US2612607A - Mass spectrometer - Google Patents

Description

Sept. 30, 1952 w. E. STEPHENS MASS SPECTROMETER Filed April 5, 1947 nid 02PM z dohqdnzu@ CJLZZL'am E'. Stephens 'rzveator Gbbornet Patented Sept. 30, 1952 vLUN-IT511D',ls'lli'rrzs PATENT MASS lSPECTRMiE'1ER. william s. Stephens, .Philadelphia Pa.

Iimplication April "5, 1947, dSerial No. 739,681'

,1 Gleim.A

The present invention relatesrto improved vmass spectrometry l.and particularly provides a .method and apparatus :for vobtaining .pulses of ions segregated according to mass-velocity `relationships, collecting the ions to obtain `pulses of current dispersed in time and recording 'the result.

vA mass .spectrometer is an apparatus for sorting ions vaccording to the .ratio .of .the mass of the ion to the charge that it carries (this ratio being 'referred to hereinafter as speciiic mass) and can be employed for various purposes, including qualitative vand 1quantitative analyses of mixtures containing different kinds of molecules, determination of isotope ratios, as a leak detector in any evacuated apparatus, etc.

A common 4form of mass spectrometer includes an ionization chamber in which molecules of the vmixture to be analyzed are converted into ions, an analyzer tube in which a ,heterogeneous beam of the ions is separated by an electric or magnetic field into a plurality of diverging homogeneous beams of ions (the ions of the same specific mass being included in a single beam), an ion collector, means for bringing the several beams suc-- cessively into contact with the collector and means for indicating separately the currents collected from the several beams.

There are numerous disadvantages to an instrument of the above type. Foremost among these is the high initial cost which is seldom less than $25,000. Furthermore the instrument is bulky due to the magnet and its associated control equipment and extremely high voltages are necessary which endanger personnel.

This invention therefore has for its objects the provision of a mass spectrometer which is W in cost. which employs means for ion segregation without the use of a magnet, and employs low accelerating voltages.

The invention possesses numerous other objects and features of advantage, some of which together with the foregoing, will be set forth in the following description of specific apparatus embodying and utilizing the novel method and apparatus of this invention.

The present invention proposes a method for overcoming the disadvantages set forth above and is based on the principle that positive ions of different masses acquire different velocities when accelerated by a voltage potential according to the equation:

Velocity= z-mvwhere le is charge, V is voltage and m is `-mass. Thus-.ions :of :different .mass -ga-in :different velocities. According to the invention, `pulses--of-ions are selected at -regular `time Iintervals 'from-"ari ordinary low voltage ion source and passed down .a vacuum tube. In travelling down such 'atu'ba :a :mixture `of rions of different .specific mass AWi-ll .separate .into rclusters lof like -mass and the' #dif- :ferent clusters will spreadaout .in yspace by virtue of their difference :in velocity. These ions are -collected :in a `suitable manner, as in a Alaraday cage, v the resulting current ampli'ed and im- ;pressed on the verical plates `,of Ian yoscilloscope whose-sweep is 'synchronized with the pulse .creatingmechanism. Aspeciiicimassfspectrum oflthe ions is thus exhibited on `the fscree'n 4of the oscilloscope. Separation of ions of different masses `does `notclepend upon slit Width -as 4in .the .case for a conventional mass spectrometer, but depends only on the length of path, the accelerating voltage, pulse length and the y'detecting device. 'Ihe proper lchoice of these variables .depends upon what region of the Vmass spec'trum is desired to be observed. i

Such a mass spectrometer presents numerous advantages over present types. All magnets are done away with, eliminating the weight of the magnets and stabilizing equipment. The resolution does not depend on smallness and alignment of slits, etc. The ionv spectrum is scanned at a rapid rate. The repetition time of the analysis is reduced to the periodicity of the pulsing frequency. The indication is continually represented visually and can easily be photographed.

These and other features of this invention will be more thoroughly understood in the light ot the following detailed description taken in conjunction with the accompanying drawing in which the drawing is a diagram of one form of the mass spectrometer.

The spectrometer shown diagrammatically in the drawing consists of a sample chamber l0, an ionization chamber Il, an analyzer tube l2, a collector I3, an amplifier I4 and an oscilloscope I5. The ionization chamber and analyzer tube are enclosed in an air-tight container which is kept under high vacuum to'avoid molecular and ionic collisions.

The gas to be analyzed is introduced into ionization chamber Il through line IB containing a capillary leak or gas inlet l1. In the ionization chamber the gases are bombarded by electrons from lament I8. This bombardment ionizes the molecules and they therebybecome positive ionsi As a result of their positive charge the ions are accelerated toward slit I9 which is kept at a small negative potential with respect to the rest of the ionization chamber by means of battery 20. After passing through slit I9, the ions are further accelerated towards slit 2| which is kept at a larger negative potential with respect to slit I9 by means of battery 22. Positioned in the path of the ions emerging from slit 2I are deecting plates 23 and entrance slit 24 to analyzing chamber I2. An alternating potential for example about 100 volts, is impressed across the defiecting plates by means of sweep generator 25, so that the ion beam is swept back and forth across the slit 24 With the result that a spurt of lons will pass through the slit as the beam passes the opening. The net result of this alternating potential on the deflecting plates is that the ions will travel down the tube as a series ofk pulses each of which is made up of ions of different masses. However, due to the mass-velocity, relationships discussed above, each pulse .Will separate into clusters made up of ions of the same massl and consequently the same velocity. In order to identify the individual ions making upa cluster, it is necessary that the interval'between pulses be great enough, for example, about one thousand times greater than the duration of the pulse, and that the length of the tube be short enough so that the fast ions of a succeeding pulse do not catch up with'the slow ions of a preceed- Ying pulse. An exit slit 26 is placed in the path of the ions at theend of the tube through which the ions pass to a collector 2l, which may suitably be a Faraday cage. The resulting current is amplied and impressed on'the plates 28 of the oscilloscope I which deflect the oscilloscope beam vertically over the vscreen 29. The oscilloscope beam is swept horizontally as a function of time *over screen 29 by connecting plates `3l) of the `oscilloscope with the sweep generator 25. Thus the horizontal sweep of the oscilloscope is synchronized with the pulse generating means connected to deecting plates 23.

In this manner the type of ion is determined sired to be secured by Letters'A Patent is:

A mass spectrometerv consisting of an elongated evacuated envelope, an inlet for the admission of a gas sample to said envelope, an electrical lament adjacent said inlet for ionizing the said gas sample, a first perforated plate spaced from said filament maintained at a negative potential whereby ions of the said gas sample are accelerated toward and through the perforation of the said perforated plate, a second perforated plate spaced from the first perforated plate, delecting plates positioned between said rst and second perforated plates maintained at periodically varying electrical potentials whereby lsaid ions accelerated through the said .first perforated plate are periodically caused to pass through the perforation ofthe said second perforated plate, and voltage sensitive ion collecting means spaced from said second perforated plate to collect ions passing through `the said second perforated plate.

E. STEPHENS.

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

,Y UNITED STATES PATENTS Number Name Date 2,331,189 Hipple Oct. 5, 1943 Winkler s Oct. 23, 1945 OTHER REFERENCES

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