US2551544A - Mass spectrometer - Google Patents

Description

May l 1951 A. .0. C. Nu-:R ET AL 2,55l,544

MAss sPEcTRoMETER Filed sept. 20, 1944 I 2 sheets-sheet 1 a E F A O C NIER ET AL MASS SPECTROMETER May 11, 1951 Filed Sept; 20, 1944 Patented May 1, 1951 UNITED STATES OFFICE MAss srEorRoME'rER: I Alfred O. C. Nier and Mark Gl' Inghram, New York, N. Y., assignors to the United States of America as rcpre'sented by' the United States Atomic Energy Commissionl Application September 20, 1944,seria'1No. 554,985 12 olaims. (cl. 250-419) The present invention relates to a mass spectrometer, more specifically, it relates to a mass spectrometer suitable for analyzing gases or isoi topes of gases of low atomic weight, for example, hydrogen and deuterium, and being adapted to analyze either a single gas or isotope, or two gases or isotopes simultaneously.

In the past, the relative abundance ratio of two light gases or isotopes has customarily been determined by placing a sample of the composite gases or isotopes in a mass spectrometer, analyzing it, first with respect to one of the gases or isotopes and then with respect to the others, that is, measuring successively the respective ion currents produced by' the separate gases or isotopes at the collecting electrode, and `thereafte'r comparing the two separately collected ion currents to determine the abundance ratio. In s'uch relative abundance determination's, errors' in measurement were introduced by virtue of the fact that the intensity of the electron source beam was not exactly the same during the two separate analyses of the respective gases.

Another outstanding objection to mass spectrometers used in the past is the very cumbersome, large external magnet used to bend the ion beam for purposes of focusing the beam onto a collector electrode. Large solenoids have been used, costing several hundred dollars'fo'r materials alone, and requiring a ste'ady source of power of several kilowatts, hence requiring thel use of extremely large batteries or generators with special compensators for the purposeV of maintaining a Constant magnetic field. Such apparatus has been cumbersome, expensiveto build, as well as expensive to operate. Another difiiculty present in mass spectrometers" used in the past is that of developing a suiciently intense ion beam and properly projecting and focusing such ion beam so that a high percentage' of the ion current developed at thelsou'r'ce cari be focused onto the collecting electrodeZ with minimum loss or' scattering such as caused by the emission of secondary electrons along the tubular' walls and slitted plates used for collimating and focusing.

An object of our invention' isto provide a mass spectrometer that is devoid of the above-mentioned disadvanta'ges and difficultie'sl and that is suitable for analyzing one or two gases or isotopes either singly or simultaneously.

A further object of our invention is to provide a' mass spectrometer of simple and relatively inexpensive construction that is capable of analyzing' lightgasesi 01' isotopes thereof with a compartively high degree of resolution. p I

Other objects and advantages will become more apparent from the study of following specification, taken together with the drawings in which; p

Figi 1l is' a schematic diagram of almass spect'rornete'r embody'ing the principles of our invention Fig.. 2 is an enlarged perspective view of the pferinarie'nt magnet located at the center of the tube' shown'in Fig.. 1; and

Figi` 3`isl an enlarged perspective view of the c'ollectin'g" electrodes shown in Fig. 1. I

g R'e'fe'rring'more specifically to Fig. 1 numeral I defio'tes 'a' vitreous tube such as, for example, glass, the extreme endsiof which are not shown. Ifithe right portion of tube I, as illustrated, there is/ip'rovided a source'ofl electrons comprising the following combinations of elements: a tungsten filament 2 in the form of a hair pin supported by a pair of lead-ins 3, only one of which appears in Fig. 1. Lead-ins 3 exte'nd through a glass stem 4 fused to theV right extremity of tube I (not shown). Upon the glass stem 4 is mounted a plate 5 having a slit 6, plate 5 being adjustably mounted toV move axially on glass stem 4 so vas to make it possible to precisely align the fila- 'ment and slit'. All of the above-described parts 2 to 3, inclusive, constitute an assembly wellknown in the art. This assembly is inserted: into tube I between a pair of spaced discs 'l and 8 of nonmagnetic material, such as, for example, a

composition of nickeland chromium containing approximately S0 per cent nickel and 20 per cent chromium. Discs 'l and 8 are inter-connected both electrically and mechanically and constitute a shield 7-8 or enclosure for the ionizing space. Plate constitutes one of the inter-connecting means between discs 'I and 8 and includes a slit 6a with which slit 6 is brought into registry. Also internally of the shield 1-8 isprovided an electron collecting electrode IO, Afor example, in the form of a cup consisting of a composition' cont'aining approximately 80 per cent nickel and 20 per cent chromium, adapted to collect a large portion of the electrons emitted by filament 2 as a beam, and collimated by slits and 6a.

An ion repelling electrode il is positioned back of the electron beam, and has a potential applied thereto which is slightly more positive than that applied to shield '5-3, for example, in the neighborhood of 2 volts more positive than the voltage on 'll-8. Filament 2 has a voltage that is more negative than the shield voltage while catcher electrode E9 has a voltage that is more positive than the shield voltage, thereby accelerating the electron beam toward the catcher electrode. A capillary leak |2 is provided constituting a glass tube With a capillary at the end thereof for the purpose of introducing a sample of the gas or gases to be analyzed into the space adjacent the electron beam and repellingr electrode Ii. As a result of electron boinbardment of the gas molecules positive ions will be formed in the space enclosed by shield 1-8. A small permanent magnet, for example, of the horseshoe type, having poles |3 and 14, is provided externally of tube l in order to produce a magnetic field parallel to the electron beam for the purpose of collimating and aligning theelectron beam so that most of the electrons will be collected by the collecting electrode 60. The ion repelling electrode H having a slightly positive voltage With respect to shield 7-5 would normally in the absence of the magnetic field produced by the magnet poles |3-l4 attract electrons. However, in the presence of the magnetic field the electrons travel in a Substantially helical path between the filament '2 and the collecting electrode iil, thus producing more ions than if they merely traveled in a straight path through the gas. If a sample of gas containing, for example, hydrogen and deuterium, is introduced through oapillary leak :2, HDi ions having a mass of 3 as well as Dz+ ions having a mass of 4 Will be created as a result of electron bombardment of the molecules f these gases. For example, small traces of ions of mass 3 are for-med with large numbers of ions of mass 4. The ion repelling electrode ll will repel the ions in a direction at right angles to the electron beam, that is, in a direction to project the ions through a slit S1 in disc 8.

VIn order to accelerate the ions produced, in a direction longitudinally of tube I, as shown by dottecl lines, a pair of spaced semi-circular beam centering plates and E5, having a slit S2 therebetween, have applied thereto an average voltage that is more negative than that of Shield 1-8. To further accelerate positive ions, shield 7-3 has applied thereto a voltage that is substantially more positive than the voltage applied to a slitted disc il' having a slit S3. Such voltage applied to shield 1-8 may be, for example, of the order of +360 volts whereas the disc i? may be at zero or ground potential. The vol*- age applied to beam centering plates |5 and (5 is selectively adjustable from O to 1/3 of the total accelerating voltage, that is the voltage beloa7 the voltage of shield '1-8, and is generally of the order of +330 volts.A The ion beam is therefore successively accelerated through tube l. Disc 8 and beam centering plates 95-18 together constitute an electronic lens that tends to focus the ion beam to a small extent, thus insuring entry of the ion beam through slit S3 in plate l'l. The main purpose of beam centering plates l-I, however, is thatof adjustably centerng the ion beam so that it Will be in reflistry With slit S3. This can be eifected by any Wellknown potential varying means (not shown) that can be used for the purpose of varying either or both of the Voltages applied to the respective beam centering plates 15 and i. For example, if it is found that the ion beam impinges on a portion of the disc H to the right of slit S3, then by making the voltage applied to plate E5 slightly more positive or by making the voltage applied to plate l5 slightly more negative, or by simultaneously varying these voltages in the mannei' stated, the ion beam can be made to come into registry With slit S3. The slit S3 may be of any suitable dimension, for example, 1 millimeter by 14 millimeters (the large:` dimension being at right angles to the plane of the drawing).

The ion beam, after emerging from slit S3 is projected substantially coaxially of tube l through a V shaped analyzer comprising a cylindrical tube S of non-magnetic material, such as, for example, a composition containing approximately per cent nickel and 20 per cent chromium, a permanent magnet M, and a metallic tube 19 (of substantially i'ectangular crosssection) and of non-magnetic material, such as, for example, a composition consisting of approximately 80 per cent nickel and 20 per cent chromium.

The specific construction of the permanent inagnet M is shown more clearly in Fig. 2 and comprises, for example, a plurality of substantially U shaped permanent magnet portions 2B having common parallel wedge-shaped pole faces 2| and 22 made of any suitable magnetic material, such as, for example. iron or a magnetic composition including approximately 8 per cent aluminum, 14 per cent nickel, 24 per cent cobalt, and 3 per cent copper and iron. Permanent magnet portions 20 may be secured to the pole faces 2i and 22 by means of metallic strps 23 acting as hangers, closely surrounding the periphery of the permanent magnet portions and having their extremities secured to pole faces 2I-22 v by suitable fastening means, such as, for example, screws 24. The upper edges of pole faces 2l-22 are inter-connected by means of a thin rectangular plate 25 of a composition consistng of approximately 8G per cent nickel and 20 per cent chromium. For example, such plate is fastened to the upper edges of pole faces ZI and 22 by suitable fastening means, such as, for example, screws 25. By virtues of the above-described construction of permanent magnet M, it is porsible to provide a magnetic field of relatively high intensity, for example, one having almost 1,000 gauss. The magnetic intensity can be varied by varying the number of permanent magnet portions 20 aixed to the pole faces 2| and 22. It Will be noted that magnet lvl is totally enclosed in tube I, hence pole faces 2 I-22 may be brought close together to give a strong field.

Referring to Figs. 1 and 2, it will be noted that the lateral, inclined edges of pole faces 21 and 22 form substantially a wedge. The angle formed by the lateral, inclined edges is preferably of the order of 60. As the positive ions enter in the space partly defined by the right, inclined edges of pole faces 2! and 22, they come under the influence of the magnetic field developed by permanent magnet M, such field being perpendicular to the plane of the drawing. The magnetic field will cause bending of the ions through meters from the point of intersection of the 60,

netic fields. as extending beyond these edges.

- disposed 'dashedlines .in Fig. 1,` andizwill ;cause .fieldtat both of the inclined edges of pole";fa-ces 2| and 22, lit. is 'necessary .to 'consider' themag- A suitable allowance is made when the width of .theair gap between the pole faces'is added to .both the inclined edges as shown'bydash Vlines in Fig. 1, which lines constitute the effective magnetic field boundaries as distinguished from .the actual boundaries Aof the magnet-M- As .that the entrance slit S3 of the analyzer, the apex A of the V formed by extending the effective edges of the magnetic field boundaries, and the exit slit will all lie in a straight line. It should be understood that the 60 is merely a convenient angleand is in no` way critical for the purpose of focusing ions. For example, the angle between the effective laterally inclined edgeslof the pole faces may be anywhere between a very small angle up to an angle of 180 so long as the ion beam both enters and leaves the inclined edges of the magnet perpendicularly. It will be noted that the objections to a very small angle are that the distances of the exit and entrance slits from the edges of the magnet will be relatively great necessitating a long tube and analyzer. The provision of such long tube has not only the disadvantage of being more cumbersome but 'has'the additional danger of providing a greater probablty of scattering of the ions, by collision with other ions or neutral molecules, particularly if there is an appreciable pressure of the gas in the tube. On the other hand, by making 'the angle very large, for example, almost 180, there will'be practically no distance between the inclined edges of the magnet and the entrance and exit slits. In other words, the slits can be cated laterally, close to the upper surface ofthe 180 magnetic field. Large angles, however, have the advantage of causing greater lateral separation at the collector of gaseous ions of different mass or isotopes in cases wheretwo of these are collected simultaneously by a pair of collecting electrodes. `Such large angles are particularly useful in separating elements of relatively high mass and relativelysmallmass difference, for example, in separating mass [T235 from mass U2, whereas small angles are more suitable for the separation of elements of light mass, such as for example, for separating deuterium molecularions having a mass of 4 from hydrogen molecular ions of mass 2 or in separating deuterium from I-ID+ ions having a mass 3.

Referring to Figs. 1 and 3, after the ions are bent by the magnetic field and separated and sorted in accordance with their different mass-to- .charge ratios, .they will emerge from the magratio measurement.

'21:6 'plel includes 'two' gases; 'such :as hydrogen anddeuterium having different: masses. .:One:'.:of?.the beams .having D2+ ions of V'mass .4 :is collected by collecting electrode Hand theother: havig.:HD+ ions of mass 3 bya. collectinge1ectrode:28, :Collecting electrodes 21 and fl'mayzbe 'supportedifat Vthe left extremityl ofv the tube' I (notshownhzand are positioned so asto be'.inregistry'rwithfexit .slits 29:and 30.1ocatedtat' the.:end jofrtubesflf.

v.Plate 32 is provided' to: act. as afib'arrier, for separating collecting electrodes 211andA 28. By :applying voltage to plate 3 I: that: is more :negativethan the voltage of the col'lector'electrodes;2'l'vandf28, secondary electrons that tend to'emergefrom c'ollecting electrodes 21 an'd 28pwhen'bom'barded1by ions, are preventedl'from emergingfromrthese electrodes. A suitable negative potential sto apply toplate 3 lV is one that =is, for example;1 about 20 volts more. negative'than .the'voltage,.,of,'the collecting electrodes 21 and- 28,:a1thoughz-aflzwide range of negative Voltagesmay be. used instead, with varying restraining effects nsofar: asfseoondary-emission -is concerned. *The Vdarrgerfizof secondary electron emission :is that the current carried by such electrons willzcause erroneous'ion current readings at the "collecting-.-electrodes. The purpose of plate 32 is to prevent travel of :any secondary electrons or ions 'that "mightz occur` in the vicinity of one collectingtelectrode from travelingto the vicinityf'of the-otherxcollecting. electrode thereby introducing errors in'sthe ion'current measurement and. the relative abundance The ,ion currents collected by collecting eleotrodes 217 and- 28;may-be:measured by any suitable well-known electroniomeains (not shown). For example, the ion'currentto the collecting electrode Z'Lfiwhichris the 'massf' collector, may be detected and s measuredbyian eiectrometer of any well-known type, particularly one having, for example, a highly Sensitive? amplifier tubetcommercially known as the ``Fly-'54 type. Collecting electrode 28, the mass 3` collector, .may havezconnectedthereto veither an'felectrometer or preferably a. negative :feed-back amplifier (not shown) consisting ofa .tube.=commercially known as. the' 959 type. vIn 'order-.to

. make. comparisons of currents collected by 'the respective collecting electrodes .easier a null method or potentiometer resstormethod. of .comparison of the collected ion currents. maybeused as shown in copending application Serial Number 548,588. of Alfred O. C. Nier and.Edward.P.Ney entitled AV Mass Spectrometer System -filed August 8, 1944, nowl Patent No.'.2,456,426. `lSuch electronic circuits form no part of the present invention.

In the design of the apparatus shown'i`Fig'; 1, the following well-known" formula is n'oted:

where r`is'the radius of-curvaturefof ions'in centimeters 'in the magnetic'ffield, wherei'mwis the mass of the 'ion in atomic mass and unitsand e is its charge'in terms'ofv the electronic'charge'as a unit,.where I-I is the magnetic' field intensityrof the magnet infgauss,v and Where'V. is the total accelerating'potential in volts. In separating. mass 2 from mass 3 for example, the value m/e'may be taken as the average of these twomasses, namely 2.5,'and if 1' is 5 centimeterssan'd'I-Il-is 1,000 gauss then V=482 .vo1ts,'which=is::.the accelerating voltage necessary lfor ;focusing .the aforementioned values of'm/e. Actually .Hi-'in'the present device isless than 1,000 gauss, giving a value of V of 360 volts. By holding the magnetic field intensity constant and by adjusting voltage V various values of m/e will fit the focusing conditions and register on the designated collecting . electrodes 27 and 28.

The analyzer may be assembled in tube l in the 'following manner: tube I, being substantially Y shaped, is provided with open ends at all the extremities of the Y. Magnet M is then inserted through the lower leg of the Y and may' be held in place in tube i by suitable fiexible supporting means such as spring clips 33 and M. The lower` leg of the Y is then sealed. Tube l is ,then introduced through the right leg of the Y shaped tube I and by means of a long screw driver tube IB may be fastened to the right inclined edge of magnet M by means of screws 35 that enter threaded holes at the right inclined edge of the magnet M. By means of a pair of pins (not shown) located along the periphery of the collar portion 31 of tube 10 and correspondingslots in the magnet, it will be possible to readily align screws 35 with their` respective screw threaded holes in the magnet. In a similar manner, tube 19 is introduced from the left extremity of the Y shaped tube I and may likewise be fastened to the magnet by means of screws 38 and pin and slot connections. Of course, after suitable baking of the analyzer for purposes of degassing, as

- is well-known in the art, the extremities of the tubes may be sealed, that is, of course, after the remainder of the ion source assembly and collecting electrode assembly is introduced in the tube l. A suitable vacuum is produced by means of a mercury diifusion pump (not shown) having a Dry Ice trap (not shown) that is effective to l 'create a vacuum of about 2x10-7 millimeters of mercury.

It will be seen, therefore, that we have provided a mass spectrometer that is suitable to analyze either a single gas 01' isotope thereof, or a pair of gases or isotopes simultaneously by the use of either one or both of the collecting electrodes, respectively. By simultaneously collecting ions of two gases or isotopes there is no danger of introduction of error due to variations in ion intensity inasmuch as both readings are taken Vsimultaneously at a single ion current intensity.

It is not important whether such intensity varie; from time'to time because of the fact that the readings are taken simultaneously at a given ion intensity. and thus will give an accurate indication of the relative abundance ratio of the two different ions representing the twe different gases `01' isotopesi It will also be seen that by virtue of beam travels, and therefore obtain a relatively high intensity of magnetic field. 'I'hus we have provided a relatively simple, rugged and inexpensive magnet requiring no power supply or cur- -rent regulating means such as is necessary in the common external electromagnet generally used for mass spectrometers. For example, the cost of external magnets may be several hundred dollars as compared to a few dollars foi` an internal permanent magnet according to our invention. It will also be noted that we have provided a relatively simple, rugged and efficient mass spectrometer that is particularly adapted for the analysis of two gases of light atomic weight simultaneously in order to determine their relative abundance ratio without error as the result of fiuctuations in electron intensity of the source or the like.

It should also be noted that we have provided a mass spectrometer that despite its simplicity of construction has a substantially high resolving power for the determination of the abundance ratio of gases having light atomic Weights such as deuterium and hydrogen.

It will be apparent that modifications will be readily suggested to others skilled in the art as the result of the teaching of our invention, hence, it should be understood that our invention is not limited by the specific construction hereinbefore described, which is merely an exemplary embodiment, and should be restricted only inscfar as set forth in the following claims.

What is claimed is:

l. A mass spectrometer comprising an evacuated tube enclosing, in combination, an ion source at one end of said tube, means for accelerating and focusing ions from said ion source and separating them in accordance with their different mass-to-charge ratios including a permanent magnet wholly contained inside and intermediate the ends of said tube, means at the other end of said tube for collecting said ions, and two tubular non-magnetic members supported on said accelerating and focusing means, one extending in the direction of said ion source and the other extending in the direction of the ion collecting means, said members enclosing a major portion of the path of the ions.

2. A mass spectrometer comprising an evacuated tube enclosing, in combination, a source of positive ions at one end of said tube, electrode means for accelerating ions developed by said source, permanent magnet means contained whclly in said tube intermediate the ends therecf for bending and focusing said ions and separating the ions in accordance with their different mass-to-charge ratios, collecting electrode means at the other end of said tube for collecting said ions, and two non-magnetic tubular members supported on the magnet means, one extending in the direction of the ion source, and the other extending in the direction of the collecting means.

3. A mass spectrometer comprising an evacuated tube enclosing, in combination, an ion source at one end of said tube, electrode means for accelerating ions from said source, and for projecting them in the form of an ion beam, a permanent magnet of substantially U-shaped cross-section contained wholly within and intermediate the ends of said tube and having wedgeshaped poles disposed with respect to said ion beam to bend said ion beam through a predetermined angle and effect focusing thereof, a collecting electrode at the other end of said tube for collecting said focused ion beam, and two nonmagnetic tubular members supported on the magnet, one member extending in the direction of the ion source, and the other member extending in the direction of the collecting electrode,

- said tubular members substantially enclosing the path of the ions.

4. A'mass spectrometer comprising an evacuwithin and intermediatethe ends of said tube and' having a common pair of confronting wedge'-- shaped poles for bending and focusing said ion beam, and two vnon-'magnetic tubular members supported by the magnetic lool'es,` one of-said members `extendin-g in the direction of the-` ionsource substantiall 'enclosing the'path of'the ions to the poles, and th'e Voth'er'of said members'4 extending from the ;oolesV along -the *path of the ions and substantially enclosing said path.`V

5. A mass spectrometer comprising an evacuated, substantially Y-shaped tube enclosing in combination, a source of positive ions at one extremity of the Y, electrode means for accelerating ions from said source and for projecting them in the form of an ion beam along one leg of the Y, a permanent magnet wholly contained within said tube at the intersection of the Y, and projecting into a second leg of the Y and having spaced parallel wedge-shaped poles between which said beam passes for bending, projeoting and focusing said ion beam into a third leg of the Y, a collecting electrode at the extremity of said third leg of the Y for collecting the ion beam, and two non-nagnetic tubular members supported by the magnet poles, one of said mem- ;ers extending from the magnet down one leg of the Y toward the source of ions, and the other of said members extending down the other leg of the Y toward the collecting electrode.

6. A mass spectrometer comprising an evacuated tube enclosing, in combination, an ion source for simultaneously developing ions of two elements of different atomic mass, electrodel means for accelerating said ions, a permanent magnet contained wholly within said tube for providing a magnetic field substantially at right angles to the direction of movement of said ions to effect bending and focusing of said ions in the form of two ion beams laterally separated in accordance with the different mass-to-eharge ratios of ions constituting said two separate ion beains, a pair of collecting electrodes arranged side by side for simultaneously collecting said two ion beams, each of said ion beams being focused on one of said oollecting electrodes, and two non-magnetic tubular members supported by the permanent magnet, one of said tubular members extending from the magnet toward the ion source and the other of said tubular members extending from the magnet toward the collecting electrodes, said tubular members substantially enclosing the path of the ions.

'7. A mass spectrometer comprising evacuated vitreous tube enclosing in combination, a substantiall,7 V-shaped analyzing tube of nonmagnetic material, an ion source at one extremity of the V-shaped analyzing tube for projecting ions through one leg of the V-shaped analyzing tube, a permanent magnet of substantially U-shaped cross-section having wedge-shaped poles between which said beam passes and forming the juncture of said v-shaped analyzing tube for bending the ions into the form of a beam and nrojecting the beam through the other leg of the V-shaped analyzing tube, said perinanent magnet being contained wholly within said vitreous tube, and a collecting electrode at the other extremity of said V-shaped anaiyzin-g tube.

8.1 A' mass rsp'ectrometer comprising i an evacuated vitreous tube enclosing, in combination, a1

substantially V-shaped'analyzing tube of nonmagnetic' material, an' ion source at one exi tremity of the V-shaped analyzing tube for projecting ions throughfone `leg of' the V-shaped analyzing tube, a permanent magnet of substantiall'y U-shaped cross-section havingwedgeshaped poles between which said' beam'passes* and formingfthe. juncture of saidwV-shaped analyzing tube for bending the ions into the form ofa beam and projecting vthe beam through the other-'leg of'the 'V-shaped'analyzing tube, said vpermanent magnetV contained wholly' within said vitreous tube;'and a 'pair of oollecting electrodesfdisposed-side'by side vat'the other extremity of said V-shaped analyzing tube.

9. A mass spectrometer comprising an evacuated envelope totally enclosing in combination, means within said envelope for establishing a magnetic field, a source of ions, means projecting ions from said source in the form of a beam through said magnetic field at right angles to the lines of force thereof, said magnetic field having a cross sectional pattern to cause said beam to be resolved into components each having ions of different mass-to-charge ratio, means for selectively collecting the ions of said components and ion shielding means attached to the means for establishing a magnetic field and substantially enclosing the trajectory of the ions between the ion source and the collecting means.

10. A mass spectrometer comprising an evacuated tube enclosing in combination, an ion source at one end of said tube, electrode means for collecting ions at the other end of said tube, magnetic means between said ion source and said electrode means being wholly Within said tube and having wedge-shaped pole-faces on opposite sides of the ion trajectory between said source and electrode means, and two non-magnetic tubular members supported on said magnetic means one extending in the direction of said ion source and the other in the direction of said electrode means to enolose the major portion of the path of ions between said source and said electrode means.

comprising evacuated envelope generally of Y-shape, enolosing, in combination, an ion source at the closed end of one arm of said envelope, an ion collecting electrode at the closed end of the opposite arm of the Y-shaped envelope, magnetic means wholly enclosed within the intermediate arm of said envelope developing a magnetic field normal to the ion trajectory, and a non-magnetic tubular element attached to said magnetic means extending into each of said first mentioned two arms of said envelope to enolose the principal length of the ion trajectory between said source and said collecting electrode.

12. A mass spectrometer comprising in combination, an evacuated tube, an ion source mounted in one end of the evacuated tube, ion collecting means mounted at the other end of the tube, means to establish a magnetic field normal to the path of the ions between the source and collecting means intermediate the ion source and the collecting means, and two non-magnetic tubular members mounted in the tube, one of said members being mounted between the ion source and the magnetic means, the other of said members being mounted between the collecting means and the magnetic ll means, said members substantially enclosing the ion ti'ajectory.

ALFRED O. C. NIER. MARK G. INGHRAM.

Rrzirlamsixicias CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,143,579 Ruska Jan. 10, 1939 2,200,039 Nicoll May 7, 1940 2,221,618 Stickney Nov. 12, 1940 15 2,230,825 Brett Feb. 4, 1941 12 Number Name Date 2,341,551 Hoover Feb. 15, 1944 2,417,79'7 Hipple Mar. 18, 1947 OTHER REFERENCES Tech. Pub., Mass Spectrum Analysis," in Physical Review, vol. 50, Aug` 15, 1936, pages 282-296.

Tech. Pub., A Mass Spectrometer for Routine Isotope Abundance `Measurements, by Alfred O. Nier, in Rev. of Sci. Inst., vol 11, July 1940, pages 212-216.

Tech. Pub., An Experimental Mass Spectromete1', by N. D. Coggeshall and E. B. Jordan. Rev. of Sci. Inst., vol 14, No. 5, May 1943, pages 125-129.

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