US2489344A - Mass spectrometry - Google Patents

Description

Nov. 29, 1949 H. w. wAsHBURN mss sPEoTRoMETRY 4 shams-sheet 1 Filed July 16, 1945 MINIMQQQWBN NN OT. QQMT..

4 Sheets-Sheet 2 Filed July 16, 1945 lllli H TTONEXS' Nov. 29, 1949 Filed July 16, 1945 N0 JECoA/a ,GRY fM/II ECTROMETRY" /ON JUPPEESlf/OA/ 4.- Sheets-Sheet 3 MICRO-AMPS.

Nov. 29, 1949 H. w. WASI-:BURN 2,489,344

MASS SPECTROMETRY y Filed July 1e, 1945 4 sheets-sheet 4,

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ya von) CA '7c/ffl? CURRENT fle) )Vl/090 H/WPI (wwwa-raf? /v 66A r/ v6 l5. i. 2 voarf) n. D. 2 2 fc Vamfmrc//E V04 mss) .su E u||| l nml l I nul l l :lul 1 l l mi l 1 m 0 l0 2O 3() 4D SO 60 f1 q# 4 .IN1/mmf Hmmm W Wwf/5MM Patented Nov. 29, 1949 MASS SPECTROMETRY Harold W. Washburn, Pasadena, Calif., assignor to Consolidated Engineering Corporation, Pasadena, Calif., a corporation of California Application July 16, 1945, Serial No. 605,262

(Cl. Z50-41.9)

12 Claims.

This invention relates to the mass spectrometry and particularly to the ionization of molecules therein by bombardment with an electron beam. It provides improvements in apparatus for producing and controlling such a beam with the result that the distribution of electron energy in the beam is made more uniform, to the end that the eilicency of ionization is increased and the calibration of the spectrometer is less subject to deviation.

Essentially, a mass spectrometer is an apparatus for producing ions and sorting them according to mass-to-charge ratio, i. e. according to specic mass. A sample to be analyzed, for example a gas mixture, is ionized (for example, by bombarding its molecules with electrons) and the resulting ions are propelled as a heterogeneous beam by an electrical potential into a magnetic eld in which the ions separate into a series of divergent homogeneous ion beams. The sorted ions are collected and discharged, the quantity of each kind of ions being measured by the amount of current that they discharge upon collection.

In certain more recent types of mass spectrometers, ionization is accomplished in a separate chamber, usually by bombarding molecules therein with a beam or stream of electrons and the resulting ions are propelled through slits in collimating electrodes into an analyzer by potentials maintained between the electrodes.

The electron beam employed to ionize the molecules in the ionization chamber conveniently originates at an electron source, such as a heated lament; and electrons of the beam are propelled across a space within the ionization chamber toward a catcher electrode. A very small portion (say l in 10,000) of the electrons in the beam are consumed in the ionization of molecules with which they collide in their movement Within the ionization chamber; the balance pass to the catcher. Many of the electrons lodge at the catcher and produce no secondary effects. Some (say half) of the electrons, upon striking the surface of the catcher, cause the throwing oit of other electrons which rebound into the electron beam to the electron source, where they strike and may cause further secondary emissions of electrons. In this Way, electrons of various energy contents are, in efect, bounced back and forth in the electron beam between the a source and the catcher, especially if electrons are held in this path by a magnetic field which encompasses both the ionization chamber and the analyzer.

I have discovered that this bouncing or "reection of electrons due to the secondary emission eiTects just described, has a deleterious effect upon the mass spectrograrn produced by the instrument and tends to cause erroneous and erratic results. Thus the secondary emissions originating at the catcher tend to produce a nonuniform distribution of electrons and of electron energy within the beam, and these non-uniformities in turn afect (1) the efliciency of ionization and (2) the peak pattern or spectrogram produced, since the effects of secondary emission are different at each peak.

In accordance with my invention, the secondary emissions are prevented from bouncing back into the electron beam within the ionization chamber (and thus tertiary, Quaternary, etc. emissions are also prevented) by disposing a secondary emission suppressor electrode in the path of the electron beam between the source and the catcher but adjacent the latter and at or near the potential of the former. Conveniently, the suppressor electrode is provided with an aperture disposed coaXially with a straight line drawn from the electron source to catcher, i. e. in line with the two, so that the electron beam passes through the aperture.

Generally speaking, the most convenient way of achieving correct polarity and potential for the suppressor electrode is to connect it directly through a low-resistance lead to the positive side of the filament which acts as an electron source. However', on occasion it may be desirable to have the suppressor electrode slightly positive with respect to the electron source and this may be accomplished by inserting a small bias battery or other D. C. current source in the circuit connecting the electron source to the suppressor electrode. Less frequently, it may be desirable to make the suppressor electrode slightly negative with respect to the electron source, and this can be accomplished by switching the polarity of the bias voltage or by inserting another bias battery of reversed polarity.

In the preferred structure of my invention both the electron source (lament) and the catcher are mounted outside `the ionization chamber and the electron beam passes from one to the other through apertures in the walls of the chamber. The beam thus passes transverse to the direction in which the ions are propelled from the ionization chamber to analyzer and parallel to the lines of force between the poles of an electromagnet which brings about sorting of the lons.

Preferably, also, the suppressor electrode is located outside the ionization chamber proper, as by being mounted within a separate chamber in which the catcher is also disposed. This separate chamber conveniently is cut in the wall of the ionization chamber itself.

These and other features of my invention will be understood more thoroughly in the light of the following detailed description', takenlin conjunctionwith'I the accompanying drawings in which:

Fig. 1 is a schematic diagram oi a mass specz trometer of a type to which the inventionv isapplicable;

Fig. 2 is a schematic transverse.;sectionthrougha the head of the mass spectrometer`y ofiE-ig; 1?,v taken along the line 2 2, withV an associated;

wiring diagram showing the1electricalf connec-y tions of filament, catcher and the secondary emission suppressor;

Fig. 3 is a graph showing the relationship of peak. height to i catcher Voltage andmatcher: cur.- rent in; a: mass spectrorneter. Whenno: secondary emission. suppressor is: employed;

Fig. 4. shows the relationship of. the catcher voltage and catcher currenty in the same mass spectrometerV employing, a secondary: emission suppressor in accordance with the invention;

Referring now to; Fig; 1; it. Will befseen; that the apparatus comprises .an cylindrical. head; or block it, therear of whichfconnects; With1a gas inlet tube Il made'of-insulatingmaterial;` Within-the head there is an inlet orpusher. electrode l2: in the form ofV a. metallicfplate; insulatedA from: the restof the apparatus:

An electron beam; I3? is. produced.` within the headin the space;immediatelyfbelow-the'pusher electrode as described: in greater.` detail4 hereinafter.

The space in the head-through which the elec.- tron beam` passes; is known asy the. ionization chamber It. The front orf lovver-portion or the ionization chambenis formed byv a; collimating electrode l5 which ist electrically; connected` to the head or block` and is-provided1with aV slit Si that is substantiallyin.linefwithwthepusher elec L` trode and with the path of;v the electron beam. A second collimatingffelectrode It provided: with a: second slit Sz; in line;v with.- the.; slit S1y and the electron beam is spacedbelowsthe rsticollimating electrode byfa ring If'I-of; insulating material.

Thehead of.A the apparatusfisY connected to an analyzer-.tuber I8; through the slit S2: The analyzer tube is bent in a=semicircle andrencloses an. analyzingchamberv I 9.-

In; the operation; ofv the-apparatus, asample ofV gas to beanalyzed is admitted'intdthe gas inlet tuberand passes: into-thespace below the pusher electrode. where itis bombarded by the electro beam. Moleculesof thefgas sampleV are thus ionized and the.- resulting lions are'vpropelled as.v an unsorted ion beamby thefpusherv'electrode through the.s1its f S1,- S2` in.thecollimating electrodes into the analyzer tube: There'the unsorted. beamis` separated'A into.` a plurality of divergingghomogeneousI ion beams B1, B2, B3, by meansof a. magnetic. eldf'.: producedinthe yhead of-the apparatusr aswell Aasin'theY analyzer tube byran.electromagnetinot shown inl'fig. 1);

Any.r oney of theediverging.V ion.-beams may be focused-en anexit slitzl.- at the end ofthe analyzer: tube so.tha.t.v it.- passes therethrough and strikes. an ion-.collector-Zl. The.;` current thus established-in theeion collector. is` amplified. and recorded by suitable amplifying and recording apparatus .-2 Zandserves asranindeX-.of the yquan- 4net shownY diagrammatically. in Fig.` 2.

tity of the particular type of ions collected from the sample.

The head I0 of the mass spectrometer and attached analyzer tube I8 are enclosed within an envelope 23 through the wall of which the sample inlet tube I I projects. A high degree of vacuum is maintained within the envelope by means of vacuunnpumps (nottshown) connected to the envelope. Electrical circuits which produce the propelling potentials within the head of the mass spectrometer are shown at the left of Fig. 1. A. battery. or other direct current supply 3G is connected to the ends of a potentiometer 3|. The. positive end of.' this potentiometer and its ;s'lider` are connected through a switch 32 to a condenser 33; A: potential dividing network is connected across this condenser and takes the form of a resistance or potentiometer 35 con nected'atits negative end to ground.

The pusher electrode I2 is connected to the positiveside of the network. Therstcollimatingv electrode I5=is-connectedtothe slider of the potentiometer 35 and the. second collimating electrode Itis connected to the negative end of the network and to ground.

The position. of the iorrbeams B1, Bz, B3 with respect to the exit` slit may be changed either by varying the magnetic eldor. the propelling potentials. The latter, method is preferred and may be practiced as follows:

The switch 32r inthe. energizing network is closed. and the condenser.V 33 is charged to an appropriate.V level. Then the. switch is opened and the condenser isY permitted to discharge through the potentialdividing network and the severalelectrodesin the-head of the spectrometer. lnthis way the propelling potentials between the severalv electrodes gradually decay while maintaining. relative values.

It will be observed that the potentials in the head may be adjustedrelative to each other by suitably setting thepotentiometer. In this way the ion beam may be adjusted to give optimum resultsat the exitV slit andthe ion collector.

Referring now` toFi'g. 2; the head of the spectrometer is shown in. a section taken along the line2-2of Fig. 1. Conveniently, the head is a thick-walled metallic cylinder of. Nichrome or thelike. and. enclosesthe cylindrical ionization chamber I4. One side. of the head has a cutout portion within whichan electrongun comprising a filament 4i! and an electron accelerating electrode lllv are mounted. A closed chamber I2 is cut into the wall of the. head on theopposite side and enclosesa, metallic catcher electrode 43 with a secondary emission` suppressor 4d mounted in front of it. The electron beam I3 produced at thev lament passes 'through an aperture l5 in the electron accelerating electrode 'andthence through an aperture or gun barrel 46- in the block into the ionization chamber. The electron beam passes parallel to andabove the rst slit S1 in. the collimating electrode I5 through a second aperture VIv in the block or head, thence through an aperture Q8 in the secondaryY emission suppressorV and strikes the catcher 43.

The lament, the electron accelerating electrode, the catcher-and the secondary emission suppressor are insulated fromV each other and from the head.

Asindicated hereinbefore, the head of the massgl spectrometer` as well as its analyzer tube are mounted between the poles of an electromag- Thus,

magnet poles 50, 5l are so disposed with respect to the head that the direction of the magnetic lines of force (indicated at 52) is parallel to the path of the electron beam I3.

To understand the manner in which the electron beam is produced and controlled, reference should be made to the wiring diagram in Fig. 2. A direct current source 6U produces current and potential for the filament 4!) of the electron gun with which it is connected through a variable resistor 6|. A center tap resistor 62 is connected -by its ends across the circuit adjacent the lila.- ment. The positive end of the lament may be connected directly to the secondary emission suppressor 44 through a triple throw switch 63. However, if it is desired to make the secondary emission suppressor slightly positive with respect to the filament, the triple-throw switch may be thrown to cut in a small direct current source 64, i. e. a bias battery. It may be desirable to make the secondary emission suppressor slightly negative with respect to the lament, in which case the triple-throw switch will be thrown to cut in another small direct current source or bias battery 65, the polarity of which is reversed with respect to current sources 60, 64. The catcher 43 is connected into the circuit through a microammeter S6 and a direct current source $1 to the center tap of the resistor 62. In this way, a potential difference to further the iiow of electrons is maintained between the catcher and the filament.

Energy and potential .for the electron accelerating electrode 4l is supplied by a direct current source 58 connected to the accelerating electrode through a milliammeter B9 and to the center tap of the resistor 62. It is shunted by a voltmeter 10.

A so-called ionizing potential maintained between the filament and the block is supplied by a direct current source 'H connected to the slider of the potentiometer 62 and to the block or head I0 through a microammeter 12. This current source is shunted by a voltmeter 13.

In the operation of the mass spectrometer electrons produced by heating of the filament 40 are projected as a beam through the ionization chamber to the catcher. When the electrons strike the surface of the catcher, there is a tendency for them to throw off electr-ons of less energy and because of the direction of the magnetic lines of force, the secondary emission thus produced tends to be driven backward as a stream of electrons toward the iilament. Some of these secondarily emitted electrons may strike the lament and set up further tertiary, etc. emissions at that point with the result that electrons are bounced back and forth in the beam between the lament and the catcher. This result, if it were permitted to prevail, would change both the distribution of electrons and electron energy in the cross-section of the beam and would make the distribution of electron energy most erratic. Ionization efficiency depends upon having a uniform energy content for the electrons, so it is apparent that the eiect of secondary emission up'on ionization efficiency is deleterious.

To prevent secondary emission, the secondary emission suppressor e!! is interposed between the catcher and the filament and adjacent the former. By maintaining the potential of the suppressor at or close to the potential of the filament, any secondarily emitted electrons are caught by the suppressor and not permitted to enter the ionization chamber in countercurrent to the main stream of electrons.

As indicated hereinbefore, it will usually be desirable to maintain the secondary emission suppressor at the same potential as the lament. Occasionally, however, it may' be desirable to make the suppressor slightly positive with respect to the lament and less occasionally, slightly negative with respect to it. A11 three of these results can be accomplished with the triplethrow switch 63 and auxiliary current sources 64, E5.

The deleterious effect of secondary emissions from the catcher of a mass spectrometer when no suppressor is employed are illustrated in Fig. 3 whereon catcher voltage is plotted as the abscissa and peak height and catcher current on the ordinate. In Fig. 3, it will be observed that catcher current varies markedly with catcher voltage, indicating that as the voltage is changed, the amount of secondary emission from the catcher varies quite erratically. The same may be said with respect to peak height which varies erratically but somewhat inversely with catcher current as catcher voltage is raised. Moreover, if a different gas is introduced, the shape of the curves becomes different, so that the effect upon the mass spectrum is altered and the calibration oi the instrument is changed.

The height of the 58 peak was chosen for illustrative purposes. Curvesl with a variety of shapes have been observed with other peaks in the mass spectrum.

Fig. 4 shows the beneficial results obtained through the use of a secondary emission suppressor of the type illustrated in Fig. 2. Again,

.catcher voltage is plotted as the abscissa and height of the peak 58 and catcher current are plotted as ordinates. The effect of employing a suppressor which is slightly negative with respect to the filament is shown by the solid lines on Fig. 4. The effect of making the suppressor slightly positive with respect to the filament is shown by the dotted lines on Fig. 4. The curves for zero difference lie somewhere between the respective dotted and solid curves.

It will be observed that whether the difference between the lament and the suppressor is slightly positive or slightly negative beyond a certain catcher voltage, the eiect of variation in catcher voltage upon peak height and catcher current becomes negligible, i. e. constant conditions of operation are obtained. This uniformity is obtained at lower catcher voltage when the suppressor is made slightly positive. However, since the catcher Voltage is ordinarily quite high during operation, this advantage may be more theoretical than real.

I claim:

l. In a mass spectrometer having an ionization chamber, means for admitting molecules into the chamber, means for bombarding the molecules in the chamber with electrons to convert them into ions, and means for propelling the resulting ions out of the chamber and sorting them according to specic mass, the combination which comprises an electron source, an electron catcher,

means for propelling the electrons as a beam between the source and the catcher, a suppressor electrode disposed substantially in the path of the electron beam adjacent the catcher, and means for maintaining the suppressor electrode at a potential that approximates that of the electron source.

2. In a mass spectrometer having an ionization chamber, means for admitting molecules into the chamber, means for bombarding the molecules-in the chamber with electrons to convert them into ions, and means for propelling the resulting ions out of the chamber and sorting them accordingV to. specific mass, the combination which comprises an electron source and an electron catcher disposed outside the charn ber on opposite sides thereof but communicating therewith, `means for propelling the electrons as a beam between the source and the catcher, a suppressor electrode disposed substantially in the path of the electron beam adjacent the catcher, and means for maintaining the suppressor electrode at a potential that approximates that of the electron source.

.3. In a mass spectrometer having an ionization chamber, means for admitting molecules into the chamber, means for bombarding the molecules in the chamber with electrons to convert them into ions, and means for propelling the resulting ions out of the chamber and sorting themaccording to specific mass, the combination vwhich comprises an electron source, and an electron catcher disposed outside the chamber on opposite sides thereof but communicating therewith through apertures in the Wall of the chamber, means for propelling the electrons as a beam between the source and the catcher, means for producing in the region of the electron beama magnetic eld having lines of force passing in the same general direction as the beam, a suppressor electrode disposed substantially in thepath of the electron beam adjacent the catcher, and outside the chamber, and means for maintaining the suppressor electrode at a potential that approximates that of the electron source.

4. Apparatus according to claim 3 in which the wall of the ionization chamber is metallic and the catcher and the suppressor electrode are located in a chamber cut into the wall of the ionization chamber.

, 5. In a mass spectrometer having an onization chamber, means for admitting molecules into the chamber, means for bombarding the molecules in the chamber with electrons to convert them into ions, and means for propelling the resulting ions out of the chamber and sorting them according to specic mass, the combination which comprises an electron source, an electron catcher, means for propelling the electrons as a between the source and the catcher, means for producing in the region of the electron beam a magnetic field having lines of force passing in the same general direction as the beam, a suppressor electrode disposed substantially in the path of the electron beam adjacent the catcher and having an aperture concentric with a straight iine drawn between the catcher and the source, and means for maintaining the suppressor electrode at a potential that approximates that of the electron source.

6. Apparatus according to claim 5 in which the source is located on one side of the chamber and communicates therewith through an aperture, and the suppressor electrode and the catcher are located outside the chamber on the side opposite the source and communicate with the chamber til-rough an aperture coaxial with that through which the source communicates.

7. In a mass spectrometer having an ionization chamber, means for admltting molecules into the chamber, means for bombarding the molecules in the chamber with electrons to convertl them into ions,.and means for propelling the resulting ions youtof the chamber and sorting them according:to'speciiic'mass, the combination which comprises anelectron source, an electron catcher, means for propelling the electrons as a beam-between the source and the catcher, a suppressorelectrode'disposed substantially in the path; of the electron beam adjacent the catcher, and means for maintaining the suppressor electrode at a potential that is slightly positive with respect to the electron source.

8. In:a mass spectrometer having an ionization chamber, means for admitting molecules into the chamber, means for bombarding the molecules in the chamber with electrons to convert them into ions, and means for propelling the resulting ions out of the chamber and sorting them according to speciiic mass, the combination which comprises an electron source, an electron catcher,fmeans for propelling the electrons as a beam between the source and the catcher, a suppressor electrode disposed substantially in the path of the electron beam adjacent the catcher, and-means for maintaining the suppressor electrode ata potential that is slightly negative with respect to that of.the electron source.

9. In a mass spectrometer having an ionization chamber, means for admitting molecules thereinto, means for subjecting the molecules therein t-o electronbombardment to form ions and means for propelling the resulting ions from the chamber, the combination which comprises an electronv source, an electron catcher, the source and catcher being so mounted with respect to the chamber that a straight line connecting them follows a substantially unobstructed path within the chamber, means for propelling electrons as a beamfrom the source to the catcher, means for producing in the region of the electron beam a magnetic field having lines oi force passing in the same general direction as the beam, a suppressor electrode mounted in the path of the beam in the neighborhood of the catcher andhaving an aperture therein through lwhich the beam passes and electrically connected to the electron source, and means for maintaining the source and the suppressor electrode at approximately the same potential.

10. Apparatus according to claim 9 in which the electron catcher and the suppressor electrode are mounted adjacent each other outside the ionization chamber, the latter being provided with an aperture through which the electron beam passes, this aperture `being in line with the electron source, the catcher and the aperture in the suppressor electrode.

11. Apparatus according to claim 9 in `which the electron catcher and the suppressor electrode are mounted adjacent each other in a second chamber formed in the 'wall of the ionization chamber.

12. In a mass spectrometer having an ionization chamber, means for admitting molecules into the chamber, means for bombarding the molecules in the chamber with electrons to convert them into ions,and means for establishing a potential gradient across the chamber to propel the resulting ions from the chamber, the combination which comprises an electron source, an electron catcher, means for propelling the electrons as a beam between the source and the catcher and transverse to the direction of the gradient,.azsuppressor electrode disposed substan- -tially in. the ipath ofthe electron Ibeam adjacent the catcher, and means for maintaining the sup` `prerssor electrode at a potential which `approxi- The following references are of record in the le of this patent:

10 UNITED STATES PATENTS mates that `of the electron source. Nzuggm H lla'ne 1 O t gte 936 HAROLD W. WASHBURN. 0 s e a- G 1 u 2,165,308 Skellett July 11, 1939 2,373,151 Taylor Apr. l0, 1945 REFERENCES CITED 2,374,205

Hoskins Apr. 24, 1945

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