US2611875A

US2611875A - Electron source

Info

Publication number: US2611875A
Application number: US52341A
Authority: US
Inventors: Harold W Washburn
Original assignee: Consolidated Engineering Co Inc
Current assignee: Consolidated Engineering Co Inc
Priority date: 1948-10-01
Filing date: 1948-10-01
Publication date: 1952-09-23
Anticipated expiration: 1969-09-23

Description

P 1952 H. w. WASHBuRN 1 ELECTRON SQURCE Filed-Oct/l, n48

an AMPL/F/ER IN VEN TOR. HAROLD m mas/mum gwww A T TOR/VE Y Patented Sept. 23, 1952 ELECTRON SOURCE Harold W. Washburn, Pasadena, Calif., assignor to Consolidated Engineering Corporation, Pasadena, Calif., a corporation of California Application October 1, 1948, Serial No. 52,341

4 Claims. (01. 250-419) I This invention relates to mass spectrometers in general and in particular to improvements therein relating to the formation of an electron beam for the ionization of sample molecule in the mass spectrometer.

A mass spectrometer is an analytical apparatus which functions to sort and measure ions. Ordinarily it includes an ionization chamber into which molecule of the sample to be analyzed are introduced. In the ionization chamber the molecules are bombarded by a stream of electrons and converted into ions. So called Dropelling or accelerating electrodes propel the ions from the ionization chamber into and through an analyzer chamber.

During passage through the analyzer chamber the ions are subjected to a transverse electric or magnetic field or both, to separate them according to their mass-to-charge ratios into a plurality of diverging beams of ions. Each ion beam is composed of ions of the same specific mass and differing from the mass in the other beams. The diverging beams are successively focused on an ion collector by varying the potential applied to the accelerating electrodes or by varying the intensity of the transverse field in the analyzer tube. The current produced by the discharge of each beam on the ion collector is a measure of the partial pressure of the molecules (from which the ions were derived) in the sample being analyzed.

In one type of mass spectrometer, ions are formed within the ionization chamber by an electron beam directed through the chamber at right angles to the path of the ions. A conventional instrument is equipped with a ribbonfilament for the production of electrons which are directed through a slit in the wall of the ionization chamber transversely across the chamber and through a slit in the opposite wall to impinge on an electron collector or target. In this arrangement the ribbon produces an electron beam with a sufiiciently large cross section that warpage or movement of the filament still permits the size of the cross section of the beam to be determined by the slit, commonly referred to as the electron slit. The filament is made in the form of a thin ribbon so that an electron beam of large cross section is produced with low end effects by a reasonable length of filament. The thin ribbon also gives a reasonably low power consumption.

I have now found that another possible arrangement is to use a filament of sufficiently small cross section so that the thickness of the electron beam is determinedby the size of the filament rather than by the size of 'the'electron slit. This arrangement has the followinggad vantages over the conventional electron..,gun,' described above: i

1. For the same emissiondensity and the, same cross section of the electron beam, the watts consumed by the small filamentare less due to the small radiation area; I y

2. The filament can be made sufficientlysmall that reasonable wattage .consumption canbeyobrtained when using a filament with a circular cross section. In this respect the circular cross section has the advantage of having a longer life.

The reason for the longer life of a:"fi lament with a circular cross section may be appreciated from the following comparison. For example, comparing the life of an 8 mil diameter filament. with the life of a 1 by 40 mil fiat ribbon filament, considering that the life is determined by evapoe ration, it will be seen that the roundfilamentwill have a life corresponding to at least ,oneymil evaporation of tungsten. Onthe'other hand, it would probably be necessary to discontinuert use of the fiat ribbon filament after 0.1 mil had evaporated.

There is one problem which becomes.,mo re. acute, but for which I have provided asimple solution when the physical size of the filament; rather than thesize of the electron slitis used to limit the thickness of the electron beam. When the thickness of the beam isto be limitedby the. size of the filament and where the filament is a magnetic field of low magnitude, say .500 gauss. or less, it is necessary toprovide some sortof. electric field focusing. 1

There are many means of obtaining this elece tric field focusing but a preferred method inac-i cordance with the invention is to mount a shield, either flat or cupped, on the side of the filament opposite the electron slit. By adjusting the voltage on the shield it is possible to .driveiall-of the electrons, emitted from the-filament,;back-' into the filament except for a narrow strip on the electron slit side of the filament. At the same time it is possible to produce an electrical field on the emitting side of the filament which is substantially parallel to the desired direction of motion of the electrons or I may produce a field which is slightly converging so that any side. energies imparted to the electrons are in such a' direction that thethickness of the electron beam is reduced. This latter feature is highly important since any side energies which will thicken the beam and make it larger than the filament will impair the resolution of the mass spectrometer.

In accordance with the invention therefore, I provide in combination with a mass spectrometer having an ion source comprising an ionization chamber, a slit in a wall thereof for introducing electrons into the chamber and a sample inlet tube, the improvement comprising an electron emittingfilament of small cross section, preferably circular, mounted adjacent the ionization chamber and aligned with the slit, and a focusing electrode disposed on the side of the filament opposite the ionization chamber and being at a potential such as to drive the electrons emitted from the filament back into the filament except for a narrow strip on the ionization chamber side of the filament. I

As pointed out above, this arangement serves to determine the dimensions of the electron beam and the size of the slit through the wall of the ionization chamber is no longer critical. The invention will be better understood from the following detailed description thereof taken in conjunction with the accompanying drawing in which:

Fig. l is: a diagrammatic. view of a conventional 180 mass spectrometer showing the ionization chamber: end of the instrument blown up to approximately three times its comparative size;

' Fig. 2 isa line diagram of the electric. field produced'by the filament shield and the electron slit when the voltages are properly adjusted.

Fig. 3 is a sectional elevation of an ion source as described by Clifiord E. Berry in United States patent application Serial No. 33,596 filed June 17, 1948, showing means of employing the electron source of the invention in conjunction therewith;

Fig. 4 is a section taken on the line 4-4 of Fig. 3; and

Fig". 5 is a perspective view of the electron source'of the invention mounted as described by Gordon'W'. Pallette in United States patent application Serial No. 40,481, filed July 24, 1948. In the drawings the invention is shown in association with a 180 mass spectrometer of the type wherein propelling electrodes are provided in the. ion source. As will be apparent from. the following description, the invention is equally adapted to incorporation in substantially any mass spectrometer wherein the sample to be analyzed'is ionized by means of an electron beam. The invention is independent of the particular focusing means, the path of ion travel or the use-of accelerating electrodes.

Referring to Fig. 1, it will be observed, that it shows a mass spectrometer having an ion source ll, an analyzer tube I2, and an ion collector l3 disposed within an envelope M which is'kept at low pressures during the operation of the instrument. The analyzer tube I2 is provided at the end adjacent the ion collector l3 with an exit slit I through which the ion beams are focused onto the collector IS.

The spectrometer is provided with a pumping system or envelope exhaust line [6 which may be connected with a mercury diffusion pump, molecular pump, or any appropriate evacuating system not shown. The analyzer tube i2 may be provided with ports 11, by means of which the ion. source and the analyzer tube are evacuated through. the envelope. Alternatively, the envelope may be ommitted by making the analyzer tube l2 gas tight and, attaching the pumps to it by means which are well known. An inlet line l8 provides means for introducing a sample to be analyzed. An electron gun 20 in accordance with the invention is mounted adjacent to or as part of the ion source and cooperates with an electron target or catcher 21 to discharge a beam of electrons through an electron slit 22 in a wall of the ion source adjacent the gun 20 and a second electron slit 23 in an opposite Wall of the source adjacent the collector 2!. A propelling electrode 25 and accelerating electrodes 26, 2'! are disposed within the ion source so as to propel the ions formed therein into and through the analyzer tube. Representation of this particular ion source is for illustrative purposes only, there being many modifications thereof, equally applicable to this invention.

As above described, the ions propelled from the source are formed in the analyzer tube into diverging beams of ions of given specific mass. These diverging beams are successively focused on the ion collector l3 through the, exit slit [5 by varying the potential applied to the accelerating

electrodes

26, 21. The ion collector i3 is linked with an amplification and sensing circuit by a lead 29 sealed through the wall of the envelope l3. The lead 28 connects the collector 20 to an amplifier 30 which is in turn linked to a sensing means (not shown)v such as av recorder or the like.

Accelerating

electrodes

26, 21 are connected through leads 32, 33 respectively to a conventional voltage supply circuit. In a similar manner the propelling electrode 25 and the electron gun 20 and catcher 2| are connected to a more or less conventional voltage supply circuit.

The electron gun 20 shown in Fig. 1 comprises a filament 34 preferably of circular cross section and disposed adjacent the first electron slit 22. An electron shield 36 is disposed adjacent the filament 34 on the side oppositethe electron slit 22. A focusing electrode 31' is disposed between the filament 34 and the slit 22 and has a slit 31A therein in alignment with the filament and electron slit 22. By establishing the proper potential between the shield 36, the filament 34, and the focusing electrode 31, the electrons emitted from the filament may be driven back into the filament except for a very narrow beam emanating from the filament from the side adjacent the focusing shield. In this manner, and by making the slit width greater than the filament diameter, the electron beam width becomes independent of the dimensions of the electron slits 22 and 31A.

The electric field established around the filament 34 is illustrated in the line diagram of Fig. 2 showing the filament 34, the focusing electrode 31 and the shield 36. Representative potentials are shown; the shield being at .50 volts, the filament at 0 volts and the focusing electrode at +20 volts. In the drawing of Fig. 2 the shield 36 is shown as a fiat surface rather than in the arcuate form shown in Fig. l. The shield electrode 36 may be either flat or arcuate, the fiat form being shown in the diagram of Fig. 2 to simplify the illustration of the field forces set up thereby.

Referring to Fig. 2 it is seen that a portion of the electrons emitted from the filament 34 are driven back into the filament. The phenomena is illustrated by lines 40, 4|, 42 etc. Another portion of the electrons emitted from the filament 34 are propelled in the direction of the focusing electrode 31 and converge towards each other. The electric field controlling the motion ofthe electrons is represented by the

lines

43, 44. and 45.

Ii Theillustration of Fig. l is: diagrammatic only in that it does not show specific means for mounting the electron source of the invention with respect to a specific ionization chamber. In Fig. 3 I have shown in sectional elevation anion :source ofthe type described by Clifford E. Berry in United States patent application Serial No. 33,596,

'filedJune 17, 1948. This particular ion source is chosen as a means of illustrating the application of the electron gun of the invention to an ion source but is not intended to limit the invention in its application to any particular source. As described above, any ion source will comprise a substantially enclosed chambenkcommonly re ferred. to as an ionization chamber, having an inlet'and an outlet and atleast one electrode for discharging ions from. the chamber. after they have been formed therein. The electron source of the invention is adapted to be employed with any ion source of this nature. In Fig. 5 the electron source is shown. in a perspective view as mounted in accordance with mounting means described by Gordon W. Pallette in co-pending respect to the ionization chamber and means'of applying differing potentials thereto is described in detail in the aforesaid co-pending application of Clifford E. Berry and forms no part of the present invention.

As shown in Fig. 4 the body of the ion source 60 is generally cylindrical in section. A portion of one wall of the source is cut away to form a fiat face extending from the lower end of the ion source (Fig. 3) to a point substantially in lateral alignment with the outlet end of the ionization chamber 6|. An arcuate shield 12 forms with the face 10 an enclosure H open at itslower end and closed at its upper end by an overhanging lip 13.

The electron source itself (see Figs. 3, 4 and 5) comprises a filament or wire 16 preferably of circular cross section attached at opposite ends to terminal posts Ti, 18. Preferably the terminal posts are sprung inwardly a small amount when the wire is afiixed'to their top end so as to produce an expanding tension on the filament. The terminal posts are sealed through an insulating base 80 and project through the opposite side of the base for connection to suitable electric leads (not shown). A post 82 is sealed in the base 80 intermediate the terminal posts I1, 18, and supports at its upper end (see Fig. 5) a positioning member 84. The positioning member is shaped to fit within a receptacle 86 formed centrally on the flat face 10 of the ion source body. As described in the aforementioned co-pending application of Gordon W. Pallette, by fitting the positioning member within the receptacle and affixing the same thereto the position of the filament is fixed with respect to a first electron slit 94 in a wall of the ion source. A diametrically opposite slit 95 opens into a cavity 96 in which is disposed an electron catcher 98.

6 As shown in Figs. 4 and 5 and as described by Pallette, the posts 82 upon which the positioning member is mountedisdisplaced forwardly from the plane intercepting'the terminalposts-H, .18. This permits insertion of the positioning member in the receptacle 96 and proper spacing; of the filament 16 with respect to its lateral displacement from the wall of the ionsource. Inaccordance with the. invention the mounting means is provided with a pair of auxiliary conductive posts I00, Hll sealed through the base and extending parallel to the terminal posts H, 18 andgin-termediate these posts and the center post 82. A semi-circular electrode 104 is mounted-to the upper end of the posts 100, NI soas to exten partially around the filament l6. I In this embodiment of the inventionthe electrical shaping field around the filament 1,6. is formed by the electrode I04 and by thewall of the ion source 60. The focusing electrodei, e. the electrode 31 (Fig. 1) is omitted. The useof additional focusing electrode 1 t optionalfeature of the invention since the; ions ource itself may be established at a potential such as to accomplish the purpose of the focusing electrode. As above described the. electrode 36 (Fig.4) or, the electrode 104 (Figs. 3, 4 and 5) may be eitherarcuate in section as shown in Figs.-;l, 3, 4 and 5 or it may comprise a fiat plate as. illustrated schematically in Fig. 2. Referring tothe electron source shown in Fig. 5it will be necessary, if using a fiat electrode to replace the electrode 104, to offset the conductive posts 100, I9! rearwardly as viewed in Fig. 5. This may be easilydone so as to disposelthelfiat electrode in a planelying behind the filament l6 adistance approximately equal to the spacing between the filament nd the arcuate electrode I04. By using an electron source of the type described, regardless of the means by which it is mounted to the ion source or regardless of the type of ion source employed, the dimensions of the first electron slit 94 communicating between the electron emitting filament and the inside of the ionization chamber are no longer critical. I-Ieretofore the shaping of the electron beam has been determined by the dimensions of this slit Which were therefore highly critical.

The invention has been illustrated and described with reference to the use of a filament of circular cross section. Although use of such a filament constitutes preferred practice, filaments of irregular section or comparatively narrow ribbon filaments are satisfactory. In any case, filament dimensions should be selected, such that the size of the electron beam will be determined by the filament in association with the shaping field irrespective of the size of the electron slit in the ionization chamber wall.

One feature of the Pallette mounting means which makes it particularly advantageous for use with the electron source of the instant invention is the accuracy with which its positioning may be fixed. Since the character of the electron beam is determined in my electron source by the size of the filament and by the shaping field produced between the shield electrode and the ion source or auxiliary focusing electrode, the positioning of these elements with respect to the ionization chamber or the slit opening thereinto is important. Although Pallette'shows one excellent means of establishing a fixed relationship between the electron emitting filament and the ion source as such, any such means are within the contemplation of the invention.

Rslitiiini a iawall'fthe'reof for: admitting: a beam of selectrons xinto the vchambenan electron: emitting filament bfismaller crossisection' than theminor we axis of the slit bpeni'ng mounted adjacent the iiionizati'om chamber and "in alignment with the slit: and means for producing aishaping electrical lifield around'i the filament toform" the electrons -='emitted from the fiiament into an electronbeam of smaller cross seetion than the 511mm directed throughthe slit.

' --"2; -Ina 'mass 'spectrometer ion' source for use in a relatively weak magnetic-'fieldythe combina- "tion comprising an ionization chamberhaving a @slit' inswan thereoffor admitting abeam of reieetrons'in'to the chamber, arr-electron emitting filament or 'cireular'- cross section and; of smaller.

diameter than 'the-minor axisof'the slit opening "mountew adjacent" the ionization chamber" and in alignm'ent' with the" slit; a: semi-cylindrical electrode mounted coaxialiy with "the filament "andorftheside thereoi opposite the ionization i chamber; and means for impressing a potential "-"ori' the electrode to direct the electrons emitted by the" filament througntheslit' as a' beam of smaller *cross section than the slitopening.

3'; "In-amassspectrometerion source for use in a'relatit'elywe'ak magnetic field; thecombinatioircomprising tan-ionization chan'iber having a slit? in the -wa1l"thereof= for admitting a beam'mf *eleactronsintd-thechamber, anelectroni'emitting -"filam'ent'of smaller cross s'ectiomthan'the minor 0 taxisfoi" the" slit opening mounted adjacent the chamber and 'in" alignment with .the "slit," "and means forv producing af'shaping electrical" field iaround the filament i r to i-.'form the r electrons 1 emittedrfromthe filament into anelectron beam 1 :of smaller cross section than the slit :and directed .throughrther slit.

* 4; In a mass. spectrometer ionsourceclforuse 1 inc; relatively weak 'magnetic field, the combina- .uslit; opening mo'unted adjacent the: ionization chamber; and in alignment with: the 's1it,na first ..zelectrode.spaced 'from the filament: Eon the side opposite the ionization -ohamber, .a second reiecl5- 'tionxcomprising an ionization chamber: having a t slit in a2wall thereof for'admitting electrons into the :chamber; an electron emitting filament of 'smallericrossfsection thanathev minor axisiof the trade rdisposed between thefilament' vand the t: ionization chamber and; having a slit thereini-in alignm'entswith the slit in the'ionizationrchamriser and defining an opening having a minor axis 7 longer; .thanfthe cross 1 section 20f the electron 20 emitting. filament, and rmeans for: 'impr'essing-:.a potential between the" first and second-electrodes toxsha'pe the electrons:emittedi from the-filament into: a beam directed through the slits: in the 9 second electrode and .in thezioniza-tion chamber 25 and :of .a width independent oi. the dimensions of the slits.

.. 2 HAROLD WASHBURN.

--REFERENCES CITED Thefollowing references are of record in the 'UNITEDTSTATES" PATENTS