US2457530A - Electron gun for mass spectrometers - Google Patents

Description

Dec. 28, 1948. N. D. COGGESHALL ETAL 2,457,530

ELECTRON GUN FOR MASS SPECTROMETERS 2 Sheets-Sheet 1 Filed Aug. 6, 1946 Norman D ljugg'eshall Nathan F- Kerr m Q m 1948. N. D. coGGEsHALL ETAL ELECTRON GUN FOR MASS SPECTROMETERS Filed Aug. 6, 1946 2 Sheets-Sheet 2 Nurman 1] fingg'eshall NaihaTL'F. Kerr Patented Dec. 28, 1948 2,457,530 ELECTRQN "GUN FOR M A'S S" '7 SPECTROMETERS' Norman D." 'Cog'geshall; Verona, and- Nathan F. Kerr,"Pittsburgh, Pa., asslgnors' to 'Gulf' Research & -Development- Company, Pittsburgh,- Pa.-.,-a corporationof Delaware- Applicatlon'Au'gust'fi, 1946, Serial No. 688,736

Claims.-(Cl. 250 413) This inventionrelates to devlces'for furnishing a plurality of electronsin motion and for employing the said electrons. l

The mass spectrometer is today'ran analytical"- and-research instrument of great economic importance. It 'is usedzfor' routineanalysis of gas and liquid samples, hydrocarbon analyses adjunct torefinery control, for followin'g isotopic' tracers,

for studying molecular energy"'leve1s; and-Tor" In the mass spectrometers in use today the ion beam is formed byrthat com other purposes.

ponent of the aparatus known as the ion source; Y

Basically, this consists of a r-arrangement of electrodes across which'the'reare'im pres'sed" direct current vo1tages.- Inth'e free space between" the electrodes an electron beam-is made t'o passi- Intraversing this region the electrons" create post tive ions from the gas molecules or atoms pres-' ent. These ions are drawn in one -directi'onby the impressed voltages, and bymeans oftwo 'slit apertures a beam of ions is defined'.' --The e1ec-= tages named for 'the tungsten filament will be common tothese othertwo types of sources.

In the application of the mass spectrometer to the problem of quantitative analysis of multicom- -ponentmixtures of hydrocarbons the cracking patternsof'the individual components are used.- Wherrra "single-compound is examined in the instrument it is ioundthat in the ion beam'there are 'ionsof-many -diiTerentmasses. That is,

theremay' be the molecule ionizedby losing an electron, but it may also dissociate by electron impactto form' neutral "and ionized fragments. For example; n-butane will give rise to ions of masses 58;-- 56,- 43; 42; and so forth. Iso-butane will giverise to ions o-fthe same masses but their relativeintensities will be' considerably diiferent. The pattern of ionic masses and their relative intensities'ior anycompound is referredto as its cracking pattern and it is the differences in -these patternsthat make'theanalysis of multitron beam has'heretofore been form'e'd by' a fi1ament and q eement:knownseen' It y" consistmerelyof 'a ma-4. ment which emits electrons and 1 g; v

electron gun.

electrodes with apertures which-meat a positive potential relative to the filament; This results in electrons-beingdrawn towards the electrodes and passing through the apertures-to form a beam. A relativly'weak magnetic field in the direction of electron travel may be employed to aid in-the forming and-maintaining of-a beam.

The filament used by the electron gun-may be tungsten, thoriated tungsten or-"the sourceof electrons may be an indirectl'y heatedoxidecoated cathode. In' any case; the electron beam o'rig inates from a source of thermionicemissionx A- thermionic emission source-of electrons; although practically the only type-heretofore usable in" mass spectrometers; has several very serious disadvantages. These disadvantages; which will be explained belowyresult in considerable exp'endi ture of time and-money 'to keep'the-instrum'ent' in operating order,

The enumerationand"description of the disadvantages of the" thermionic emission typ'ei'of cathode may be limited toa discussion of "ordi' nary tungstenfilaments, inasmuch as theyare easier to use than 'oxide'coatedcathodesortho The latteritwo are very susceptible to' poisoning? 'of the emitting riated tungsten filaments.

surfaces by improper vacuum conditions and are not considered practical t use in a routine instrument. Furthermore, mostotthe disadvancomponent mixtures possible. The instrument is calibrated for any particular mixture by examiningin a pure'form each of the components con-- tained 'in the mixture. This data,"that is, the

relative 'intensities'of the various ions formed,"

must -be known' accurately in order to perform accurate" quantitative analyses. Furthermore, theins-trument must be'so designed and operated as torender the cracking patterns as independent aspossible of such parameters within the ion source asmay-change with time. This condition is very important for if notsatisfied it may be necessary to recalibrate the instrumentquite oft-en, perhaps once a week or every few days,

typeof electron source is very troublesome in causing changes-in the cracking patterns.

Although. the-major part of the ions coming from the ion'source are due-directly to electron impact on the gas molecules there are a substan tial number that owe their origin partly or wholly Orlereason for this is that the to the filament. filament which operates in the neighborhood of 2000 Kelvincauses thedissociation of some of the'molecules' that impinge on it. The neutral fragments may then by diifusion enter the ionization region where they may be converted into ions. Alsothe process may take place wherein some of the parent molecules or their fragments may be ionized on the surface of the filament and subsequently .diffuse into the ion beam. The" two above effects, although not responsible for the major number of ions in the ion bea'm, can contribute a substantial number." Furthermore,

and this is -an expensive and-time consuming operation.-- As will .be explained, the filamentmode of operation','a highly undesirable situa 1 effects by placing a separate vacuum pumping; connection on the filament assembly sothat the r through which the electrons pass ones explained above comes about from th output from the filament.

; its heat outputiis" absorbed as'radiant ener'gy-iby the metal parts ofthe electronfigun and 10113525 source. This results in a temperature of these- 1 face condition which change with time and mode of operation. This causes the cracking patterns oi the pure compounds to change with time and tion.

Attempts hav e been made to remedy the above 7 the aperitini from the fila ment into the ionization region. T successful, due to the high mQbl1l'ty{ is a pressure differential across cules and ions at the tempertures-encountei e and due to long mean free paths at the pressures used.

Another disadvantage somewh Z1) .w'il if .l at akin p titre" e heat As the filament is surrounded onlallsides. b-y'electrodes o'r shielding, i and as the-gas pressure surrounding it is 10w, be

ingot the order; of 10* mm. of mercury,:mo'st of parts higher' than the ambient one). Molecu1esimpingingon the" walls of the-ion source will a'ssume a nearly equivalent":temperature:and-this" afiects theirimobility' oriaverage velocity." Tl'llS g latter quantity is an important one in controlling? the speed of pumping of the gas through the ion source, {This means-then that the temperatureof the ion source affects the molecular densitywithin it and this in turn affects the intensity of theiionbeam which it is desirable to maintain veryl constant'f If the temperature of the ion source" cculd be maintained-veryconstant; the aboveefiect w'oul-dnot be very important. I-Iow-J ever, due to the' relatively" poor vacuum in whichi 40 the filament must operate and to the varietyi compounds with which it'has contact, thesurface conditions on it'change insuch-a manner that it operates ati'difierent temperatures at difieren't times; 7 This gives rise'tq the effects just explain ed and it constitutes a serious" disadvantage iniusing this t pe of electron -source.- 'Fo'r example? we have experimentally found-that the sensi-" tivity of the mass spectrometermay varyas much as" 0,3;per cent'per' de'gree Centigrade change 02 0 ion source temperature. r "Sensitivity jas here used means the ion intensities per'unit p're's'sure of ad mi-ttdgas. As we hav'eseenab ove, there are strong cisadvantages of a filament or-heated-catho'de typefof electron source when it is desired to operate mass spectrometer -under very-constant conditions. There are in addition-certain other disadvantages due to the frequent maintenancethis, type of electron sourc requires. Since the gas atmosphere in a mass spectrometer is crth order of 10 mm. of mercury,fwhi'ch is much-" higher pressurethan in a sealedvacuum tub e the? filament is subjected tothe emission iinhibitin effects of thedifferent gases Asa 'result the op at n i n et t mlj t mid e-ih ti a filament in avacuum tube of 'thef conventi-on type, in order to get sufficient emission. ;At .this" temperature; fan appreciable amount of tungstent evaporates; weakening the filament and 'ev ent u' 7 ally necessitating achange 'of filamentsya costly joband one which puts the instrument-outer 0P eration for at least several hours. I v Often, the gas being-eX-amin'edby am 3 spectrometer has oxygen'as'one of its constituents. 'It" I theinstrument.istoutofioperation.

, the

, photoelectrons to is well known that oxygen can poison certain electron emitting surfaces and that it can cause oxidation of tungsten filaments, thus shortening their lives. Another disadvantage of the tunesten filament is its fragility. For example, a filament-that has been ope gated fqr some time become' iivdyibrittl'land acslightfdisturbance will break it. This means that practically each time the'rfilament assembly is removed from the ion e fqr ihspection or repair, the filament must edbefore re-assembly. This is very undesirable; as it increases the time during which 'olo'ji'a'ct 0f Tthis invention to provide an gunf amas spectrometers, cathode ray sclllographs;"television equipment, electron minospop esa otl er devices, that does not have 7 sad ntage-s'named above which :are attendant te a thermionic emission type of electron source. i

:It sis 'a; further ilobj-ect of this invention to :pro-

vide': anielcc'trontgunsiori theabovenamed instruments which has certain,positiveadvantages;as

V will'Jbetapparent from the-followin p anatio over:conventional:"types ofauelectron gunswhich employ ith'ermionicverhissio sthe source of 616 trons-5.1 .111- a M H m- QT :1

It .is another object of thisinventiont provide I apparatus and .severalzxalternative means i er, the =C10SBI3YldafiOOl1TM1E CODtIfOITZOf anj electronstream, emergingirorn' thene'w ypeof, electron gun; describedlbelowa 1 A fu lther object dit s in on. s t eras means: of using on a cel a n vo ta e;

supply; ofz agrn'assspectrometer-to operate ournew t pe lectr nsunrwheoitis used. o e m s c-c ,1 thu lim'n tins ,t e-need- -ex a q m t fit lllanebhe eq iect .0. th in nti i top o- 1 ce..al asslspecirqms er av n an x al i t source rcaistinghlight throu hta Window in ,an envelope cohtainingr a photocathode, electron mul p ier-and.e ec ron e s f r o mi a stream or electrons f-orionizing gaseousand gaslike matins id nre o e. .Qfillfilhfiblfifii and: d nieces tog w t ntainwd ai'ls of,c n 'mfuctiona dmod ofrop re ion, lli ejz p al n irom heqi ol w s d 1 criptlq i a re rred emb d men o lth ai ne nsi lu trat d i th e accompanying drawings, nd nwh qhl. i F I i- I. Fi ur i aidia'sram illus ra n -th a l cas; ion otcur,,-in nti ras z qu l e'c e c o s n: a mass peptr mQ nd: 3 flieu evz sliows egty-p cfacont l i uitt att may ben d tore atez-t e:c r 'o p t'f m he lectroneu i m'a -c zlinz o i llrs 1 .51 1 raquumtisht. env p s ns". e onzswme-iandizelec rqn un s-v r pre n ed known 'mannemof I from'the lamp iffa'lli efemtt Thesephotoelectrons are drawn towards electrode" 8 by the potential difference between the cathode and the electrode 8. These electronsstrike electrode 8 with considerable velocity and cause secondary electrons to be emitted. The secondary electrons are emittedwith a small initial energy, of the order of a few volts, and as a result are drawn to electrode 9 by the potential difference between electrodes 8 and 9. At the surface of electrode 9, these electrons in turn cause the emission of more secondary electrons. As the surfaces chosen for the electrodes 8, 9, and so on, are such that the secondary electron emission coefficient is greater than unity, being preferably from 4 to 8, there is a multiplication at each successive electrode. As a result the electron emission from electrode i6 is very much larger than that from electrode '1. There are a number of factors that control the final current from electrode I6 and they will be discussed below when automatic controlling methods of the unit are described.

The operating voltages for the electrodes of the electron multiplier are obtained by tapping off suitable fractions of the resistances I1 and i8. The tapping off contacts are made so that each successive electrode is at a higher potential,- that is, 8 is higher that l, 9 is higher than 8, and so forth. These voltages may alternatively be obtained by suitably tapping ofi from a single resistor.

Electrons leaving the last secondary emission electrode it are focussed by means of an. electron lens. One way in which this maybe done is to accelerate them towards the cylindrical electrode 19 and towards the electrodes 20 and 2!. The three electrodes i9, 29 and 2| may be so arranged and have such potential distribution as to form a beam of electrons from those originating from electrode Hi. This illustrates only one of several types of electrostatic and/or magnetostatic electron lenses which may be employed.

The beam of electrons is made to pass between electrodes 22 and 23 and through an aperture in 23, to the electron collector electrode 24. A potential is maintained between electrodes 23 and 24 so that 24 functions essentially as a trap and secondary electrons from electrode 24 are drawn back to it. The voltages for the. focussing electrodes I9, 20 and 2! may be obtained-by taps from the resistor I8 and the potential between electrodes 23 and 24 may be obtained from taps on resistor 50.

In operation, the electron beam traversing the region between electrodes 22 and 23 creates ions by impact of the individual electrons on the gas molecules or atoms present. These ions, which are mainly positive, are drawn by the existing potential differences towards electrode 23. A slit in electrode 23 allows some of the ions to emerge into the region between electrodes 23 and 25. A large potential difference, of the order of a thousand volts is maintained between electrodes 23 and 25. The ions are thereby further accelerated and as a result pass through a slit in electrode 25 in a spatially well defined beam ofions 26 having the form of a IlbbOI'L' This beam of ions 26 goes to the magnetic analyzerthrough tube 44 and is separated into individual beams of different mass characteristics in the usual manner of a mass spectrometer. The gas to be analyzed enters the envelope I along the path indicated by arrow 4i through inlet tube 42 which is normally small in cross section, a vacuum pump being connected to tube 43.

As seen in Figure 1, all the voltages required to operate the ion source and the associated electron multiplier electron gun are obtained from one voltage supply shown as 21. It is of great advantage to be able to use the same high voltage supply for both the acceleration of the ions and also for operation of the electron gun.

In one mode of operation, the different ion masses to be found in the ion beam 26 are successively brought to the mass spectrometer collector by varying the magnetic field of the magnetic analyzer in a manner well known in mass spectrometry. Another mode of operation -is to bring the different ion masses to the collector by varying the total ion accelerating voltage. The former mode is to be preferred, however, as it lends itself to overall simpler operation.

It is very important to keep the electron gun current quite constant as the intensity of the ion beams will depend upon it. As mentioned above, there are a number of ways in which the current output of the electron gun may be controlled and in Figure 1 is to be seen one preferred manner. In this case, the electron collector 24 collects a certain fraction of the total electron emission from electrode it. The electron current collected by 24 is that which traverses the entire length of the ionization region. Current collected by electrode 24 returns to electrode 55 after passing through the high resistance 28. Variations in the current from electrode 24 will cause voltage variations across resistance 23 and these variations are conducted by lead wires 29 to the low voltage power supply 30.

The power supply 30, which is shown in more detail in Figure 2, responds to voltage variations in leads 29 in such a way as to maintain constant the electron current that passes through the ionization region and which is collected by electrode 24. The control is through the temperature of the filament of lamp 4, that is, an increase of voltage across resistor 28, due to an increase in electron current, will cause a drop in the filament temperature and conversely a decrease in voltage across resistor 28 will cause an increase in filament temperature of lamp 4. These temperature changes may be made in the direction to maintain a constant electron flow, since the number of photoelectrons initially leaving electrode 7 depends upon the light output of lamp 4.

Figure 2 shows the controlling circuit which utilizes and amplifies the signal on leads 29 in such manner as to control the heating current in lamp 4. Here 3i represents a first amplifier tube and 32 a second. When the electron current falling on collector 24 (Figure 1) exceeds its normal value it causes an increase of potential drop in resistance'28 (Figure l) which causes an increase of negative grid bias in tube'3l' of Figure 2. Grid bias battery 39 and potentiometer 40 provide grid voltage and adjustment thereof in tube 3] in conjunction with resistor 28 of Figure 1. The change in control grid bias of tube 3|, due to the increase of potential drop in re.- sistor 28 decreases the plate current of tube'3l, with the result that the potential drop across resistor 33 is decreased. As this drop controls the bias of the control grid of tube 32, the result of the above changes is an effective positive increase of the control grid bias in tube 32 with a consequent increase in its plate current. This increase of plate current will cause an increased potential drop across resistance 34. Resistance 34 controls the bias of the grids in both tubes 35 and 36, the net result being a decrease in the plate currents of both. The triodes 35 and 36,

together with the secondary windings of trans gamete formers .31 and I which .supply..the plat an heater, voltages, i form ,a, dissipative circuit,- controlled only by the grid voltage which is th e volt- .age drop across resistor 34.; V.

It is to, be noticedthatthe ,ransformer 3 which supplies the platevoltage for tubes 35 and 36, has its primary in serieswith thezprimaryof transformer 38 which suppliesthe, current ,for lamp 4., Thus, if .thecurrentfiow throughtubes 35 and 36 is reduced, it is equivalent to increasing theimpedance of the secondary of transformer 37., This in turn increases the impedance of the primary of transformer .31, withthe result, that the primarycurrent of both 31 and 38 is reduced. A reduction of, primary current, ,.of,transformer .38 reduces the secondary current with theresult that the light intensity of lamp 4 is reduced. This is the desired end effect, as itwill reduce the number of photoelectrons emitted by electrode] of Figure l.

If one assumes the opposite condition, that ,is, a decrease of electron current reaching electrode 24 and follows out the resulting changes of operation of the components of Figure 2, one finds that the end result is an increase of light from lamp 3 which would cause more initial photoelectrons to be emitted. Thus, We have the conditions satisfied forself regulation, that is, the electron current or stream of electrons creates a signal which is utilized to maintain the current constant. In Figure 2, power is supplied to tubes 3! and 32 through powertransformer 52 with rectifier 53 and filter components 56 in conventiori'alv manner. Screen voltages are obtained through resistors 55, 56, 51, 58whose voltages are i regulated by regulator tubes 45.

' The above circuit represents one methodwo'f achieving self control but it is not byany'mean's to be construed to be the only method by'which the result may be achieved, since it is intended to represent only one operable method ofmany. Other well knowntype's of regulatingscircuitsjmay be'used, and it is intended that their use;fall within the scope of this invention.

Although we have shown the'control sign'al coming from the current to the collector electrode 24 of Figure 1, there are other means of obtaining a control signal and it is intended that they all fall within the scope of this invention. For example, that portion of the electron current which is collected by electrode'Zl, Figure 1, may be used for'a control signal and similarly for the currents collected by electrodes 23, or 59. Furthermore, the final means of control, which is the temperature of the filament of the lamp 4 in the modeof operation explained above, can be any of several possibilities each of which falls within the intended scope of this invention. For example, the control signal may be utilized to control a power supply so that a decrease of electron current would cause an increase of voltage betweenthe electrodes of the electron multiplier and, conversely, an increase would cause a decrease (of voltage. The circuit for such a control-would-be familiar to those versed in the art.

Although we has illustrated our invention with one arrangement of electrodes, it is not to 'be limited to this arrangement but rather applies to all such configurations as admit multiplication of an electron current by secondary electron emission. Also, we have shown our new type electron gun as applied to a mass spectrometer; however, it is not to be construed that this places a limitation on its use, as other applications, as in elecitelevisiomequipment and other useaarc nte ded- ,Qur illustration also shows an electron multi-. plier, the initial electrons of ,which are thosedue torhotor s o it. is 'not...i.ntended that this invention be limited to electron multipliersof this typ ut-sh u d em ra a l typ s, i udin thos wherethe initialelectrons are due to thermionic emission, electrons from radioactive transforma ions.=-andsoiorth..... V

.Some positiveadvantages of, our new type of electron gun. may be mentioned. Our improved new electron gun has unlimited lifetime and does not need to: be periodically replaced as does a tungstem or, other typeoi filament. Our new type of electron gun mayrun cold, thus eliminating two; undesirable ieatures, of thefilament type gun, namely, the changeof crackingv pattern due to pyrolysis onthe filament. surface and the overall, rise of-gtemperatureof the ion source which af ects instrument'sensitivity. Our .new

type of electron gun issturdier. -If it is desired to remove, it for alteration of the instrument, it contains no sensitive, brittle, filament that. is

easily broken. ,Our new type of electrongun, can

be put into operation immediately ,afterapplying thoylight vsignalandthe necessaryyoltages; it requiresno warm-up time for proper emission, as do some thermionic emitters; also, it does not necessitate a lengthy wait for overall temperature equilibrium, as do some thermionic emitters.

In our Figure l, we have shown an electron gun a applied to'a mass spectrometer. While we have shown-no separatepvacuum pumping line direct to the electron gun, it may, under certain conditions, be advisable and it is intended that our invention. cover such modification. "Also," it is intended that the'scopeof this invention en compass the situationwherein-the electrodes of the new type electronrun hot, that is,'at whatever elevated temperature is desired. Often it is desired to operate a mass spectrometer with the ion source hot and in such a case the electron gun mu'st b'e at the same temperature. The heating in this instance could be attained either using suitable insulated internal heaters or by using an external heaterof high resistance wire suitably applied'to thevacuum jacket.

Although We have described "a preferred'embodiment of our invention in specific terms, itis to be understood that various changes may be made in the size, shape, materials and arrangement without departingfrom from the 'spiritand scope of the invention as claimed.

tron microscopes, cathode ray o'scil-lographs, 7

What we claim as our invention is:

l. A mass spectrometer comprising an envelope havin a. light permeable window, a photosensitive cathode in said envelope for receiving light entering through said window, a plurality of multiplying electrodes in said envelope, means comprising at least one electron'lens element for focusing. electrons from said multiplying electrodes into an. electron stream, a collector electrode disposed in the path of said streamfor collecting" electrons therefromgmeans for admitting molecules into said envelope into the path of said electron stream for ionizing said molecules, and analyzer means including a magnet for analyzing said molecules.

2. A mass spectrometer comprising an envelope, light permeable means for permitting light to enter said envelope, a photocathode disposed in said envelope in the path of said entering light, a plurality of electron multiplying electrodes in said envelope in cooperative relationship to said photo- 'cathodafmeans comprising at leas'tone electron lens element for focusing electrons from said multiplying electrodes into an electron stream, a collector electrode disposed in said envelope for collecting electrons from said stream, means for admitting gaseous and gas-like matter into said envelope, means for moving said matter through said electron stream whereby at least a portion of said matter becomes ionized, analyzing means operating upon at least a portion of said ionized matter for analyzing said matter, and self regulating means constructed and arranged for maintaining the magnitude of said electron stream at a predetermined level by regulating the light entering said envelope.

3. In a mass spectrometer, the improvement which comprises an external light source, a light permeable window in the envelope of the mass spectrometer, at photocathode within said envelope for receiving light from said external source, electrodes for forming an electron stream, self regulating means constructed and arranged for maintaining the magnitude of said electron stream at a predetermined level by automatically regulating the light entering said envelope.

4. A mass spectrometer comprising an envelope, means for admitting molecules to be ionized into said envelope, means for producing and forming an electron stream in said envelope for ionizing said molecules, said means including a photocathode actuated responsive to light entering said envelope from an external source, an electron collecting electrode upon which said electron stream impinges self regulating means actuated responsive to the current from said collecting electrode and constructed and arranged for maintaining the flow of electrons in said stream at a predetermined level by regulating the said light that enters the envelope, means including a magnet for analytically operating upon said ionized molecules, and means for moving at least a portion of said ionized molecules into cooperative relationship with said last named means.

5. A mass spectrometer comprising an envelope, means for admitting molecules to be ionized into said envelope, an external source of light, means for producing and forming an electron stream in said envelope for ionizing said molecules, said means including a photocathode actuated responsive to light entering said envelope from said external source, a plurality of electron multiplier electrodes in the path of electrons emitted by said photocathode, electron lens means cooperating with said multiplier electrodes for forming a fast moving, relatively concentrated electron stream in the path of said molecules being admitted whereby ions are produced, an electron collecting electrode upon which said electron stream impinges, means including a magnet for operating upon and analyzing at least a portion of said ions, and regulating means actuated responsive to the current from said collecting electrode for maintaining the magnitude of said electron stream at a predetermined level by regulating the intensity of said external light source.

NORMAN D. COGGESHALL. NATHAN F. KERR.

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

UNITED STATES PATENTS Number Name Date Re. 21,907 Balsley Sept. 30, 1941 1,961,703 Morrison June 5, 1934 2,149,080 Wolff Feb. 28, 1939 2,181,720 Barthelmy Nov. 28, 1939 2,240,713 Orthuber et al May 6, 1941 2,373,151 Taylor Apr. 10, 1945

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