US2768305A - Mass spectrometer - Google Patents

Description

Oct. 23, 1956 w. H. WELLS MASS SPECTROMETER 2 Sheets-Sheet l Filed March 24. 1952 1N VEN TOR. VV/LL/AMH. WELLS ATTORNEY.

W. H. WELLS MASS SPECTROMETER Oct. 23, 1956 2 Sheets-Sheet 2 Filed March 24, 1952 INVENTOR.

WILLZAMHQ WELLS ATTORNEY United States Patent MASS SPECTROMETER William H. Wells, Detroit, Mich., assignor to Bendix Aviation Corporation, Detroit, Mich, a corporation of Delaware Application March 24, 1952, Serial No. 278,215

8 Claims. (Cl. 2504 1.9)

This invention relates to mass spectrometers and more particularly to mass spectrometers for providing a relatively sharp delineation between ions of diiierent mass by measuring the time required for the ions to travel through a predetermined angular distance. The invention also relates to methods of providing such a relatively sharp delineation between ions of different mass.

Mass spectrometers are adapted in general to measure the molecular masses of the different gases in an unknown mixture and sometimes to indicate the relative abundance at which the molecules of the different gases in the mixture occur. Some spectrometers operate on the principle of producing a pulse of ions from molecules of the diilerent gases and applying a predetermined force to accelerate the ions through a predetermined distance. i

This distance may be either linear or curvilinear. The ions of relatively light mass have a greater acceleration imparted to them by the force than the ions of heavy mass and, therefore, travel through the predetermined distance before the ions of heavy mass. By providing an indication of the relative times at which the ions travel through the predetermined distance, the masses of the ions can be determined,

Spectrometers subjecting the ions to a curvilinear travel path are used for a number of reasons. An important reason for the use of a curvilinear travel path is that the spectrometer can be smaller than if a straight travel path of the same distance is imposed. This is especially true where the ions travel in a repetitive manner such that they describe a predetermined number of revolutions before they are collected. In such spectrometers, the ions of different mass become separated relatively sharply in space and produce relatively sharp indications on the basis of their mass when they are collected.

However, spectrometers subjecting the ions to curvilinear paths are disadvantageous for several reasons. They require the use of a magnetic field as well as an electrical field to subject the ions to such a curvilinear path. The requirements for magnetic as well as electrical fields increases the power required to operate the equipment as well as the weight of the equipment and its complexity of construction. Furthermore, when the ions travel through a plurality of revolutions the ions of different mass tend to travel axially down the spectrometer as well as radially. Because of such axial travel, many ions are collected after a different number of revolutions than the desired plurality. These ions tend to cloud the measurements which are obtained and to reduce the accuracy of these measurements.

This invention provides a mass spectrometer in which the ions describe orbital movements from a common starting position and in which all of the ions return to their starting position at the end of each orbit. Since all of the ions return to a common position even after a plurality of orbital revolutions, strong output signals are produced by the collection of all of the ions after a predetermined number of revolutions. The spectrometer produces such focu ssing of the ions without requin ing the use of any magnetic field to direct the ions through a circular path. Because of the elimination of a magnetic field and the focussing action inherent in the operation of the spectrometer, the spectrometer is relatively simple, compact and inexpensive.

An object of this invention is to provide a mass spectrometer for measuring the masses of different ions by determining the relative times required for .the ions to travel through a predetermined angular distance.

Another object of the invention is to provide a mass spectrometer of the above character for producing an angular rotation of the ions through a predetermined number of orbits and for collecting the ions after their rotation through the orbits.

A further object is .to provide a mass spectrometer of the above character for subjecting the ions to a predetermined electrical field to produce orbital rotations in which each orbit of the different ions commences at substantially the same position.

Still another object is to provide a mass spectrometer of the above characterwhich does not require the use of any magnetic field to subject the ions to a curvilinear travel path.

A still further object is to provide a mass spectrometer which is simple, compact and inexpensive without sacrificing any accuracy in measurement in comparison to spectrometers now in use.

Another object is to provide methods of subjecting the ions of difierent mass to orbital movements and determining the relative times required for the ions of diiferent mass to travel through a predetermined number of orbits,

Other objects and advantages will be apparent from a detailed description of the invention and from the appended drawings and claims.

In the drawings:

Figure 1 is a perspective view, with some parts broken away and other parts shown in block form, illustrating one embodiment of the invention; and

Figure .2 is a sectional viewschematically illustrating the operation of the embodiment shown in Figure 1.

In one-embodiment of the invention, a wedge-shaped cathode 10 is provided and is formed from a suitable material such as tungsten to emit electrons upon the application of heat. A control grid 12 having a vertically disposed slot 14 is disposed relatively close to the cathode It) with the median position of the slot lying in substantially the same horizontal plane as the cathode 10. An accelerating grid 16 having a-slot 18 corresponding in shape and position to the slot 14 is in turn disposed relatively close to the grid 12 and in substantial alignment with the grid. A collector plate 20 is provided in parallel relationship with thegrids 12 and 16 at a relatively great distance from the grids.

A backing plate 22 is positioned between the grid 16 and the collector plate 20 slightly in back of a line drawn from the tip of the cathode 10 through the median positions of the slots 14 and 18 to the collector plate 2% In like manner, a grid 24 having a slot 26 is positioned slightly in front of this line in parallel relationship with the backing plate 22. A conduit 28 communicates at one end with a slot (not shown) in a bottom plate 30 connected between the backing plate 22 and the grid 24, and at the other end the conduit communicates with a receptacle 32 adapted to hold the molecules of the different gases in an unknown mixture.

The grid 24 faces a suitable window 34 in a conductive spherical electrode 36. The electrode 36 is concentric with an inner' spherical electrode 38 also made from a suitably conductive material. The electrode as is maintained in fixed position relative to the electrode 38 by an insulating bushing 40 suitably secured in spaced relationship to the electrodes.

A suitably insulated wire 44 is wound on one of the bushings 46 such that the greatest concentration of turns appears adjacent the electrode 38 and the concentration of turns progressively decrease along the axial length of the bushing. The end of the wire 44- contiguous to the electrode 38 is connected to the electrode, and the other end of the wire is connected to the electrode 36. Connections are also made from the electrodes 3d and 38 to appropriate output terminals of a suitable direct power supply 46. The lead extending from the power supply 47 to the electrode 38 is preferably threaded through a hole in the center of the bushing 46.

A pair of substantially aligned deflecting plates 5% and 59 are provided in the region between the electrodes 36 and 33. The plate 48 is connected to a termin l in the winding 44 positioned from the outer terminal by distance corresponding to the radial distance between the plate and the electrode 36. Similarly, the plate 5 3 is connected to a terminal in the winding 44 disposed at a distance from the inner terminal corresponding to the radial distance between the plate and the electrode 33. The plates are disposed in perpendicular relationship to the radial line extending through their median position and are preferably slightly curved to adopt the position of a circular arc.

The deflecting plates 48 and 50 face a window 52 disposed in the electrode 36 at an intermediate position between the projections of the deflecting plates. A collcctor plate is provided adjacent the window 52 on the exterior side of the electrode 36 and is disposed within a shield 56 in recessed relationship to the shield. The side of the shield 56 facing the electrode 36 made from a suitable insulating material and is covered by a suitable screen 58 made from a wire mesh material. A time indicator 60 such as an oscilloscope is connected to the collector plate 54 to provide an indication of the relative times at which the ions of different mass reach the plate.

In the steady state condition, the grid 12 has a positive voltage applied to it from the power supply 46. and the collector plate 21] and the receptacle 54- have slightly positi e voltages applied to them from the power supply. The electrode 16 may also have a slightly positive potentia app ed to it from the power supply 46. or it may be substan iallv grounded by connection to a suitable terminal in the power supply. The collector plate 26 is at a slightly positive potential to attract the electrons flowing from the cathode 1!), and the collector 54 is at a slightly positive potential to attract back to it electrons secondarily emitted from it by the impingement of ions. The cathode 10, the backing plate 22, the grid 24 and the shield 56 are substantially at ground potential in the steady state condition.

Because of the positive voltage on the grid 12 relative to the voltage on the cathode 1i), electrons emitted by the cathode are attracted towards the grid. The electrons are not further accelerated after reaching the grid 12 since the grid 16 is at a lower potential than the grid 12. Therefore, any electrons that do reach the region between the backing plate 22 and the grid 24 do not have sutficient strength to produce an ionization of gas molecules introduced into the region from the receptacle 32.

Upon the application of negative pulses of voltage from a pulse forming circuit 62 to the cathode and the control grid 12, the voltage on the grid 16 becomes more positive than the voltage on the grid 12. Because of the acceleration imparted to the electrons by the grid 16, the electrons are given a velocity which is considerably greater in the region between the plate 22 and the grid 24 than their velocity in the steady state condition. As a result, the electrons strike molecules of gas in the region between the plate 22 and the grid 24 with sufficient force to ionize the molecules. After striking the gas molecules, the electrons continue on to the collector plate 20.

Since the ions produced by the collision between electrons and gas molecules have a charge opposite to that of the electron stream, they are retained within the stream. The number of ions that can be so retained is limited only by the charge produced by the stream, and the width of the pulse is determined by the width of the stream which is collimated by the slots 14 and 18 and which may be collimated by a magnetic field (not shown). In this way, a considerably greater number of ions can be retained in a relatively confined space over that produced in spectrometers now in use. The operation and advantages of an ion source for retaining ions within an electron stream are disclosed in detail in copending application, Serial No. 221,554, filed April 18, i951, by Ian H. McLaren and William C. Wiley, now latent No. 2,732,500.

When the number of ions retained within the electron stream approaches saturation, the electron stream is cut off by discontinuing the voltage pulses on the cathode 10 and the grid 12. At the time that the electron stream is cut off or at a slightly later time, a positive pulse is applied from the pulse forming circuit 62 to the backing plate 22. Since the ions are no longer restrained by a negative charge from the electron stream, they are repelled by the backing plate 22 towards the grid 24. This repelling force is of sui'licient magnitude to produce a movement of the ions through the slot 26 and the window 34 into the region between the spherical electrodes 36 and 38.

Since the electrode 38 has a negative voltage with respect to the electrode 36, the ions entering into the region between the electrodes are initially attracted radially inwardly as they move towards the window 52. This causes the ions to enter the channel between the deflecting plates 48 and 50 at a position approximately intermediate the plates.

At approximately the instant that the ions move into the region between the spherical electrons 36 and 38, a positive pulse of voltage relative to the voltage on the deiiecting plate 56 is applied on the deflecting plate 43 by the pulse forming circuit 62. This voltage produces in the region between the plates 43 and 50 an electrical field which alters the electrical field produced by the voltage on the electrode 38 relative to the voltage on the electrode 36. The resultant electrical field between the plates 43 and 50 is of sufficient magnitude to deflect the ions so that they will commence to move in a circular or elliptical path dependent upon their mass. Because of the deflection of the ions in the region between the plates 48 and 50, the ions move in orbital paths through the region between the electrodes 36 and 33 without passing through the windows 34 or 52 and without striking any other part of either electrode. The voltage pulse on the deflecting plate 48 relative to the voltage to the plate 56 is removed after the ions have passed through the passageway between the plates.

After passing through the passageway between the plates 48 and 50, the ions enter the electrical field produced by the voltage between the electrodes 36 and 38. Because of the manner in which the turns are spaced on the bushing 40, a potential of is produced at any position in the region between the electrodes, where:

V=the potential of the field at that position;

K1=a constant; and

r=the radial distance to that position from the center of the electrodes.

E=the strength of the electric field acting on each ion at any position having a radial distance of r; and K2=a constant. The above relationship for E is produced since where 2Z=the partial derivation of V with respect to r In accordance with a well-known relationship, the kinetic energy of each ion at any position is K.E.=1/2 m1) where K.E.=the kinetic energy of the ion at that position; m=the mass of the ion; and v=the velocity of the ion at that position.

The potential energy of each ion at any position is as follows:

where P.E.=the potential energy of the ion at that position.

' If the potential energy of each ion at any position between the spherical electrodes 36 and 38 is assumed to equal the kinetic energy, the following relationship is obtained:

in Equation 7 is the centrifugal force exerted by an ion in travelling through a circular path, the relationship is the centripetal force exerted by the electrical field to maintain the ions in the circular path. Actually, only ions of a predetermined mass will rotate through circular orbits, while ions of all other masses will rotate through elliptical orbits. However, the time required for the ions of different mass to pass through a complete orbit is dependent only upon the mass of the ion, provided that each ion has the same electrical charge as a result of the removal of only one electron from the gas molecule and provided also that all of the ions of a particular mass enter into the space between the electrodes 36 and 38 with a velocity dependent upon their mass.

After the ions have rotated through a predetermined number of orbits suflicient to produce a material separation in time between the ions of difierent mass, at voltage pulse of a predetermined amplitude is reapplied between the electric plates 48 and 50 to distort the electric field produced between the plates by the electrodes 36 and 38. The distortion of the electric field in the passageway between the deflecting plates is of such a nature as to cause the ions to pass through the passageway and to continue towards the window 52. After travelling through the window 52, the ions pass to the collector plate 54. Some of the ions may produce a secondary emission of electrons from the plate 54 when they impinge on the plate, but these electrons return to the collector plate because of the positive voltage on the plate relative to the voltage on the shield 56. The ions impinging on the plate 54 produce signals which appear on the indicator 60. The relative times at which the signals are produced by the ions of different mass provide an indication of the ion masses.

The mass spectrometer disclosed above has several important advantages. One advantage is that all of the ions are collected at the end of the predetermined number of orbital movements so as to produce strong and clear signals. This results from the fact that all of the ions are somewhat focussed as they pass through the passageway between the deflector plates 48 and 50 and thus start their orbital movements from substantially the same position. Furthermore, because of the spherical electrodes 36 and 38 and the particular electrical field applied between them, the ions return to the same starting position after each orbital rotation.

Another advantage resides in the fact that finite and material separations in space between ions of different mass are produced in a mass spectrometer which occupies a relatively small space. The mass spectrometer operates in such an advantageous manner in' spite of its size because of the relatively long travel paths provided for the ions by the rotations imparted to the ions and, furthermore, by the movement of the ions through a predetermined plurality of orbits before their collection. The clear-cut separation between ions of different mass also results from the fact that the ions all travel through the same number of orbits before being collected. In addition to the above advantages, the mass spectrometer disclosed above is relatively simple and inexpensive. These advantages are obtained because of the elimination of any need for a magnetic field to produce a rotation of the ions.

Although the pulse forming circuit 62 is shown in block form in the drawings, it can be purchased or it can be easily built by a person skilled in the art. For example, Model 902 of the double-pulse generator manufactured by the Berkeley Scientific Company of Richmond, California, may be used to produce pulses having the time spacing disclosed above. Furthermore, such equipment may also be built in accordance with principles outlined on pages 223 to 238, inclusive, of volume 20 entitled Electronic Time Measurements" of the Radiation Laboratory Series prepared by the Massachusetts Institute of Technology.

It should be realized that pulses of ions can be prd duced in difierent ways than that disclosed above for introduction to the mass spectrometer. The ions of difierent mass may also be collected in difierent ways than that disclosed above and may even be detected without actually being collected. Furthermore, difierent voltage pulses than those disclosed above can probably be applied between the deflecting plates 48 and 50. As a matter of fact, no pulse of voltage need be applied between the deflecting plates under certain circumstances. For example, if it is desired to collect ions of only one mass, an electric field may be applied between the electrodes 36 and 38 for a period of time to produce a rotation of the ions through a predetermined number of orbits. When the ions have rotated through the desired number of orbits, the electric field may be reduced or eliminated. Since no centripetal force is now exerted upon the ions, the ions of the desired mass travel outwardly and pass through the window 52 to the collector plate 54. However, the ions of different mass fail to pass through the window 52 since they are not properly positioned with respect to the window when the electric field is cut oflf. By varying the time at which the electric field is cut off, ions of diflerent mass can be collected in a constant sampling procedure.

Although this invention has been disclosed and illustrated with reference to particular applications, the principles involved are susceptible of numerous other applications which will be apparent to persons skilled in the art. The invention is, therefore, to be limited only as indicated by the scope of the appended claims.

What is claimed is:

l. A mass spectrometer, including, aninner spherical member, an outer spherical member, means for introducing a plurality of ions into the region between the two spherical members, means for applying an electric field between the members to produce a rotation of the ions through orbital paths dependent upon their mass, means for detecting the ions after their movement through a particular orbital path, and means for determining the relative times at which the ions of different mass reach the detecting means.

2. A mass spectrometer, including, an inner spherical member, an outer spherical member, means for introducing a plurality of ions into the region between the two spherical members, means for acting upon the ions after their introduction to the region between the spherical members to alter the course of the ions, means for providing between the spherical members an electric field to produce an orbital movement of the ions, means for collecting the ions after a particular orbital movement, and means for indicating the relative times at which the ions of different mass reach the collecting means.

3. A mass spectrometer, including, an inner spherical member, an outer spherical member, means for introducing a plurality of ions into the region between the two spherical members, means for applying between the spheres an electric field having at any position a substantially inverse relationship to the radial distance to that position so as to produce an orbital movement of the ions, means for detecting the ions after their movement through an orbital path of particular length, and means for indicating the relative times at which the ions of different mass reach the detecting means.

4. A mass spectrometer, including, an inner spherical member, an outer spherical member concentric with the inner spherical member, means for introducing a pulse of ions to the region between the spherical members, means for applying in the region between the spherical members a radially directed electric field having at any position an intensity substantially inversely proportional to the square of the radial distance to that position so as to rotate the ions through orbital paths in which the ions return to their starting position after each orbit, means for collecting the ions after their rotation through an orbital movement of particular length, and means for indicating the relative times at which the ions of different mass reach the collecting means.

5. A mass spectrometer, including, an inner spherical member, an outer spherical member concentric with the inner spherical member, an ion source for introducing a pulse of ions into the region between the inner and outer spherical members, a winding disposed between the members and having a turn density at every radial position to produce an electrical field inversely related to the square of the radial distance to that position, a circuit for energizing the winding to produce the particular electrical field for an orbital movement of the pulse of ions to produce a time separation of the ions on the basis of their mass, means for detecting the ions after a particular orbital movement in the region between the spherical members, and means for indicating the detection of ions of different mass.

6. A mass spectrometer, including, an inner spherical member, an outer spherical member concentric with the inner spherical member, a Winding disposed between the members and having a plurality of turns with a density at each radial position to produce a potential inversely related to the radius at every position, an ion source for introducing a plurality of ions into the region between the inner and outer spherical members, a circuit connected to the winding for energizing the winding for a movement of the ions through orbital paths having a substantially constant initial position defining the commencement of each orbit, means for detecting the ions after their movement through a particular orbital path, and means for indicating the detection of ions of different mass.

7. A mass spectrometer, including, an inner spherical member, an outer spherical member concentric with the inner spherical member, a winding disposed between the members and having a turn density at every radial position to produce an electrical field at every position inversely related to the square of the radial distance to that position, an ion source for introducing a pulse of ions into the region between the inner and outer spherical members, a circuit for energizing the winding to produce the particular electrical field for an orbital movement of the ions to obtain a time separation of the ions on the basis of their mass, a pair of plates positioned between the inner and outer spherical members, a circuit for applying an instantaneous pulse to the plates to produce a sufficient variation in the electric field upon the introduction of the ions into the region between the spherical members to provide a deflection of the ions for subsequent orbital movements of the ions, an ion detector, the pulsing circuit also being operative to apply an instantaneous pulse to the plates after a particular orbital movement of the ions to produce a movement of the ions towards the detector, and means for indicating the detection of ions of different mass.

8. A mass spectrometer, including, an inner spherical member, an outer spherical member concentric with the inner spherical member, a first window in the outer spherical member, an ion source for introducing a pulse of ions through the first window into the region between the spherical members, a winding disposed between the members and having a turn density at every radial position to produce an electrical field inversely related to the square of the radial distance to that position, a circuit for energizing the winding to produce the particular electrical field for an orbital movement of the ions to obtain a time separation of the ions on the basis of their mass, a pair of plates positioned in the region between the spherical members, a circuit for applying an instantaneous pulse of voltage between the plates upon the introduction of the ions into the outer spherical member to produce a variation of the electrical field for a deflection of the ions for subsequent orbital movements of the ions, a second window in the outer spherical member, the pulsing circuit also being operative to apply an instantaneous pulse to the plates after the rotation of the ions through a particular number of revolutions to produce a movement of the ions through the second window, an ion detector positioned to detect the ions after their movement through the second window, and means for indicating the detection of ions by the detector.

References Cited in the file of this patent UNITED STATES PATENTS 2,576,601 Hays Nov. 27, 1951 2,582,216 Koppius Jan. 15, 1952 2,698,905 Goudsmit Jan. 4, 1955 2,709,750 Smith May 31, 1955

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