US2947868A - Mass spectrometer - Google Patents

Description

Aug. 2, 1960 R. F. K. HERZOG MASS sPEcTRm-:TER

Filed July 27, 1959 INVENTOR R/cuAna-A Kif/kzou 528mm v KENWAY, MONEY, WITTER HILDRETH ATTORNEYS United States MAss srncrnoivrnrnn Filed July 27, 1959, ser. No. 829,683V

z3 Claims. (c1. 25o-441.9)-

The invention relates Vto a new and improved mass spectrometer, in particular to a mass spectrometer of high sensitivity for the analysis of minute trace impurities in solids.

Extensive research on the properties of materials in the solid state has revealed the importance of minute amounts of impurities which may be present in these materials. Concentrations of impurities as small as one part in 109 or l0 may exert a very large influence on the properties of a material, particularly on its electrical properties. Presently available mass spectrometers are generally incapable of providing reliable measurements of the presence of such small amounts of impurities. This is due in large part to their limited sensitivity and resolution, as well as to the practical diiculties encountered in measuring extremely small current intensities.

The sensitivity of an instrument is primarily a function of the existence of background which may be considered analogous to noise in conventional electrical measurements. Background can be considered as falling into one of two types, ie. general or continuous background and line background. Thus, ina plot of current intensity (number of ions per second) against atomic mass, the general 'background Willappear as a more or less c ontinuous quantity of varying amplitude superimposed on the general plot. The line background, on the other hand, will appear as separate lines opposite the respective atomic weights of the impurities which yare present in addition to the particular trace impurity analyzed. The latter, of

course, also appears as a line opposite its atomic Weight. The general background is produced in large part by ions which have collided with molecules of the residual gas in the spectrometer enclosure, as well -as by ions which have been reflected from the walls of the enclosure. This type of background can be reduced by improving the vacuum within the spectrometer and by traps and baffles which either prevent ions from hitting the walls or which catch reflected ions before they can reach the collector.

The line background is produced primarily by impuri-` ties released from the walls and gaskets of the instrument,

particularly those of the ionsource. It may also derive from the residual gas in the ionization chamber. This background can be reduced by thoroughly de-gassing the instrument at high temperatures and by eliminating all organic materials such as occur in gaskets, insulators, stopcock grease and vacuum pump fluid. A considerable improvement can be brought about by employing mercury diifusion pumps in place of the more conventional oildiffusion pumps. The mercury background is limited to a few mass numbers and therefore produces very little interference with the detection of the impurities under investigation. -On the other hand, the line background of oil dilusion pumps covers -many mass numbers and characteristically varies strongly from one mass number to another. Such a condition is undesirable since it produces uncertainity in the detection limit of the different impurities analyzed. l

vThe prior an; discloses many examples .Ofinstrumentsv g 2,947,868 Pete-*ed Aeg.; t 19??,

ice

. 2 which employ -techniques aimed at improving the performance of standard mass spectrometers. In general, these measures are directed at improving the sensitivity and the resolution of theinstrument. Thus, Thomsons well-known parabola spectograph was improved in two ways- (1) vby AJ. Dempster who employed a single magf netic field transverse to the ion beam `for the purpose of obtaining optical focusing by means of beam gdeection, (A New Method of Positive Ray Analysis, Physical Review, vol. 11, page A3 16, 1918); and l(2) b yFfW, Aston who added a transverse electric field in onder'tov obtain velocity focusing, ('A Positive Rayv Spectrographff Philosophical Magazine,'vol. 38, page 70, 1919). Velocity focusing compensates forthe unequal energies of the ions in the spectrometer which may be due tothe finite size of thesource or due to collisions of theio'nsV with residual gas molecules. k'Another' expedient which has been employed to improve spectrometer'performancev is the use of special iieldsv to obtain anastigmatic focusing.v (Mass` Spectrograph with Anastigmatic Focusing,l Zeitschrift fr Naturforschung, vol. 8a, page 191, 1953.) y It has also been proposed to combine ytwo electric fields and one magnetic Iield lin order to improve the resolution of mass spectrometers. (Increase of the Resolving Power of Mass Spectrographs, Physikalischel Zeitschrift, vol. 39, page 463, 1938-.) Two-stage mass speetrometers, wherein each stage employs a single mag netic. eld, vhave been used for special purposes. (Handbook of Mass Spectroscopy, by Inghram and Haydn, Publication 311, page 24. -National Research Council.-

, Published 1954.) In one application of this instrumentr the general background' was reduced bythe pre-selection of one ion species with an attendant increase in the sen.= One of. the disadvantagesv of.; this 2-stage instrument, which is attributed to Irlghramv and Hess, is that the second stage must be'at a consider-j ably higher potential than the lirst stage because of the additional acceleration potential applied to the ion beam:

in the second stage."V Such an arrangement 'represents' 'a' serious limitation-which impairs the generalvutility and'- the flexibility of the instrument. Another purpose for. whichV the above instrument has been successfullyused .isf the investigation of collision processes. Inrthisk case, thedellection Vfield ofthe first stage `was vused to select a spe" cial type of bombarding ion (monochrometer) `and the deflection i'ield ofthe second stage `was used to analyze the collision products (spectrometer). y v None of the foregoing techniques has been entirelyV successful to prov-ide 4al1 instrument which is capable. of

carrying out reliable measurements of minute-amounts of impurities. YAn instrument which is adaptedfor su'ch measurements mustv not only have .an extremely high sensitivity coupledwith acoeficient of transmission that ap# proaches unity, but must further provide vgood resolutiony and a wide dynamic range.

' Accordingly, it is the primary object of this invention; to provide an improved mass 'spectrometer which over comes the vdisadvantages of prior art instruments andi which is adapted to carry out reliable measurements of.

minute amounts of impurities to trace impurities in solids.V` f I This and other objects 'of the invention,l together withf the novel features and advantagesvthereo'f, will become, apparent'from the following `detailed speciication with aid ,in the analysis of;

reference to the accompanying drawing, in which; preferred embodiment ofthe in- Fig. l-illustrates a vention; and

Fig. v2 is` a cross-sectionall View Fig. 1 taken along line 2 2; g In brief, the invention herein comprises .a multi,-stage, high-resolution mass spectrometergwhich hasva coeiiicientf;

. of transmission that. appraachesunity... Optical as yvel1faS.-.-

of the apparatus 'of 3 5 velocity focusing is carried out in each stage by the use of transverse magnetic and electric elds separated by a slitted diaphragm in order to limit the energy spread. Thestages preceding .the-.tinal stage `serve to reducethe background in Ath e.tinalstage Vby pre-selecting the desired: ionspecies., They-are further voperative to focus. a twodimensional Vimage of-thevdiaphragm .slit whichinitiates the iirst stage, atthe llocusoithe initial diaphragm slit of. the inal` stage.v With this instrument, veryrhigh sensitivity measurements canbecarried out without compromisingv eitherthefhigh resolution or the wide dynamic rangerequired.` f. y

Fig: 1 illustratesa preferredembodiment of the inventionherein; -A-ptunpvlZ.v provides a medium vacuum region` of about-v.5 of-mercury in an. enclosed chamber-.11e Ions from :anaargonw ion `sour-ce 1'3 pass tfhroughthe opening ofa diaphragm 14Yto form an intenseargon beam whichiis accelerated to an energy of abouti kev. The yargon ion beam is directed at a sarnple .15 whose trace impurities are to be analyzed and produces'sputtering and'ionization'. Ions of the sample whichf are released asa result of the'bombardment by the -argonbeam are drawn'trom the sample byV a small electric iield and pass through opening 16 to form an ion beami An-acceleration potential of the order to 10 to 20 keV-.ris applied across-ian insulator 10 in thedirection ofqthebeamto accelerate theV ions. The apparatus required-to apply the-aforesaidv electric field and acceleration potentialfis conventional and well known in the art-:and has been omittedfrornthe drawing for the sake ot'fclarity.- A Adiaphragm-17,V which has a discriminating slit 18, separates the medium vacuum region from the' rststa'ge which is generally indicated by the reference numeral 21. A pumpl 23" maintains the stagei21, which is definedby annenvelope 22, at a vacuum pressure of the order Aof 10J" mm. :Hg Thestage is sealed with metal gaskets in order to .reduce Ithe general background `and topermit thorough degassing. of the system at thigh temperatures; A toroidcondenser 24, adapted to apply an electric field'transverse to the ion beam, is positioned as shown .inthe drawing with respect `to the beam path. This path is indicated by the optical beam axis'25 which generallycoincides with the true beam axis. The condenserfconsistsiofL-apair of'spaced, concentrically curved,

conductive surfaces. The. radius of the condenserv sur-r faces vinthe plane .ofthe drawing, i.e. aboutthe point 19:'011 thelbroken: line 20, diiers from the radius of curvaturelin `a planenormal thereto, i.e., a plane whose traceV in the plane of theldrawing is indicated by the line 20. This is shown more clearly4 in Fig. 2. A diaphragm'. 26 whichhas aidiscriminating slit 29 is disposed in theA envelopei22 subsequent to the toroid condenser. A pair.` of .identically.shaped, parallel pole pieces 27-is .stationed-subsequent'to the diaphragm 26 along the pathI of the'ionbeam. yIn the preferred embodiment shown, thepolepieces are spaced from eachother in a direction normal tosthe'plane .of the drawing so that the envelope 22 is disposed between them. With this arrangement vthe necessity for withdrawing the pole pieces during thedegassing procedure fis obviated. A diaphragm` 2'8 .fterminatesthe stage 21.. VA discriminating slit 30 in the diaphragm communicates with the subsequent stage 21.. An ion collector 31 which consists of a conventional lFaraday cage, is shown in its withdrawn position butrma-y'bepositioned in the path of the ion beam behind the'l slit 30` as indicated by the double-ended arrow in they drawing. I t is connected to one terminal of the switch 39.

The stage 21 -s substantially identical in construction to stage y21, corresponding parts having been labeled with prime. reference' numerals. A pump 23 maintains -a vacuum pressure of the order of 1'0-9 mm. Hg in stage 21fin1order to reducethe continuousl background asfar aspossiblei f Alion-collector 31V is positioned subsequen tofaidia'phragmfZS' which--terminatestheistage 21.# Y Y ing'the walls'of the envelope 22.

The ion collector 31 is also retractable as indicated by the double-ended arrow in the drawing and is connected to the other terminal of the switch 39. The switch is coupled to a vibrating reed electrometer 34 which, in turn, is connected to -a recorder 35. An electron multiplier 36 is positioned to receive ions through the discriminating slit of diaphragm 28 when the ion collector 31 is in its withdrawn position. The electron multiplier `is connected to an amplifier 37 which, in turn, is connected to a counter` 33 in operation, theions trom the yargon source 13 bombard the sample `15 to cause sputtering andionizationat the surface thereof. The ions released from the sample are drawn away by the application of the small electric iield and are accelerated toward diaphragm 17 by the application of an acceleration potential in this direction. The two-dimensional discriminating slit 1S of the diaphragm -17Y serves as-the object of the ion-optical lens systemand therebyfdeterminesthe shape ofthe ion beam insubsequent spectrometer stages.

A transverse deiiecting force, i.e. a force normal tothe optical .beam axis .25, is applied tothe ion beam which passesbetween the plates of Ytoroid. condenser 24.y This dellectingwforce is produced by the electric Iiield which is applied. `between the plates. It is operative through .an-angle centered about the plane of the drawing which dependsvon the' curvature of the condenser plates. The eiTectof the electric deiiecting force on the beam in the plane of'the drawing is to bend the beam in an upward` curvature of the toroid condenser surfaces in the plane which is `normal to the plane of the drawing and which is represented by the 'trace\20, cooperates with the curvature in the plane of` the drawing Ato concentrate the ion.

-beam inall directions and prevents the ions from strik- This. action is dier ent Vfrom `that which takes place in more conventional mass spectrometers wherein the deecting fields act only as-cylindrical lenses. As such, they concentrate the ion beamffin only one directionand permit it to strike the walls of the envelope dueto beam divergence in other directions. 11n contradistinction, the toroid condenser-in the present invention concentrates the beam.inall directions. This concentrating eiiect becomes particularly important in the narrow space between the pole pieces 27.

The actual amount of optical focusing which takes place in the toroid condenser alone is relatively small.l

As can be seen from the drawing, a certain amount of dispersion actually takes place after the beam leaves the toroid condenser. The ion beam is also subjected to a certain'amount of velocity dispersion in the con-` denser alone.- The primaryfunction of the toroid condenser` is to cooperate -with the subsequently disposed magnetic fie-ld to bring about velocity focusing of ions of the same: mass and of different energies. As a result of this cooperation a transmission coeflicient with ap-` proaches unity for the stage .is obtained. The transmission coefficient may bedeiined as the ratio between the number of `ions passing through slit 30 to the number of ions' entering slit 18 when the iields are adjusted for the passage of a given ion species. By the proper use of a'toroid condenser in cooperation with suitably shaped pcie pieces the effective lens opening is enlarged by approximately two orders of magnitude. This is equivalentlto ydoubl-ing the beam intensity without the loss of any resolution. Where trace impurities of the order of `1 l09 or 1x10-10 are to be detected, the ability to double the beam intensity represents a very important tadvaneeover the prior art.

An improvement in the resolution as well as in the thelslitg129. offidiaphragm 26;..The vdynamic range-can be'deiined as the ratio' of the largest and-smallest in be focused by the latter to produce a sharp image at slit` 30. The action of Aslit 30 in diaphragm 28 is similar to that of slit '29. Its primary function is to reject ions which have been in collision'in the magnetic eld. The improved resolution and dynamic range obtained contributes materially to the increased sensitivity of the instrument, vj

VThe transverse magnetic field set up by the spaced pole pieces 27 in a directicnrncrmal to the optical beam axisi V25 applies a deflecting force in the plane of the drawingto the-ion beam which is opposite in direction to that applied by the toroid condenser. The pole pieces are shaped so as to provide the maximum beam width forithe vminimum image distortion, as lWell as to account for thelelfects of the. fringing magnetic fields. In addition .to itsV function in cooperation with the toroid'condenser. to bring about velocity focusing of the ion beam, the magnetic deecting eld is shaped to bring about optical focusingat the discriminating slit 30 of the ion beam received from Vthe toroid condenser. It will be understood in this context,that the beam deilecting forces of lthe shaped magnetic field are not only applied to the plane ofthe drawing. A dedlect-ing force normal to the plane of-the drawing is exerted on all ions, which fail to enter the magnetic field in a direction normal to a tangent to its edge.

The action of the mass spectrometer which forms the subject -matter of this invention is, to a certain extent, analogous-to the action of an optical lens sysem. The properties of such a system can be more readily improved by combining a large numberl'of single lenses than by lattempting to incorporate all the desiredcorrections in a single lens. Indeed, it is frequently impossible to obtain the desiredcharactenistics iwithin the practical limitations `of 'a single lens. The same situation holds true for. la mass spectrometer wherein the action of the individual deflecting lields for the purpose of this discussion, maybe considered as being analogous to thatrofthe-,lenses in the optical system. In the case of an optical lenssystern the sensitivity of the instrument depends on :the largest opening of the discriminating slits whichV may be used while retaining vsuicient resolution. Y SinceV 'a' wider aperture requires better correction of any spherical aberrations, a larger number ofV lenses is generally called for.l Similarly, in the' case of the mass spectrometer, the larger the effective aperture, the greater-is the number of deecting fields required. Since, in order to measure minute trace impurities, the maximum intensity consistent with high resolution is required, a large effective aperture is necessary. i

The-overall effect resulting from the cooperation of the toroid condenser 24, the slit 29 iand the magnetic pole `pieces 27 of thestage'Zl, is to act as an ion-optical lens v.system which produces a sharp, two-dimensional imageV of the .initial discriminating slit 18 at the locus ofl the terminating discriminating slit 30. 'Ihe action which takesrplace in .the stage 21 thus serves a dual function.VK

While keeping the stage21 as clean as possible of the background-producing ions by admitting only properly selected ions, it simultaneously permits an extremely` This. condition again Ytends to vreduce the background at thehigh .vacuum to be maintainedfin the stage 21.

outputiand thus results in an improved sensitivity.

The collector 3-1, which is positioned immediately behindthe slit 30, is used only for purposes of adjustment in order to obtain the proper output from the stage Z1.

During thisV measurement the collector 31 is Vconnected to.the,e1ectrometer 34 by means of the switch 39. After` Suitable' adjustments have been made, the switch 39 is returned to the Vposition shown in the drawing. 'lfheL collector 311-is withdrawn and the beam is permitted to,

enter the stage 21' where substantially the sameprocess is yrepeated as was described in connectionrw-ith stage 21. The ion collector 31 is'also retractable isilsed only for the purpose of measuring the principal con-4 stituents of the sample. When use, the ion current actuates the Vibrating reed electrometer 'which in tum produces a measurement at the recorder 35. Magnitudes of the order of 10ml amperes can be measured in this way. For the purpose of obtaining a measure'-r ment of the minor constituents of the sample acounting technique is employed with the aid of the electron multiv plier 36. The output of the multiplier is amplified and is applied to the counter 38. With this technique rthe smallest ion current which can be measured with fairly goed statistical accuracy.(about 10%) after a counting time of the order of two minutes, is approximately oneI ion per second, or l019 amperes.

It will be readily understood that the use of more than one instrument stage brings about a material improvement in the sensitivity of measurement. Thus, if

the continuous background in each stage superimposedI on the trace element under analysis is 10-5 of the main peak (the latter being defined as the intensity level at the atomic mass of the main constituent), a combination of two stages reduces this background to about, l0"lo of the main peak. The addition of more stagesl will bring about a further reduction of the background.,4 Due to the extremely small ion current which is to be' measured, it is important that the transmission coefiicient 'of each stage approaches unity. If this condition is, not met, the loss of amplitude of the output current.V of a multi-stage instrument may conceivably oisetthe reduction in the general background.

Numerous modiiications may be made in the multistage mass spectrometer disclosed herein without departingrfrom the scope of the invention. It will be readily understood that the transverse deflecting fields applied by the toroid condenser and by the magnetic pole pieces respectively, need not necessarily deflect the beam inA For example, beam curvature in`- the same sense may be eifected to cause the beam toi opposite directions'.

double back on itself.

' It is not necessary for the envelope of each stage vto conform to the beam path as shown in the drawing.,V

"'Various -ion sources mayl be used, provided only that v a substantially mono-energetic, parallel beam of high and constant intensity is obtained. Thus, the sample may be evaporated in a small oven wherein ionization 1s brought about by electron impact or by a hot filament. The latter method is applicable for samples of low and medium melting points. Alternatively, a high` frequency` spark may be passed between two electrodes of the sample. This method produces a strong beam,A

even from samples having a high melting point. Holwever, the intensity of thei beam is subject to severey iluctuat-ron which, under certain conditions, may makel precise quantitative analysis diicult. The embodiment illustrated in Fig. l employs the method of sputtering by ion bombardment.

It will be seen from the foregoing disclosure that thek multi-,stage mass spectrometer which Iforms the subject matter of the invention herein combines the multiple.'

In this techm'que the ion current is relatively constant sinceit does not depend to anyv advantages of relatively higlr'output current, a coeicient of transmission whichis substantially unity, high resolution and wide Vdynamic range. An extremely high sensitivity is thus obtained which' makesl possible the precise and-reliable measurementof minute trace impurities in solids `Which havedefied reliable analysis heretofore.

Although a specific'embodimentof the invention has been described andillustrated; the foregoing disclosure is forthe purposek of illustration only. It will be understood that numerous modifications, departures and equivalents-will now occur to those skilled in the art, all of which fall within'the truespirit and scope of the invention'.

I claim:

1. A mass` spectrometer for operating `on an ion beam, comprising at least two pairs of deflecting fields spaced alongthe path'of said beam,`each of said deflecting fields beingadapted to apply transverse deflecting forces to said beam, means precedingsaid deiiecting fields for applying anaccelera-tion' potential to said beam, and means posit'ioned intermediate successively positioned deflecting fields to permit beam'transmission through a predetermined opening only.

2. A mass spectrometer .for operating on an ion beam, comprising at least two pairs of deflecting fields spaced along the path of said beam, said fields being adapted -to concentrate said beam in a plane `transverse thereto', and means preceding Vsaid deliecting fields for applying an acceleration potential to said beam.

3. A mass spectrometer for operating on an ion beam, comprising `at least two pairs of transverse fields spaced along the path of said beam, said fields being adapted to concentrate said beam in a plane transverse thereto, yand means positioned intermediate successive pairs of said fields to limit beam transmission to a predetermined opening. Y

4. A mass spectrometer for operating on an ion beam, comprising at least two pairs of transverse fields spaced along the path of said beam, said fields being adapted to concentrate said beam in a plane transverse thereto, and means positioned intermediate successive ones of said fields to limit beam transmission to a predetermined opening.

5. A mass spectrometer for operating on an ion beam, comprising a plurality of magnetic and electric deflecting fields spaced in alternation to determine the path of said beam, said fields being adapted to concentrate said beam in a plane transverse thereto.

6. A mass spectrometer for operating on an ion beam, comprising a plurality of magnetic and electric deflecting fields spaced inalternation to determine the path of said beam, said fields being adapted to concentrate said beam in a plane transverse thereto, and means positioned intermediate successive ones of said deiiecting fields tol confine beam transmission to a predetermined opening.

7. A mass-spectrometer for operating on an ion beam, comprising at least two pairs of `defiecting fields spaced along the path of said beam, each of said pairs including one magnetic and on electric field adapted to apply transverse `deflecting forces to said beam.

8. A mass spectrometer `for operating on an ion beam, comprising at least two pairs of deiiecting fields spaced lalong the path of said beam, each of said pairs including one magnetic Vand one electric field adapted to apply deilecting forces transverse to said beam, and means positioned intermediate successive ones of said deiiecting fields to permit beam transmission through a predetermined opening only.

9. A mass spectrometer for operating on an ion beam, comprising means `for applying an acceleration potential to said beam, a plurality of sequentially disposed stages following said -last recited means, a first discriminating slit separating said -last recited means from the first one of saidstages, each of said stages comprising velocity focusing means and optical focusing means and being termi- 8 nated by a discriminating slit, said focusing means being adapted to produce a two-dimensional image of said first discriminating slit at the locus of thel discriminating slit which terminates the-last stage.

10. A massv spectrometer for operating on an ion beam, comprisingv4 a pluralityy of sequentially disposed stages, means includingl av discriminating slit adapted to terminate each stage and* couple it to` the subsequent stage of said spectrometer, initial coupling means including an'initial discriminating"slitpreceding said first stage, each of said stages including a shaped magnetic field adapted to apply transverse magnetic ldefiecting forces to bring about optical focusing of the ion beam entering said magnetic field, saidmagnetic deflecting yforces being applied substantially parallel to a first plane to al1 ions entering said magnetic field within a predetermined angle of incidence, saidrmagnetic deflecting4 forces being additionally applied in a planenormal to said first plane to all-ions enteringl said magnetic deld' outside of said predetermined angle of incidence, each of said spectrometer stages further including an electric field adapted to apply transverse deliectingV forces to said beam, said electric deecting forces .being applied throughout a predetermined angle centered labout'said first plane, said electric field being adapted to cooperate with said magnetic field to bring about velocity focusing of said beam, whereby a twodimensional image of said initial discriminating slit is focused. at the locus of the terminating discriminating slit of each stage.

l1. The apparatus of claim l0 wherein said electric field of each stage precedes said magnetic field, and further comprising means including a `discriminating slit `disposed intermediate the respective ldeiiecting fields of each stage, said last-recited discriminating slit admitting only that portion of said beam to said magnetic field which can be focused by the latter.

12. The apparatus of claim 10 wherein Vsaid spectrometer is housed in an envelope, each of said discriminating slits being disposed in a diaphragm, said diaphragms separating. successive stages within said envelope from eachother while permitting communication through said discriminating slits, and means for maintaining successive stages of said spectrometer at an increasingly higher vacuum.

13. A mass spectrometer for analyzing the trace impurities of an ion beamV derived from a sample in the solid state, comprising a plurality of stages each defined by an envelope` and separating diaphragms, each of said diaphragms comprising a discriminating slit adapted to pass a predetermined portion of the impinging ion beam, means for maintaining successive spectrometer stages at increasingly higher vacuum pressures, means for preselecting the ions of the desired trace impurity against the background in the spectrometer section prior to the nal stage, said pre-selecting means including optical and velocity ion focusing means, said' focusing means being adapted to `produce an image of the initial discriminating slit of they first one of said spectrometer stages at the locus of the initial discriminating slit of said final stage, said final stage including electric and magnetic fields to apply transverse deflecting forces to said beam, said deflecting forces being adapted to produce anastigmatic velocity focusing and optical focusing of said beam, a diaphragm disposed intermediate said fields, said last-recited diaphragm including a discriminating slit adapted to admit only those ions to said magnetic field capable of being focused by the latter, whereby the two-dimensional image of the initial discriminating slit of said first stage appears at the locus of the terminating slit of said final stage substantially free from Vsaid background.

14. The apparatus of claim 13 and further comprising means for applying an acceleration potential to said ion beam prior to said Vfirst stage only, and means for measuring the ions passing through the terminal slit of said final stage.

15; A multi-,stage mass spectrometer for operating on an ionbeam kwhichemanates from aninitial discrimination Y.slit coupled to the first stage, .successively disposed stages being coupled to each otherV by means of respective discriminating slits, each of said stages comprising means for providing a magnetic deilecting iieldpreceding theterminating.gdiscriminating slit thereof, said magnetic deflecting field being shaped yto produce optical` focusing of the entering ion beam by the application of trans-verse magnetic dellecting forces, said magnetic deflecting forces being substantially confined to a first plane when the angle of incidence of said beam with respect to said magnetic field falls within a predetermined range, said magnetic eld being adapted to apply magnetic defiecting forces in a plane normal to said first plane when the beam angle of incidence falls outside said range, said last-recited means comprising a pair of parallel magnetic pole pieces spaced from each other in a direction normal to said first plane to contain said ion beam therebetween, said magnetic field being applied between said spaced pole pieces, means for providing an electric field disposed intermediate the initial coupling slit of said stage and said magnetic eld to apply transverse electric deflecting forces to said beam, said electric deecting forces being substantially confined to an angle centered about said first plane, said last-recited means comprising a toroid condenser consisting of a pair of spaced, concentrically curved surfaces disposed 4to contain said ion beam therebetween, said electric field being applied between said spaced surfaces, said electric field being adapted in cooperation with said shaped magnetic field to produce velocity focusing of said ion beam, whereby a two dimensional image of said -initial discriminating slit is focused at the locus of the terminating discriminating -slit of the final spectrometer stage.

16. The apparatus of claim 15 wherein respective analyzer stages of said mass spectrometer are maintained at substantially the same potential, and' further comprising a discriminating slit interposed between respective defleeting fields of each stage.

17. The apparatus of claim 16 and further comprising means for maintaining successive ones of said spectrometer stages at increasingly higher vacuum.

18. The apparatus of claim l5 and further comprising means for positioning an ion collector subsequent to the terminal coupling slit of each stage, each of said ion collectors being adapted to be withdrawn from the path of said beam, and means for measuring the ions obtainedat each of said collectors.

19. A multi-stage mass spectrometer' for operating on an ion beam, each of said stages comprising a 'nrst deflecting field adapted to apply transverse beam deecting forces in an angle centered about a first plane, a second deecting field for providing optical focusing of the entering beam, said second deecting eld being adapted to apply transverse beam deflecting forces substantially in said first plane upon beam incidence within a predetermined angular range, said second deflecting field being additionally adapted to apply transverse beam -deecting forces substantially perpendicular to said first plane upon beam incidence outside said angular range.

20. A multi-stage mass spectrometer for operating on an ion beam, each of said stages comprising a first deflecting field adapted to apply transverse beam deecting forces in an angular range centered about a first plane, a second detiecting field for providing optical focusing of the entering beam, said second deflecting field being adapted to apply transverse beam deliecting forces substantially in said first plane upon beam incidence within a predetermined angular range, said second deflecting field being additionally adapted to apply transverse beam deecting forces substantially perpendicular to said firstv plane upon beam incidence outside said angular range, the focal lengths of said first deecting field being different in mutually perpendicular directions, and a discriminating slit coupling successively disposed stages.

10 2.1,.- A multi-stage mass spectrometer for operating on an' ion beam, respective stages'being maintained at sub' stantially the same potential, each of ysaid stages comprising a first deecting ,field adapted to 'apply transverse` beam'deflectingforces in an angle centered about aiirst plane, raz secondjdeecting field for 'providing optical focusing off-the, entering beam, said second dellecting elsi biagyadapted to apply transverse beam deecting forces substantially in said first plane upon beam incidence within a predetermined angular range, said second deilecting field being additionally adapted to apply transverse beam Ideilecting forces substantially perpendicular to said first plane upon beam incidence outside said angular range, the focal lengths of said first deliecting field being different in mutually perpendicular directions, and a discriminating slit coupling each pair of successively disposed stages.

22. A multi-stage mass spectrometer for operating on an ion beam, respective stages being maintained at substantially the same potential, each of said stages comprising a first defiecting field adapted to apply transverse beam dellecting forces in an angle centered .about a first plane, a second deflecting field for providing optical focusing of the entering beam, said second deflecting field being adapted to apply transverse beam deiiecting forces substantiallyein said first plane upon beam incidence within a predetermined angular range, said second deflecting field being additionally adapted to apply transverse beam defllecting forces substantially perpendicular to said first plane upon beam incidence outside said angular range, the focal lengths of said first defiecting field being different in mutually perpendicular directions, a discriminating slit coupling successively disposed stages, said successively disposed stages being maintained at an increasingly higher vacuum.

23. A multi-stage mass spectrometer adapted for the analysis of minute trace impurities in solids, comprising a first cham-ber containing the sample to be analyzed, means for evacuating said chamber to maintain a predetermined vacuum pressure therein, means disposed in said chamber for producing ions of said sample, means for applying an accelerating potential to said ions to produce an ion beam, each of said stages comprising an envelope portion separated from the subsequent stage by a diaphragm, each of said daphragms having a discriminating slit which communicates with the subsequent stage, means for evacuating each of said stages to a predetermined vacuumpressure, said last-recited means being adapted to maintain increasingly lower vacuum pressures in the successive stages of said spectrometer, a diaphragm separating `said irst enclosed space from the first stage, said diaphragm having a discriminating slit disposed in the path of said ion beam, each of said stages comprising a toroid condenser consisting of a pair of spaced, concentrically curved surfaces, each of said surfaces having different radii of curvature in mutually perpendicular first and second planes, means for applying an electric field between said surfaces, said electric field being adapted to apply transverse deecting forces to said beam in an angle centered about the first one of said mutually perpendicular planes, cach of said stages further comprising a pair of shaped, spaced magnetic pole pieces parallel to said first plane, means for applying a shaped magnetic field between said pole pieces, said magnetic field being adapted to apply transverse deflecting forces to said beam in said first plane when the beam angle of incidence is substantially said magnetic field being additionally adapted to' apply transverse beam deflecting forces in a plane normal to said first plane when the beam angle of incidence is not 90, said fields being adapted to deflect said beam in said first plane substantially through mutually opposite angles, said electric field being adapted to bring about velocity focusing of said beam in cooperation with said shaped magnetic field, said magnetic field being adapted-'t0 bring about-optical yfocusing oft-lie entering beam to produce a two-dirri'e'nsionalV imageof4 the 'initial slit`of each stage-at the tenninalslit' the'eo'i,4 a diaphnagm; ha'v'ing a discriminating -slit disposedfin the'patl'ofvsaid beam intermediate said toro'id condenser` ande-said -pai'rv of pole pieces, said last-recited discriminating-- slit being adapted -to admit only that beam portion to saidl magnetic field-whichv is adapted tabel focusedy-a retractable ion 12 collector ladapted to -be disposed inthe pathosaid beam immediately l fol-lowing -thediaphragmtermin-ating. each of'said stages, Vand'measuring"means adapted-to life-"c011-v nected to'thefion collector-df each of said stages'.Vv v

References `CitedI in fthe. tile patent;

UNITED STATES PATENT OFFICE CERTIFICATE 0F CORRECTION -Patent No., 2Y947868 August 2 1960 Richard F, Kn Herzog It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 3Y line 25, for to,v first occurrence? read of column IY line 53y for with read which #-5 column 7Ai line 59Y for '*on" read one Signed and sealed this 4th day of April l9l (SEAL) v Attest; ERNEST W. SWIDER XXMXX/XME@ ARTHUR W. cRocKER Attesting Ocer Acting Commissioner of Patents

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