US2472870A

US2472870A - Mass spectrometry

Info

Publication number: US2472870A
Application number: US564424A
Authority: US
Inventors: Harold W Washburn
Original assignee: Consolidated Engineering Co Inc
Current assignee: Consolidated Engineering Co Inc
Priority date: 1944-11-21
Filing date: 1944-11-21
Publication date: 1949-06-14
Anticipated expiration: 1966-06-14

Description

June 14, 1949. H. w. wAsHBURN MASS SPECTROMETRY Filed Nov. 21, 1944 5 Sheets-Sheet l K A TTORNEYS June 14, 1949. H. w. wAsHBURN 2,472,870

MAss sPEcTRoMETRY Filed Nov. 21, 1944 s Sheets-sheet 2 le le v L M MMV-mm INVENTOR. HAROLD W. WASHBURN BY www A TTORNE YS .lune 14, 1949. H. w. wAsHBuRN 2,472,870

MASS SPECTROMETRY Filed Nov. 21, `1944 3 Sheets-Sheet 3 /NvENToR HARO/ D w. wAsHBuR/v BY www ATTORNEYS Patented June 14, 1949 lf.Uurige :STAT-Es PATENT -oFFl-cs *.HaroldW. Washburn,- Pasadena, Calif., assigner to Consolidated Engineering Corporation, `Pasadena, Calif., a corporation of California .Application 4November-21, 1944, `Serial No. 564,424 7 claims. "(ol. 25o-41.9)

This .invention is lconcerned .with mass `spectrometry .-and. particularly with the* .analysis of mixtures .With-aimass spectrometer. It provides improvements .in .methods .of .and :apparatus for mass spectrometryto the .end -that mass .spectra of improved uniformity .may loe-produced, 4and-.is a continuation-impartfof.my-co-pendingapplication Serial .No. 513,528, led December :9, 1943.

The mass spectrometer is adapted for. use .in both qualitative .and `quantitative analysis. Ithas been :employed tfor `scientific studies such .as .the measurement -of `isotope -ratios..and, more recently, ,tor .other problems such tas ,analysis of complex mixtures .of hydrocarbons. :It is, essentially, an apparatus :for producing .ions -and .sorting them according to -the -ratioof their mass-to their charge, .-i. e. -zacoording-.to-.their .specific mass. .A sample to .befanalyzed -for example aa gas mixture, is ionized in a chamber, tor ,example-@by heating -or .by electron. bombardment, .and `.the resulting ions are propelled-.by an :electrical field through san-.aperture cinto an. analyzer. There the ions, under the inuence .of .amagnetic -or electric .field .are :sorted in-accordance to their specific mass, .ions .of low .specic .-mass pursuing a path difieren-t trom .those-of high speci-fic mass. The sorted -ions are 'collected and-discharged, the quantity .of .each :kind of Aions being measured. by the amount-of current. that they A.discharge upon collection.

one typepf mass spectrometer, ionization is accomplished by .bombarding molecules-of the sample in 1an `ion-ization chamber .Wi-th .a beam or stream of electrons, .and the resulting ions are forced through .anf ion Voutlet aperture in A:a rst electrode by .means -of an electrical eld produced by .impressing-a .potential-across --the region in lWhich-the ions .are produced from a pusher .electrode to the firstfelectrode. 'Ihe -beam of ions .passing .through .the-.aperture inthe r-stelectrode may be `further focussed or col-limated by passing-it. through-1an aperture vin -a second electrode, the .bea-m .bei-ng propelled `through the aperture in the .second Pele1':trode'.loy means ofa potential impressed :across the Yspace between the first and .second electrodes .In va,.preier-rediorlfn of this type of mass spectrometer, --the rsty and. second electrodes ,have ionslits that-are elongated in cross-section and disposed substantially in line, i.'-e. .oriented .with each other.. Means are provided. .for 4directing an .electron beam fthroughtheispace .between xthe pusher :electrode-.and the. Iirst electrode, preferably, substantially parallel. .to-the .two 'sl-its zfor at least .in the. plane Adonned-.by the two slits. In this form -of apparatus ionsare-iormed :in the space .between the ,-pusherfand .first electrode. and collimated into lan ion fbeam for.:1ibbon -vvhich passes .through the slits into the analyzer .of the apparatus.-

marked-'ly with .the result that .spectra of. im-..

proved intensity .and stability can be produced, if the voltage which .is .employed to .force the ions as a beam through .the `tvv-o fcol-limating slits or apertures has a -component in-.adirection transverse :to the axis of the apertures. By .altering the magnitude of. this transverse component, it is possible to obtain a ,spectrum of maximum inten.- sity. I

The crossed .-eld. or transverse .component can also-be-.adjusted to counteract .so-called surface effects, such for example .as those-produced by vagran-t ions which charge insulating surfaces which may beter-med .by deposits `on .the walls .of the ionization chamber and which .tend 4to introduce irregularities .and errors in the spectrum. Thus, .the .use .ofthefinven-tion makes possible .an improvement in .the consistency-of results obtained with the spectrometer,

The crossed field has further .advantages in that:

1. The transverse component .can he .adjusted to selections .originating in the uniform part of the electron beam;

2. The adjustment can be l.used .to ascertain that the ions arebeing selected from `the 4uni-term part .of the beam.

3. The .apparatus pan be .so set that .small changes in focussing of the vion beamwill not affect recordedbeam intensity, .so that .Spectra of improved uniformity .and intensity ,are .obtained.

Moreover, during the course wof an analysis .in

Whichpeaks Whichrepresent -the amounts .of ions of different specific `mass .are produced .successivelyon a record, the transverse component may be changed .from .time .to time 7so that each .successive-peak is reproduced .at maximum intensi-ty.

As `disclosed in the .aforementioned .copending application, .one way `of ,providing .a propelling or .accelerating voltage with a transverse .component .for .the ions in the region in which the beam is formed is teem-ploy a 4plurality .oi pusher electrodes together with means for impressing different potentials fbetvveen .the .respective pusher electrodes. and .the .first electrode. ...In this Waytrodes, this transverse `componen-t may be .altered to .obtain .amass .spectrum of maximum intensity. In accordance wththe instant .invention .a

l transverse component-for .the .accelerating voltage is provided-in another way, which is .particularly applicable -in the -case-of .an apparatus in which -the ion -slits are elongated in cross-.sec..

tion. Thus, I provide .at least-,one auxiliary iocussing electrode mounted between the first and second electrodes (which contain the slits) adjacent the end of the slits and also providing means for impressing a potential on the auxiliary electrode to affect the focussing of ions passing through the space between the slits. In this way the ion beam formed when the ions pass through the slits may be adjusted, particularly with respect to the width. Thus, the invention may be utilized to avoid undesirable spreading of the ion beam. Accordingly, my invention contemplates in a mass spectrometer having an ionization chamber provided with a rst electrode having an ion outlet slit that is elongated in cross-section, and means for propelling ions through the slit, the combination which comprises a second electrode disposed outside the chamber and provided with an elongated ion outlet slit disposed substantially in line with that of the first electrode, means for impressing a potential between the first and second electrode to propel ions issuing from the chamber through the slit in the second electrode, at least one auxiliary focussing electrode mounted between the first and second electrodes adjacent an end of a slit, means for impressing a potential on the auxiliary electrode to affect the focussing of ions passing through the space between the slits, means for introducing molecules into the ionization chamber, and means for directing an electron beam against the molecules in the chamber.

In the preferred form of the apparatus of my invention a plurality of auxiliary focussing electrodes are provided, these being mounted opposite each other between the rst and second electrodes adjacent the ends of the slits and. on opposite sides of the beam.

In a further modification of my invention, a third electrode is disposed beyond the second electrode and has a slit of elongated cross-section in line with the other two slits. A second set of auxiliary electrodes is mounted so that the electrodes are opposite each other between the second and third electrodes adjacent the sides of the ion beam and means are provided for impressing a potential between the second set of auxiliary electrodes to affect the focussing of the beam in this region. When three slitted electrodes are employed, the second set of auxiliary electrodes should be so disposed that a line drawn between them makes a different angle with the beam than that made by a line drawn between the first set of auxiliary electrodes.

. The apparatus of the invention preferably is mounted within a magnetic field, i. e. means should be provided for creating a magnetic field in the space which includes the slits, In some instances this means will be the magnet Iwhich exerts its influence on the analyzer of the apparatus.

These and other features of my invention will be understood more thoroughly in the light of the following detailed description taken in conjunction with the accompanying drawings in which:

Fig. 1 is a schematic diagram of an ionization chamber adapted for the practice of my invention and including two collimating or slit electrodes with a set of auxiliary or focussing electrodes disposed therebetween;

Figs. 2 and 3 are longitudinal sectional views through a preferred form of the ionization chamber of Fig. 1, the sections being taken at right-angles to each other;

Fig. 4 is a wiring diagram of apparatus adapted to change the ratio of voltages applied to the two auxiliary or focussing electrodes of the apparatus of Figs. 1 to 3, during the recording of a mass spectrum with such apparatus; and

Fig. 5 is a schematic diagram of the ionization chamber of a mass spectrometer adapted for the practice of my invention and provided with three slit or collimating electrodes with a rst set of focussing electrodes between the first and second slit electrodes and with the second set of focussing electrodes between the second and third slit electrodes.

Referring now to Fig. 1, it will be observed that the instrument comprises an ionization chamber I0 disposed adjacent an analyzer tube Il which is followed by a conventional ion collector (not shown), all disposed within an envelope I4 that is adapted to be maintained at low pressure by pumps (not shown). A gas sample to be analyzed may be introduced into the ionization chamber through an inlet con-- duit I6 which passes through the envelope wall at one end thereof.

Molecules of the gas sample are converted into ions in the ionization chamber by being bombarded with electrons of an electron beam Il which is shot across the ionization chamber from an electron gun HA to an electron catcher IIB. The ions thus formed are moved toward a slit Si in a first propelling electrode I8 by means of a potential imposed across the gap between this electrode and pusher electrode P1, disposed within the ionization chamber.

The electron beam passes through the space between the first propelling electrode and the pusher electrode and parallel to the slit Si in the rst propelling electrode.

As the result of the potential impressed be` tween the pusher electrode and the first propelling electrode the ions pass through the slit Si4 with substantial velocity. The ions are iurther accelerated at this point by an additional potential impressed between the first propelling electrode and a second propelling electrode I9 having a slit S2 parallel to the rst slit S1. The two slits act as a collimator and cause the ions to be projected into the analyzer tube as a narrow beam or ribbon. This beam is caused to follow a curved path in the analyzer tube due to the presence of a strong magnetic ield impressed by an electromagnet (not shown). The magnetic field has another effect in that it causes the heterogeneous ion beam that passes through the slits Si, Sz to be separated in the analyzer into a plurality of separate diverging ion beams according to the specific mass of the ions. By varying the accelerating potential or the magnetic field, or both, each of the ion beams may be caused to focus at the outlet slit of the analyzer tube, so that the ions of this beam impinge upon the ion collector and are there discharged. The resulting current is amplified and recorded to produce the mass spectrum of the gas being analyzed.

The apparatus of Fig. 1 is provided with a pair of auxiliary or focussing electrodes A1, A2 disposed between the two collimating electrodes adjacent the edges of the ion beam. By separately energizing these auxiliary electrodes and by impressing different potentials upon the two, the ion beam or ribbon may be focussed, i. e. expanded or contracted in the region between the two col-` limating electrodes. In this way a transverse component of the voltages is created and it is this transverse component. :which vaflects `the ffocus.-

sing, AThe l:transverse l"pomponent imay. be varied atffwill to `minimize changes inf sensitivity thatfre,

strumentzby vagrant ions.

11n4 theapparatusfof-KFg. 1, .a 'battery or other constant voltage supply "30 is provided and is con.

neot'ed ftoi theviendsrof.: a v"potentiometer '32. The Positive V:endof vthis ipotentiometer y. and its u'slider are. 'connected through a. switch- 36 to jafcondenser 3.4.. .'Aipotcntial dividen-circuit is 'connectedlacros's this lcondenser fand .takes the `norm Iv'of ia resistor 38 :Connected in: series Withitwo otherfs'eries rreE sisters 'orpotentiometers `39, 40, Whichfarein parallel with each other 'fand 'are .in Hturn icons nectedin 'series with -a fourth resistor 41. The negative end -of fthe potential dividingnetwork is .connected to ground. The 1 pusher -felectrode Pi vis .connected .to lthe positive side of `thonet-- work, i. e. to the upper end of the resistor '3'8"in" Fig. 1.v The vlrstf'collimating#electrode'lfd isfconnected to the'otherendofthe resistor 68. One of the vau-Xiliz'try:er .focussingelectrodes-Ai is icon.- neoted-,as lthe slider fof-a potentiometer on the resistenti) the otherf'auxiiiaryy lelectrode A2 being connected tosthe'slider on the resistor vorpotentiometerlil. .The-second apropellin'gor collimating electrode |.9 is @'connected to `the negative Aend 1 of the network and vto ground;

lf.- `the potential 1 across the condenser changes, thepotentials onvtheeelectrodesfconnected theretoA Will- Changeprhortionally. .Similarly shifting-tof the `sliders .ofthe .potentiometers -3 9, -lll Willfchange the ratio v. of the .potentiels .applied 1 to` zthe vfocust sing. electrodes A1, A2.. vIn the'operatien of the apparatus the sliders on the tpotentiomet'ers 39,.'450 areshifted toproduoe .maximum intensity'fOr-then particular peakl of the'speetrum beinglxneasured.

A. preferred form of ionizationchamber'of my invention .isshown in Figs. 2 and 3. Referring: to theseguresVit Willbe observed that the apparatus comprises-a oylindricalfhead'vvll', theirear of Whichconnects With .thegassampleinlet conduit T6. lWithin .thehead .there laan-extension' ofthis conduit in theiorm of .gas :passages 51,552 whichare enclosed'byoonductors: 53,. 54l thatter.- rninateinv the pusher electrodePi. This pusher electrode 'is/spaced .from the rstpropelling elec'.

trode |8`W1iich vis lprovided with. the slit S1. An

' thereto. This third collimating electrode has lan I electron beamlis projectedin thespace between thel'uher electrode-and the flrstpropelling electrode and substantially parallel. to thezslit inthe latter from an .electron gun 55 mounted outside the'head and connected with theionization chamber through 'anapertureor window. Theother propelling 'electrode l9.provided with the slit Sg is disposed parallel to. the rst propelling-electrodebut spaced therefrom, The slitssi 4and S2. are parallel/With eachotherand With the lelec?A tron beam. The 'head of the ionizationchamber is fastened to the adjacent analyzer tube hl by "means oi'a flange.

lThe auxiliary ior focussing electrodes; A1, Azare mount'ediwithin ,the ionization chamber between the slit electrodes IB, .i9 l and are .insulated therefrom. '.The auxiliary electrodesfare thus disposedvr adjacent. theends of the slits and work upon the Cil edges ofthe electron beamer` ribbon formed-as i the' ions pass through the slits S1, S2. 1n many instances 4it isidesirable .to 4change--theratio rof the voltages applied` tothe -two auxiliary electrodes during the -vreeordingoi a :mass spectrum rsince fa' 'potential by closing theswitch 36. yswitch is open, the condenser gradually discharges through'thefpotential dividercircuit inbe obtained.

'stant voltages V1, V2 into the leads to the auxil iary electrodes. To 4'automatically record a mass spectrum-the vcondenser 34 is: charged to `a highv When fthis cluding the resistance 38, the potentiometers`39`, 40 andthe resistance lfill. While acondenser is thus'd iscl'iarging,y beams comprising ions of lower "and lower specic mass fall successively on the collector (not shown) Simultaneously, the trans--v verse-voltage component introduced by? the .electrodes A1,.Az varies. By suitably adjusting v'the' potentiometers 60, `62 as -Well asthe potentiome- 'tel's y39', `lll the ratio of. the Various propelling voltages'aas va funoti'on'of specific mass may 'ibo' varied vin a Wide variety of Ways, .By suitable settings of :those ,potentiometers the optimum operating point forfions of each specific massm'ayJ v1`teerringnew"to Fig. '5, it will be :observed that thefieni nationL ehamberand the 'wiringadi gram of this apparatus vare the lsame as that of Fig. -1f,.1ike parts being identiedby like '-ref.

eretico. numlcersandy letters exceptv in the 'follow'- ing.particulars:l

The ionization chamber is provided Withvan additional collimating Lelectrode |00 1 disposed be@ yondthe collimating electrode I9 and parallelelongatedslit-Siparallel to-slits S1,Sz.. Thesecondlpair of'foc'ussing .electrodes A3, A4l is' disposed between.thefsecondcollimating electrode I9 and the :third vcolliinatingelectrode |00. `This sec ond..pairo f auxiliary :'electro'des; instead of being` disposedfadjacentithe edges of the ion'beam as are A1, A2,:fare `disposed at rightiangles thereto and 4Work rupon the thin section ofthe ion' be'an'r or ribbon. This second pair of focussing electrodes :likewise laids fin 'focussing' -and 'collimatin'g' theelectronbeam.

.The 'wiring diagram Aof Fig. 15, as 'indicated' above, is similartothat of'Fi'g. 1 except 'that itis l provided with anotherpair of potentiometers |01, .i021 connected inseries in the potential dividing inetwork, but vin `parallel With each other; Theslider-'ofthepotentiometer IUI is connected to 4'auxiliary-velectrode A4 fand: the slider of the potentiometer' I 02-1is 'connected' to the auxiliary electrode fAa; By adjusting the. potentials applied to -the secondset yoi auxiliary' electrodes with fthe second setof potentiometers, the focussing of the 'beam -Inay befaiected.

tIn operation, thezsettings v4of lthe potentiometers 39,'-40-1andof :.thefpotentiometers lola, `|ll2` are` adjustedlto-givefmaximum sensitivity.

Ln-many instances. it :isdesirable'r te "chan'getheratio :ai-pusher voltagei. "eithe ratio-of the volt- -ages applied to" the ltwo misher electrodes duringv the .recording Jef l'a mass spectrum, `'since a dif# ferent ratio mayibeoptimum forfea'chp'eak.

Ifclaim: 1

-1=. :Ih'afmassspectrometerhaving an"ionization chamber-'provided with a first electrode 'having any fion outlet slit Vthat' is elongated v:in crosssection, and means ffor propelling 'ions through theslit, the combination which comprises a secondfeiectrcde di-sp'osedfoutside 'the chamber and provided wanten-elongated ion loutlet slitfdis posed substantially in line with that of the first electrode, means for impressing a potential between the first and second electrode to propel ions issuing from the chamber through the slit in the second electrode, at least one auxiliary focussing electrode mounted between the first and second electrodes adjacent an end of a slit, means for impressing a potential on the auxiliary electrode to affect the focussing of ions passing through the space between the slits, means for introducing molecules into the ionization charnber, and means for directing an electron beam against the molecules in the chamber.

2. In a mass spectrometer having an ionization chamber provided with a first electrode having an ion outlet slit that is elongated in cross-section, and means for propelling ions through the slit, the combination which comprises a second electrode disposed outside the chamber and provided with an ion outlet slit elongated in cross-section and disposed substantially in line with that of the first electrode, means for impressing a potential between the first and second electrode to propel ions issuing from the chamber as a beam through the slit in the second electrode, auxiliary focussing electrodes mounted opposite each other between the first and second electrodes adjacent the ends of the slits and on opposite sides of the beam, means for impressing a potential on the auxiliary electrodes to aiect the focussing of ions passing through the space between the slits, means for introducing molecules into the ionization chamber, and means for directing an electron beam against the molecules in the chamber to form the ions.

3. In a mass spectrometer having an ionization chamber provided with a first electrode having an ion outlet slit that is elongated in cro-sssection, and means for propelling ions through the slit, the combination which comprises a second electrode disposed outside the chamber and provided with an ion outlet slit elongated in cross-section and disposed substantially in line with that of the first electrode, means for im# pressing a potential between the first and second electrode to propel ions issuing from the chamber as a beam through the slit in the second electrode, auxiliary focussing electrodes mounted opposite each other on the two sides of the beam between the first and second electrodes adjacent the ends of the slits, means for impressing a potential between the auxiliary electrodes to affect the focussing of ions passing through the space between the slits, a third' electrode disposed beyond the second electrode and having a slit of elongated cross-section inline with the other two slits, a second set of auxiliary electrodes mounted opposite each other between the second and third electrodes adjacent the sides of the ion beam, means for impressing a potential between the second set of auxiliary electrodes to affect the focussing of the beam between the slits in the second and third electrodes, means for introducing molecules into the ionization chamber, and means for directing an electron beam against the molecules in the chamber.

4. In a mass spectrometer having an ionization chamber provided with a first electrode having an ion outlet slit, and means for propelling ions through the slit, the combination which comprises a second electrode disposed outside the chamber and provided with an ion outlet slit disposed substantially in line with that of the rst electrode, means for impressing a potential between the rst and second electrode to'propel ions issuing from the chamber as a beam through the slit in the second electrode, auxiliary focussing electrodes mounted-between the first and second electrodes on opposite sides of the beam, means for impressing a potential between the auxiliary electrodes to affect the focussing of ions passing through the space between the slits, a third electrode mounted beyond the second electrode with a slit disposed in line with those in the first and second electrode and in the path of the beam, another set of auxiliary electrodes mounted between the second and third electrodes on opposite sides of the beam and at a different angle to the beam than the angle made by the first auxiliary electrodes, means for impressing a potential between the auxiliary electrodes of the second set, means for introducing molecules into the ionization chamber, and means for directing an electron beam against the molecules in the chamber.

5. In a mass spectrometer having an ionization chamber provided with a first electrode having an ion outlet slit that is elongated in crosssection, and means for propelling ions through the slit, the combination which comprises a second electrode disposed outside the chamber and provided with an ion outlet slit elongated in cross-section and disposed substantially in line with that of the first electrode, means for impressing a potential between the first and second electrode to propel ions issuing from the chamber through the slit in the second electrode, at least one auxiliary focussing electrode mounted be-1 tween the first and second electrodes adjacent the ends of the slits, means for impressing a potential on the auxiliary electrode to aiect the focussing of ions passing through the space between the slits, means for introducing molecules into the ionization chamber, and means for directing an electron beamagainst the moleculesV in the chamber, and means for creating a magnetic field in the space including the slits.

6. In a mass spectrometer having an ionization chamber provided with a rst electrode having an ion outlet slit that is elongated in cross section and means for propelling ions through the slit, the combination which comprises a second electrode disposed outside the chamber and provided with an elongated ion slit disposed substantially in line with that of the first electrode, a pair of oppositely faced focusing electrodes mounted between the first and second electrodes adjacent the opposite ends of the slits, means for impressing a potential on the first and second electrodes, means for impressing an adjustable potential on the focusing electrodes independent of the potential of the first and second electrodes, and means associated with said last named means for proportionately varying the potential on all of said electrodes.

7. In a mass spectrometer having an ionization chamber provided with a iirst electrode having an ion outlet slit that is elongated in cross section, and means for propelling ions through the slit, the combination which comprises a second and third electrode spaced from the first electrode and cach having an ion outlet slit elongated in cross section and disposed substantially in line with that of the first electrode, a pair of oppositely faced focusing electrodes disposed between the first and second electrodes adjacent the opposite end of said slits, a second pair of oppositely faced focusing electrodes disposed between said second and third electrodes and adjacent to the opposite 9 ends of said slits, means for impressing a potential between the rst and second electrodes to propel ions issuing from the chamber as a beam through the slit in the second electrode, means for impressing a potential between the :second and third electrodes for accelerating the ion beam passing therebetween, means for impressing an adjustable potential on the rst pair of focusing electrodes, means for impressing an adjustable potential on the second pair of electrodes, the means for impressing the adjustable potentials on the two pairs of electrodes being independent of the means for adjusting the potential impressed on said iirst, second and third electrodes, and means associated with said potential impressing means for proportionately varying the potential on all of said electrodes.

HAROLD W. WASHBURN.

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

UNITED STATES PATENTS Number Name Date 2,228,958 Hinsch Jan. 14, 1941 OTHER REFERENCES Tech. publication, The Mass-Spectrograph and Its Uses by Walter Bleakney, in American Physics Teacher, Vol. 4, Feb. 1936, pgs. 12 to 23. Photostatic copy in Div. 36, U. S. P. O.