US2762926A - Mass spectrometer - Google Patents

Description

p 11, 1956 w. c. WILEY 2,762,926

MASS SPECTROMETER Filed Oct. 25, 1951 IN V EN TOR.

VV/LL/AM CL. WILEY gwmk-m ATTORNEY TVXASS SPECTRGMETER William C. Wiley, Detroit, Mich, assignor to Bendix Aviation Corporation, Detroit, Mich, a corporation of Delaware Application October 25, 1951, Serial No. 253,136

15' Claims. (Cl. 25041.9)

This invention relates to apparatus for and methods of distinguishing between ions of different mass. More particularly, 1e invention relates to apparatus for and methods of distinguishing between ions of different mass by measuring the time required for the ions to travel through a predetermined distance. The invention is especially adapted to provide a relatively sharp delineation between ions of dififerent mass as compared to apparatus and methods now in use.

Mass spectrometers are adapted to determine masses of the difierent gases constituting an unknown mixture and sometimes the relative abundance of each gas in the mixture. One type of mass spectrometer makes these determinations by applying a predetermined force to a pulse of ions to accelerate the ions through a predetermined distance. The ions of relatively light mass have a greater velocity imparted to them by the predetermined force than the ions of heavy mass and, therefore, travel through the predetermined distance before the ions of heavy mass. By measuring the time required for the ions of difierent mass to travel through the predetermined distance the masses of the ions can be determined.

Certain operational characteristics inherent in the spectrometers now in use tend to cloud the measurements which are obtained. For example, since the ion pulse has a finite width, some of the ions of the given mass are positioned in back of the intermediate position adopted by the majority of the ions having the same mass and other ions are positioned in front of the intermediate position. Furthermore, the ions have kinetic and other energy which causes some of the ions to be travelling towards the collector plate at the instant that the predetermined force is applied and other ions to be travelling away from the collector plate at that instant. The different positioning and random motion imparted by the kinetic energy causes individual ions of a given mass to reach the collector plate at difierent times.

This invention provides a mass spectrometer having a plurality of grids to compensate for the differences in positioning and random motion of the individual ions. As part of the invention, electric fields of diiferent intensity are applied between successive pairs of grids. Thus, a field of moderate intensity is applied between a backing plate and a first grid, a field of low intensity between the first grid and a second grid and a field of considerable intensity between the second grid and a third grid. In this way, a relatively sharp delineation between ions of difierent masses is obtained.

An object of the invention is to provide a mass spec trometer for determining the masses of the different gases in an unknown mixture by measuring the relative times required for the ions of the dillerent gases to travel through a predetermined distance.

Another object of the invention is to provide a mass spectrometer of the above character for producing a relatively sharp differentiation in space between the ions of different mass after the ions have travelled through a predetermined distance.

2,762,926 Fat-tented Sept. 11, 1956 A further object is to provide a mass spectrometer of the above character having a plurality of grids to compensate for differences in positioning and random motion imparted to individual ions of a given mass.

Still another object is to provide a mass spectrometer of the above character in which electronic fields of di-lferent intensity are applied between successive pairs of grids to compensate for differences in positioning and random motion of individual ions.

A still further object is to provide a method of compensating for any difierences in positioning and random energy of individual ions so as to obtain an accurate measurement of the ion masses.

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

The single figure is a somewhat schematic view of one embodiment of the invention, the mechanical features of the invention being shown in perspective and the elec trical features being shown in block form.

In one embodiment of the invention a wedge-shaped cathode 10 made from a suitable material such as tungsten is provided. A control grid 12 is separated by a relatively short distance from the cathode and is provided with a vertical slot 14 whose intermediate position is at substantially the same horizontal level as the cathode 10. The grid 12, as well as the other grids in the spectrometer, may actually be a plate with a meshed material covering the slot 14. An accelerating grid 16 is positioned relatively closely to the control grid 12 on one side and to a shield grid 18 on the other side and is substantially in alignment with both grids. Slots 2i) and 22 corresponding substantially to the slot 24 are provided in the grids i6 and 18, respectively. A collector plate 2.4 is disposed at a relatively great distance from the shield grid 18 in substantial alignment with the grid.

A backing plate 26 is positioned slightly to the rear of the stream of electrons flowing from the cathode towards the collector plate 24 and is substantially in parallel with the direction of electron ilow. The backing plate is substantially in parallel with a grid 28 positioned slightly in front of the electron how and having a horizontal slot 36 The distance between the backing plate 26 and the grid 28 may be approximately two millimeters. The plate 26, the grid 28, the shield grid 18 and the collector plate 24- form a compartment, the top and bottom of which are defined by plates 32. A horizontal slot 34 is provided in the bottom plate 32 substantially directly below the electron stream and is in communication with the flared mouth 3i) of a conduit 38. The conduit in turn extends from a receptacle 40 adapted to hold molecules of different gases constituting an unknown mixture.

A grid 42 is positioned substantially in parallel with the grid 28 at a relatively short distance such as two millimeters from the grid and is provided with a horizontal slot 44 corresponding in position and shape to the slot 30 in the grid 28. In like manner, a grid 46 having a slot 43 corresponding to the slots 30 and 44 is substantially in parallel with the grid 42 at a relatively short distance such as two millimeters from the grid. A detector such as collector plate St) is positioned substantially in parallel with the grid 46 at a predetermined distance from the grid, this distance being preferably relatively great. For example, the plate 50 may be separated from the grid 46 by approximately forty centimeters. A time indicator 52 such as an oscilloscope is connected to the collector plate 5i) to show the relative times at which the ions of different mass reach the plate.

The control grid 12 and accelerating grid 16 normally have positive potentials of substantially equal magnitude applied to them from a suitable power supply 54. The collector plates 24 and Stl have slightly positive potentials applied to them from the power supply 54, a positive 3 potential being applied to the collector plate 50 to attract back to the plate any electrons produced by secondary admission when the ions impinge on the plate. The cathode 10, the shield grid 18, the backing plate 26 and the grids 28, 42 and 46 are normally at substantially ground potential.

Because of the positive voltage on the grid 12 relative to the voltage on the cathode 10, the electrons emitted from the cathode are accelerated towards the grid. The electrons receive no further acceleration after travelling past the grid 12 since the grid 16 is at substantially the same potential as the grid 12 and the grid 18 is at ground. Therefore, any electrons that may travel through the region between the grid 18 and the collector plate 24 do not strike molecules of gas with sufiicient energy to ionize the molecules.

To accelerate the ions with considerable energy towards the collector plate 50, voltage pulses of negative polarity and of approximately equal amplitude are ap plied to the cathode and the control grid 12 from a pulse forming circuit 56. This causes the electrons to be considerably accelerated towards the grid 16 because of the positive voltage on the grid 16 relative to the voltage on the cathode during the pulses. The added acceleration causes the electrons to travel with suiiicient energy through the region between the backing plate 26 and the grid 28 to ionize molecules of gas introduced into the region from the receptacle 40.

Some of the ions produced by collision of gas molecules with electrons are retained within the electron stream since they have an opposite charge relative to that of the electrons. The ions are thus retained in a space having a relatively narrow width because of the collimating action which is provided on the electron stream by the slots 14, 2t) and 22 and the collimating action which may be applied by a magnetic field (not shown). The ions are further retained in the region adjacent to the slot 30 by the repelling action produced by the positive potentials on the grid 16 and the collector plate 24. The potential well created by the electron stream and the repelling action provided on the ions by the grid 16 and the collector plate 24 cause a considerably greater number of ions to be concentrated in the region adjacent the slot 30 than is available in spectrometers now in use.

The formation of an ion pulse similar to that disclosed above is disclosed in detail in co-pending application Serial No. 221,554, filed April 18, 1951 by me and Ian H. McLaren.

The electron stream becomes saturated fairly quickly by the formation of ions from the gas molecules. At approximately the instant of saturation, the voltage pulses on the cathode 10 and the grid 12 are cut off to discontinuethe flow of electrons towards the collector plate 24. When the electron stream is cut oif, the ions are released for acceleration towards the collector plate 50. The acceleration is produced by voltage pulses applied to the backing plate 26 and the grids 28 and 42 at the instant that the electron stream is cut ofi or at a slightly later time. The ions of relatively light mass are given a greater acceleration by the voltage pulses than the ions of heavy mass and are, therefore, collected at the plate 50 before the ions of heavy mass. The masses of the different ions collected by the plate 50 can be determined from the relative times at which the ions reach the plate.

I have found that the accuracy of measurements of ion masses by mass spectrometers of the present type may be. improved by compensating for the eifects of differences in the positioning and random motion of ions at the instant of the application of an accelerating voltage pulse. For example, individual ions of a given mass may be positioned in back of an intermediate position occupied by a majority of the ions of that mass, and other ions of the same mass may be positioned in front of this intermediate position. This causes ions of the same mass to arrive at the collector plate at different times so that the measurements become somwhat clouded.

In addition, individual ions of a given mass have a random motion produced by thermal and other energy before the ions are accelerated towards the collector plate. For example, individual ions of a given mass may be moving towards the collector plate before the ions are accelerated and other ions of the same mass may be moving away from the collector plate at that.instant. Thus, the random motion of the ions prevents all of the ions from reaching the collector plate at the same time.

It has been found that the errors in measurements resulting from the difierent positioning and random motion of individual ions can be minimized in spectrometers now in use by placing the collector plate at a predetermined distance from the grid before the pulse is accelerated. This distance in spectometers employing only one grid is approximately twice the distance between the grid and the ion pulse before the pulse is accelerated. Since the distance between the ion pulse and the grid is relatively small, the distance between the grid and the collector plate is also small. This prevents the ions from travelling through a sufiicient distance to become completely separated in space on the basis of their ditferent masses and, therefore, prevents the measurements from being truly accurate.

This invention provides a mass spectrometer which compensates for any differences in the positioning and random energy of individual ions without any decrease in the distance through which the ions travel. The invention provides a first electric field of moderate intensity between the plate 26 and the grid 28, a second electric field of relatively low intensity between the grids 28 and 42 and a third field of considerable intensity between the grids 42 and 46. For example, a positive pulse of approximately 200 volts may be applied on the plate 26; positive pulses of approximately volts may be simultaneously applied on the grids 28 and 42; and the grid 46 may be maintained at ground.

By providing a field of moderate intensity between the plate 26 and the grid 28, the individual ions of a given mass which are in back of the intermediate position adopted by the majority of ions of that mass are given a somewhat greater velocity than the ions in the intermediate position. This results from the fact that the ions retarded in position are in the electric field between the plate 26 and the grid 28 for a greater period of time than the ions intemediate in position. Similarly, the ions of a given mass in front of the intermediate position are given less velocity than the ions in the intermediate position.

Compensation is also provided in the electric field between the plate 26 and the grid 28 for the differences in random motion of individual ions. Thus, ions of a given mass moving away from the grid 28 at the instant that the electric field is applied are given an acceleration for a longer time than ions moving towards the grid at that instant. The increased period of acceleration is obtained because the ions initially moving away from the grid must first be stopped and then accelerated in the proper direction from a standstill position.

Since the grids 28 and 42 have pulses of approximately the same amplitude applied on them, the ions travel through the region between the grids with substantially the same velocities as those imparted to them in the region between the plate 26 and grid 28. This causes the compensatory action initially imparted to the ions in the first region to be continued in the region between the grids 28 and 42. The grid 42 is disposed at the position of maximum compensation or is preferably slightly in front of this position so that a slight additional compensation can occur in the region between the grids 42 and 46.

The field-free region between the grids 28 and 42 acts to isolate the electric field produced between the grids 42 and 46 from the field between the backing plate 26 and the grid 28. This is important in producing an optimum focussing action on the ions of each mass, especially when the electric fields are of considerably different magnitude. As used herein, the term focussin refers to the action of bringing the ions of each mass together into relatively tight groups in the direction of their movement. As such, the term focussing refers to the action of bunching the ions of each mass so that they are collected in a minimum amount of time.

The field-free region between the grids 28 and 42 is also important when the ions are withdrawn from their place of formation before the electron stream is cut off. Since a compensatory action for differences in the positioning of individual ions occurs in the region between the grids 28 and 42, the field imposed on the ions in the region between the backing plate 26 and the grid 28 can be minimized. By minimizing this electric field, the electrons in the stream are subjected to only a minimum deflection from a linear path as they travel through the region between the backing plate 26 and the grid 23.

Because of the difference of 150 volts between the grids 42 and 46, a considerable increase in energy is imparted to the ions in this region. Since all of the ions of a given mass are subjected to substantially the same electrical field in the region between the grids 42 and 46, they receive substantially constant increases in energy while travelling through the region. Because of the considerable voltage differences between the grids 42 and 46, the increases in energy imparted to the ions between the grids are considerably greater than the thermal and other energies inherent in the ions and the energies imparted to the ions during their movement towards the grid 42. This causes any differences in the random motion in the ions of a given mass to be substantially dwarfed by the substantially constant increment in energy imparted to the ions in the region between the grids 42 and 46. As a result, any errors resulting from the random energy of individual ions is minimized.

The considerable increments in the energy imparted to the ions between the grids 42 and 46 also produce a projection in the position at which the ions receive their optimum compensation for differences in the initial positioning of individual ions. Since the ions travel through a relatively great distance before they receive their optimum compensation, the ions become materially separated on the basis of their mass.

After travelling through the region between the grids 42 and 46, the ions travel through the distance between the grid 46 and collector plate Sit. Since this distance is relatively great, the ions become sharply separated in space in accordance with their different masses, the light ions reaching the plate ahead of the heavy ions. The separation between ions of diiferent mass is further accentuated since the ground potential on the grid 46 prevents the ions from being disturbed by the other grids during their movement towards the plate 59.

It should be realized that ion pulses may be formed in different ways than that disclosed above. Different voltages those disclosed above may also be applied on the different accelerating members as long as electric fields of predetermined intensity as disclosed above are created between the members.

As previously disclosed, a first group of pulses is initially applied to the cathode it and to the grid 12, and a second group of pulses is subsequently applied to the backing plate 26 and to the grids 23 and 4-2. Equipment for producing two groups of pulses separated from each other by a relatively short time is known to persons skilled in the art and may be either purchased or built. For example, Model 902 of the Double-Pulse Generator manufactored by the Berkeley Scientific Company of Richmond, California, may be used to produce two groups of pulses having a variable spacing from each other. Or equipment for producing two groups of pulses having a variable time separation may be built in accordance with the 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.

There is thus provided a mass spectrometer which compensates for the different positioning and random motion of individual ions by employing a plurality of accelerating members and providing electric fields of predetermined intensity between successive pairs of members. The apparatus also provides methods of accomplishing such compensation.

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, means for forming an ion pulse, means for moderately accelerating the ions through a first region to provide a focussing action on the ions, means for maintaining the ions in flight through a second region at a relatively low acceleration compared to the acceleration in the first region, means for consider ably accelerating the ions through a third region to provide an extension of the position of optimum focussing of the ions, means for collecting the ions at a distance past the third region corresponding to the position of optimum focussing of the ions, and means for indicating the relative times at which the ions of different mass reach the collecting means.

2. A mass spectrometer, including, means for forming an ion pulse having a relatively narrow width, means for imposing an electrical field on the ions of sufficient magnitude and duration for a moderate acceleration of the ions through a first region to provide a focussing action on the ions, means for coasting the ions through a second region having a width of the same order of magnitude as the first region, means for imposing an electrical field on the ions of sufiicient magnitude and duration for a considerable acceleration of the ions through a third region having a width of the same order of magnitude as the first region to provide an extension of the position of optimum focussing of the ions and to provide a compensation for differences in the random motion of the ions, means for collecting the ions at a distance past the third region corresponding to the positioning of optimum focussing of the ions, and means for determining the relative times at which the ions of different mass reach the collecting means.

3. A mass spectrometer, including, means for forming an ion pulse, a backing plate disposed a relatively short distance in back of the ion pulse, a first grid disposed a relatively short distance in front of the ion pulse, a second grid disposed a relatively short distance from the first grid, a third grid disposed a relatively short distance from the second grid, a collector, means for imposing an electric field upon the ions in the region between the backing plate and the first grid to impart moderate velocities to the ions to compensate for differences in the positioning of individual ions, means for imposing substantially no electric field upon the ions in the region between the first and second grids to electrically isolate the region between the backing plate and the first grid from the region between the second and third grids, means for imposing an electric field upon the ions in the region between the second and third grids to impart substantially constant and considerable increments in energy to the ions to provide a compensation for differences in the random motion of individual ions, the collector being disposed at the position of optimum focussing of the ions, and means for indicating the relative times at which the ions of different mass reach the collector plate.

4. A mass spectrometer, including, means for forming .ing plate disposed to the rear of the ion pulse and substantially in parallel with the pulse, a first grid disposed in front of the ion pulse and substantially in parallel with the backing plate, a second grid disposed in front of the first grid and substantially in parallel with the grid, a third grid disposed in front of the second grid and substantially in parallel with the grid, a collector disposed substantially parallel to the third grid, means for applying a voltage pulse of moderate amplitude between the backing plate and the first grid for a moderate acceleration of the ions towards the first grid to provide a compensatory action for difierences in the positioning of individual ions in the relatively restricted width, means for maintaining the second grid at substantially the voltage of the first grid for substantially no material acceleration of the ions between the first and second grids to electrically isolate the region between the second and third grids from the region between the backing plate and the first grid, means for applying a voltage pulse of considerable amplitude between the second and third grids for an increased acceleration of the ions towards the third grid to provide a compensatory action for ditferences in the random motion of individual ions, the collector being disposed at the position of optimum focussing of the ions, and means for indicating the relative times at which the ions of different mass reach the collector plate.

5. A mass spectrometer, including, a backing plate, a first grid disposed substantially in alignment with the backing plate, means for collecting a plurality of ions in the region between the backing plate and the first grid, means for providing between the backing plate and the first grid a voltage pulse of a magnitude and polarity for a movement of the ions with moderate velocity towards the first grid, a second grid disposed substantially in alignment with the first grid, means for providing substantially the same voltage on the second grid as on the first grid during the movement of the ions towards the second grid, a third grid disposed substantially in alignment with the second grid, means for providing between the second and third grids a voltage pulse of a polarity and magnitude for a movement of the ions towards the third grid with a considerable and substantially constant increment in energy for the ions in each mass, means disposed substantially in alignment with the third grid at a distance from the grid corresponding to the position of optimum focussing of the ions to collect the ions flowing past the grid, and means for indicating the relative times at which the ions of difierent mass reach the collecting means.

6, A mass spectrometer, including, a backing plate, a first grid disposed substantially in alignment with the backing plate at a relatively short distance from the plate, means for forming between the plate and the first grid 21 pulse of ions, means for applying a pulse of voltage between the backing plate and the first grid of proper polarity and of sufiicient duration and magnitude to moderately accelerate the ions past the first grid, a second grid disposed substantially in alignment with the first grid at a relatively short distance from the grid, means for applying substantially the same voltage on the second grid as on the first grid to cause the ions to coast past the second grid, a third grid disposed substantially in alignment with the second grid at a relatively short distance from the grid, means for applying a pulse of voltage between the second grid and the third grid of proper polarity and of sufficient duration and magnitude to considerably accelerate the ions past the third grid, means for collecting the ions at a distance past the third grid corresponding to the position of optimum focussing of the ions, and means for indicating the relative times at which the ions of different mass reach the collecting means.

7. A mass spectrometer, including, means for obtaining a plurality-of ions, an ion detector, means for proan ion pulse having a relatively restricted width, a backducing a kinetic energy of moderate intensity in the ions during movement of the ions through a first region towards the detector, means for maintaining the kinetic energy of the ions substantially constant during a movement of the ions through a second region towards the detector, means for producing a kinetic energy of considerably increased intensity in the ions during movement of the ions through a third region towards the detector, the detector being disposed at the position of optimum focussing of the ions, and means for providing a delineation between ions of different mass in accordance with the actuation of the detector.

8. A mass spectrometer, including, means for obtaining a plurality of ions, an ion detector, means positioned between the ion means and the detector to create a first region for an acceleration of the ions, means positioned between the first region and the detector to create a second region for the coasting of the ions, means positioned between the second region and the detector to create a third region for an acceleration of the ions, means for providing a moderate acceleration of the ions through the first region to provide a focussing action on the ions, means for providing an increased acceleration of the ions through the third region relative to the ion acceleration in the first region to provide an extension of the position of optimum focussing of the ions, the detector being disposed at the position of optimum focussing of the ions, and means for indicating the relative times at which ions of difierent mass reach the detector.

9. A mass spectrometer, including, means for obtaining a plurality of ions, an ion detector, means for imposing an electric field of moderate intensity on the ions in a first region for an acceleration of the ions in the direction of the detector to provide a compensation for difierences in the positioning of individual ions in the direction of their movement towards the detector, means for imposing substantially no electric field on the ions in a second region to maintain the ions at substantially a constant velocity relative to their velocity at the end of the first region, means for imposing an electric field of considerable intensity on the ions in a third region for an increased acceleration of the ions in the direction of the detector to provide a compensation for differences in the random motion of individual ions and to provide an extension of the position of optimum focussing of the ions, and means for indicating the relative times at which the ions of ditferent mass act upon the detector.

10. A mass spectrometer, including, a backing plate, a first grid disposed substantially in alignment with the backing plate at a relatively small distance from the plate, means for introducing a pulse of ions into the region between the backing plate and the first grid, a second grid disposed substantially in alignment with the first grid at a relatively small distance from the grid, a third grid disposed substantially in alignment with the second grid at a relatively small distance from the grid, means for applying pulses of voltage on the backing plate at periodical times, means for applying pulses of voltage on the first grid at substantially the same time as the application of the voltage pulses on the backing plate and for at least a period of time corresponding to the movement of the ions past the second grid, the first grid having a voltage of the proper polarity and of moderate magnitude relative to the voltage on the backing plate during the pulse to produce a flow of ions towards and past the first grid, means for applying on the second grid a voltage pulse of the proper polarity and magnitude with respect to the voltage pulse on the first grid to maintain the velocity of the ions substantially constant during the movement towards the second grid and for at least a period of time corresponding to the movement of the ions past the third grid, means for imposing on the third grid a voltage of the proper polarity and magnitude 'with respect to the voltage pulse on the second grid to provide a considerable acceleration of the ions towards the third grid, means disposed at a relatively great distance from the grids and at substantially the position of optimum focussing of the ions to detect the ions flowing past the third grid, and. means for indicating the relative times at which the ions of ditferent mass are detected.

11. A mass spectrometer, including, means for providing a plurality of ions in a relatively confined region, means for imposing an electrical field on the ions in a first region to impart moderate velocities to the ions and slightly greater velocities to the ions of each mass at the back of the confined region than to the ions of the same mass at the front of the region, means for imposing an electrical field on the ions in a second region to impart considerable and substantially equal increments of energy to the ions of each mass during the travel of the ions through the region, means for imposing substantially no energy changes on the ions in a third region between the first and second regions, a detector positioned at the position of optimum focus of the ions, and an indicator for determining the relative times at which the ions of different mass are detected.

12. A mass spectrometer, including, a backing plate, a first grid disposed in substantially parallel relationship to the backing plate and in front of the plate, a second grid disposed in substantially parallel relationship to the first grid and in front of the grid, a third grid disposed in substantially parallel relationship to the second grid and in front of the grid, means for providing a plurality of ions in the region between the backing plate and the first grid, means for applying voltages on the backing plate and the first grid until the movement of the ions past the grid to impart moderate velocities to the ions and slightly greater velocities to the ions of a given mass closest to the backing plate relative to the velocities imparted to the ions closest to the grid, means for applying a voltage on the second grid approximately equal to the voltage on the first grid until the movement of the ions past the second grid to impart relatively little changes in velocity to the ions during their movement towards the second grid, means for applying a voltage on the third grid relative to the voltage on the second grid until the movement of the ions past the third grid to impart substantially constant increases in energy to the ions during their movement towards the third grid, a detector disposed in front of the third grid at the position of optimum focussing of the ions, and an indicator for determining the relative times at which the ions of different mass are detected.

13. A mass spectrometer, including, means for providing a plurality of ions, means for imposing an electric field on the ions in the first region until the movement of the ions past the region for the imposition of moderate velocities to the ions in the region and for the imposition of differences of velocities to the ions of each mass dependent upon the initial positioning of the ions, means for imposing substantially no electric field on the ions in a second region contiguous to the first region until the movement of the ions past the region, means for imposing an electric field on the ions in a third region contiguous to the second region until the movement of the ions past the region for the imposition of substantially constant energies to the ions of each mass and energies greater than those imparted to the ions in the first region, a detector disposed at the position of optimum focus of the ions, and an indicator for determining the relative times at which the ions of different mass are detected.

14. A mass spectrometer, including, a backing plate, a first grid disposed in front of and substantially in alignment with the backing plate, a second grid disposed in front of and substantially in alignment with the first grid, a third grid disposed in front of and substantially in alignment with the second grid, means for providing a plurality of ions between the backing plate and the first grid, a circuit for producing an electrical field between the backing plate and the first grid for at least the period of movement of the ions to the grid to impart moderate kinetic energies to the ions and slight differences in kinetic energy to the ions of each mass inversely dependent upon the positioning of the ions from the backing plate, a circuit for producing substantially no electrical field between the first and the second grids for at least the period of movement of the ions to the second grid, a circuit for producing an electric field between the second and third grids for at least the period of movement of the ions to the grid to impart substantially constant and considerable increases in energy to the ions of each mass, a detector disposed in front of the third grid at substantially the position of optimum focussing of the ions, and an indicator for determining the relative times at which the ions of each mass are detected.

15. A mass spectrometer, including, means for providing a plurality of ions in a relatively confined region, means for imposing a force on the ions in a first region to impart moderate velocities to the ions in the region and slightly greater velocities to the ions of each mass at the back of the region than to the ions at the front of the region, means for imposing substantially no force on the ions in a second region contiguous to the first region, means for imposing a force on the ions in a third region contiguous to the second region to impart substantially constant increases in energy to the ions of each mass and material increases in energy to the ions relative to the energy imparted to them in the first region, a detector disposed at the position of optimum focus of the ions, and an indicator for determining the relative times at which the ions of different mass are detected.

References Cited in the file of this patent UNITED STATES PATENTS Bennett Dec. 26, 1950 Koppius Jan. 15, 1952 OTHER REFERENCES

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