US2662184A

US2662184A - Mass spectrometry

Info

Publication number: US2662184A
Application number: US228219A
Authority: US
Inventors: Clifford E Berry
Original assignee: Consolidated Engineering Co Inc
Current assignee: Consolidated Engineering Co Inc
Priority date: 1951-05-25
Filing date: 1951-05-25
Publication date: 1953-12-08
Anticipated expiration: 1970-12-08

Description

Dec. 8, 1953 Filed May 25, 1951 AMPLIFIER 30 c. E. BERRY,

MASS SPECTROMETRY SHIELD 36 3 Sheets-Sheet l INVENTOR. CLIFFORD E. BERRY ATTORNEY Dec. 8, 1953' c. E. BERRY 2,662,134

MASS SPECTROMETRY I Filed May 25, 1951 3 Sheets-Sheet 2 FIG. 3.

A SH/ELD CONNECTED J 70 F/LAMENT 1/0- 0 m0 8 SH/ELD AT /2V. W/TH RESPECT k T0 F/LAME/VT Q 90- L] 1 g 80- U 50 T0 REGULATED SHIELD an I F/LAMENT CURRENT- AMPS JNVENTOR,

. CLIFFORD E. BERRY ATTORNE V Dec. 8, 1953 c. E. BERRY MASS SPECTROMETRY Filed May 25, 1951 3 Sheets-Sheet 5 Fl 6. 4. FIG. 5.

E /5 ELECTRON BEAM T 7 if 46 s4 \-/36 ELECTRON BEAM ELECTRON BEAM INVENTOR. CL IFFORD E. BERRY D. C. AMPLIFIER ATTORNEY Patented Dec. 8, 1953 MASS SPECTROMETRY Cliflord E. Berry, Altadena, Calif., assignor to Consolidated Engineering Corporation, Pasadena, Calif., a corporation of California Application May 25, 1951, Serial No. 228,219

12 Claims. 1

This invention relates to mass spectrometry and mass spectrometers and particularly to ionization of sample molecules by an electron beam and methods and apparatus for regulating the intensity of the ionizing electron beam.

A mass spectrometer is an analytical apparatus which functions to sort and measure ions. Ordinarily it includes an ionization chamber into which molecules of the sample to be analyzed are introduced. In the ionization chamber the molecules are ionized as by bombardment with a stream of electrons, the extent of the ionization being dependent in part upon the intensity of the electron beam. So-called propelling or accelcrating electrodes propel the ions from the lOIllzatio'n 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 with each beam being composed of ions of the same specific mass and differing from the specific mass of ions in the other beams. The diverging beams are generally successively focused on an ion collector by vary ing the potential applied to the accelerating electrodes or by varying the intensity of the transverse field in the analyzer tube. In a fixed focus or monitoring instrument a particular beam of interest may be continually focused on the collector electrode and there is no need to scan the ion spectrum by variation of the parameters-affooting the focus thereof. The current produced by the discharge of an ion beam on the ion collector is a measure of the partial pressure of those molecules in the sample from which the given ions were derived.

In one type of mass spectrometer, in comparatively general use, ions are formed within an ionization chamber by an electron beam directed through the chamber at right angles to the path of the ions and parallel to the direction of the magnetic field. The ionizing electrons may be developed by an electron emitting filament either in the form of a ribbon or as a thin wire, the electrons being collimated as a beam and directed through a slit in a wall of the chamber, transversely across the chamber and through a slit in the opposite wall to impinge on an electron target electrode.

In co-pending application, Serial No. 52,341, filed October 1, 1948, now Patent No. 2,611,875, by Harold W. Washburn, consideration was given to the problem of collimating the electron beam.

In that application the advantages of using a thin wire filament over a ribbon filament were explained, and means was disclosed for developing a collimating electrical field to make possible the use of the preferred filament.

In mass spectrometry it is desirable to hold electron emission from the heated filament of the electron gun at a substantially constant value independent of variations in the filament power source and in the emitting properties of the filament. Variation in electron emission and hence in beam intensity introduces a corresponding error factor into the analysis as a consequence of correlative variation in the degree of ionization. conventionally the desired control of electron emission is accomplished by electronic feedback means which automatically applies a compensating-adjustment to the power supplied to the electron emitting filament. Such an electronic system adds appreciably to the cost and complexity of the instrument; 'Moreover, any change in power to the filament results in a corresponding change in filament temperature. Such result is undesirable as placing an increased load on the ion source temperature control means.

I have now found that electron emission can be controlled by the'development of a negative field adjacent the electron filament and on the side thereof opposite the ionization chamber. The invention in one aspect comprises a method of producing ions within a space which comprises admitting molecules to be ionized into the space, causing emission of'electrons from a filament adjacent the space, directing the electrons as a beam through the space, collecting electrons at a target electrode spaced from the filament, applying-a negative voltage to a shield electrode adjacent the filament and ion the side thereof opposite the target electrode and controlling the negative voltage as a function of the current developed at the target electrode.

Several means are illustrated and describedlfor carrying out the foregoing method, the invention contemplating in addition to the method an ion source comprising an ionization chamber having a first slit in a wall thereof for admitting electrons and a second slit in a wall thereof for exit of electrons, the combination comprising an electron emitting filament mounted exteriorly of the ionization chamber in alignment with the first slit, a target electrode. mounted exteriorly of the ionization chamber in alignment-with the second slit, a shield electrode mounted adjacent the filament and on the side thereof opposite the first slit, means for impressing a negative voltage on the shield electrode and means for varying the magnitude of the negative voltage as a function of the current developed by electron discharge at the target electrode.

In the aforementioned co-pending application it was proposedfitoruse a shield electrode in substantially thesame' position as the shield electrode in the presently described apparatus and a focusing electrode intermediate the filament.

and the first slit, and to apply to the shield electrode a negative voltage and to the focusing electrode a positive voltage with respect: to, theJfila=- ment so as to develop a shaped fieldfforfocusing the electrons emitted from a thin wire filament through the slit in the ionizationchamber. wall. The purpose of focusing the electrons by means of a shaped field was to avoid the necessity off The invention will-be. more clearly understood from the following detailed-description taken in conjunction with the-accompanying, drawings in which:

Fig. 1 is a-Ldiagrammatic view" of a conventional 180- mass spectrometem Fig. 2 is an, enlarged section taken on the line 2'---2 of Fig. 1;. c

Fig. 3.-is a graph showingxthe effect on anode or target electrode *currentofavarious connections of theshield electrode; j

Fig; 4" is a: circuit diagram showing; one. means of connecting the'shieltl electrode to regulate filament, emission in. accordance withrthe: invention;

Fig:v 5 is; a; circuit. diagram of: an alternative means for accomplishingthis regulation;

Fig. 6 is a circuit diagram of another means of accomplishing; this: regula'tidm; and' Fig: 7 is a diagraniiiofc amalternative form; of regulator circuit? including: amplification meansz.

Im the; drawingsthe'. inventiom iSTShOWIl'. irr. as sociatiorr with: a. 180"" massi spectrometer; of' the type wherein propelling electrodesi are: provided i111 theI-ioir source. 'Aswill betapparentt from the following; description; me invention? is equally adapted: to incorporationzin substazrrtially' any mass: spectrometer: whereimtiie. sample: to be analyzed isionized by meansorzanielectrombeann It: is also. apparent that the: invention. is: not limitedito-thefieldcoffmass spectrometry but: will find: application in. ion source. iniwhich: uniformity of an ionizing-:eledtrmr beautis sought-i.

- InaFigsr. L and; thermis shcwnialmass-spectrometerilnhavingaan iorcsourcer than analyzer tube t2; an: ion: collector. [$.1a112disposdz within anenvelop e: I 4 whiclris keptrat lowpressuresi'dur ing' the.- operation-0f. the; instrument. A trans:- verse magneticifield isxestahlishediin1 the; analyzer tub e by conventionalmagnet: means: (not shown).- mounted exteriorlyr ofi the. envelope; The analyzer tube l2: provided atzitsend: adjacent the ion; collecton BJWith: an; exit: slit 55 through which; thezionibeamsareafomsed?omthacollectmi electrode; I3.;-

The; spectrometer is; with: a. pumping system or envelope exhaust line [5 which may be connected with a mercury diffusion pump, molecular pump, or any appropriate evacuating system (not shown). The analyzer tube 12 may be provided with ports I! by means of which the ion source and the analyzer tube are evacuated through the envelope. Alternatively the envelope may be; omitted by making the analyzer tube gastight and by attaching the evacuating system directly to the analyzer tube 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 mountedf adjacent to or as part of the ion sourceand cooperates with an electron target 1 2iI-to discharge-a beam 19 of electrons through a first.v slit 22,in.a.wall of the ion source adjacent the gun 2D and a second slit 23 in an opposite wall of the source adjacent the target electrode 2|.

Bropelling. electrodes 25 and accelerating electrodes26g21 are disposed within the ion source to expel the ions formed therein into the analyzer chamberand at a sufficient velocity to carry them through theanalyzer chamber. Representation of this particular ion source is for illustrative purposes only, therebeing many modifications thereof equally applicable/to this invention.

Asabove described, the ions propelled from the source are formed in-the analyzer tube under the influence of the transverse magnetic field into diverging beams of ionsof given specific mass. Thesediverging beams arev successively focused on'the ion collector l3 through the exit slit l5 by varying the-potentials applied to the accelerating electrodes 26, 2.1. The ion collector I3 is linked with an amplification and sensing circuitvby a lead 29 sealed through a wall of the envelope l3 andconnectedexteriorly to an amplifier 30 which is,1in turn; linkedto sensing means (not shown) suchas arrecorder or the like.

Acceleratingelectrodes

26, 21 are connected throughleads 32, 33'toaconventional voltage supply circuit (not shown). In a similar manner tliepropelling electrode 25 may be and generally is connected into the-same voltage supply circuit.

The electron-gun shown: inFig. 1 comprises a filament 34' preferably of circular. cross section and. disposed adjacentthe first electron slit 22. A shield electrode 3.6 is disposed adjacent the filament 3.4 onthe. side opposite the slit 22. The shield: electrode 36. is=shown :as arcuate insection. However; there isno requirement in the present invention that: this' electrode assume any particular shape:

The filament34-,-shield electrode 36 and electronitar'getrzll are interconnected in accordance with the invention in onet-of a-number of circuits whereby the shield 36$is maintained at a negative potential with respect to the filament. 34 and the magnitude:ofthenegative voltage applied to the shield at isdetermined as-a function of a bias voltage'source and 'of.the1current collected at the target: '21:: Various circuits for accomplishing thisz'purposeareiillustrated in Figs. 4,5, 6 andfl, hereinafter discussed 2 Reference to; Fig. 3 will giVeTa clear understanding'of the objectives of the. invention. Fig. 2 shows-"the variation in anode current. (abscissa) as-a functionof filamentcurrent (ordinate) unthe, relationship. between. filamentcurrent and anode current with the shield held at a constant potential of minus 12 volts with respect to the filament. Curve C shows the relationship of filament current and anode current with the shield regulated in accordance with the present invention. The slope of the plateau in curve C is such that a 3.3% change in filament current causes a 1% change in anode current.

It is plain from curves A and B of Fig. 3 that for a given filament current, the anode current decreases rapidly as the shield is made more negative with respect to the filament. It is this characteristic that enables the shield electrode to be used as a regulating element to control the anode current.

One circuit for energizing the electron gun and controlling the voltage applied to the shield electrode is shown in Fig. 4. In Fig. 4 and in the succeeding figures, the shield, filament, and

target electrodes are given the

numbers

35, 34, 2|,

respectively, to conform these figures with Fig. 1. The target electrode 2| is connected directly to the shield 36 through a bias battery All which delivers a negative voltage to the shield 36. A resistor 42 and batteries 54, 36 are connected between the target 2| and the filament 34 with the ion source connected between the

batteries

44 and 46.

The operation of the circuit of Fig. 4 is as follows: The filament is operated so as to supply an excess of electrons, and the filament shield is 4 biased negative with respect to the filament to a suitable operating point by th battery 40. The anode current flows through the resistor 42 which is of comparatively high resistance, variations in the anode current causing corresponding variations in the voltage across the resistor. These voltage changes are directly coupled to the shield electrode 36 and are in such a direction that an increase in anode current causes an increase in the negative shield voltage. The effect of an increase in the negative shield voltage is to decrease the potential of the space between the filament and electrode I, which has the effect of decreasing the current to the target 2|. Thus any tendency for the target current to change is counteracted by the action of the shield.

A somewhat different circuit arrangement is shown in Fig, in which target electrode 2| is connected directly through

batteries

44 and 40 to the shield electrode 35. The resistor 52 and the filament battery 46 are connected to the anode 2| intermediate the batteries llland 44, and the ion source H is connected intermediate resistor 42 and battery 56.

The operation of the circuit of Fig. 5 is similar to that of Fig. 3, the only differenc being that the particular arrangement reduces the total battery voltage required.

Fig. 6 shows yet another arrangement of the same components including the three batteries 40. 44 and 46 and the resistor 42. The circuit of Fig. 6 again has an advantage of minimizing the battery voltage requirements. As an example of the operating characteristics of the system of Fig. 6, battery 40 may be a 22.5 volt battery, battery 44 a 2'70 volt battery and battery 46 a 90 volt battery. Resistor 42 has a value of 1.75 megohms, so that with a current of 58 ra, the nominal shieldto-filament voltage is minus 12 volts.

Th circuit of Fig. 7 differs from that of Fig. 1 in the inclusion of a D, C. amplifier in the lead connecting target electrode 2| with the shield electrode 36. An amplifier may be included in any of the illustrated circuits to increase the sensitivity of the system to changes in collected ponents may be altered, the specific values given for voltage sources and resistors being by way of example only, and in no way limiting. In the same respect illustration of batteries in the several described circuits is intended to be representative of any D. C. voltage source, it being ob.- vious that there is no limitation to the derivation of power from any particular type of source.

I claim:

1. In an ion source comprising an ionization chamber, the combination comprising an electron emitting filament, a target electrode for collecting electrons, a shield electrode mounted adjacent the filament on the side thereof opposite the target electrode, means for directing the electrons through a portion of the ionization chamber to the target electrode, means for impressing a negative voltage on the shield electrode, and

means for varying the magnitude of the negative voltage as a function of the current developed by electron discharge at the target electrode.

2. In an ion source comprising an ionization chamber having a first slit in a wall thereof for admitting electrons and a second slit in a wall thereof for exit of electrons, the combination comprising an electron emitting filament mounted exteriorly of the ionization chamber in alignment with the first slit, a target electrode mounted exteriorly of the ionization chamber in alignment with the second slit, a shield electrode mounted adjacent the filament on the side thereof opposite the first slit, means for impressing a negative voltage on the shield electrode, and means for varying the magnitude of the negative voltage as a function of the current developed by electron discharge at the target electrode.

3. In an ion source comprising an ionization chamber having a first slit in a Wall thereof for admitting electrons and a second slit in a wall thereof for exit of electrons, the combination comprising an electron emitting filament mounted exteriorly of the ionization chamber in alignment with the first slit, a target electrode mounted ex teriorly of the ionization chamber in alignment with the second slit, a shield electrode mounted adjacent the filament on the side thereof opposite the first slit, means for holding the shield electrode at a negative potential with respect to the filament, and means for varying the shield electrode to filament potential as a function of the current developed by electron discharg at the target electrode.

4. In an ion source comprising an ionization chamber having a first slit in a wall thereof for admitting electrons and a second slit in a wall thereof for exit of electrons, the combination comprising an electron emitting filament mounted exteriorly of the ionization chamber in alignment with the first slit, a target electrode mounted exteriorly of the ionization chamber in alignment with the second slit, a shield electrode mounted adjacent the filament on the side thereof opposite the first slit, a first source of bias voltage connected to maintain the shield electrode at a negative potential with respect to the filament, and a second source of bias voltage operable to vary the shield electrode to filament potential as; a function of the current developedby ment with the first slit, a target electrode mounted exteriorly of the ionization chamber in alignment with the second slit, a shield electrode mounted adjacent the filament on the side thereof opposite the first slit, a first source of bias voltage connected to maintain the shield electrode at a negative potential with respect to the filament, means for developing a voltage responsive to and as a function of the current developed by electron discharge at the target electrode, means for amplifying this voltage, and means for applying the amplified voltage to vary the shield electrode to filament potential.

6. In an ion source including an ionization chamber, the combination comprising an electron emitting filament, a target electrode for collecting electrons, means for directing electrons from the filament through a portion of the ionization chamber to the target electrode, means developing a potential between the filament and ionization chamber to direct electrons emitted from the filament toward the ionization chamber, and means for varying this potential as a function of the current developed by electron discharge at the target electrode.

'7. In an ion source including an ionization chamber having a first slit in a Wall thereof for admitting electrons and a second slit in a wall thereof for exit of electrons, the combination comprising an electron emitting filament mounted exteriorly of the ionization chamber in alignment with the second slit, means developing a potential between the filament and ionization chamber to direct electrons toward the first slit, and means for varying this potential as a function of the current developed by electron discharge at the target electrode.

8. In an ion source including an ionization chamber having a first slit in a wall thereof for admitting electrons and a second slit in a wall thereof for exit of electrons, the combination comprising an electron emitting filament mounted exteriorly of the ionization chamber in alignment with the first slit, a target electrode mounted exteriorly of the ionization chamber in alignment with the second slit, means developing an electronfocusing field in the region of the filament to focus electrons through the first slit, and means for varying the focusing characteristics of the field as a function of the current developed by electron discharge at the target electrode.

9. In an ion source including an ionization chamber, the combination comprising an elec; tron emitting filament, a target electrode for collecting electrons, means for directing electrons from the filament through a portion of the ionization chamber to the target electrode to develop a current at the target electrode proportional to the number of electrons discharging thereon, means developing a potential between the filament and ionization chamber, and means for varying this potential responsive tovariations in the current developed by electron discharge at the target electrode whereby this current is maintained substantially constant in magnitude.

10. Apparatus for producing ions within a space which comprises means for introducing molecules to the space, electron producing means, means developing an electrical field for directing electrons from the electron producing means through the space, means for collecting electrons after passage thereof through the space and developing a current proportional in magnitude to the number of electrons collected, and means for varying the magnitude of the field responsive to variations in the magnitude of the current so as to vary the number of electrons directed through the space to hold the current at a substantially constant value.

11. Apparatus for producing ions within a space which comprises means for introducing molecules to be ionized into the space, electron producing means, means developing an electrical field for directing electrons from the electron producing means through the space, means developing a negative electrical field in the region of electron production, means for collecting electrons after passage through the space, and means controlling the negative field as a function of the current developed by collection of the electrons.

12. Apparatus for producing ions within a space which comprises means for introducing molecules to be ionized to the space, an electron emitting filament disposed adjacent the space, means directing electrons from the filament through the space, means for collecting electrons after passage through the space, an electrode disposed adjacent the filament and on the side thereof op,- posite the space, means maintaining said electrode at a negative potential with respect to the filament, means controlling the magnitude of the negative potential as a function of the current developed at the electron collecting means responsive to discharge of electrons thereon. V

CLIFFORD E. BERRY.

References Cited in the file of this patent UNITED STATES PATENTS Number Name I Date 2,373,151 Taylor Apr. 10, 1945 2,457,530 Coggeshall et al Dec. 28, 1948