US2587575A - Mass spectrometry - Google Patents

Description

Feb. 26, 1952 H. W. WASHBURN MASS SPECTROMETRY 5 Sheets-Sheet l Filed March 24, 1951 IN VEN TOR. HA ROL D W WASHBURN ATTORNE Y Feb. 26, 1952 H. W. WASHBURN 2,587,575

MASS SPECTROMETRY Filed March 24, 1951 3 Sheets-Sheet 5 IN V EN TOR. HAROLD W. WASHBUR/V BY jm 5M I AT TORNEY Patented Feb. 26, 1952 UNITED STATES PATENT oEElcE MASS SPECTROMETRY Harold W. Washburn, Pasadena, Calif., assignor to Consolidated Engineering Corporation, Pasadena, Calif., a corporationA of California Continuation of application Serial No. 513,528,

December 9, 1943. This application 1951, Serial No. 217,355

to be analyzed is ionized in a chamber orionv source, for example by heating or by.. electron bombardment, and the resulting ions are ,propelled by an electrical eld from the ion source into an analyzer. AIn the analyzer, the ions, une der the inuence of a magnetic or electrical field, are sorted in accordance with theirspecic mass, ions of differing specic mass pursuing different paths through the analyzer. The L ion beams eachcompose'dof ions of a given specific mass may be collected andv discharged, the discharge' current giving a measure of the partial pressure in the original sample of the parent molecules from which the particular ions derived. l

,the outlet electrode. Frequently one or more additional apertured electrodes are interposed in the ion'source between the point of ion formation and the outlet electrode, the ion propelling potential in such case being established between the pusher and successive ones of these electrodes including the outlet electrode. The several apertured electrodes function not only to control distribution of the propelling field established across the ion source, but additionally to dene or collimate the ions into a beam of a cross section generally similar to the shape of the electrode apertures.' The velocity of the ions as they pass through the outlet velectrode isa function of the potential gradient .between the point of their formation andthe outlet electrode and inv dependent of the distribution of this gradient as determined by the position and potentials of intervening collimating electrodes.

7 Claims. (Cl. Z50-41.9)

March 24,

I have discovered that the sensitivity and stability of the spectrometer can be materially inf,- creased with a resultant improved accuracy if the potential established across the ion source (i. e. between the point of ion formation and the outlet electrode) to expel ions from the ion chamber includes a, component transverse to the axis of ion travel through the outlet aperture and normal to the major axis of the aperture.

The aperture in `the outlet electrode as Well as aperturesin preceding collimating electrodes are elongated relatively narrow slits and are herein delineated as having major and minor axes asy well as a longitudinal axis, the latter axis being inthe direction of ion travel through the electrode and normal to the major and minorA` axes.

The propelling potential. having both longitudinal andtransverse components, is referred to asa crossed eld and is useful in adjusting the focusof the ion beam on the outlet aperture. By variation of the magnitude of the transverse eld either simultaneously with or `independently of variation in the magnitude of the longitudinal component of the field, the ion beam may be shifted', in the direction of the minor axis of the aperture. l

` The crossed field or transverse component can also be adjusted to counteract so-called surface eiects such, for example, as those produced by vagrant ions which charge insulating surfaces as may be formed by deposits on the walls of the ionization chamber and which tend to introduce irregularities and errors in the spectrum. Thus the use of the invention improves not only the accuracy of the instrument but also the consistency of results obtained. The crossed eld eiiect resulting from application of propelling potential having both longitudinal and transverse` components has further advantages in that: l

' 1. The transverse component can be adjusted to select ions originating in a uniform portion of the electron beam;

2. The adjustment can bevused to ascertain that the ions are being selected from such uniforml part of the electron beam,v since in general the variation of peak intensity with propel-V ling. potential is relatively small when ions are thus selected; and

3. The propelling potential can be so adjusted that smallchanges infocusing the ion/beam will not affect recorded beam intensity, so that spec.- tra of improved uniformity and intensity arcobtained.

A further advantage of the use of a propelling potentialhaving transverse components resides in the fact that, in an analysis of mixtures having components susceptible to cracking under the prevailing conditions in the ion source, the potential may be adiusted so as to give optimum sensitivity for a given component sought to be determined."Y Forexample, the transverser com,` ponent ofthe propelling potential may be ad-` justed to set the focus of the ion beam on the outlet aperture to emphasize that portion of aM mass spectrum of the hydrocarbon mixture charnlv acteristic of one or more of the ingredients, say

a heavy hydrocarbon such as heptane More-"- over, during the course of analy'sis'in which peaks` representative of ions of different specic mass are successively produced on .a record, the-ratio 'p15` of the transverse and longitudinal*:components-z of the propelling potential may be changed 'from time to time so that each successive peak is re.- produced at maximum intensity.

YTheinvention will be more clearly understood in"thelighttof"the following` detailed description wh1`ch=istakenA in conjunctionwiththe accom= panying 'drawingsA inv `which:

Fig'fl is'a schematicv diagram oa massspecf rometer. adapted for the practiceV oflmy inven`` ioni;

Fig-2 is a graphiillustrating the` effect onthe intensity'iof a peak in a massspectrum produced byfvarying"the'transverse componentrof pro-f peiling potential; i

"Fig3 isa partial view of the ion source of a mass spectrometer' showing another errlbodilrient of "th'e'v invention 4'with -associated electrical circuitry;

Fig; 4 is'a similar' partial View y"showing an-A other embodiment ofl the-inventionVA and f 'Fi'g;;5 isa similar partialview V showing still another` embodimentv of'A the invention.

Referring ito'ig.' l, the -mass spectrometerthere 1shownrcomprises 'an' ionization chamber` orion'source I0 disposed Vadjacent anlarralyfzer-r tubei #which` lis followedA by an ion collector-12,1 all-)disposed within' an envelope^|4 The spec trometer isadapted to be maintainedatflow vpreslv sure by evacuating-means (not shown) connected Y V to the envelope through a spherical joint'= I5 gassample to be analyzed may be' introducedinto the'` ionization chamber through' an finletf con duitfIIi` passing through the envelope wall lat one end thereof.

` The ion sourceor chamber Illis separated' from-1 the"analyzer tube -II by an'outletfelectrode- IE- having an aperture Szf'thereinz The'ionfsourcealsov encloses anelectrode 18" having' anfaper tlle'S therein alignedwith the aperture Safin therboundary electrode I9.- A pair ofisoecalledf repellerror 'pusherf'electrodes =P1,` vPifare:` spaced;- front-theT electrode I8 on the opposite'side of an electron beam I7: projected acrossthe ion source by`anelectron'gun `(not shown) Molecules of the gas-sample "introduced into the Y ion source. through conduit I6 are converted into ions with injthe source bybombordment withthe electron beam I'I; Theions thus formed are urged toward aperture Sr in the v-1lrst propelling electrode 'i3 S65 under "the influence -of a potential established across^theregion of ion formation between the.` pusher electrodes and the outlet electrode la."

`The ions lpass through 'the slit 'Sifin electrode l'withfappreciable velocityand are further ac- 7()IV adaptedrorvarying the potentials applied to the pu'sherzan'd.apertured electrodes while maintaim,

celeratedbetweentheelectrode I8 and the barrier electrode ISAl The accelerated-ions issuevthrough` aperture S2 in the barrier electrode into the analyzer tube.-` The -twoiaperturesln electrodes I3` land" ISfunction to collimate tandpropel the'" ions to be projected into the analyzer tube as a narrow beam or ribbon. Additional electrodes similar to the electrodes I8, I9 may be included in the ion source, the final Velocity of ions issuing into the analyzer tube being a function in any lease 'of/the V-potential `established `in the ion source between thepusher or repeller electrodes andthe terminal or barrier electrode separating the ion source from the analyzer and independent of. thefdistribution of the eld between the several collimating electrodes.

The ion beam is caused to follow a curved path in... the analyzertube due to the presence of a strongy magnetic eld impressed across the spectrometerbya magnet (not shown) transverse to the path of `the ions. Under the inuence of the magnetic'field a heterogeneous ion beam entering the analyzer is separated into a plurality of separate beams B1, B2, B3 according to the specific mass of the ions. By varying the accelerating potential, .'by; varying .themagnetic field Yor both, each-Lof :the ionr beams :may be focused on the collector through. anaperture 2li at the end ofthe analyzer: tubeopposite, the ion source. The current.developed;.at the-collector responsive t0 ion discharge: isamplii-led.` and recorded to .produce the mass spectrum of the gas being analyzed. 1

The:two'pusherelectro'des P1', P2.' disposed on oppositefsides of'thezlongitudinalaxis of the ion: sourcezaraseparately energized so that the poV` tential impressed: acrossi the'. ion 'source between.` thei pushery electrodes'z'and the terminal electrode f I9 maybe given a transverse-as welli as a longis tudinalvcomponent'by impressing different voltagesion the'two @pusherelectrodesx This vtransverse vcomponent maybe varied at; will by.vary

ingstthe :ratio ofwvoltages supplied to .the twov pushera electrodes toi :minimize changes' in sensitivity resulting"fromizdisturbance :of ion focus The transverseV compof' nentfmayalso'bevaried in the same manner to minimizerchanges'fofsensitivity due to fluctuations:in. the' emission characteristics of the elec-v tron source notsh'own). and may be altered so that' the; peaks krepresenting `the currents cor-V responding to discharge of the several ion beams*V at-athe .collector maybe emphasized and recorded withlemaximumr intensity.`

A voltage supply'circuitas.illustrated diagram-V matically in Fig. 11 maybe used to supply po-V tentials tozr the several electrodes .within the ion source..This circuit includes a battery'or` other constant" voltage source` 3B across: which is con-i e nectedla potentiometerSZ.; The positive end of' this,potentiometerisconnected through a slider anda switch '.36 toa capacitor'.34,.also connected tor--ther-negativeV sidegof the Voltage-source, A potential dividerI circuit is` connected across the capacitor` and takes'theform of. a large resistor 38'connected in series with two parallel coupled smaller resistors 46,42.' The negative end of the potentialudivi'ding network is connected to the negative'sideLof; thevoltage source 3) (in this casergroun'd)` and toth'e' outlet electrode I9. l The electrode: I8 is 'connected tothe'potential dividing networkibetween the resistor 38 and the parallel coupledy resistors 43,342, and the two pusher electrodes are connected respeotivelyto sliders 44, d'5

' tappingtheiresistors'l, 42 respectively.

Thewcircuit: asfillustrated land described ing a1constanti ratio between vthese potentials, such variation of' thezpropelling potentials serving to scan the `io'nJeams;` acrossthe exitor foeusir'ig sin zo of tne'anaiyzertube. The relative potentials applied to the pushers Pi and P2 maybe varied independently of the potentials on the other electrodes by manipulation of the respective sliders 44, 46.

Fig. 2 illustrates the effect of different settings'- of the sliders 44, 46 in the intensity of a spectrum peak corresponding to a specific mass 58. The ordinate of the graph in the figure representsthe intensity of the peak as determinedby they amplification and recording system connected to the collector. The abscissa represent the settings of the slider 46 (connected to electrode Pi)r as the percent of resistor 42 tapped oi by the slider.

The various solid curves of the diagram? show how the intensity varies with the setting of the slider 44 of the other pusher electrode Pi and a second collimating electrode 66 interposed between the point of ionization, i. e. electron beam 68 and the terminal electrode 62. In this embodiment a transverse component of propelling potential is developed by means of elec'- trodes 1 0, 'Il mounted on opposite sides of the connected to the junction of the parallel coupled -resistors 40,l 42 opposite the resistor 38, and4 aslider 'I5 connected to the pusher electrode .60: Electrodes 10, 'H are connected respectively to sliders 44, 46 of the resistors 40, 42 respectively again in terms of the percentile ratio of the tapped oi portion of the respective resistor 46.. Particular settings of the slider 44 are noted on the graph adjacent the minimum point o f the several solid curves and are designatedv No. 1.-30 etc. Fig. 2 shows that for each setting of the first pusher P1, a .curve representing the peak against the P2 pusher control setting is obtained. Each of these solid curves has a maxigraph as the operating point.

The maxima of the various individual curves v 'f and the transverse potential component may be adjusted by independentvariation of the potentials impressed on these two electrodes in""the v manner described with relation to Fig. l.

It should be emphasized that the propulsion of ions from the ionization chamber through the aperture in the terminal electrode 82 depends upon the potential diierences established across the chamber between the point of ion'formation,

i i. e. the electron beam, and this electrode and is independent of the number of intervening elecrepresent sensitivities which are greater than is jl obtained if bothpusher electrodes P1 and Pz are at the same potential. Dotted line curve B on lthe graph shows the sensitivity'forvarious pusher settings wherein the pusher electrodes are operated at the same potential. This curve lies well below the operating point of curve A joining the maxima. v -If the pusher electrode settings are such that the operating point is displaced appreciably from curve A, such as for example the point Q at which the settings are respectively 50 and 54 for the two pusher electrodes, slight voltage disturbance at the electrodes, induced for example by changes in contact potentials at the surface, will cause relatively large changes in sensitivity compared to any such change that will occur if the pusher electrode settings generally correspond to the ratio'at the operating point of curve A.v Optimum operating conditions are obtained by operating at the point of curve A in the shallow flat region thereof designated as the operating-.13j`

point.

For this particular mass and with an ioni" source in which this data was obtained, the operating point corresponds to relative pusher settings-of 60 and 'l0 respectively for pusherelec`-` trodes P1 and Pz. f From th-e foregoing analysis it is evident that by establishing in the ion source apotential hav-1 ing a transverse as well as a longitudinal component, and in which the transverse componentl is normal to the major axis of the collimatingv apertures, improved sensitivity and stability can be obtained by suitable adjustment of the magnitude of this transverse component.

` v-Other equally satisfactory means of establishing a potential across the ion source having both i electrode 62 giving access to an analyzer tube v64 l trodes employed to establish this potential. The acceleration is likewise independent of thev distribution of the longitudinal component of this potential. For example, the potential established between electrodes 66 and 62 may be, and generally is, considerably in excess o f that established between the p-usher 60and electrode 66, and yet the over-all acceleration of the ions is solely a function of the potential difference be.- tween the pusher electrode and the nal electrode.

It should also be emphasized that the beneicial effect of the transverse component is in the ability as a result of this component, to focus the ion beam as desired on the aperture of the final electrode, electrode` 62,in Fig. 3. This is inherent vin the application of a transverse potential component and holds true regardless o f whether the component is developed byisplit pusher electrodesby the electrodes 10, 'H preceding the first apertured electrode or by electrodes otherwise positioned in the ion source, as for eX- ample as shown in succeeding gures. The ef- Afeet of the transverse potential on intensity, sensitivity and the minimization of the effects of vsurface charge is due solelyto its 'eiect on the ion beam as related to thisaperture through which the ions are propelled into the analyzer tube. Any effect that the transverse component might have with relation to the aperture in preceding electrodes is only incidental to the'effect Thus by `-and for the same reasons as the transverse com"- ponent produced by the two pusher electrodes of the embodiment of Fig. 1. The ion source shown diagrammatically in Fig. 4 is similar in most respects to that shown in Fig. 3, including a pusher electrode 60, means (not shown) for forming an electron beam 68, a first apertured electrode 66 and a terminalpr barrier the .transverse potential component produced by application of different potentials to electrodes 18 and 80 will be normal to the major axis of th apertures in electrodes 66, 62.

Again the voltage supply circuit for impressing appropriate potentials on the various electrodes and the ion source is essentially similar to that shown in Fig. l including the battery 30, potentiometer 32, switch 36, capacitor 34, resistor 38, parallel coupled resistors 40, 42 connected in series between resistor 38 and a resistor 14, with the potential dividing network produced by the several resistors 38, 40, 42, 14 being connected across capacitor 34,. In this instance pusher electrode 60 and the first apertured electrode 66 are connected to taps 82, 83, respectively, of resistor 14. The terminal electrode 62 is connected to the negative side of resistor 38, in this case ground, and the electrodes 18, 30 are connected respectively through taps 44, 46 to the parallel coupled resistors 40, 42 respectively.

The construction shown in Fig. 4 is essentially similar to that illustrated and described in greater detail in my co-pending application Serial No. 103,730, iiled on July 8, 1949, now abandoned, insofaras electrode positioning within the ion source is concerned. With respect to the advantages accruing from production of a transverse component of the propelling field established in the ion source, the present application as well as the aforementioned co-pending applica- Y tion Serial No. 103,730 derived from discovery of the benets of such a transverse potential as set forth in application Serial No. 513,528, of which the present application is a continuation.

, The ion source shown in Fig. differs from that shown in Fig. 4 only in the inclusion of an additional apertured electrode 86 interposed between rst apertured electrode 66 and the terminal electrode 62. The voltage supply circuit in this embodiment is likewise substantially similar to thaty of Fig. 4 differing only in the inclusion of an additional tap 88 connecting the additional electrode 86 to resistor 14 for impression on the electrode 86 of a voltage intermediate the Vvoltage impressed on electrodes 66 and 62.

There is no difference in the operation of the ion source as shown in the various illustrations, the desired transverse potential component being achieved in each case by applicationof different potentials to the split electrode Whether the split electrodes be pusher electrodes, electrodes preceding the first apertured electrode or following the iirst apertured electrode. In some instruments, because of the construction thereof, use of split or dual pusher electrodes is preferredgenerally in circumstances where the collimating or apertured electrodes are so arranged as to preclude insertion of split electrodes therebetween. In other situations the transverse potential component is most conveniently achieved by electrodes disposed between the collimating electrodes, the eiect in any case being the saine and the inherent benets ofthe transverse eld being realized in each case.

I claim:

l. In a mass spectrometer having an ionization chamber provided with a rst electrode having an ion outlet aperture, and means for impressing a potential across at least a portion of the chamber --tol the aperture for propelling ions fromV the chamber through the aperture, the combination which comprises a plurality of electrodes mounted in the chamber opposite the aperture, means for impressing different potentials between the respective electrodes of the plurality and the first electrode, means for introducing molecules into the vspace between the rst electrode and the plurality of electrodes, and means for directing an electron beam into said space.

2. In a mass spectrometer having an ionization chamber provided with a first electrode having an ion outlet aperture, and means for impressing a potential across at least a portion of the chamber to the aperture for propelling ions from the chamber through the aperture, the combination which comprises a plurality of electrodes mounted in the chamber opposite the aperture, means for impressing different potentials between the re.- spective electrodes of the plurality and the iirst electrode, meansfor varying the potentials thusl impressed, means for introducing-molecules into the space between the rst electrode and the plurality of electrodes, and means for directing an electron'beam into said space.

3. In a mass spectrometer having an ionization chamber provided With a rst electrode having an ion outlet aperture, and means forimpressing a potential across at least a portion of the chamber to the aperture for propelling ions from the chamber through the aperture, the combination which comprises a plurality of electrodes mounted in the chamber opposite lthe aperture, and respectively on opposite sides-of the axis thereof, means for impressing dilerent potentials between the respective electrodes of the plurality and theiirst electrode, means for varying at least one of,v said potentials, means for introducing molecules intothe space between the first electrode and the plurality of electrodes, and means `for directing an electronbeam into said space.

4. In a mass spectrometer having an ionization chamber provided with a first electrode having an ion outlet aperture; and means Afor impressing a potential across at least a, portion of the chamber to the aperture for propelling ions from the chamber through the aperture, the combination which comprises a plurality of electrodes mounted in the chamber opposite the aperture and disconnected from each othery within the chamber, means for impressing potentials between the respective electrodes of the plurality and the rst electrode, means for altering the vratio between said potentials, means for introducing molecules into the space between the rst electrode and the plurality of electrodes, and means for ionizing the molecules in said space.

5. In a mass spectrometer having an ion source in which ions are produced from molecules, an analyzer chamber, and a communicating aperture between the analyzer chamber and the ion source and through which the ions produced in the ion source are introduced into the analyzer chamber, the combination which comprises a plurality of electrodes disposed in the ion source, means for introducing molecules into the ion source whereby ions formed from said molecules will be subject to the iield produced by said plurality o f electrodes, and means for producing between the electrodes an electrical potential having a component parallel to the longitudinal axis of the aperture kfor forcing the ions throughthe aperture into the analyzer chamber and a component transverse to the major axis of the aperture! 6. In ion source for a mass spectrometer having means for receiving into the source a sample to be analyzed, means within the source for ionizing the sample and an outlet electrode having an aperture for egress of ions from the ion source, the combination which comprises a plurality of electrodes disposed in the ion source and means for producing between said electrodes,including said outlet electrode a potential having a component parallel to the direction of ion iiow through the aperture in said outlet electrode and a component normal to the major axis of said aperture.

7. In an ion source for a mass spectrometer having means for receiving into the source a sample to be analyzed, means within the source for ionizing in an ionizing region molecules of the sample to be analyzed, anoutlet electrode having an aperture for egress of ions Ifrom the source, and at least one collimating electrode disposed between said ionizing region and the outlet electrode, the combination comprising at least one pair of electrodes disposed on opposite sides of the ion source in transverseA alignment and means for impressing independently variable voltages on said pair of electrodes to develop in the source a potential transverse to the major axis of the aperture in the outletelectrode.

REFERENCES CITED The following references afwofrecord in the file of this patent: f"

UNITED STATESA nTENTs Name l Date Berry Mar. 6, 1945 OTHER REFERENCES Mass-Spectra and Isotopes.' byQF. W. Aston. 1933, pages 28, 29, and 69,pi'1blished by Edward Arnold 8: Co.. London, England. v

A Mass-Spectrographi'c" S udyvof the Isotopes of Hg, xe, Kr, Be, I, As, andesgby' Alfred o. Nier. pages S33-937. Physical Review, vol. 52,2Iyflov. l, 1937.

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