US2486452A - Mass spectrometry - Google Patents

Description

Nov. l', 1949. l H. w. wAsHBuRN E-rAL 2,486,452

' MAss sPEcTRoMETmf' y VFiled April 50, 1945 2 sheets-sheet 1 lll/ NOV. 1, 1949. H. w. wAsHBURN ETAL 2,486,452

MASS SPECTROMETRY 2 Sheets-Sheet 2 Filed April 30, 1945 INVENTORS MOLD W. Wn

F VA' AVAVAVA' Patented Nov. 1, 1949 l n UNITED sri-irasN fior-Fiel:

della, Calif., assignors to 2inist),liilated "Engineering Corporation, Pasadena,v r`Calif., 1a. cbrporation of'California Appucatinn Aprirac, 1945,1seriaNo.91h10 comms. (c1. chorale) l 2 This invention is concerned with mass specspaced fr'inbo'th and lconductive means for dis.,- trometers. It provides improvements in apparacharging A current from the baffle. In one form 'of tus for protecting an ion beam to be analyzed the invention, electrodes are spaced from each from the'effects of static charges built up withotherwitliin thespect'rovmeter along the ion beam in-thespectrometerto the-end that -the calibra- 5 path by 'an insulator, and the ion beam is protion of the apparatus remains unchanged over tectedrfrom the charge at the insulator by baflonger periods, so that the accuracy of analysis fles spaced lfrom each other between the el-ecis improvedl trodes and attached respectively to them. A pre- A mass spectrometer is essentially an apparaferred form of the invention contemplat-es in a tus for producing ions and sorting them accord l0 mass spectrometer the combination which cornine tothe ratio of their mass to their charge, prises collimating electrodes, each. having an i. e. according to their specificmass. A sample aperture therein through which an ion beain is to -be analyzed, for example a gas miXture, is propelled, an insulating spacer between the colionized (for example, by electron bombardment) limating electrodes, a shield 'comprising everand the resulting ions are propelled by an elecl5 lapping bailies spaced from 'each vother and districal potential into an analyzer. The ions thus posed between thecollimating electrodes and beenter the analyzer as an unsorted beam, and in tween theinsulator and the beam, 'one of the the analyzer are sorted yunder the in-uence of bales being connected to (and preferably formed an electromagnetic field in accordance with their integrally with) one collimatr'ig electrode and Speciiic mass into a series of divergent homo- .0 the other baille being connected to (and preferageneous ion beams. The sorted ions are collected bly `fo`rrned integrally with) the other collimatand discharged, the quantity of each kind of irri; electrode. The shield ,thus obstructs the iOnS being measured by the amount of current straight line paths between the insulator and the that they discharge upon collection. ion bea'm and e'iectively shields the latter from In one type 0f mass Spectrometer, CHZMOn l5 the eiect of static 'charges built up ony the inof the molecules is accomplished in an ionization Sulatoi; 'A's'indiceted above, the bellies Should be Chamber, OI @Xa'mple by 'bOmbaIdment With an of conductive material and preferably are formed electron beam. The resulting OHS are fOTCed integrally with the electrodes. The bailles overthrough a plurality of collimatirl'g electrodes haV- lap in a direction substantially parallel with the ine oriented slits 0r apertures therein, and S0 beiii 'and transverse to substantiallyany straight emerge into the analyzer tube 24S an Unsorted line which could be drawn between insulator and beam. L ion beam.

It has been fOul'ld that Some types 0f. 1095 These 'and other features of the invention will formed in ,a mass Spectrometer tend, t0 lqdge, be understood more thoroughly in the light of upon .dielectric surfaces Within. the apparatus an@ the "fc'lltviig detailed description, taken in congiO build @Static merge-,S ntheseslefeeee The miraron with the maintaining drawings in gradual accumulation of such charges as are'- Which: l y sult of this and other phenomena affects th'ecalifyi-@ 1 "aha 2 are Ingt'udinal Sections taken mation of th ppaa'tus' The effect S part'muf at right angles to each other through the head muy marked at an insulator which is emplgyed 40 of 'a spectrometer 'adapted to the practice as a spacer between the collimating orpropelling -ur invention; md

electrodes, and we have discovered that these ge 3 'is' L SChmatc diagram 'of a mags Speemay serve a double function if they are con'- tyl'te'rqbp' Azimuth-e head of pagg; and 2,

structed in the form of a Shield 0r ble Whll Referring t6 the drawings, it 'will be seen that obstructs the space between the ion beam, and the apparatus comprises a cylindrical heed ym the insulator. In this manner the effeto the the rear df which -ccimects with e ges inlet tribe static charge upon the ion beam is minimized. Vof lisulfjrig materials, l Within lthe head The baille, however,` may be an integral element. there is eh irilet electrode l2 Ain the form .ofy a Accordingly, our invention contemplates the come inetallictube which v'c onfimunicates with the gas bination in a mass spectrometer which comprises inlet c'oi'icluitv at its .rearand which is partially an insulator disposed within the apparatus inl closed at its front end by a cylindrical electrode such position that ions may lodge upon it plate IKS having a transverse slot to accommodate create a static charge, means for producing'ah an electron beam ll. v y Y n ion beam, a conductive baille disposed between The electron beam is produced by an electron the irlulatl" and the path f the beal and gun Iin'o'ilhtd 'Outsidea metallic blOCk Il Which 3 encloses an ionization chamber I8 within which the slotted electrode plate is disposed, (See Figs. 1 and 2).

The electron beam enters the ionization chamber through an aperture I 9 which is in line with the slot in the inlet electrode, passes through the slot, and thence through an aperture in the opposite side of the block to an electron catcher 2 I.

The front of the ionization chamber is formed by a propelling electrode 22 separated from the block by an insulator 23 and provided With a slit S1 that is substantially in line with the slot in the inlet electrode. A second propelling electrode 24 provided with a slit S2 is disposed in front of the first propelling electrode with the slit S2 in line with the slot in the inlet electrode and the slit S1. The two propelling electrodes are spaced from each other by a ring 25 of quartz or similar insulating material enclosing a chamber 26.

The insulating ring 25 separating the two slit electrodes 22, 24 tends to accumulate static charges from vagrant ions which adhere to its inner surface. These static charges are prevented from exerting an influence on the ion beam passing from the slit S1 through the slit S2 by means of a shield comprising an -inner baille 2 and an outer baille 28. These two baiiles are spaced from each other but overlap slightly so that they obstruct straight-line paths from the insulator ring to the space through which the ion beam passes.

It desirable to introduce a transverse component in the potential employed to propel the ions from the ionization chamber through the slits S1, S2 into the analyzer tube. In the type of apparatus illustrated this is accomplished by making the rst propelling electrode in the form of two sections 22A, 22B. These sections define the slit S1 but are spaced from each other and are adapted to be energized separately as described hereinafter. In consequence the outer baille 23 is likewise slit into two opposed segments 28A, 23B separated by a gap adjacent the slit S1, and these segments are integrally formed respectively with the segments 22A, 22B of the rst propelling electrode.

The inner baille is a continuous ring formed integrally with the second propelling electrode.

The ballied chamber communicates with an analyzing chamber 29 of the mass spectrometer through the slit S2 in the second propelling electrode, which latter is fastened to an analyzer tube 36 that forms the wall of the analyzing chamber.

Referring now to Fig. 3, it will be observed that the head l0 of the mass spectrometer and the attached analyzer tube 30 are enclosed within an envelope 3l, through the wall of which the gas inlet tube l I projects. A high degree of vacuum is maintained within the; envelope by means of vacuum pumps (not shown) connected to the tube.

The molecules of a gas sample admitted into the ionization chamber through the inlet electrode are bombarded by the electron beam passing through the slot at its end and thus converted into ions. These ions are propelled as an unsorted beam into the analyzer tube by electrical potentials established between the several electrodes in the head by an electrical system shown schematically at the left of Fig. 3.

A battery or other direct current supply 4U is connected to the ends of a potentiometer 4|.

The positive end of this potentiometer and its slider are connected through a switch 42 to a condenser 43. A potential dividing network is connected across this condenser and takes the form of a resistor or potentiometer 44 in series with a parallel pair of resistors or potentiometers 45, 46 which are in turn connected to ground at the negative end of the network.

The inlet electrode is connected to the positive side of the network at the slider of the Ipotentiometer 44. One segment of the rst propelling electrode is connected to the slider of the potentiometer 45; the other segment of this electrode is connected to the slider of the potentiometer 46, and the second propelling electrode is connected to the negative end of the network and to ground.

In the operation of the apparatus a sample of gas to be analyzed is admitted through the gas inlet tube and passes through the slot in the inlet electrode, where it is bombarded by the electron beam. For optimum results (as described and claimed in co-pending application Serial No. 590,917, led April 28, 1945 now Patent No. 2,475,653 issued July l2, 1949) the current producing this electron beam should be maintained constant, which result can be obtained by observing the current at the electron catcher by a conventional meter (not shown) and adjusting the electron current (for example, by the means shown in co-pending application Serial No. 590,917, liled April 28, 1945).

Molecules of the gas sample are thus ionized and the resulting ions are propelled as an unsorted ion beam from the inlet electrode through the slots S1, S2 into the analyzer tube. There the unsorted beam is separated into a plurality of diverging homogeneous ion beams B1, B2, Ba by means of an electromagnetic lield produced in the analyzer tube by an electromagnet (not shown). Any one of the diverging beams may be focussed on the exit slit at the end of the analyzer tube, so that it passes therethrough and strikes an ion collector. The current thus established in the ion collector is amplied and recorded by suitable apparatus and serves as an index of the quantity of that particular type of ions produced from the sample.

The position of the ion beams with respect to the exit slit may be changed by varying the electromagnetic iield in the analyzer tube or by varying the propelling potentials. The latter method is preferred and may be practiced as follows:

The switch 42 in the energizing network is closed and the condenser 43 is charged to an appropriate level. Then the switch is opened and the condenser is permitted to discharge through the potential dividing network and the several electrodes in the head of the mass spectrometer. In this way the propelling potentials between the several electrodes gradually decay, while maintaining relative values.

It will be observed, however, that the potentials may be adjusted by suitably setting the potentiometers. Thus the potential at the inlet electrode may be adjusted by varying the setting of the potentiometer 44. Likewise, the potentials at the two segments 22A, 22B of the rst propelling electrode can be varied by varying the setting of the potentiometers 45, 46. In this way the ion beam may be adjusted to give optimum results at the exit slit and ion collector.

If it is desired to operate without employing a transverse component of potential for focussing purposes at the slit S1, the rst propelling elecs trode can be built as a single electrical unit, in which case the outer bailie 28 attached thereto may be a continuous annulus.

It should be observed that static charges may also tend to accumulate on the insulator ring 23 which separates the electrode segments 22A, 22B (that define the slit S1) from the block Il. As described and claimed in (zo-pending application Serial No. 590,917, led April 28J 1945, the tendency of these charges to de-focus the ion beam may be overcome by a conductive ring 50 disposed inside the insulator ring and substantially covering it, save for a gap which separates the conductive ring from the segments. The conductive ring is mechanically and electrically connected to the block so that it is under substantially the same potential.

We claim:

1. In a mass spectrometer, the combination which comprises electrodes spaced from each other along the path of an ion beam, an insulator holding the electrodes apart, and a shield disposed between the insulator and the ion path and comprising conductive bailies spaced from each other and attached respectively to the electrodes.

2. In a mass spectrometer, the combination which comprises collimating electrodes each having an aperture therein through which an ion beam is propelled from one electrode to the other, an insulating spacer disposed between the collimating electrodes, and a shield comprising overlapping conductive baflies spaced from each other and disposed between the electrodes and between the spacer and the path of the beam.

3. Apparatus according to claim 2 in which one bale is fastened to one electrode and supported thereby in electrical contact therewith and the other baille is similarly fastened to the other electrode.

one baiile is formed integrally with one electrode and the other baiile is formed integrally with the other electrode.

5. In a mass spectrometer, the combination which comprises a first collimating electrode comprising two segments insulated from each other and adapted to be separately energized, the two segments being separated from each other by an aperture, a second collimating electrode having an aperture matching that between the two segments, means for propelling the ion beam through the two apertures, an insulator disposed between the two electrodes and holding them apart, and a shield disposed between the two electrodes and between the insulator and the path of the ion beam and comprising a first conductive baille fastened to and in electrical contact with the second collimating electrode, and a pair of conductive baiiles spaced from each other and the first conductive baille, but overlapping the latter and fastened respectively in conductive relationship with the segments of the first collimating electrode.

HAROLD W. WASHBURN. CLIFFORD E. BERRY.

REFERENCES CITED I The following references are of record in the le of this partent:

UNITED STATES PATENTS Name Date Hipple July 18, 1944 OTHER REFERENCES Number

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