US3010017A

US3010017A - Mass spectrometer

Info

Publication number: US3010017A
Application number: US817258A
Authority: US
Inventors: Wilson M Brubaker; Lawrence G Hall; Joe E Slay
Original assignee: Consolidated Electrodynamics Corp
Current assignee: Consolidated Electrodynamics Corp
Priority date: 1959-06-01
Filing date: 1959-06-01
Publication date: 1961-11-21
Anticipated expiration: 1978-11-21

Description

gld.; A

Nov. 21, 1961 w. M. BRUBAKER ETAL 3,010,017

MASS SPECTROMETER Filed .June 1. 1959 s sheets-sheet 1 Nov. 21, 1961 w. M. BRUBAKER ETAL.. 3,010,017

MAss sPEcTRoMETER Filed June l, 1959 5 Sheets-Sheet 2 Nov. 21, 1961 w. M. BRUBAKER ErAL 3,010,017

MAss sPEcTRoMETER Filed June l, 1959 5 Sheets-Sheet 3 Jaf- E. .9x/fr INVENToRs NOV. 21, 1961 w. M. BRuBAKl-:R Erm. 3,010,017

MASS sPEcTRoMETER Nov. 21, 1961 w. M. BRUBAKER ETAL 3,010,017

MAss SPECTROMETER Filed June l, 1959 5 Sheets-Sheet 5 "limited States parent fornita Filed June l, 1959, Ser. No. 817,258 25 Claims. (Cl. Z50-41.9)

The present invention relates to a device for passing charged particles through magnetic fields. More particularly, charged particles of a selected mass moving in a magnetic field having an intensity gradient may be directed and focused either -with respect to angular dispersion in their direction of movement or with respect to both angular dispersion in their direction of movement and dispersion in their velocity of movement.

The movement of charged particles in a magnetic field is in a curved direction. The movement of charged particles in an electrical field is in a parabolic path. When a magnetic and an electric field are combined by crossing so as to be perpendicular to each other, charged particles immersed in these crossed fields move in a direction perpendicular to the magnetic field and along the electric field.

A charged particle subjected to crossed electric and magnetic fields normally moves in a cycloidal path. The average velocity Vector of the charged particle in the direction perpendicular to the magnetic fields, v, is given by the ratio of the electric field, E, to the magnetic field, B, with units in the meter-kilogram-second system of measurement.

Generally, charged particles enter the crossed fields with a velocity different from that given by E/B. Even if a particle enters the crossed fields with a velocity of E/B, its direction of movement is not normally in a straight line mutually perpendicular to the electric and magnetic fields. However, a particle with such characteristics of motion, that is, moving in a direction perpendicular to the electric and magnetic fields with an actual velocity of E/ B, continues to travel in the straight line at the E/B velocity so long as the crossed electric and magnetic fields remain constant. Furthermore, if both the electric and magnetic fields change in intensity so that E/B remains constant, the particle continues to move in the straight line.

lt has been found that in crossed electric and magnetic elds in which E/B is a constant, particles moving in an angular direction slightly deviating from a line mutually perpendicular to the electric and magnetic fields periodically cross this line. The distance between such crossings for such angularly dispersed particles is nvm/L1B, where m is the particle mass, q is the particle charge, and the particles have the same mass and velocity.

lt has further been found that particles initially moving in a direction perpendicular to the crossed electric and magnetic fields at a velocity only slightly differing from the constant E/ B follow a path which deviates from a straight line mutually perpendicular to the electric and magnetic fields and periodically return to this line. rIhese points of return or focus of velocity or energy dispersed particles have a spacing of 2mm/QB. Therefore, it is apparent, by comparing the distance between angularly dispersed particle focuses, nvm/QB, to the distance between velocity dispersed particle focuses, 21rvm/qB, that there are twice as many focal points for angularly dispersed particles within a given distance as for like velocity dispersed particles.

inasmuch as the E/B ratio gives the vector of velocity of all charged particles along the line mutually perpendichil? Patented Nov. 2l, i361 ular to the crossed electric and magnetic fields, like charged particles emanating from a point source, Whether angularly dispersed or velocity (energy) dispersed, bunch or focus at those points corresponding to the velocitydispersed focal points. Furthermore, those points corresponding to angular dispersion crossings which are not velocity dispersion crossings correspond to the points at which the physical dispersion between the paths of the velocity dispersed particles is a maximum and the physical dispersion between angular dispersed particles is a minimum.

There exists a need for a device which is operable to introducev charged particles into a magnetic tield. Since charged particles normally follow a curved path in a magnetic field, such introduction utilizing conventional means is difficult and often impossible. For example, in a device such as a mass spectrometer, an intense magnetic field exists in the device itself, and radiating outwardly from the device is a fringing magnetic field which has an intensity gradient roughly inversely proportional to its distance from the device. The fringing magnetic field has this intensity gradient due to the magnetic permeability of the substance surrounding the mass spectrometer and its magnet. Thus, charged particles passing through this fringing magnetic field prior to their introduction into the mass spectrometer tend to assume a circular path which makes their introduction into the mass spectrometer difficult. For this reason, the conventional practice is to initially utilize particles having a neutral charge. These neutral particles can pass through the fringing magnetic field at a relatively low velocity without experiencing path deviation caused by the fringing magnetic field, since they have no charge. When immediately adjacent the mass spectrometer inlet aperture, the particles are ionized and subjected to an electric field and are accelerated. This electric field accelerates the charged particles and forces them into the mass spectrometer.

According to the present invention, an electrical field is set up which is perpendicular to the fringing magnetic field and which varies in intensity with the variations in intensity of the fringing magnetic field.

ln its preferred embodiment, the invention comprises a device consisting of two field-forming electrodes between which an electrical potential difference exists, so as to form an electrical field therebetween. The device is utilized to pass charged particles through a fringing magnetic field without the particles assuming a circular path.

The field-forming electrodes are positioned about an axis along which charged particles travel. These electrodes are shaped so as to diverge asymtotically from the axis and oriented so as to have an increasing divergence as the fringing magnetic field decreases. With such a configuration, for a given electrical potential difference existing between the field-forming electrodes, the electrical field strength along the axis varies inversely with the amount of divergence of the electrodes from the axis. The variation in strength of the electrical field is therefore proportional to the variation in the fringing magnetic field strength.

By properly shaping the electrodes with respect tothe strength of the fringing magnetic field, a constant E/B ratio between the electric field of the field-forming electrodes and the fringing magnetic field is obtained.

ln an alternate embodiment, the same result is achieved by positioning a plurality of electrode pairs adjacent each other about the axis. The potential difference between opposite pairs of the field-forming electrodes is then selected so as to maintain a substantially constant E/B.

As is stated above, charged particles follow a substantially straight line path in a constant E/B field, with traidor? particles of like mass passing through angular dispersion and velocity dispersion focusing points. Also, the actual points of focus for a particular particle mass are determined by the velocity and the mass of the particle. One or more additional electrodes are therefore utilized adjacent either end of the field formed by the field-forming electrodes to control the velocity of the particles passing between the held-forming electrodes. Potentials appro priate to give the charged particles the velocity required to focus particles of a selected mass at` a desired point are then applied to these velocity-control electrodes.

The intensity and intensity gradient of the electrical tield required to be set up by the field forming electrodes in a given application of the invention is determined by the intensity and intensitygradient of the fringing magnetic field. The intensity and intensity gradient of the fringing magnetic iield may be measured by any one of a variety of conventional methods.

|In the preferred embodiment, the held-forming electrodes are shaped so that the intensity of the electrical. 4'ield therebetween changes in proportion to changes in the intensity of the ringing magnetic field. The actual coniiguration of the electrodes is determined by the gradient of the intensity of the magnetic tield. The electrodes are constructed to diverge in proportion to the weakening of the magnetic iield. The held-forming electrodes of the preferred embodiment, which are utilized to give substantially straight line motion to the charged particles, have heretofore been described as diverging. However, these electrodes may also be considered to converge asymptotically to parallel lines as the fringing magnetic ield strength increases. vllt is to be understood that, as is used herein, the word diverging refers to the physical coniiguration of the electrodes with respect to the point of closest physical proximity of the electrodes,l andincludes electrodes which can conversely be described as asymptotically converging.

ln the alternate embodiment of the invention, the required electric ield is set up by applying Vappropriate potential differences between opposite pairs of electrodes so as to give a substantially constant E/B for the measured B of the `fringing magnetic lield.

Other methods or' setting up the required electrical field and field gradient will be apparent to those skilled in the art. For example, the electrical iield and iield gradient can be set up by utilizing semiconductor materials for the ield-forming electrodes. The eld is then set up by the potential difference between opposite portions of the electrodes, and the iield gradient is set up by the potential drop along the electrodes.

Cycloidal mass spectrometers, that is, mass spectrometers in which, due to mutu-ally perpendicular electric and magnetic fields, ion beams transverse cycloidal paths to a resolution point, are Well known. Cycloidal mass spectrometers may be divided into two classes: those in which the ion beam follows a curtate cycloidal path, and those in which the ion beam follows a prolate cycloidal path. A mass spectrometer of the former type is described in U.S. Patent No. 2,844,726, issued July 22, 1958 to C. F. Robinson, and assigned to Consolidated Electrodynamics Corporation, the assignee of the present application. Such a mass spectrometer is `also described in 27 Review of Scientiiic Instruments 504 (1956).

Conventional cycloidal mass spectrometers have a comparative disadvantage of being somewhat insensitive. This insensitivity is due to the fact that the collector electrode utilized to measure the magnitude of the ion beam is located Within the cycloidal tube. Therefore, thc collector electrode must be of comparatively simple configuration and construction in order to avoid disturbing the electric and magnetic iields within the cycloid tube. This collector electrode is conventionally placed at the 360 point of the ion beam path, that is, the point at which `the ion -beam of a selected mass of charged particles constitutes an image of the ion beam of the selected mass of charged particles as they entered the cycloid tube.

Utilizing the present invention, the conventional co1- lector electrode within the cycloid tube is replaced by a resolution aperture or slit, through which pass ions of the beam having a selected mass, so as to accomplish mass resolution of the ion beam. These ions then commence an additional cycle of their cycloidal paths, identic-al to their liirst cycle insofar as the crossed electric and magnetic fields remain constant.

The desired mass resolution of the ion beam having taken piace, so that only ions of the selected mass remain in the ion beam, the resolved ion beam is removed from the cycloid tube. In order to prevent the dispersion of the resolved ion beam by the fringing magnetic iield, the resolved ion beam is passed through a charged particle device constructed according to the invention. The charged particle device serves to cause the ion beams to move in a substantially straight line path, and also focuses the ion beam.

The focused and resolved ion beam then is measured by a conventional ion measuring device. For example, an electron multiplier may be used. Such an electron multiplier as is described in US. Patent No. 2,854,583, issued September 30, 1958 to C. F. XRobinson, and assigned to Consolidated Electrodynamics Corporation, the assignee of the present application, may be utilized.

In most applications, Vthe particles to be analyzed are neutral in charge. Neutral charges can be introduced into the magnetic field of the mass spectrometer through the fringing magnetic iield surrounding the mass spectrometer Without having their paths affected by the fringing magnetic field. Once introduced into the magnetic field of the mass spectrometer, the particles are ionized by au electron beam. Mass resolution of the charged particles then is accomplished.

If the particles to be analyzed in the mass spectrometer are already charged, their introduction into the mass spectrometer through the fringing magnetic iield is diiii cult. If the charged particles are in a beam, the beam tends to lose its coherent nature on transmigration of the fringing magnetic field. In order to overcome this diiiiculty, thecharged particle device of the present invention is utilized in conjunction with mass spectrometers to ionize the particles to be resolved, if necessary, to pass the charged particles through the iield gradient of the fringing magnetic field, and to focus the charged particles at the mass spectrometer inlet aperture with respect to either angular dispersion, or both angular and velocity dispersion. The focused charged particle beam is then introduced into the mass spectrometer in the conventional manner.

The invention may be more readily understood by reference to the accompanying drawing in which:

FIGURE l (parts (a) and (b) taken together) illustrates curtate cycloidal paths of charged particles in crossed electric and magnetic iields;

FIGURE 2 (parts (a) and (b) taken together) illustrates the paths of charged particles of like mass moving in crossed electric and magnetic elds which increase in intt'ensity while maintaining a substantially constant E/B ra 1o;

FIGURE 3 (parts (a) and (b) taken together) illustrates the paths of charged particles of like mass moving in crossed electric and magnetic iields which decrease in intensity while maintaining a substantially constant E/B ratio;

FIGURE 4 is a sectional view of a device according to the invention utilized to introduce particles into a strong magnetic iield through a magnetic field increasing in strength;

-FEGURE 5 is a schematic sectional View of a mass spectrometer employing the device of the invention to remove charged particles from the mass spectrometer through a fringing magnetic field; and

FIGURE 6 is a schematic sectional View of a mass spectrometer employing a device of the invention to introduce charged particles into a mass spectrometer through a fringing magnetic .field yand a device of the invention to remove charged particles from the mass spectrometer through the fiinging magnetic field.

Referring to FIG. im), cycloidal paths for three charged particles having the same velocity and of mass M, M-f-AM, M-AM are shown. A mass resolving plate 10 has a 180 point aperture il and a 360 point aperture 12 therein. Charged particles `from a source 13 follow curtate cycloidal paths due to crossed electric and magnetic fields (not shown). Particles of mass M follow path .t4 and pass through the apertures l1 and i2. These particles of mass M continue to follow a curtate cycloidal path so long as they remain in the crossed electric and magnetic fields. Particles f mass M-AM follow a curved path i'. These particles of mass M-AM strike the resolving plate and impinge thereon. Similarly, particles of mass M-j-AM follow a curved path 16 and strike the resolving plate l0. The

curved paths

15 and 16 are the 0 to 180 and 0 to 360 portions, respectively, of curtate cycloids.

In FIG. 1(b), the curtate cycloidal paths of charged particles having identical mass but different velocities are illustrated. These particles, from a source 13', move in crossed electric and magnetic fields (not shown) and pass through the 180 aperture l1 of the mass resolving plate 10. The particles continue in their curtate cycloidal paths, and pass through the 360 aperture l2 in a true focus. The 360 aperture 1 2 serves to mass-resolve the charged particle beam. These mass resolved particles commence a second curtate cycloidal cycle after passing through the 360 aperture l2.

In FIGURE 2, a device according to the invention for injecting charged particles into a strong magnetic field through a fringing magnetic field, that is, a magnetic field having a strength gradient, is illustrated.

FIGURE 2(a) illustrates the paths followed by charged particles in a fringing magnetic field perpendicular to which an electric field is superimposed so as to maintain a substantially constant E/B ratio along a line located between two held-forming electrodes. A first field-forming electrode 2d is connected to a lirst variable electrical potential source 2l. A second field-forming electrode 22 is connected to a second variable potential source 23. rl`he charged particles passing between the field-forming

electrodes

2,0 and 22 differ from each other only in having angular dispersion.

Assume that the charged particles enter the electrical field formed by the field-forming electrodes with a velocity of ElBl from a point or narrow slit source aperture. Assume also that the crossed fields along a line or axis midway between the field-forming electrodes have a value of ElBl. Those entering the crossed electric and magnetic fields along the axis between the field-forming electrodes 2) and 22 with no angular dispersion with respect to this axis follow a straight line path 24 which coincides with the axis. Particles entering the crossed iields along the axis, but whose motion is angularly dispersed from the axis, follow paths such as 25 and 26, thek particular path followed depending upon the magnitude and direction of the angular dispersion with which the particle enters the crossed electric and magnetic elds. The eiect of the crossed electric and magnetic fields on the angularly dispersed charged particles is such as to focus all of these particles at a first angular dispersion focal point 29 and again at a second angular dispersion focal point 30. lt will be noted that the focal length between the point at which the particles enter the crossed iields and the first focal point 29 is much greater than the focal length between the first focal point 29 and the second focal point 30. This change in focal length is due to the fact that the focal length varies inversely with B, the -fringing magnetic eld strength.

FIGURE 2(b) illustrates the paths followed by velocity dispersed particles in the charged particle device. The charged particle device consisting of lfirst and second electrodes 2f# and 22 and two potential sources 2l and 23 is the same as is illustrated in FIG. 2(0). Also, the charged particles whose paths are shown in FlG. 2(b) have the same mass and charge as the particles whose paths are shown in FIG. 2(a). Those charged particles entering the crossed electric and magnetic fields on the axis between the field-forming

electrodes

20 and 2,2 at a velocity of ElBl follow the straight line path 24. Those particles entering the crossed electric and magnetic fields at the axis, but whose velocity is greater or less than ElBl follow paths 3l and 32. The field-forming electrodes Ztl and 22 have a configuration which is determined by the intensity gradient o-f the magnetic field in which they are immersed. The most common gradient of such fields is a decrease in the intensity with distance in an inverse square relationship. All of these velocity dispersed particles are focused at a velocity dispersion focal point 35, which corresponds to the second angular dispersion yfocal point 30 of FIG. 2(0).

ln FIG. 3 a device according to the invention for extracting charged particles from a strong magnetic field through a fringing magnetic field is illustrated. The components illustrated in FIG. 3 are identical to the components illustrated in PEG. 2.

in FIG. 3(0) the paths of angularly dispersed ions of like mass, charge, and velocity are illustrated. Those particles entering the crossed electric and magnetic fields along the axis between the two field-forming electrodes Ztl and 22 with a velocity of ElBl follow the straight line path 24. rlhose charged particles entering the crossed electric and magnetic fields along the axis, but with angular dispersion with respect to the axis, follow

paths

25 and 26. All of these velocity dispersed particles are focused at a iirst velocity dispersion focal point 29 and at a second velocity dispersion `focal point. 30.

ln PIG. 3(b), particles of like mass and charge entering the crossed electric and magnetic fields lof the extraction device with velocity dispersion are illustrated. Those particles entering the crossed fields along the axis etween the field-forming

electrodes

20 and 22 follow paths such as 3l and 32. The field-forming

electrodes

20 and 22 have a configuration which is determined by the intensity gradient of the magnetic eld in which they are immersed. All of these velocity dispersed ions are focused at a velocity dispersion focal point 35 which corresponds to the second angular dispersion focal point 36' of FIG. 3(a).

FIGURE 4 is a schematic sectional view of a charged particle device, together with velocity control electrodes and electrical potential sources, such as may be used to ionize particles and inject the ionized particles into a strong magnetic field through `a fringing magnetic field. The fringing magnetic field is indicated by an arrow indicating the direction of increasing field intensity, and the strong magnetic field exists between two pole faces 62 (only one of which is shown in dotted lines) of a mass spectrometer magnet, for example.

Nonionized particles indicated by the arrow l0 are ejected from a chamber al. The nonionized particles pass through an electron beam 42. The electron beam ionizes the particles passing therethrough. T wo accelerating electrodes 43 and 44 accelerate the ionized particles in the direction of a charged particle device 45. Alternatively, the ionized particles may be accelerated and directed into the charged particle device by use of only a single accelerating electrode i3 or 44. The use of two accelerating electrodes enables the ions to be formed in a -low electrical intensity, -while still providing for a sufficient velocity control to focus the charged particles with respect to a broad range of masses, as will be subsequently described. Y

The charged particle device i5 consists of two iieldforming electrodes 46A and del?, to which electrical potentials are applied so as to form the appropriate electrical iield between the field-forming

electrodes

46A and 46B. During their passage between the field-forming

electrodes

46A and 46B, charged particles of the same mass have two focal points: an angular dispersion focal point for ions of common mass and velocity but varying in initial angular direction of movement; and, a velocity focal point for ions of the same mass having velocity dispersion.

A path indicated by the solid line 47 corresponds to the path of anion entering the charged particle device in a direction perpendicular to the crossed ieldg .therein and moving at a velocity of B21/B1. lons differing from ions following the path 47 only in angular dispersion follow, for example, paths indicated by either of two lines i8 or 49, depending upon the direction of the angular dispersion. The three

paths

47, 48 and 49 meet at a tirst focal point 50, diverge therefrom, and again -meet at a second focal point l. Ions having velocity dispersion with respect to the ions following the straight line path 47 follow, for example, paths indicated by either of two lines 52 or S3. The angular and velocity dispersed ions following the paths S2 and S3 and the ions following the straight line path 47 first meet at the second focal point 5l. Thus, at the second focal point 51;, ions having a common ymass are focused both as to angular dispersion and as to velocity dispersion.

At the tirst focal point Si?, ions having a common velocity are focused with respect to angular dispersion. However, the physical spread between velocity-dispersed ions is a maximum at the focal point Sil. Thus, by selecting either the focal point 50 or the focal point 51 to coincide with an inlet aperture Sli of an appropriate apparatus, all charged particles having a common mass can be injected into the apparatus; alternatively, a selected portion of those ions exhibitimy velocity dispersion with respect to the median ion path can be excluded from the apparatus. In addition, by changing the electrical iield of the charged particle device, while `maintaining the same fringing magnetic held, the spectrum of velocity dispersion may be scanned at the inlet aperture S4.

In order to select which of the two focal points will be utilized, and in order to accommodate a wide range of velocities of charged particles, a variable electrical potential source 55 is utilized. The variable electrical poten-tial source 55, as illustrated in FIG. 4, consists or" a battery 56 and potentiometers 53, 59 and 66. The potentiometer 5S is utilized to adiust the potentials on the iieldforming electrodes 416A and 46B.

The potential of the iinst accelerating electrode 43 is adjusted by means of potentiometers S9 and 60 to set up the proper electrical held with ionization of the particle stream dit by the electron beam e2.

A velocity cont-rol electrode 611 is located between the charged particle device 45 and the inlet aperture Se. The potential difference between the second accelerating electrode 44 an-d the velocity control electrode 61 is adjusted by means of potentiometers S8, 59 and 60 to give a selected velocity to the charged particles passing through the charged particle device 45. r{"his velocity is selected so that the required focal points of the charged particles are brought with-in the charged particle device, since, as stated above, the focal points are determined by the velocity, v, and the mass, m, of the particle. Thus by utilizing a wide range of potential differences between the second accelerating electrode 44 and the isolating electrode 61, a wide nange of masses of charged particles may be selectively focused by the charged particle device.

FlGURE 5 illustrates an embodiment of the invention in which mass resolved ions are extracted from a cycloid tube at the 450 point of the cycloid path. A cycloid tube 70 has electrodes 7l to which electrical potentials are applied in order to establish an electrical potential gradient lor held across the tube in a direction perpendicular to a magnetic field B. Such tubes are wel-l known in the mass spectrometry art.

A charged particle device '72 is positioned about an outlet aperture '73 for the mass-resolved ion beam. An electron multiplier 74 and a conventional mult-ipl-ier output measuring means 75 measure the magnitude of the masseresolved ion beam after its passage through thecharged particle device 72 and focusing elect-rode 74.

A beam of particles or molecules (illustrated by a dotted line 76) is ionized by an electron beam 77 and introduced into the cycloid tube 79 through an inlet aperture 7:8 by any conventional means. For example, a repeller electrode 7-9 so charged as to deilect the ion beam into the cycloid tube may be utilized. The cycloid tube 70 contain-s an ion resolving plate lil having a 180 aperture l1' and a 360 aperture 12'.

Ions of a mass determined by the magnitude of the crossed electric and magnetic iields pass through the apertures ll and 12' and continue through the ion beam outlet aperture 73 into the charged particle device 72. Iihe ion beam outlet aperture '73 is positioned at the 450 point of the cuntate cycloida-l path followed by the resolved ion beam. The ions passing through the 360 point aperture l2' are in focus at that point, as explained previously with respect to FIG. l(b). However, at the 450 point of the path, the ions have diverged, due to velocity differences, and therefore the outlet aperture 73 is comparatively wide.

Tlhe charged particle device 72 serves to cause the ion beam to travel in a straight line path rather than in a cycloidal path. Thus the electric and magnetic fields within the charged particle device are of such a magnitude that the ratio of E, the electric field, to B, the magnetic iield, equals v, the velocity of the ion beam, all dimen sions being in the m-ks system. The electrical field, E, is provided by an electrical potential difference between the field-forming electrodes of the charged particle de` vice 72. The magnetic eld, B, is the fringing magnetic field which exists adjacent the cycloid tube. Since the actual magnetic eld surrounding a particular mass spectrometer is dependent upon the physical coniiguraton of the spectrometer and its magnet, the dimensions of the charged particle device 72 must be deter-mined with respect to the particular mass spectrometer and magnet in order to maintain the E/B ratio constant.

As previously stated, small variations in the velocity of the ions exist. By adjusting the electrical potential difference between the field-forming electrodes of the charged particle device 72, the fields therein are made to be of the proper magnitude so that E/B along the axis equals the median resolved ion velocity. Those resolved ions having differing velocities then oscillate about the median velocity ions as the ion beam moves thnough the charged particle device. The ions periodically pass through velocity and angular dispersion focal points.

A focusing electrode serves to regulate the velocity of the ion beam emerging from the charged particle device 72. The ion beam is focused either with respect to angular dispersion or angular and velocity dispersion by a potential applied to the focusing electrode 80 so that the ions are in a narrow beam as they strike a iirst dynode 79 of the electron multiplier 74. Each ion striking the first dynode 79 causes a plurality of electrons to be emitted therefrom. Each of these emitted electrons strikes the next dynode of the multiplier, causing a plurality of electrons to be emitted therefrom. By this process, the number of electrons emitted from the first dynode '79 is multiplied in the succeeding stages of the electron multiplier 74. An electrical current is thereby produced, the magnitude of which is proportional to the number of ions striking the first dynode 79. This electron multiplier output current is measured by the multiplier output meas.-

9 uring means 75 to indicate the current and thus the number of ions striking the rst dynode 79.

The mass spectrometer illustrated in FIG. is relatively compact. However, the positioning of the charged particle device at the 450 point of the curtate cycloidal path may cause the crossed electric and magnetic fields within the spectrometer to become non-uniform at points on the curtate cycloidal path prior to the 360 point, the portion in which mass resolution occurs. Such non-uniformity in the crossed fields causes the mass spectrometer to give fallacious results. In order to overcome this difficulty, alternative configuration of a mass spectrometer and charged particle device as shown in FIG. 6 may be utilized.

In FIG. 6, a cycloid tube 90 has electrodes 91, to which electrical potentials are applied to establish an electrical potential gradient or field across the tube. A magnetic field B perpendicular to this electrical gradient or field is established by magnets (not shown). The cycloid tube 90 contains an ion resolving plate l0" having a 180 aperture Il and a 360 aperture 12". A charged particle stream 92 is introduced into the cycloid tube 90 at an inlet aperture 93 through the fringing magnetic field by a charged particle device 94.

A charged particle device 95 is positioned about an outlet aperture 96 for the mass-resolved charged particle beam. The outlet aperture 96 is positioned to correspond to the 540 point of the cycloid path followed by the mass-resolved particles. A focusing electrode 97 is positioned adjacent the outer extremity of the charged particle device 95. An electron multiplier '74 and a multiplier output means (not shown) measure the magnitude of the mass-resolved ion beam after its passage through the charged particle device 95 and the focusing electrode 97.

The operation of the mass spectrometer of FIG. 6 is similar to that described above with respect to FIG. 5. The beam of charged particles 92 is introduced into the mass spectrometer of an inlet aperture 93. The charged particle device 94 is utilized to pass the charged particle beam through the fringing magnetic field in substantially a straight line. The dimensions and potentials of the charged particle device 94 are selected to give a focusing of charged particles of a selected mass at the inlet aperture 93. The mass of the particles injected into the mass spectrometer is selected by adjusting the velocity of the particles, as is described with respect to FIG. 4. The velocity or energy dispersion of the particles of the selected mass is then scanned by focusing the beam on the inlet aperture of an angular dispersion focal point which is not a velocity dispersion focal point. By varying the E/B ratio of the charged particle device, the velocity dispersion spectrum may be swept across the inlet aperture 93.

Rather than extracting the ions from the cycloid tube at the 450 point of the cycloid ion path, the mass-resolved ions are extracted at the 540 point of the path. Positioning of the charged particle device 95 at this 540 point removes the charged particle device 95 from the proximity of the unresolved ion beam. Therefore, any effects which the charged particle device 95 has upon the electric and magnetic fields in its immediate vicinity are not refiected back into the unresolved portion of the charged particle beam.

We claim:

l. A device for passing charged particles through a magnetic field having a field strength gradient comprising a first and a second electrode, each having a pre-determined configuration positioned so as to diverge from each other about an axis, a source of electrical potential, means for applying a potential difference from said source of electrical potential between the said first and second electrodes to produce an electrical potential gradient therebetween, and accelerating electrodes positioned adjacent the divergent ends of said first and second electrodes, a velocity control electrode positioned adjacent the ends of the first and second electrodes remote from the divergent ends thereof, and means connected to said source of electrical potential for applying a potential difference between said accelerating electrode and said velocity control electrode.

2. A device for injecting neutral particles into a magnetic fieldthrough a fringing magnetic field having a field strength gradient comprising a source of nonionized particles, a beam of electrons positioned so as to intercept and ionize at least a portion of the nonionized particles, a first and field-forming second electrode, each having a predetermined configuration and positioned so as to diverge from the other along an axis, a source of a selected electrical potential difference, means for applying the selected electrical potential dierence to said first and second electrodes to produce an electrical potential gradient therebetween, said first and second electrodes being positioned so that the divergent ends thereof are adjacent the electron beam.

3. A device as defined in claim 2 and including velocity control electrodes positioned adjacent the ends of the first and second electrodes and an electrical potential difference applied between said velocity control electrodes.

4. A device as defined in claim 3 and including an ionization field forming electrode positioned adjacent the electron beam and an electrical potential connected to said ionization field forming electrode.

5. In a mass spectrometer, the combination of an ion beam, a first and .a second electrode, each having a predetermined configuration and each positioned so as to diverge from the other about an axis, a source of a selected electrical potential difference, means for applying the electrical potential difference to said first and second electrodes so as to produce an electrical potential gradient therebetween, said first and second electrodes being positioned so that the ion beam emerging passes between the first `and second electrodes.

6. In a mass spectrometer, the combination as defined in claim 5 in which lthe mass spectrometer is of the cycloid tu e type and the resolved ion beam is removed from the cycloid tube at the 450 point of its curtate cycloidal path.

7. In a mass spectrometenthe combination as defined in claim 5 in which the mass spectrometer is of the cycloid tube type and the ion beam is removed from the cycloid tube at the 540 point of its curtate cycloidal path.

.8. In a mass spectrometer having an ion beam inlet aperture, the combination of an ion beam, a first and a second field-forming electrode, each having a predetermined configuration and each positioned so as to diverge from the other about an axis, a source of a selected electrical potential difference, means for applying the electrical potential difference between said first and second field-forming electrodes so as to produce an electrical potential gradient, said first and second electrodes being positioned so that the ion beam passes therebetween'and through the mass spectrometer inlet aperture;

9. In a mass spectrometer, the combination of an ion beam, means for introducing the ion beam into the mass spectrometer, means for removing the resolved ion beam from the mass spectrometer, a first and a second fieldforming electrode, each having a predeterminedv configuration and each positioned so as to diverge from the other about an axis, a source of a selected electrical potential difference, means for applying the electrical potential difference to said first and second field-forming electrodes so as to produce an electrical potential gradient therebetween, said first and second electrodes being positioned so that the ion beam emerging from the mass spectrometer passes between the first and second electrodes, and means for measuring the ion beam subsequent to its passage between the first and second electrodes.

l0. In a mass spectrometer, the combination of an ion beam, a cycloid tube adapted to provide for mass resolution of an ion beam entering the tube at an ion beam entrance aperture and following a curtate cycloidal path therein at the 360 point of the path and a resolved beam outlet aperture for removal of the resolved beam from the tube, means for introducing the ion beam into the tube at the ion beam entrance aperture including a first and a second field-forming electrode, each having a substantially inverse square configuration and each positioned so as to diverge from the other about an axis, a source of a first selected electrical potential difference, means for applying the first electrical potential difference between said first and second electrodes so as to produce an electrical potential gradient, said first and second electrodes being positioned adjacent said entrance aperture, a third and fourth field-forming electrode, each having a substantially exponential configuration and each positioned so as to diverge from the other about an axis, a source of a second selected electrical potential difference, means for applying the second electrical potential difference between the third and fourth electrodes so as to produce an electrical potential gradient, said third and fourth electrodes being positioned so that the resolved ion beam emerging from the tube passes between the third and fourth electrodes, and means for measuring the resolved ion beam subsequent to its passage between the third and fourth electrodes.

l1. In a mass spectrometer, the combination as defined in claim in which the resolved ion beam is removed from the cycloid tube at the 450 point of its curtate cycloidal path.

l2. In a mass spectrometer, the combination as defined in claim 10 in which the resolved ion beam is removed from the cycloid tube at the 540 point of its curtate cycloidal path.

13. In a mass spectrometer, the combination of a source of particles to be analyzed, a cycloid tube adapted to provide mass resolution of an ion beam entering the tube atan ion beam entrance aperture and following a curtate cycloidal path therein at the 360 point of the path and a resolved beam outlet aperture for removing the resolved beam from the tube, means for ionizing the particles, a first and a second electrode, each having a substantially inverse square configuration and each positioned so as to diverge from the other about an axis, a source of a first selected electrical potential difference, means for applying the first electrical potential difference between said first and second electrodes so as to produce an electrical potential gradient, said first and second electrodes being positioned adjacent the ion beam entrance aperture so as to separate the entrance from the ionizing means, whereby at least a portion of the ionized particles pass between the first and second electrodes and enter the cycloid tube, a third and fourth electrode, each having a substantial exponential configuration and each positioned so as to diverge from the other about an axis, a source of a second selected electrical potential difference, means for applying the second electrical potential difference between the third and fourth field-forming electrodes so as to produce an electrical potential gradient, said third and fourth electrodes being positioned adjacent the resolved beam outlet aperture so that the resolved beam emerging from the cycloid tubeV passes between the third and fourth electrodes, and means for measuring the ion beam subsequent to its passage between the third and fourth electrodes.

14. The combination as defined in claim 13 in which the resolved ion beam is removed from the cycloid tube at the 450 point of its curtate cycloidal path.

l5. The combination as defined in claim 13 in which the' ion beam is removed from the cycloid tube at the 540 point of its cui-tate cycloidal path.

16. In a mass spectrometer, the combination of a source of particles to be analyzed, a cycloid tube adapted to provide for mass resolution of an ion beam entering the tube atan ion beam entrance `aperture and following a curtate cycloidal path therein at the 360 point of the path and a resolved beam outlet aperture for removal of the resolved beam from the tube, means for ionizing the particles, a first and a second field-forming electrode, each having a substantially inverse square configuration and each positioned so as to diverge from the other about an axis, a source of a first selected electrical potential difference, means for applying the first electrical potential difference between said first and second electrodes so as to produce an electrical potential gradient, said first and second electrodes being positioned adjacent the ion beam entrance aperture so as to separate the entrance from the ionizing means, means including at least one accelerating electrode positioned between the ionizing means and the first and second electrodes for causing atleast a portion of the ionized particles to pass between the first and second electrodes and enter the cycloid tube, a third and fourth field-forming electrode, each having a substantially exponential configuration and each postioned so as to diverge frorn the other about an axis, a source of a second electrical potentiaf difference, means for applying the second electrical potential difference between the third and fourth electrodes so as to produce an electrical potential gradient, said third and fourth electrodes being positioned yadjacent the resolved beam outlet aperture so that the resolved beam emerging from the cycloid tube passes between the third and fourth electrodes, and means for measuring the ion beam subsequent to its passage between the third and fourth electrodes.

17. in a mass spectrometer, the combination as defined in claim 16 in which the resolved ion beam is removed from the cycloid tube at the 450 point of its curtate cycloidal path.

18. In a mass spectrometer, the combination as defined in claim 16 in which the ion beam is removed from the cycloid tube at the 540 point of its curtate cycloidal path.

19. In a mass spectrometer, the combination of a source of particles to be analyzed, a cycloid tube adapted to provide for mass resolution of an ion beam entering the tube at an ion beam entrance aperture and following a curtate cycloidal path therein at the 360 point of the path 'and a resolved beam outlet aperture for removal of the resolved beam from Ithe tube, means for ionizing the particles, a first and a ysecond field-forming electrode, each having a substantially inverse square configuration and each positioned so as to diverge from the other `about an axis, a source of a first selected electrical potential difference, means for applying the first electrical potential difference between said first and second electrodes so as to pro-duce an electrical potential gradient, said first and second electrodes being positioned adjacent the ion beam entrance aperture so as to separate the entrance from the ionizing means, means including at least one velocity-control electrode positioned between the first and second electrode and the ionizing means and at least one velocitycontrol electrode positioned between the first and second electrodes and the ion beam entrance aperture for causing at least a portion of the ionized particles to pass between the first and second electrodes and enter the cy cloidal tube at a selected velocity, a third and a fourth field-forming electrode, each having a substantially exponential configuration and each positioned so as to diverge from the other about an axis, 4a source of a second selected electrical potential difference, means for applying the second electrical potential difference between the third and fourth electrodes so as to produce an electrical potential gradient, said third and fourth electrodes being positioned adjacent the resolved beam outlet aperture so that the resolved beam emerging from the cycloidal tube passes between the third and fourth electrodes, and means for measuring the ion beam subsequent to its passage between the third and fourth electrodes.

20. In a mass spectrometer, the combination as defined in claim 19 in which the resolved ion beam is removed from the cycloid tube at the 450 point of its curtate cycloidal path.

2l. In a mass spectrometer, the combination as defined in claim 19 in which the ion beam is removed from its cycloidal tube at the 540 point of the curtate cycloidal path. t

22. A charged particle device for passing charged particles through a magnetic eld having a field strength gradient comprising at least two opposed electrodes and an electrical potential applied to each electrode, in which said electrodes are positioned with respect to the fringing magnetic field so that the electrical field set up between opposed electrodes by the application of the electrical potentials thereto is perpendicular to the magnetic field, and the electrical field strength has a gradient which establishes a line through the fringing magnetic field whose locus is defined |by a substantially constant ratio of electrical field strength to magnetic field strength.

23. A charged particle device as defined in claim 22, in which the line through the magnetic field is a substantially straight line.

24. A device for injecting charged particles into a magnetic field through a fringing magnetic field having a field strength gradient comprising opposed electrodes each of which converges on an axis in a substantially inverse square relationship as the fringing magnetic field strength increases and a source of electrical potential 14 applied to each electrode, whereby the ratio of the electric field strength between the electrodes to the fringing magnetic field strength between the electrodes remains substantially constant along the axis of convergence of the electrodes.

25. A device for extracting charged particles from a fringing magnetic field through a fringing magnetic field having a field strength gradient comprising opposed electrodes each of which diverges from an yaxis in a substantially square relationship as the fringing magnetic field strength decreases and a source of electrical potential applied to each electrode, whereby the ratio of the electric field strength between the electrodes to the magnetic field strength lbetween the electrodes remains substantially con stan-t along the axis of divergence of the electrodes.

References Cited in the file of this patent UNITED STATES PATENTS 2,473,031 Larson June 14, 1949 2,780,729 Robinson Feb. 5, 1957 2,794,126 Robinson May 28, 1957 2,806,956 Hall Sept. 17, 1957 2,880,356 Charles et al May 31, 1959 UNITED STATES PATENT oEEIcE CERTIFICATION OF CORRECTION Patent Noi,v .39010,017 November 217 1961 Wilson M. Brubaker et al.`

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 111 line 7]7 strike ou t "fringing" first, occurrenceQ7 and insert the Same after "the'HI in line 13 second occurrence6 Signed and sealed this 24th day of April 1962..

(SEAL) Attest:

EsToN 6 JOHNSON DAVID L. LADD Attesting Officer Commissioner of Patents