US2794125A - Mass spectrometer - Google Patents

Description

May 28, 1957 w. c. WILEY 2,794,125

MASS SPEQTROMETER Filed Dec. 3, 1953 Off/LL 0.56015 w wag S' u 3 Kb IN VEN TOR.

MAL/AM MLA'V ATTORNEY U ited States p n MASS SPECTRGMETER William C. Wiley, Detroit, Mich, assignor to Bendix Aviation Corporation Application December 3, 1953, Serial No. 355,980

Claims. (Cl. 250 -413) This invention relates to mass spectrometers and more particularly to mass spectrometers for providing an enhanced resolution between ions of different mass.

In some types of mass spectrometers, a plurality of ions are formed from molecules of the different gases and vapors in an unknown mixture. When a considerable number of ions have been formed, the ions are Withdrawn in a pulse from their place of formation by,

the imposition of a force on the ions. The ions of light mass receive a greater velocity than the ions of. heavy mass. This causes the ions of light mass to travel through a particular distance before the ions of heavy mass. By measuring the relative times at which the ions of different mass are detected, the masses of the ions can be determined.

Ideally, all of the ions of a given mass should travel through the particular distance in substantially the same time. In actual operation, this does not always occur. Variations in the travel times of individual ions result in part from diflierences in the positioning of the ions before the imposition of the force and in part from differences in the thermal and other energies in the ions. Several attempts have been made to compensate for differences in the positioning and in the thermal energy of individual ions. Such attempts have been largely but not entirely successful.

This invention provides a mass spectrometer for pro ducing a further compensation for differences in the initial positioning and in the thermal energy of individual ions. The mass spectrometer provides such compensations by initially producing a first field of zero intensity and a' second field of considerable intensity. Since the ions are maintained in the first region, means are provided for preventing any leakage from the second region into the first region. The mass spectrometer subsequently adjusts the fields for a brief interval to produce a field of moderate intensity in the first region and a field of considerable intensity in the second region.

The particular electrical fields are so produced that they rise instantaneously to optimum values and remain substantially at these optimum values until the elimination of the fields. Because of such electrical fields, a fairly rigid control can be provided over the energies imparted to the different ions so that all the ions of a particular mass reach a particular position at substantially the same instant of time. In this'way, an enhanced resolution is obtained over mass spectrometers now in use.

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

Another object is to provide a mass spectrometer of A the above character for providing a compensation for differences in the positioning and random motion of individual ions before their withdrawal from their place of formation.

A further object is to provide a mass spectrometer of 'ice the above character for producing an enhanced resolution between ions of diiferent mass by maintaining the ions in a field-free region and subsequently subjecting the ions to forces having fast rise times and substantially constant maximum values.

Still another object is to provide a mass spectrometer of the above character which requires relatively simple electrical circuits to produce a relatively sharp resolution between the ions of different mass.

A still further object is to provide a mass spectrometer of the above character which utilizes a plurality of electrodes and applies particular voltages to the electrodes to obtain an enhanced resolution between the ions of different mass in an unknown mixture.

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, partly in block form and partly in perspective, illustrating one embodiment of the invention.

In one embodiment of the invention, a wedge-shaped filament 10 made from a suitable material such as tungsten is adapted to emit electrons when heated. An electrode 12 is disposed at a relatively close distance such as millimeter from the tip of the filament 10. The electrode 12 is provided with a vertical slot 14, the median position of which is at substantially the same horizontal level as the filament 10.

An electrode 16 is positioned relatively close to the electrode 12 and in substantially parallel relationship to the electrode. For example, the electrode 16 may be 1 millimeter away from the electrode 12. The electrode 16 has a vertical slot 18 corresponding substantially in shape and disposition to the slot 14 in the electrode 12. A collector 20 is disposed in substantially parallel relationship to the electrode 16 at a relatively great distance,

such as 4 centimeters, from the electrode.

A backing plate 22 is positioned between the electrode 16 and the collector 20 in substantially perpendicular relationship to these members. The backing plate is disposed slightly in back of an imaginary line which extends from the tip of the filament 10 through the slots 14 and 18 to the collector 20 in a direction substantially parallel to the backing plate. An electrode 24 is substantially parallel to the backing plate 22 at a relatively short distance such as 2 millimeters from the plate. Because of the spacing between the backing plate 22 and'the electrode 24, the electrode is slightly in front of the imaginary line disclosed above. A horizontal slot 26 is provided in the electrode 24.

Top and bottom slats 28 made from a suitable insulating material extend between the backing plate 22 and the electrode 24 to form a compartment with these members. A horizontal slot 30 is provided in the bottom slat 28 at a position directly below the'imaginary line disclosed above. A conduit 32 communicates at one end with the slot 30 and at the other end with a receptacle 34 adapted to hold molecules of the different gases and vapors in an unknown mixture.

A shield electrode 36 made from a suitable wire mesh is positioned relatively close to, and in substantially parallel relationship to, the electrode 24. An electrode 38 is parallel to and slightly in front of the shield electrode disposed at a relatively great distance such as 40 centime I ters from the electrode 38. An indicator such as an oscilloscope 46 is connected to the collector 44 to indicate the relative times at which the ions of ditferent mass reach the collector.

A direct voltage of positive polarity is applied to the electrode 12 through a resistance 50 from a suitable power supply 52. Slightly positive voltages are respectively applied to the collectors 20 and 44 through suitable resistances 54 and 56 from the power supply 52. Positive voltages are applied to the collectors 20 and 44 so that the collectors will attract back to them electrons secondarily emitted from them upon the impingement of charged particles.

The backing plate 22 and the electrode 24 also have direct voltage of positive polarity applied to them. These voltages are respectively applied to the backing plate 22 and the electrode 24 through suitable resistances 58 and 60 from the power supply 52. For reasons which will be disclosed in detail hereinafter, the voltages applied to the backing plate 22 and the electrode 24 are of substantially equal and relatively great magnitude. For example, the backing plate 22 and the electrode 24 may each receive +400 volts from the power supply '52.

The voltage applied to the shield electrode 36 is dependent upon the voltage difference between the electrodes 24 and 38. For example, since the shield electrode 36 is equidistant from the electrodes 24 and 38 and a voltage difierence of approximately 400 volts exists between the electrodes 24 and 38, a positive voltage in excess of 200 volts is applied to the electrode 36. This voltage is applied through a suitable resistance 62 from the power supply 52. Preferably, the voltage is slightly greater than that applied to the backing plate 22 and the electrode 24. Thus, a potential of +420 volts may be applied to the shield electrode 36. The filament is connected to a grounded resistance 64, and the electrodes 16 and 38 are directly grounded.

The filament 10 and the electrode 12 are connected through coupling capacitances 66 and 68, respectively, to a suitable pulse forming circuit 70 so as to simultaneously receive voltage pulses having a negative polarity and a substantially equal magnitude from the circuit 70. Voltage pulses of negative polarity are also applied to the electrodes 24 and 36 a particular time after the imposition of the voltage pulses on the filament 10 and the electrode 12. Preferably, the voltage pulses are applied to the electrodes 24 and 36 immediately after or a relatively short time, such as a variable period up to two microseconds, after the termination of the pulses on the filament 10 and the electrode 12. The voltage pulses are respectively applied to the electrodes 24 and 36 through suitable coupling capacitances 72 and 74 from the pulse forming circuit 70. A voltage pulse is also applied to the oscilloscope46 at substantially the same time as the imposition of the pulses on the electrodes 24 and 36 so as totrigger the horizontal sweep of the oscilloscope.

Although the pulse forming circuit 70 is shown in block form, its construction and operation are known to persons skilled in the art. For example, Model 902 of the Double Pulse Generator manufactured by the Berkeley Scientific Instrument Company of Richmond, California, may be used to produce a plurality of pulses separated from one another by variable periods of time. This model generator is fully described in a publication entitled Instruction Manual, Berkeley Double Pulse -Generator, Model 902 issued by the Berkeley Scientific Instrument Company in August 1950. The pulse forming circuit disclosed in co-pending application Serial No. 288,014, filed May 16, 1952, by Macon H. Miller and William C. Wiley, can also be conveniently adapted for use.

The electrons emitted by the filament 10 are attracted towardthe electrode 12 because of the positive potential on the electrode with respect to the voltage on the filament. The electrons are decelerated in the region between the electrodes 12 and 16 since the electrode 16 is at substantially the same potential as the filament 10. This prevents any electrons from travelling into'the region between the backing plate 22 and the electrode 24 with a sufiicient amount of energy to ionize molecules of gas and vapor introduced into the region from the receptacle 34.

Upon the imposition of the negative voltage pulses on the filament 10 and the electrode 12, the voltage on the electrode 12 becomes negative with respect to the ground potential on the electrode 16. This causes the electrons passing through the slot 14 to receive an additional increment in energy in the region between the electrodes 12 and 16. Because of this additional increment in energy, the electrons travel into the region between the backing plate 22 and the electrode 24 with a suflicient amount of energy to ionize molecules of gas and vapor introduced into the region. Most of the ions which are produced have a unitary positive charge.

The ions produced from the molecules of gas and vapor are retained in the electron stream since they have an opposite charge to that of the stream. The ions are retained in a region having a relatively narrow width because of the collimating action which is provided on the electron stream by the slots 14 and 18 and which may be provided by a suitable magnetic field (not shown). Since the electron stream is formed from a considerable number of electrons, a relatively large number of ions can be retained in the stream before the stream becomes saturated.

Because of the voltages applied to the backing plate 22 and the electrode 24, substantially no electrical field is produced between these members to produce a movement of the ions towards the electrode 24. Since there is a voltage difference of approximately 400 voltages between the electrodes 24 and 38, an electrical field of considerable intensity is produced between these members. Some of this electrical field would leak into the region between the backing plate 22 and the electrode 24 if the shield electrode 36 were not present. However, such leakage is prevented by disposing the shield electrode 36 between the electrodes 24 and 38 and by applying to the electrode 36 a voltage for reducing or even reversing the electrical field between the electrode 24 and the shield electrode 36. The electrical field is reduced by imposing on the shield electrode 36 a potential greater than +200 volts and less than +400 volts. The electrical field is actually reversed by the application of +420 volts on the shield electrode. Upon the application of such a voltage, any field which does leak past the electrode 24 is in a direction to prevent the ions from moving towards the electrode. In this way, most of the ions are retained in the negative potential Well created by the electron stream.

The pulse forming circuit 70 is set to apply the pulses to the filament 10 and the electrode 12 for a particular period of time to allow a considerable number of ions to be produced for retention "in the stream. When the pulses are cut off the ions are available for easy withdrawal upon the application of voltage pulses to the electrodes 24 and 36 from the pulse forming circuit 70. The pulse forming circuit 70 is set to apply the pulses to the electrodes 24 and 36 at a particular time after the pulses applied to the filament 10 and the electrodeg12 are cut oil. The particular pulses applied to the electrode 24 and the shield electrode 36 cause an electrical field of relatively moderate intensity to be produced between the backing plate 22 and the electrode 24 and an electrical field of considerably increased intensity to be produced between the electrodes 24 and 38. For example, a voltage pulse having a negative polarity and a magnitude of approximately 20 volts may be applied to the electrode 24 and a voltage pulse having a negative polarity and a magnitude of approximately volts may be applied to the shield electrode 36. The pulse forming circuit 70 is set to apply these voltage pulses for a time duration suiticient for all of the ions to travel into the region between the electrode 38 and the collector 44.

Since the electrical field imposed on the ions between the backing plate 22 and the electrode 24 continues until after the movement of the ions past the electrode, the

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ions receive difierences in energy dependent upon their initial positioning. Thus, the ions positioned in the back of the electron stream receive a slightly greater amount of energy than the ions positioned in the front of the stream. By imparting'such slight differences in energy to the individual ions, compensation is provided for differences in the initial positioning of the ions.

As has been previously disclosed, the voltage pulses applied to the electrode 24 and the shield electrode 36 continue until after the movement of the ions past the electrode 38. Since all of the ions of a given mass travel through the same distance between the electrodes 24 and 38, they receive substantially constant increments in energy even though they may have differences in velocity. These increments in energy are not affected by the voltage imposed on the shield electrode 36 provided that the voltage on the shield electrode is less positive than the voltage on the electrode 24 during the imposition of the voltage pulses on the electrodes.

Because of the considerable and substantially constant increments in energy imparted to the ions of each mass in the region between the electrodes 24 and 38, any diflerences in the random motion of individual ions becomes considerably dwarfed. For example, one ion of a given mass may have a relative velocity of 13 upon its movement past the electrode 24 and another ion may have a relativevelocity of 16 at this position. The differences in velocity between the two ions of the same mass result in large part from the random motion imparted to the ions by the thermal and other energies in the ions. Because of the considerable energies imparted to theions in the region between the electrodes 24 and 38, the two ions may have relative-velocities of 113 and 116 as they move past the electrode 38. As may be seen, the percentage difference between velocities of 113 and 116 is considerably less than the percentage difference between velocities of 13 and 16.

The imposition of a relatively great electrical field becrease since the collector is positioned at substantially the position of optimum focus of the ions. Because of the increase. in the distance through which the ions travel, the separation between the ions of different mass becomes enhanced. In this way, relatively sharp signals indicative of the ions of different mass are produced by the oscilloscope 46.

As previously disclosed, the electrical fields between the backing plate 22 and the electrode 24 and between the electrodes 24 and 38 cause differences in velocity to be imparted to the ions such that the differences are depend ent upon the masses of the ions. These differences in velocity cause the ions of each mass to become materially separated from the other ions as the ions travel through the substantially field-free region between the electrode 38 and the collector 44. By measuring the relative times at whichthe ions of different mass reach the collector, the masses of the different ions can be determined.

The mass spectrometer disclosed above has several important advantages. It produces a substantially fieldfree region in a transverse direction relative to the backing plate 22 and the electrode 24 during the time that the ions are being formed. This field-free region is produced by the imposition of uniform voltages on the backing plate 22 and the electrode 24 and by the disposition of the shield electrode 36 between the electrodes 24 and 38 at a voltage for reducing'or even reversing the electrical field between the electrode 24 and the shield electrode 36. Because of the field-free region between the backing plate 22 and the electrode 24, no force is imposed on the ions in the direction of the electrode 24 to withdraw them from the electron stream and the ions are retained in a region having a relatively narrow width. Furthermore, by including the shield electrode 36, the electrical field between the electrodes 24 and 38 can be made large withoutany electrical field of large intensity is imposed in the region between the electrodes 24 and 38 to provide an optimum focusing action on the ions of each mass.

Since the backing plate 22 and the electrode 24 are both :at' relatively high positive potentials, only a pulse of moderate magnitude has to be applied to the electrode 24 to produce an electrical field of moderate intensity between-the. backing plate 22 and the electrode 24. By applying voltage pulse in the order of 20 volts to the electrode 24, the pulse can be provided with a relatively fast rise time and with a flat top. In this way, a substantially rectangular pulse is produced. A rectangular pulse is advantageous inthat the electrical fields required for optimum focusing of the ions are instantaneously produced and are thereafter maintained.

Although a voltage pulse in the order of volts is applied to the shield electrode 36, this pulse does not require the same rectangular wave shape as the pulse on the electrode 24. Thus, the rise time of the voltage pulse on the electrode 36 can be slow and the amplitude can be somewhat variable. The reason for this is that the voltage diflYerence between the electrodes 24 and 38 remains constant in spite of any variations in the voltage on the shield electrode36. Because of this constant voltage difierence, the ions of each mass receive substantially constant increments in energy between the electrodes 24 and 38.

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 providing a first region, means for providing a second region past the first region, means for providing a third region past the second region, means for providing a plurality of ionsin the first region, there being substantially no force in the first region to move the ions into the second region, means for initially imposing on the ions in the first region a force for maintaining the ions in the first region, means for initially imposing on any ions in the third region a considerable force in a direction to move the ions through the region, means for subsequently imposing on the ions in the firstregion a moderate force to impart to the ions of each mass an energy dependent upon the positioning of the ions in the region, means for simultaneously imposing on the ions of each mass in the second and third regions a total force for producing a substantially constant increment in energy to the ions of each mass and an increment greater than that imparted to the ions in the first region, a detector disposed at substantially the position of optimum focusing of the ions, and means for indicating the relative times at which the ions of different mass are detected.

2. A mass spectrometer, including, a backing plate, a first electrode disposed at a particular distance from the backing plate, a second electrode disposed at a particular distance from the first electrode, a shield disposed between the first and second electrodes, means for providing a plurality of ions in the region between the backing plateand the first electrode, an electrical circuit for applying substantially constant voltages to the backing plate and the first electrode and a voltage of considerable magnitude and of a first polarity on the second electrode relative to the voltage on the first electrode to provide an attraction of the ions towards the second electrode upon their movement past the first electrode, an electrical circuit for applying to the shield a voltage of a magnitude and of a polarity to impede the movement of the ions past the first electrode, an electrical circuit for applying a voltage pulse of moderate magnitude between the backing plate and the first electrode to produce a movement of the ions'past the first electrode, an electrical circuit for applying a voltage'pulse of increased magnitude be tween the first electrode and the shield to provide for the movement of the ions through the electrical field between the first and second electrodes, a detector disposed at substantially the position of optimum focus of the ions, and means for indicating the relative times at which the ions of diflerent mass are detected.

3. A mass spectrometer, including, a backing plate, a first electrode disposed at a particular distance from the backing plate, a second electrode disposed at a particular distance from the first electrode, a shield disposed between the first and second electrodes, an electrical circuit for imposing an electrical field on the ions in the region between the backing plate and the first electrode to retain the ions within the region, an electrical circuit for imposing an electrical field on the ions in the region between the first electrode and the shield to impede any ions from moving'past the first electrode, an electrical circuit for imposing an electrical field of moderate intensity on the ions in the region between thebacking plate and the first electrode to withdraw the ions from their place of provision and to impart slight difierences in velocity to individual ions dependent upon the positioning of the ions in their place of provision, an electrical circuit for imposing an electrical field of increased intensity on the ions in the region between the first and second electrodes to impart substantially constant increments in energy. to the ions of each mass and increments in energy greater than those imparted to the ions in the region between the backing plate and the first electrode, a detector disposed at substantially the position of optimum focus of the ions, and means for indicating the relative times at which the ions of different mass are detected.

4. A mass spectrometer, including, means for providing a first region, means for providing a plurality of ions in the first region, means 'for providing an electrical field of an intensity and polarity in a second region to impede the ions from moving from the first region into the second region, means for providing an electrical field of considerable intensity and of polarity in a third region to impart a considerable increase in velocity to the ions upon their movement into the region, means for simultaneously adjusting the electrical fields in the first and second regions to produce an electrical field of moderate intensity and of a polarity in the first region and an electrical field of a polarity in the second regionto produce a movement of the ions'through the regions and a separation of the ions on the basis of their mass, a detector disposed past the third region at substantially the position of optimum focus of the ions, and means for indicating the relative times at which the ions of diiferent mass are detected.

5. A mass spectrometer, including, means'for providing a first region, means for providing a second region past the first region, means for providing a third region past the second region, means for providing a plurality of ions in the first region, there being substantially no electrical force in the first region to produce a movement of the ions, an electrical circuit for imposing a moderate electrical force upon the ions in the second region to maintain the ions within the first region, an electrical circuit for imposing a considerable electrical force upon the ions in the third region to impart a considerable energy to any ions moving through the region, an electrical circuit for imposing a moderate electrical force upon the ions in the first region until the movement of the ions past the region, an electrical circuit for imposingan electrical force upon the ions in the second region to produce in combination with the force in the third region a considerable electrical force until the movement of the ions past the third region, a detector disposed past the'third region at substantially the position of optimum-"focus of the ions, and means for indicating the bi relative timesat which the ions of difierent mass are detected- 6. Amass spectrometer, including, means for providing a first region, means for providing a second region past the first region, means for providing a third region past the second region, means for providing a plurality of ions in the first region, means for providing an electrical'field in the first region to retain the ions in the region, means for providing an electrical field in the second region to impede the ions from entering into the second region, means for providing an electrical field in the third region to produce a movement of any ions through the region with considerable increases in energy, means for providing an electrical field of moderate intensity in the first region until the movement of the ions past the region to impart moderate energies to the ions and slightly greater velocities to the ions in the back of the region than to the ions closer to the second region, means for providing an electrical field of increased intensity in the second and third regions until the movement of the ions past the third region to impart substantially constant increments of energy to the ions of each mass in the second and third regions and increments in energy greater than those imparted to the ions in the first region, a detector disposed at substantially the position of optimum focus of the ions, and means for indicating the relative times at which the ions of different mass are detected 7. A mass spectrometer, including, a backing plate, a first electrode disposed at a particular distance from the backing plate, a second electrode disposed at a particular distancefrom the first electrode, a shield disposed between the first and second electrodes, means for providing a plurality of ions in the region between the backing plate and the first electrode, means for biasing the backing plate and the first electrode with particular voltages relative to the voltage on the second electrode to retain the ions in the region between the backing plate and the first electrode, means for biasing the shield relative to the voltage on the first electrode to impede the movement of ions past the first electrode, means for imposing voltage pulses of particular magnitude between the backing plate and the first electrode and between the first electrode and the shield to impose a force of moderate intensity on the ions in the region between the backing plate and the first electrode and a force of increased intensity on the" ions in the region between the first and second electrodes, a detector disposed at substantially the position of optimum focus of 'the' ions, and means for indicating the relative times at which the ions of difierent mass are detected 8. A mass spectrometer, including, a backing plate, a first electrode disposed at a particular distance frorn the backing plate, a second electrode disposed at a particular distance from the first electrode, a shield disposed between the first and second electrodes, means for providing a plurality of ions in the region between the backing plate and the first electrode, an electrical circuit for biasing the backing plate and the first electrode to retain the ions in the region between these members, an electrical circuit for biasing the shield to maintain the ions in the region between the backing plate and the first electrode, an electrical circuit for biasing the second electrode to impart a considerable amount of energy to ions upon their movement into the region between the shield and the second electrode, an electrical circuit for providing an adjustment of relatively short duration of the relative bias between the backing plate and the first electrode to impart a moderate amount of energy to the ions and a slightly greater amount of energy to the ions of each mass positioned relatively close to the backing plate than to the ions of the same'mass positioned relatively close to the first electrode, an electricalcircuit for providingan adjustment of relatively short duration of the relative bias between the first electrode and the shield to produce a movement of the ions towards the second electrode and to impart substantially constant increments of energy to the ions of each mass in the region between the first and second electrodes and increments greater than those imparted to the ions in the region between the backing plate and the first electrode, a detector disposed past the second electrode at substantially the position of optimum focus of the ions, and means for indicating the relative times at which the ions of difierent mass are detected.

9. In a mass spectrometer, a first electrode, a second electrode disposed relative to the first electrode to provide a first region between the electrodes, a third electrode disposed relative to the second electrode to provide a second region between the electrodes, means for applying substantially constant voltages of considerable magnitude to the first and second electrodes to produce an electric field of substantially zero magnitude in the first region and an electric fiield of considerable magnitude in the second region, means for providing a plurality of ions in the first region, a shield electrode disposed between the second and third electrodes, means for applying a voltage to the shield electrode to reduce in the second region the electric field between the second electrode and the shield electrode for preventing a leakage of the electric field in the second region into the first region, means for applying a voltage pulse of moderate magnitude to the second electrode to produce a moderate electric field in the first region for a movement of the ions past the second electrode and for applying a voltage pulse to the shield electrode to increase considerably the electric field between the second and shield electrodes for a movement of ions past the shield and third electrodes, and means disposed past the third electrode for detecting the ions.

10. In a mass spectrometer, a backing plate, a first electrode disposed relative to the backing plate to provide a first region between the electrode and the plate, means for providing a plurality of ions in the first region, means for biasing the backing plate and the first electrode at substantially the same potential to retain the ions in the first region, a second electrode disposed relative to the first electrode to provide a second region between the electrodes, means for biasing the second electrode relative to the first electrode to provide an electric fieid of considerable magnitude in the second region, a shield disposed between the first and second electrodes, means for biasing the shield relative to the first electrode to minimize any electric field leakage into the first region, means for applying a voltage pulse of moderate magnitude to the first electrode for producing a movement of the ions through the first and second regions, and means disposed past the second region to detect the ions.

References Cited in the file of this patent UNITED STATES PATENTS 2,685,035 Wiley July 27, 1954

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