisxslit .to

strike .the collector electrode; Discharge oigthe;

of the-spectrometer.-

beamsat the collector-electrode develops a discharge current which.isamplified and recorded.

in the-conventional manner. .Theipath of the ion beams respect-toithe resolving slit may potentials between the several electrodes graduall'y decay while maintaining "constant relative- The potentials .in the'head may be ad-- justedrelative to each other by suitable adjust-- values ment of'the slider-34A= of the-potentiometer 34.

As above described, the series of slits'provided by the electrodes 29* intermediate the resolving slit-and collector' electrode makes it possible to vary'electric'a-lly theapparent width of the resolving slitif these'electrodes are maintained at a certain potential with respect to the potential of the ion source. At some point near the axis of symmetry of theelectrodes 29 there exists a saddle point in'the-electrical potential, that is,

J the'potential is ata maximum in the longitudinal direction and a minimum in the transverse direction.

The character of the-electrostatic fieldbetween the resolving'slit 20 and the shield 21, as

1: established by the-electrodes 29,- is illustrated in Fig. 2 which is an enlarged View of the collection end of the-spectrometer. Equipotential lines 48 are sketched-in the diagram of Fig. 2 showing the saddle-point to be approximately in the region oi'the point S. If a single electrode is used in place'ofthe plural electrodes 29, the configuration of thefield is altered and different operation is" obtained. The relationship of the resolving slit tothe ion beams Bl, B2, B3 is shown schematically above the resolving slit 20.

A plot-ofthetransverse potential in the region of 'theisaddle'point'is shown in Fig. 3 for two values'Vi, V2'of' voltage applied. to the electrodes 29% The'horizontal axis of the plot rep-resents the energy of theions of a given beam in Volts. Hence any ofthe-ions of this. particularbeam which approach the saddle'point at a distance from the axis of symmetry such that the potentia1 curve Viis above-the energy of the ions, will not beable to penetrate the field. With the potentiaLVl, only those ions willpenetrate the fleldwhose physical distance'from the axis of symmetry is less than where D1 is the transversedim-ension'ofthatpart of ithes fiel'dzwhi'ch is "at a potential' less than .the potential energy of the particular" ion. If the transversepotential;isfatthe V2 level, only those ions ofapotentialenergy rep-resented by the horizontaliaxis willpass, whose distance from theiaxis of symmetry is less than Hence; by; increasing the voltage: applied to .theelectrodes '2'9-so:that:.the ztransverseccomponent of. potentia-liisalterechfromivl to V2,; as represented onltherdiagramaofeFig; 3; thezvirtual: resolving: slit developed: by the-:- electrostatic :field; will..:be;.de.--

In this way the propelling creased by an amount equal to D1 minus D2 and the resolving power of the mass spectrometer will be increased proportionately provided that D1 is shorter than the minor axis of the resolving slit 20.

Fig. i is a plot oi the resolving power of the mass spectrometer and the ion beam intensity at the collector electrode as the voltage on the electrodes 2'5 is increased. For a time, the efiective width of the virtual slit formed by the electrostatic field is greater than actual width of the resolving slit 26 so that an increase in voltage applied to the electrodes 29 has no effect on the resolving power. At a certain critical potential represented by the point X on the horizontal axis of the plot of Fig. 4, the effective width of the virtual slit becomes equal to the width of the resolving slit. Any increase in potential applied to the electrodes 29 above the critical value X increases the resolving power by reducing the width of the virtual slit to less than the width of the resolving slit. The ion beam intensity curve is seen to remain constant considerably beyond the critical potential X. This is so because in the region between potential X and potential Y the virtual slit, although narrower than the resolving slit, is still wider than the ion beam itself. When the potential on the electrodes 25 is raised above the point where the transverse component is at the poten tial Y, the virtual slit becomes narrower than the ion beam and the ion intensity drops rapidly to zero. Thus there is a range of operation between potentials X and Y in which the resolving power is a function of the voltage applied to the electrodes 2E) and yet in which there is no loss in ion beam intensity.

In addition to providing means for varying the resolving power of the mass spectrometer, the e trodes 251 also act as an energy filter to supmeta-stable ions in the manner tau ht in the p.101 art, this function however being of secondary importance to the primary function of providing means for varying the resolving power.

The advantages of being able to vary resolving power of a mass spectrometer without altering the construction of the instrument are not immediately apparent. It might seem that if a high resolving power is desirable in one set of circumstances, that an instrument having such a high resolving power would be satisfactory under all circumstances. However, this is not the case and the reasons why this is not so are illustrated in One important reason why the resolving power necessary to separate components of large mass (see Fig. 5A) is not ideal for components of low mass see Fig. 5) is the recording speed reduction necessitated by an increase in resolving power. The spatial separation between ions of low mass is sufficiently great to tolerate low resolving power and high recording speed which is desirabl as resulting in faster analysis.

In the analysis of the gases of comparatively low molecular weight, say of '70 or 80, or below, ions of the same mass number are formed from more than one type of molecule. Consider for example the mass number 4 1. Ion of this mass number are formed from ionization of carbon dioxide and from ionization of propane, the atomic weights of the respective ions being 44.04: and 44.076 respectively. In any mass range it is desirable to operate at such a resolving power that all ions having the same mass number can pass through the resolving slit at one time. In Fig. 5 trace A shows an actual trace obtained from the spectrum of ions of mass number 44 derived from carbon dioxide and propane with the eifective resolving width being greater than the spectrum width. The superposition of the CO2 and CsI-Ia peaks is evident, the solid trace being a composite of the individual peaks shown in dotted lines.

Trace B of Fig. 5 shows the result of analyzing the carbon dioxide propane mass number 44 spectrum in an instrument in which the effective resolving slit is narrower than the spectrum width. In this circumstance, the two peaks are partially resolved but the overlapping precludes determination of the relative concentrations since it is difficult to tell the true height of either peak. This overlapping effect is shown by the dotted lines which have been sketched into the trace B, the height of each peak being due in part to the height of the adjoining peak.

When it becomes a question of separating between ions of difiering mass number, the reso1ution required is a function of the distances between adjacent masses for any given magnetic field strength. This distance is a function of the reciprocal of the molecular weight of the ions and hence as the mass increases, the distance between adjacent peaks decreases. In the lower molecular weights, represented by 5, there is sufficient spread between ions of differing mass number that they may be adequately resolved in an instrument having a resolving power sufiicient to obtain the l near superposition illustrated by trace A in that figure. However, if, for example, it is desired to analyze the isotopes of mercury which have mass numbers of 196, 198, 199, 200, 201, 202 and 204, the distances between the ion beams of adjacent masses is so small that a resolving power of the order of magnitude of that employed to obtain the trace A of Fig. 5 is not sufiicient to resolve the mercury isotopes. This situation is illustrated by the trace A in Fig. 5A wherein, with the possible exception of the mass 204 peak, it is evidently impossible to determine the concentration of the other mercury isotopes from the trace. This is because the effective resolving slit is wider than. the physical spacin between adjacent masses.

N ow if this effective resolving slit is reduced so that it is narrower than the spacin between adjacent masses, a trace B is obtained wherein complete resolution between each of the mercury isotopes is achieved. The isotope 196 is present in such small proportions; in order of magnitude of one-tenth of one percent, that it does not show up on either of the traces A and B.

It should be obvious from the foregoing explanation that the utility of a mass spectrometer for analyzing mixtures over a wide mass range is greatly increased by the ability to vary its resolving power in accordance with the mass range of the particular sample under investigation and that the same increase in utility is not achieved by pre-establishing the resolving power of the instrument at the high value necessary to resolve ion beams of comparatively large mass.

I claim:

1. In a mass spectrometer having an ionization chamber, means for ionizing molecules in the chamber, an analyzer chamber, means for pro pelling ions from the ionization chamber as a beam through the analyzer chamber, means for sorting the ions of the beam in the analyzer chamber accordin to specific mass, and a target electrode for collecting ions impinging thereon, the combination comprising a conductive barrier disposed between the analyzer chamber and the target electrode and having a slit therein through which ions must pass in travelling from the analyzer to the collector, an electrode disposed between the barrier and the target and having a slit therein aligned with the resolving slit, and means for impressing on said electrode a potential of a polarity opposing ion travel from the barrier to the target and of such magnitude as to prevent ion fiow through the slit adjacent its boundaries.

2. In a mass spectrometer having an ionization chamber, means for ionizing molecules in the chamber, an analyzer chamber, means for propelling ions from the ionization chamber as a beam through the analyzer chamber, means for sorting the ion of the beam in the analyzer chamber according to specific mass, and a target electrode for collecting ions impinging thereon, the combination comprising a conductive barrier disposed between the analyzer chamber and the target electrode and having a slit therein through which ions must pass in travelling from the analyzer to the collector, a plurality of interconnected electrodes disposed between the barrier and the target and having slits therein aligned with the resolving slit, means for impressing on said plurality of electrodes a potential opposing ion travel from the barrier to the target.

3. In a mass spectrometer having an ionization chamber, means for ionizing molecules in the chamber, an analyzer chamber, means for propelling ions from the ionization chamber as a beam through the analyzer chamber, means for sorting the ions of the beam in the analyzer chamber according to specific mass, and a target electrode for collecting ions impinging thereon, the combination comprising a conductive barrier disposed between the analyzer chamber and the target electrode and having a slit therein through which ions must pass in travelling from the analyzer to the collector, a plurality of interconnected electrodes disposed between the barrier and the target and having slits therein aligned With and wider than the resolving slit, and means for impressing on said plurality of electrodes a potential opposing ion travel from the barrier to the target.

4, In a mass spectrometer having an ionization chamber, means for ionizing molecules in the chamber, an analyzer chamber, means for propelling ions from the ionization chamber as a beam through the analyzer chamber, means for establishing a magnetic field across said analyzer for sorting the ions of the beam in the analyzer chamber accordin to specific mass, and. a target electrode for collecting ions impinging thereon, the combination comprising a conductive barrier disposed between the analyzer chamber and the target electrode and having an elongated slit therein through which ions must pass in travelling from the analyzer to th collector and oriented with its major axis parallel to the magnetic field, a plurality of interconnected electrodes disposed between the barrier and the target and having slits therein aligned with and wider than the resolving slit, means for impressing on said plurality of electrodes a potential opposing ion travel from the barrier to the target, and means for varyin the potential impressed on said electrodes.

5. In a mass spectrometer having an ionization chamber, means for ionizing molecules in the chamber, an analyzer chamber, means for propelling ions from the ionization chamber as a beam through th analyzer chamber, means for sorting the ions of the beam in the analyzer chamber according to specific mass, and a target electrode for collecting ions impinging thereon, the combination comprising a conductive barrier d sposed between the analyzer chamber and the target electrode and having a slit therein through which ions must pass in travelling from the analyzer to the collector, an electrode disposed between the barrier and the target and havin a slit therein aligned with and wider than the resolving slit, means for impressing on said electrode a potential to develop an electrostatic field across said slit preventing ion flow except through a central region of the slit, and means for varying the magnitude of the potential to vary the dimensions of said central region.

References Cited in the file of this patent UNITED STATES PATENTS liuznber Name Date 2,370,673 Langmiur Mar. 6, 1945 2,427,484 West Sept. 16, 1947

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