US2955204A - Non-magnetic mass spectrometer - Google Patents

Description

Oct. 4, 1960 w. H. BENNETT NON-MAGNETIC MASS SPECTROMETER 6 Sheets-Sheet 1 Original Filed Nov.

INVENTOR WILLARD H. BENNETT 2 BY W M0 ATTORNEY6 Oct. 4, 1960 w. H. BENNETT NON-MAGNETIC MASS SPECTROMETER 6 Shets-Sheet 2 Original Filed Nov. 16, 1950 INVENTOR WILLARD H. BENNETT BY 8 Ad ATTORNEYj w. H. BENNETT NON-MAGNETIC MASS SPECTROMETER Oct. 4, 1960 6 Sheets-Sheet 3 Original Filed Nov.

INVENTOR 1 6/0 WILLARD H BENNETT ATTORNEYS Oct. 4, 1960 w. H. BENNETT 2,955,204

v NON-MAGNETIC MASS SPECTROMETER Original Filed Nov. 16, 1950 6 Sheets-Sheet 4 INVENTOR WILL Am) /i HEM/77 BY 970-048 Zl/a- ATTORNEYS Oct. 4, 1960 w. H. BENNETT NON-MAGNETIC MASS SPECTROMETER 6 Sheets-Sheet 6 Original Filed Nov. 16, 1950 AMPLIFIERT 03:11 1 A TOR L aana INVENTOR WILLARD H. BENNETT BY 0770M! ATTORNEYS United Sttes 2,955,204: NQNMAGNETIQMASSJSREC TRDME'IER.

Willard H: Bennett, 400$ lstSta SW. Washington 24,,DLC.

Substituted 01: abandoned applicatiouSer. 180,196,024, N v- 1 9 hispn cat m u 1955, 7- No. 531,337.

'Hi'GlaiIIlSn (c1; 25.0. '4h2):

This invention: relates. ion mass spectr ometers; including a velocity selector type of" tube and. associated, apparatus which; may be employed in the analysis; on separation: of ions having different isotopic mass and in theanalysis or: separation of ions-having difierent chemical identities;

This. application is. a continuatiomimpart of my co.- pending application, Radio frequency mass spectrometen, Serial. Number- 456,983, filed September.- 20, 1954 (now US. Patent No. 2;7 2t,2'7:1)', which was. a. substittute for and. cQ- ending-Withsthe application. of'the same name,- Serial: Number. 19.6,Q24 filed'November 16, 195.0; and now abandoned, and this application is also. a continuation-impart of my cospending application, Leak Primarily for massspectrorneters, Serial Number.- 241,813, filed August 14, 195i, now US. Patent No. 2,721,270.

An. object of this invention. is to provide a novel tube and associated apparatus. wherewith a mixture of substances having diiferent chemical or isotopic identities may be separated into its components and the amount off each suchcornponent in said mixture may be measured.

A further object Oh this. invention is. the proyision of new and novel apparatus employing radio-frequency alternating fields for; the separation of ions having difieren't masses. or chemical: identities,

Still another object of this invention is the provision of a mass, spectrometer which. is capable of' scanning a mass range rapidly enough toryield a. continuing presentation of the mass spectrum on a cathode ray oscilloscope screen.

Another object of this invention isthe provision of a method and apparatus for accurately determining the isotopic composition of each element present in. a. mixture.

Still another object of this, invention. is the proidsion of a rugged reliable apparatus for; the continuing admission of gases. at a controlled rate into a system.

These and other objects may be seenfrom the following description and correlative drawings in which:

Figure 1 is a top. viewoi a three-stage tube in accordance with my invention;-

Figure 2 is a schematic cross=section view. of the same tube; J

Figure 3 shows the structure of one of the. grids used in these tubes;

Figure 4. is a. schematic representation ofone form of the leak used for admitting gasesv into a tube;

Figure 5. is an end view ofi a four-stage tube;

Figure. 6 is a, schematic cross-section view of a fourstage tube with a cylindrical collectoraud separately pumped cathode;

Figure 7' is another form of-a. leak for: admitting gases at a controlled rate. to the tube;

Figure 8. shows a schematic crossesectional view. gf a four-stage tube wrapped with a baking oven; I

' Figure. 9 is a schematic. cross-sectional view of the same tube with the section taken at right angles to. the sectiontaken in the above; I

2,955,204 Cg iatented Oct. 4, 1960 F gure .10;sh vy. ..th ..s r of he n n hirq ttqf: the c hod in Figur 8;

- Figure 1;1- shows the shape of the cathodes and-boat inthe ionsource. of Figure 8;

Figure 12; is a, schematic, representation ofT a mass spectrometer tube with its-associated vacuum system and leak;

Figure 13 is a schematic representation of'the. cir cui y w ic may. .s d w th-a et ube n. a corda c w h. my n ention;

Figure. 14 isa; schematic I representation of an; alternativ fo m o c sui rywhish may e use F gure 15- is. a, schem t t; r pr se on. St l er mat ci uitry W shmay e ssd withra l rr st ge. ube. n accordance w th my n nt In a yp c smhedimsn o my n ent o a. mple 0f a s ntrqd sedr ta ery o y. eqf o ma es a d of. the tube h ou hthe med um of a a l t ss? be unde t d hat; ot er e hod o ht qdhs ha e ma ia to. e analyzed m y e d ly e u plpy d- Th a r po zed materia n he u s ni ed; by ls trons which are flowing near, one end; of the tube and h ystem. produci g such i9Il. -.W :b mpn s e th ou hqht h fe l W n de cr ti n s. h 1011 ur e The. ma s. pectr me e hiss itself fishes he rm. o n. snsates i ube;- au ollsc o ys em s'plased tthe. o her end; o e ube em. 11. wiss nd; a plur ity at r dsspaces bet een he ha e Tasha ds a apa ed ppr pr te D.- ra -fi squ nsy A-C- potentials. these: potent a s be n s el ct d. a e ion of diiters t. as s whi h P ss hrou h a d; ends l i asquire ary he) amohs s f e y- By-th P es e e ionf pstent a s a P d' oh mass; w ll: h acce erates ma s n the. the masses PESQQL he col ts. y te s or o t e collec o pl te s applied; a pote t al .tli esshpe. 2 m he. i 11 car e which fisi i t epel al ions pproachin i xcep t es of PIQ- E d: ma s whi h alot si av th a h 1: ener t each he qq l on r A ma s. spsshqmste tubs, on sts; o th crim nal p r io same he a aly r and he o ec al of which ar e s os. in he tu ?.1.1V1.P-. uh s s usua y at g a s b t; wh ch m y h mad f; othe mat r al One. f -11 0t .9 $9 is s ow n Figure 2. wherein s1sctran .f otn.sa h9ds .ll a e sssler ts ow r s s s. 1 1 by applyin a e t cal potent al; fers. su h t at lestr ds '11 an 16 are ach positiv ela v o he, pa h a o he a hode l ctrqd :7 is a r d; s mila to. the-other ri s he tube ic wil be described in greater detail later. Electrodeltif consists of tw r d d. a me all c cy nd w shown in c e -section in. Fi ure 2- 't Q grids nd. cy i dric W l a e. m ch n ca l and e ectr ly int rcon e ted.

ns lded. in the. 9. s ur e ar a so; rid 18 alien o which electrical potentials are applied such that grid'lflS s. at a ma l; ne a ve eh aldifi tsnc j oni'cage. 1 an id-19 s a a pq a h ch is n ive r lative o h t delh leq rens com ug rom cathq e are ee e r tssl towa ds. e 15. a d m t'o t m P rou h he a m itia some; Q the gas: which is. in the cage. The elegtrons-continueqon' their way and are turne hack: b th ms fi e pq eh al :1 grid y 9f them mak n anothe rip. hrou h the c and ionizing St more of the gas; molecules inthe'cage. In this way the n a v pq ehti l 01. r sl'lapre ents any of h e t qhs r m-the 9a .Qde x om. p ssing up through the tube beyond grid 19.. The. small negative potential- 011 grid .18 relative to the potential-6% the cage 16 causes the rq i e ions be ng produ ed n ue the g 16 to'emerge towards grid .18 to. pass upward into. the analyzer hedess i e u'ggeat r detail later.

Another form of ion source is illustrated in Figure 6 in which the cathode 20 is partially covered with an electrode 21 which is shaped in the form of a cylindrical cup with an aperture in the end of thecup. Covering electrode 21 is electrode 22 which is also shaped in the form of a cup with an aperture in the end of the cup; Either electrode 2 1 or electrode 22 is fitted closely to the cylindrical glass wall 23 so that the only way that the gases in the tube can get to the space around the cathode is by passing through the small apertures in 21 and 22 In this ion source electrode 22 is held at a potential which is more positive thanthe cathodebut which is not more positive than the potentials on grid 17 or cage 16. The potential on electrode 22 draws electrons away from the cathode while the potential on electrode 21 is held at" a valuewhich is equal to or only a little difierent from the potential on the cathode. The potential on electrode 21 may be either a little positive or a little negative or equal to the potential on the cathode 20. In this Way the potential applied to electrode 21 tends to guide the electrons through the aperture in electrode 22 after which the electron stream spreads outsomewhat while passing through grid 17 and cage 16. The cage 16 and the grids 17, 18 and 19 serve the same purposes and are operated in the same ways as before. 7

Still another form of ion source is illustrated in Figures maybe an electrode 24 shaped in the form of a cup in end of which is a rectangular-shaped aperture like that shown in Figure 10. In Figure '11 are shown the top view of the boat 27 and two parallel cathodes and 26. In'this ion source electrons from the cathodes 25 and 26 are accelerated towardsthe boat 27 which is held at a potential which is positive relative to the cathodes. The electron current bombarding the boat heats up the boat. If this heating is insufficient to vaporize thesolid sample material in the boat an electrical heating current can be passed through leads 28 and '29 to further heat'the materials. The electrode 24 is held at a potential which is negative relative'to the cathodes and the potential on grid 19 is also negative relative to the cathodes. The vaporization of'the' material in the boat produces a vapor through which the electrons from the cathodes must pass and in so passing some of the vapormolecules are ionized. The positive ions so produced are accelerated towards the negative electrodes, that is to say they are accelerated through the space 'betweenthe two cathodes 25 and 26,'then throughthe rectangular aperture in electrode 24 and then through the negatively'charged grid 19. l

and so into the analyzer.- 7 p The analyzer in a mass spectrometer tube consists of three or more groups of grids each group consisting of three grids'and the space between groups of gridssurrounded by cylindrically-shaped electrically; conducting '1 shields.

One form of analyzer is illustrated in Figure 2 in which grids 1, 2 and 3 comprise the first group or stage, grids 4, 5 and 6 comprise the second stage, and grids 7, 8 and 9 comprise the third stage of this three-stage tube; Mechanically'and electrically connected to'grids 3 and 4 is the electrical shield 13'and similarly'connected'to grids 6 and 7 is the shield 14.; It is convenient but not necessary to electrically connect'grid l to electrodes 3,

13 and 4 and similarly'toconnect grid 9 to electrodes6, 14 and 7. It is alsoconvenient' but not necessary to inter.- connect electrodes 2, Sand 8, The distance from grid 2 to 5 divided by the distance from grid 5"to grid 8 equals the ratio of two integral' numbers. These are preferably h two prime numbers and 'goo d numbers to 'use for this threestage tube showhin'Figure-Z are 7 and 5; {That is tosay the distance from grid ZtogridS dividedby the separation of ions having difierent masses. The distance from grid 2 to grid 5 divided by the corresponding integer (which is 7 in the example given above) is equal to the distance from grid 5 to grid 8 divided by its corresponding integer (which is 5 in the example given above) and these two equal quotients will be called the interval and is equal to the distance an ion travels during a period of the radio frequency'potential for that species of ion having the particular mass which permits it to acquire enough energy from the radio frequency fields to reach the collector as will be explained later. The distance between the three grids in each stage is equal approximately to three eighths of the interval. In the example given above the distance from-grid 1 to grid 2' equals the distance from grid 2 to grid 3 equals the distance from grid 4 to grid 5 equals the distance from grid 5 to grid 6 equals the distance from grid 7 to grid 8 equals the distance from grid 3 to grid 9and equals three fitty-sixths of the distance from grid 2 to grid 5 and equals three fortieths the distance from grid 5 to grid 8. In measuring these distances between grids these various distances are all measured to the mid-plane of a grid and not to the nearer side of a grid and more particularly the distances are measured to the mid-plane of the knitted portion of the grid rather than any part of the supporting structure of that grid. For example if the-knitted portion of the grid has a thickness of 0.003 and it is desired to make the distance between grid 1 and grid 2 equal to 0.121" then the distance between the nearer portions of the knitted portions must be made equal to 0.118.

Oneform of analyzer for a four-stage tube is illustrated in Figure 6. In this analyzer grids? and 4 are mechanically and electrically interconnected with the shield 13, grids 6' and 7 are interconnected with shield 14 and grids 9and 10' are interconnected with shield 15. It is desirable but not necessary for grid 1 to be electrically connected to 3, 13, 4 and for grid 12 to be electrically connected to 9, '15, 10. Grids 2 and 5 are electrically interconnected and areconnected to one of the lead-outs for the application of radio frequency potential (in addition to DC. base'potential) and grids 8 and 11 are interconnected and are connected to another lead-out for a similar application of radio frequency and DC. potential. Here again in the four stage tube' the distance fiom grid 2 to grid 5 divided by an integer equals the integer and this equals the distance from grid 8 to grid 11 divided by an integer which is difierent from either of the other two integers, and these three equal quotients are equal to the interval three-eighths of which equals the distance between any two'adjacent grids in each stage. A good choice of integers for a four-stage tube is 13, 11 and 7 and another good choice of integers is 17, 13 and 11. Other choices of integer combinations may be'used with good. results and not all the integers need be prime numbers. One form of collector system is illustrated in Figure 2 wherein a collector plate 30 is surrounded by an electrical shield consisting of a backing plate 31 which has a small cylindrical sleeve '32 surrounding the supporting post 33, a grid 34 and a cylindrical shield 35 where 31, 32, 34, and 35 are electrically and mechanically interconnected, and grids 36 and 37. Either or both of grids 36 and 37 may be replaced with a pair of grids electrically and mechanically interconnected for improved operation and where such grid doublet is used a reference to grid 36 or grid 37 respectively will he meant to mean sucha doublet. 7

An alternative form of collector system is illustrated in Figure 6. In this collector the cylindrical electrode 40 constitutes the gollector electrode and is not electrically -D.C. potential V is 100 volts.

as s-9e connected to any. other part of the tube but is provided withits-own separate andfs'hi elded'lead out; One of the metallic supports of this-cylinder may be usedas the lead-out and that lead-out is surrounded byandinsulated from a shielding sleeve 41.' The plate 42looks muchfllike the collector plate 30 of the previous formot collector but the plate- 42 serves a different purpose. This ,electrode 42 has attached to its center a post 43.. A shielding sleeve 35, a backing annular ring 44 and grid 34' are electrically and mechanically interconnected-and extend ingfrom grid 34 to ring 44 are a number of small cylindrical shields 45, 45 one for each of the leads extending from the head of the tubeto the respective electrodes below. The purpose of these shielding. sleeves is: to prevent any electrical fields due to'fluctuations in the electrical potential on any of these leads from. reaching the collector cylinder 40. Grids 36 and 37 and a grid 38 are also a part of'thiscollector system.

In the construction ofthese tubes a convenient material to use is knitted fine wire mounted in a stretched condition on metal rings as illustrated in Figure 3 in which a plan-view and cross-sectional vieware shown. Inthis figure theknitted wire is shown with rectangular crossed lines at 47 but in actual factthe knitted Wire consists of interlocking'knitted loops which are more or less curvedeverywherein-a manner whichis quite similar to thestructure in a-knitted nylon stocking. This material is stretched over a-narrow' annular metalring: 48 lying ona stretching form for the knitted wire and a formed grid-- ring 46 Y which a has been punched with at number of holes as shown, is laid over the knitted. material and Weldedfast by electrie'welding to the ring 48. This weldingof ring 4640- 48' is done at many places as closely spaced-as possible all aroundthe rings so-t-hat the knitted material; is crushed and held firmly between the welded rings. There is of course some electric welding of the wire to'either or both rings since the wire lies between the two rings. After such welding theexcess, knitted material'lying'outside ring 48is trimmed away. A'suitv able' knitted material is one mil tungsten wire-knitted withabout 16 loops per inch. Another suitable material is- 6 mil tungsten wire knitted withabout 25-loops per inch. j-

In the fabrication of these various forms of mass spectrometer tubes, there are many diiferent methodsfor supporting the electrodes inside the envelope and for making electrical connections through the wall of, the envelope to the electrodes, which will'be evident-to anyone skilled'in the vacuum tube art.

-In the operation of this mass spectrometer an ion which passes the first grid of anythree-grid stage while the middle grid has a negative charge on it momentarily due to the radio frequency potential being applied to that middle grid, and which passes through the middle grid at approximately the time when the charge on that middle grid passes through zero so that the charge on that middle grid becomes positive while the ion is passing from the secondto the third grid, the ion is said to be resonant with that stage. Such aresonant ion is accelerated by the radiofrequency field both before" and after passing through the middle grid and by virtue of such acceleration a resonant ion acquires energy in addition to the energy that it would have had by virtue of the steady potentials applied to the grids. For example, suppose that potentials are to be applied in the spectrometer represented in Figure 2 to make an ion travel through each successive drift space with 100 electron-volts of kinetic energy. In this case it will be said that the principal Suppose also that the radio frequency potential being applied to grids 2, 5 and 8 is 5 volts root mean square, in which case the potential difierence to -apply between the cage and the electrodes 1, 3 and 4 is 90 volts. An ion produced in the cage 16 emerges through grids I8 and 19 to arrive at grid 1 with a kinetic energy equal to 90 electron-volts,- if the ion is singly charged. An ion which can be resonant in all threestages offthistuhe wouldhave to pass each of'grid's 2',5'and.81just when the chargeoneach of those grids isichangiirg', from negative to positive and is passing througttze'ro. Such resonant ion in passing f'rom grids 1 to 3 acquires an. additionallkinetic. energy of approximately 10 electron-volts (th'atis. about twice the root mean squarevalire of'th'e radio frequency. potential.) Thus the ion.ent'ers. th'e. drift" space between grids3 and 4 witha kineticenergy of l00ielectron-volts. There is a bias potential applied. between drift space .3; 13, 4 and drift space.6,114 Tequal to 10 volts so that if no radio frequency potential wereapplied to grid. ,5 the ionv from the first. drift space would enter at second drift space with its kinetic energy reduced to electron-volts. Such an ion would also have 10"el'ectron-volts of potential en ergy so thatjits total'energy is still electron-volts. However-,Ibeoause the radio frequency. potential is. applied to grid; 5, the resonantion passing through the second stage acquiresan additional-kinetic energy of 10 electronwolt'sso that'that ion. enters the second drift spacewith'a'ki'netic energy of 100-electron-volts in addi: tion to..its IO volts of potential energy. This resonant ion in passing through the. third stage acquires an,addi-' tional kinetic energy of 1'0felect'ron-vo lts so that-this ion emerges through grid 9 with' a kinetic energy of electron-volts inadd-ition. to its IOZelctron-volts o'fpotential energy andthe total. energy of the. ion is 30 electron. volts greater than.w hen it'passed through grid 1 and entered the analyzer. If" thecoll'ector plate 30 is held at the same potential as the ion source such a resonant ion will reach-the collector plate and strike it with a kinetic energy equal to that 30 electron-volts of, energy which'the' ion acquiredf'rom the three stages. If the collector plate 301s heldata potential of approximately 25 volts positiverel ati've'to the ion source, the resonantions can" still reach the collector plate striking, it with. a kinetic energy of'about 5 electron-volts. Ions having other massesthan the massiwhich an ionmust have. in order to be resonant. acquire less energy than'30. electron-volts and in fact acquireless than 25 electron-volts of energy from the three stages so that the, only kind ofions, that can" reach :the collector are theions' having. just the. massto-charge ratio of the resonant ions.- If the tube is.a.7 and 5 cycletuhe the resonant ions. have justthe rightmas sto-cha'rge' ratioto-have:iust the right velocity sorthat they travel from grid}? to 5T-while the radiofrequency poten? tral executes 7*cycles' and from grids 5' to 8 while. the radio frequency potential executes 5 cycles. The above has been given as an illustrative. example only and the tube can be operated in other ways; As one such. example in.- stead of applying the retarding potential of, 25 volts on the collector 30,v a retarding. potential could be" applied to one or more of the intervening grids 37, 36, or 34 whilesome ofithe other gridsand possiblythe collector plate 30 are held at a negative potential for the prupose of suppressing: secondary electron emission. In the appended'clairns the termenergy will be used to designate total energy including both kinetic and potential energies.

As. another illustrative-example of the opera-tion ofa spectrometer tube the structure in- Figure 6- may be considered. Let it be supposed that this is a 13, 11 and 7 cycle. tube. Here again for an ion to be a resonant ion the ion must pass each-of grids 2, 5, 8 and 11 just'as' the charge on that grid is changing from negative to positive and that ion must pass from grid 2 to 5 while the radio frequency potentialon each executes exactly 13 cycles, must pass from grid 5- to 8 while the radio frequency potential on each executes 11 cycles and'must pass from grid8 to 11 while 7 cycles areexecuted. Anion passing from the ion source into the drift space 3, 13, 4 loses potential energy equal to the chargeonrthe ion times the potential diflierence between the ion source andthe-drift space and gains kinetic energy equal to that loss in potential energy plus the energy gained from the first stage the .resonant ion gains kinetic energy from each of the other stages. One way of connecting the electrodes in the collector system is to interconnect grid 38 with 1, 13, 4 and because of the bias potentials applied between successive drift spaces, grid'38 is negative with respect to grid 12 by four times the root mean square 'value of the radio frequency potentiaL. Grid 37'can be connected to the retarding potential which in this four-stage tube may be equal to approximately 6 /2 times the root mean square value of the radio frequency potential such that grid 37 is positive relative to the ion source. Grid 36 may be held at a potential equal to that applied to grid 38 or if rapid scanning of mass range is being performed grid 36 may be held at the maximum negative potential which is applied to 38. Grid 34 and its interconnecting electrodes may be held at a steady potential approximatelyequal to that of the ion source. Plate 42 with its associated'plate 43 is held at 8 times the root mean squarevalue of the radio frequency potential; positive relative to the ion source so that resonant ions from the ion source cannot quite reach that plate 42. The collector 4% is held at the retarding potential which may be about 6% timesv the root mean square of the radio frequency, positive relative to the ion source. -In this way,

the resonant ions are deflected by thepotential on electrode 42 and go to the collector 40. V

The gases or vapors to be analyzed must be admitted to the mass spectrometer tube at a. slow enough rate in order that the pumps which are evacuating the mass spectrometer tube can keep the gases or vapors to be analyzed at a low'enough density in the mass spectrometer tube'for the mean free path of the ions produced in the ion source to be long compared with the length of the mass spectrometer tube. The pressure of the gases or vapors in the mass spectrometer tube should in most instances be three hundredths of a micron of. pressure. or less. One method ofadmitting gases or vapors at such a slow rate is illustrated in Figure 4. Two thick walled capillary glass tubes 51 and 52.have in them wires 53 and 54 respectively.

A convenient size is capillary tubes each five centimeters long with bores equal to 0.50 millimeterv containing platinurn ortantalum wires 'eaoh 0.48. millimeter in diameter. It is convenient but not necessary to spring-mount each wire at the upper end and to attach a. piece of soft iron at the lower end. ilfa'piece of soft iron is attached at the '7 lower end of the wires in the fine capillaries; a coil ofwire may be placed around the tube and a little below the level of the piece 'of iron. 1 Passing 'a current'throu'ghthe coil draws the iron down and loosens any obstructions which may have collected between the wire and the fine capillary wall. The gas or vapor is admitted at the opening 55 or if the, material to be analyzed is a liquid whose vapor is to be used the liquid may be laid in therecess 56. Below the lower ends of the two capillaries a closed glass connection is made with another thick-walled capillary tube '57 convenient dimensions for which Would be length '5 centimeters and l millimeter here. It is convenient but not necessary to make connection from the upper end of the large bore capillary and the mechanical pump through a trap 58 the lower end of which 59 is connected to a mechanical pump. The upper end 60 of capillary tube 52 is connected to the mass spectrometer tube. An alterna- V tive form of this kind of leak is shown in Figure 7 in which the same wire passes down through both fine capillaries. In place of the coarse capillary may be used a A number of small bore thin-walled glass tubes filling the cross-section of a large diameter tube. V

The above-stated dimensions of the capillary tubes 51 and 54 together with those of wires 53 and 54 result in viscous flow of the gases in the tubes. Whether the flow is molecular or viscous depends mainly on the relative 7 length and cross-section of the opening rather than the "gas 'ilo'wing therethrough. This invention contemplates fibreglass; or any one of theequivalent materials is applied over the glass envelope of the mass spectrometer tube oi. This application of insulating material is shown in Figures 8 and 9 at 6 2. A coil of heating wire 63 is Wrapped over the layer ofinsulating material oZ after whichmore heat insulating material is applied over the heating coil at 54. Thermal insulationis applied at both ends of the tubetaking care not to cover up the lead-outs such as 65, 65 md 65 or any tapered glass joints such as 66 which may be used as waxed joints and which must be kept exposed to the air in order to keep them cooled to below the softening point of the wax while the rest of the mass spectrometer tube is being baked. The lead-outs from the heating coilare shown" at 67, 67. The entire baking structure may be enclosed in a thin metal case 68.

It will be recalled that one formof electron source used in the ion source is illustrated in Figure 6. This tube is evacuated through pumping arm 69 while the space around the filament which serves as the electron source is evacuated by a separate diffusion pump through pumping arm 70. One convenient form of the associated leak and pump systems is illustrated in Figure 12. The mass spectrometer tube is evacuated through arm 69 by a thinsion pump 71 while the space around the filament in the electron source is evacuated through arm 70 by diffusion pump 72. Associated with the diifusion pumps are traps 73 and 74 which are optional if oil difiusion pumps are used but whichare necessary if mercury difiusion pumps are used. Both diifusion pumps canbe backed by the same backing pump 75 and interposed may be an additional vapor trap 76 andtWo-way stopcock 77. The gases or vapors to be analyzed are admitted from a leak 78 in which a needle valve 79 may be interposed between the coarse capillary S7 and the'mechanical'pump 80. A further simplification is that the two pumps 75 and 80 may be driven by the same motor. I Various methods may be used for applying the proper potentials to the electrodes in the mass spectrometer described in thisinvention as may be seen from the following. one kind of circuit for use with a three-stage mass spectrometer tube is shown in Figure 13'. In this form of the invention the filament 20 which serves as the electron source is shown heated from'the transformer 96 but a battery could just as well be used for this purpose. A potential difierence between the filament 2t) and the cage 16 is obta'ined from a battery 81. The electron current passing from the filament to the cage can be controlled and held steady by use of a grid 17 whose potential diiference from the filament is obtainedfrom the same battery 81. The drawing-out potential applied to grid 18 is obtained from the same battery 81 and the potential on grid 19 which is negative relative to the filament is also obtained from the battery 81' as shown. The positive end of battery 81 which is connected to the cage is also connected to the positive end of battery 82 from which the principal DC. voltage V i'sbbtained and measured on the meter 85. A variable resistor 88 may be connected as shown for the purpose of varying the valueof V. The bias potential a'pp'lied between the successive drift spaces and the DC. base potential for the radio frequency grids is obtained from battery 83 connected with its negative end toward 7 the negative end of the meter 85. The resistance of each the spectrometer tube:

potential output of the oscillator 98 as read by the volt meter 99. The retarding potential applied between the cage 16 and the collector 3G isobtained'fronr the battery 84 in series with which is the variable resistor 97. The retarding potential so applied is measured by the stopping potential.meter 87 and is usually between 6 times the root mean square vaiue of the radio frequency potential. Interpos ed' between the" oscillatdr' 98 and the radio frequency grids 2, 5, and 8 is a condenser 94 aridiriterposed between those radio frequency grids and the point between resistors 90 and 91 is a radio frequency choke 93.- Any or all of the condensers 95', 95, 95, 9'5; 95,- 95, may be connected between the respective steady potential electrodes and ground for thepurpo'se of lay-passing the -a'p'iacitive leakage of radio frequency potential which c'ithe'rwise unintentionally might appear on those electrodes.

An alternative form of circuit is illustrated in Figure 14. In this circuit the" output of the RF oscillator 98 is passed through condenser 103 into the cascaded rectifier 102 the positive end of which is grounded. The negative end of the outputof the rectifier is passed through a variable re sistance 104 and the fixed resistance 105 the values of which are so selected that the potential of the point between them is approximately times the root mean square value of the output of the oscillator 98. The point between the resistors at that potential is connected to the grid of the cathode follower tube 106 which is driven from a DC. power supply 110 which is connected as shown to resistors 107, 108 and 109. Connected in this way the DC. power supply 110 also provides the principal DC. voltage V as measured by volt meter 85 and selected by the variable contact on the resistor The other connections are similar to the connections used in the previous figure.

Still another kind of circuit which can be used is illustrated in Figure in which a four-stage tube is shown operated entirely by electronic means. In this figure the D.C. power supply 110 supplies the potential to drive the cathode follower tube 106 whose cathode resistance is made up of the three resistors in series 11 1, 112 and 113 the points between which are used also for applying the appropriate potentials to the various electrodes in the ion source as shown. The center grounded DC. power supply 110 is also used to supply the potential for the sweep circuit 116 which may be either a sawtooth or a zigzag sweep circuit. The output of the RF oscillator 98 is used to driveeach of the two cascaded rectifiers 117 and 125, and it is also used for supplying radio frequency potential through the condensers 123 and 124 to the radio frequency grids as shown. The output of the cascaded rectifier 117 supplies potential to the four equal resistors in series 118, 119, 120 and 121 which are in series with the variable resistance 122. The resistance 122 is adjusted until the potential drop across each of the four fixed resistors is equal to the bias potential which as explained before is equal to the root mean square value of the radio frequency potential from oscillator 98. The cascaded rectifier 125 supplies potential to the three resistors in series 127, 128 and 129. The total resistance of 127 is about equal to the total resistance of 129 and the resistance of 128 is about equal to three times either of these. The variable contact on 129 is adjusted to give complete cut-off of resonant ions reaching electrode 4-2 and the variable contact on 127 is adjusted to give the correct retarding potential in the collector system as explained previously. A part of the sweep potential is taken off from potentiometer and applied to the horizontal plate pair of oscilloscope 131 and the output of the DC. amplifier 100 is applied to the vertical plate pair in the oscilloscope.

The above circuits are intended to be illustrative only and many other forms of circuit and combinations of components of such circuits appropriately devised to operate mass spectrometer tubes of three or more stages will be evident to anyone skilled in the art. For example instead ofthe DC. amplifier with ameter orinsteadiof the oath: ode ray oscilloscope there may besnlfi'stituted any one of a variety of kinds of recorders, with appropriate connecftio'nsrto the control" means for the principal DC; voltage V.

Because of'the large. efliciency with which this kind of mass spectrometer utilizes the output of the ion source it is practic'alto use large voltagej differences, between the cathode and the cage, say in the orderof .100 to" 1000 volts instead of the more, usual 50 to 100. volts with the result that there is an enhanced relative yield of multiply charged ions. This makes possible the use of stable isotopes for tracer investigations. ,For example tracer in.- vestigationscan be performed in biological and physiological research using materials containing enriched stable isotopes 9f such elements as carbon, nitrogen, oxygen and others. By comparing the intensities of mass lines appear ing at positions 6 and 6 /2 an accurate and, reliable'indication is obtained of the'amount of carbon 13 in the enriched material which has entered the sample. This of course is not possible at mass positions 12 and 13 because the intensity of carbon 13 is partially masked by the ion consisting of carbon 12 in combination with hydrogen. In like manner an accurate measure of the enrichment of nitrogen is obtained by comparing the intensity of mass position 7 with 7 /2, and the measure of the oxygen enrichment is obtained by comparing mass positions 8 and 9. These comparisons are aided by use of the rapid sweep permitted by circuits of the general kind illustrated in Figure 13 which involve rapid scanning.

I claim to have invented:

1. The method of determining the relative abundance of the isotopes of an element in a sample which includes doubly ionizing the atomic material from the sample and measuring the relative intensities of the mass lines at the mass positions corresponding to the doubly charged atomic ions of the isotopes of the element.

2. The method of mass spectrometry which comprises applying sufficient ionizing potential to the ionizing electrons to produce doubly charged ions, effecting separation of the resulting doubly charged ions of different masses from each other and from singly charged ions, and measuring the relative abundance of the different doubly charged ions.

3. A mass spectrometer comprising an ion source including means for increasing the energy of the ionizing electrons to produce doubly charged ions in gases fed thereto, means for separating doubly charged ions of difierent masses from. each other and from singly charged ions, and indicating means for indicating the relative abundance of different doubly charged ions.

4. In combination, evacuated means and a system for feeding gases to said evacuated means comprising conduit means having first and second restrictions for leading the gases to said evacuated means both of which restrictions are much longer than their transverse dimensions and both of which have such large cross section that the flow therethrough is primarily viscous, means for exhausting the conduit means between the two restrictions to a degree of evacuation less than that of the evacuated means, and a fine wire passing through the first restriction.

5. In combination, evacuated means and a system for feeding gases to said evacuated means comprising conduit means having first and second restrictions for leading the gases to said evacuated means both of which restrictions are much longer than their transverse dimensions and both of which have such large cross section that the flow therethrough is primarily viscous, means for exhausting the conduit means between the two restrictions to a degree of evacuation less than that of the evacuated means, and a fine wire passing through the second restriction.

6. In combination, evacuated means and a system for feeding gases to said evacuated means comprising conduit means having first and second restrictions for leading the gases to said evacuated means both of which restrictions are much longer than their transverse dimensions and both of which have such large cross section that the flow thcrethrough is primarily viscous, means for exhausting the conduit means between the two restrictions to a degree of evacuation less than that of the evacuated means, and a fine wire passing through both restrictions.

7. The combination of claim 4 having in addition means operable to moveone end of the fine wire.

8. The combination of claim 5 having in addition means operable to move one endof the fine wire.

9. The combination of claim 6 having in addition means operable to move the fine wire.

10. In combination, evacuated means and a system for feeding gases to said evacuated means comprising conduit means for effecting viscous flow therethrough and having first and second restrictions for leading the gases to said evacuated means both of which restrictions are much longer than their transverse dimensions and both of which have such large cross section that the flow therethrough is primarily viscous, and means for exhausting the conduit means between the two restrictions to a degree of evacuation less than that'of the evacuated means including a pipe having a needle valve therein.

11. The combination of claim 4 having a second fine wire passing through "the second restriction.

I V lfleferencesCitled in the file of this patent UNITED STATES PATENTS 787,287 Gardner .AIPI'. 11, 1905 1,126,070 Pasman Ian. 26, 1915 1,296,865 Anthony Jan. 14, 1919 2,355,658 Lawlor Apr. 15, 1944 2,412,236 Wa'shburn Dec. 1(),' 1946 2,497,223 Landon Feb. 14, 1950 2,535,032 Bennett Dec. 26, 1950 2,601,097 Crawford June 17, 1952 2,769,912 Lupfer et al Nov. 6, 1956 OTHER REFERENCES Jones et al.: Two Ion Sources for the Production of Multiply Charged Nitrogen Ions; Review of Scientific Instruments, vol. 25, No. 6, June 1954, pp. 562-66.

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