US2774883A - Mass spectrometer - Google Patents

Description

Dec. 18, 1956 w. c. WILEY MASS SPECTROMETER Filed Dec. 17. 1953 IN VEN TOR.

' MLLMM 6. MAE) ATTORNEY United States Patent C) MASS SPECTROMETER William C. Wiley, Detroit, Mich., assignor to Bendix Aviation Corporation, Detroit, Mich, a corporation of Delaware Application December 17, 1953, Serial No. 398,718

6 Claims. (Cl. 25041.9)

This invention relates to mass spectrometers and more particularly to mass spectrometers for providing, an enhanced resolution between ions of different mass.

Certain types of mass spectrometers operate to produce a plurality of ions from the molecules of different.

gases and vapors in an unknown mixture. After a relatively great number of ions have been produced in the mass spectrometer, a force is imposed. upon the ions to accelerate the ions in a pulse from, their place of provision. Since the ions of light mass will be accelerated at a greater rate than the ions of heavy mass, the ions of light mass will attain a higher velocity than the ions of heavy mass at the time of removal of the force. As

a result, the ions of light mass Will travel through a par at different spacial positions in their region of retention before a force is applied to accelerate the, ionsv towards, a detector. Furthermore, thermal and other energies initially possessed by the. ions, before the. application of a. force, impart a random motion to the ions sothat some ions of a given mass may be moving towards the detector and other ions may be moving away from the. detector- As. a result of the differences in the initial positioning.

and in the thermal and other energies of the, ions, individur a1 ionsof a given mass will attain slightly difierent velocities at the time of removal of the force and therefore will reach the detector at slightly different times tov cloud I the measurements. In the, past, attempts have been made. to compensate for this difference in velocities. These.

attempts have been largely, but not. entirely, successful.

This invention provides a mass spectrometer for. producing an improved compensation for difierences in the initial positioning and in the. thermal and. other energies of ions of a given mass. Such a. compensation is pro.- vided by initially producing a field of zero. intensity ina first region and a field of considerable intensity in. a second region. These fields are subsequently adjusted toprovide a field of moderate intensity in the first. region and a field of considerable intensity in the second region.

The fields are so produced that they reach optimum values instantaneously and remain substantially constant at. these optimum values until the fields are removed. Since the energy imparted to individual ionsv of a particular mass can be better controlled with such fields,v the. ions can be made to reach a, detector. at substantially the same instant of time. Inthis way, an enhanced separation is obtained between ions of different mass so ice 2. that relatively sharp indications of the ions of difier'ent mass are obtained.

An object of this invention is to provide a mass spectrometer for distinguishing between ions of different mass by a measurementof the time required for the ions to travel through a particular distance.

Another object is to provide a mass spectrometer of the above character for obtaining an enhanced resolution between the ions of ditferentmass in comparison with the resolution obtained in spectrometers now in use.

p A further object is to provide a mass spectrometer of the above character for obtaining an improved. resolution between the ions of difierent mass by initially maintaining the ions in a substantially field-free region and subsequently subjecting the ions to an accelerating force of moderate intensity in the region and an accelerating force of a considerable intensity in a second region adjacent to the first region.

Still another object is to provide a mass spectrometer of the above character which utilizes a plurality of electrodes and applies particular voltages to the electrodes for producing fields which reach optimum values instantaneously and remain substantially constant at these optimum values.

A still further object is to provide a mass spectrometer of the above character which requires relatively simple electrical circuits for producing an enhanced resolution between ions of different mass.

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. 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 1 2. 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 tothese members. The backing plate is disposed slightly to the rear of an imaginary line which extends from the tip of the filament ll) 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: space 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 in aposition. directly below. the imaginary line disclosedaboye. A conduit. 32 connects at one end with the slot 30 and at the other end with a receptacle 34 For example, the electrode 16 may be.

which is adapted to hold molecules of the difierent gases and vapors in an unknown mixture.

An electrode 36 is substantially parallel to the electrode 24 at a relatively short distance such as 2 millimeters from the electrode. The electrode 36 is provided with a slot 38 corresponding substantially in shape and position to the slot 26 in the electrode 24. A suitable detector such as a collector 40 is disposed at a relatively great distance such as 40 centimeters from the electrode 36. An indicator such as an oscilloscope 42 is connected to the collector 40 to indicate the relative times at which the ions of different mass reach the collector.

A direct voltage of positive polarity is applied to the electrode 12 through a resistance 44 from a suitable power supply 46. Slightly positive voltages are respectively applied to the collectors and through suitable resistances 48 and 50 from the power supply 46. Positive voltages are applied to the collectors 20 and 40 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 voltages of positive polarity applied to them. These voltages are respectively applied to the backing plate 22 and the electrode 24 through suitable resistances 52 and 54 from the power supply 46. For reasons which will be disclosed in detail hereinafter, the voltages applied to the backing plate 22 and the electrode 24 are substantially equal and relatively great in magnitude. For example, the backing plate 22 and the electrode 24 may receive +400 volts from the power supply 46. The filament 10 is connected to a grounded resistance 56 and the electrodes 16 and 36 are directly grounded.

The filament 10 and the electrode 12 are connected through coupling capacitances 58 and 60, respectively, to a pulse forming circuit 62 so as to simultaneously receive voltage pulses having a negative polarity and substantially equal magnitude from the circuit 62. A voltage pulse of negative polarity is also applied to the electrode 24 a particular time after the imposition of the voltage pulses on the filament 10 and the electrode 12. Preferably, the pulse forming circuit 62 is set to apply the voltage pulses to the electrode 24 immediately after or a relatively short time, such as a variable period up to 2 microseconds, after the termination of the pulses on the filament 10 and the electrode 12. This voltage pulse is applied to the electrode 24 through a suitable coupling capacitance 64 from the pulse forming circuit 62. A voltage pulse is also applied to the oscilloscope 42 at substantially the same time as the imposition of the pulse on the electrode 24 to initiate the horizontal sweep of the oscilloscope. The voltage pulse is applied to the oscilloscope 42 through a suitable coupling capacitance 66 from the pulse forming circuit 62.

Although the pulse forming circuit 62 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 disclosed in a publication entitled Instruction Manual, Berkeley Double Pulse Generator, Model 902 issued by the Berkeley Scientific Company in August 1950. The pulse forming circuit disclosed in copending application Serial No. 288,104 filed May 16, 1952 by Macon H. Miller and William C. Wiley can also be conveniently adapted for use.

When the filament 10 is heated, a plurality of electrons are emitted by the filament. These electrons are attracted towards the electrode 12 because of the positive potential on the electrode with respect to the potential on the filament. Upon passing the electrode 12, the electrons become decelerated in the region between the electrodes 12 and 16 since the electrode 16 is at substantially the same potential as the filament 10. Because of this decelerating action in the region between the electrodes 12 and 16, the electrons are prevented from travelling into the region between the backing plate 22 and the electrode 24 with a suflicient amount of energy to ionize the molecules of the different gases and vapors 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 grounded potential on the electrode 16. As a result of this difierence in potential, the electrons passing through the slot 14 receive an additional increment in energy in the region between the electrodes 12 and 16. Therefore, the electrons will travel through the region between the backing plate 22 and the electrode 24 with sufiicient energy to ionize the molecules of the different gases and vapors introduced into the region. Most of the ions which are produced have a unitary positive charge.

Because of their positive charge, the ions are attracted to and retained in the electron stream, which has an opposite charge to that of the ions. The region of retention of the ions is of relatively narrow width because of the collimating effect produced by the slots 14 and 18 on the electron stream. Such a collimating action may also be implemented by a magnetic field (not shown). The pulse forming circuit 62 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 oif, the ions are available for easy withdrawal upon the application of a voltage pulse to the electrode 24 from the pulse forming circuit 62. The pulse forming circuit 62 is set to apply the pulse to the electrode 24 at a particular time after the pulses applied to the filament 10 and the electrode 12 are out 01f.

As previously disclosed, the voltages applied to the backing plate 22 and the electrode 24 are of substantially equal and relatively great magnitude such as +400 volts. Because the backing plate 22 and the electrode 24 are equally biased, substantially no electrical field is produced between these members to withdraw the ions into the region between the electrodes 24 and 36. In this region, there is provided an electrical field of considerable intensity since a difference of 400 volts exists between the electrodes 24 and 36.

Upon application of a particular pulse to the electrode 24 from the circuit 62, an electrical field of relatively moderate intensity is produced between the backing plate 22 and the electrode 24 and an electrical field of considerable intensity is produced between the electrodes 24 and 36. The intensity of the electrical field produced between the electrodes 24 and 36 will be slightly lower than the intensity of the field initially produced between these members. For example, when a voltage pulse having a negative polarity and a magnitude of approximately 20 volts is applied to the electrode 24, the electrode voltage will be reduced to a magnitude of +380 volts. In this way, the difference between the backing plate 22 and the electrode 24 will be 20 volts to provide an electrical field of moderate intensity between the members and the difference between the electrodes 24 and 36 will be 380 volts to provide an electrical field of considerable intensity between these members. The pulse forming circuit 62 is set to apply to the electrode 24 a voltage pulse having a time duration suflicient to produce a movement of all of the ions into the region between the electrode 36 and the collector 40.

Since the electrical field imposed on the ions between the backing plate 22 and the electrode 24 continues until a movement of the ions past the electrode, the individual ions of a particular mass receive differences in energy dependent upon their initial positioning. Thus the ions positioned to the rear of the electron stream receive a greater amount of energy than the ions positioned in the front of the electron stream. However, this difierence in energy is only slight because the intensity of the electrical field between the backing plate 22 and the electrode 24 is relatively moderate. As a result individual ions of a given mass will attain slightly different velocities upon passing the electrode 24.

As previously disclosed the voltage pulse applied to the electrode 24 continues until after the movement of the ions past the electrode 36. Since all of the ions of a given mass travel through the same distance between the electrodes 24 and 36, they receive substantially constant increments in energy even though the ions may have different velocities imparted to them upon movement past the electrode 24.

Because of the considerable and substantially constant increments of energy imparted to the ions of each mass in the region between the electrodes 24 and 36, the differences in the velocities of the ions become considerably reduced. For example, one ion of a given mass may have a relative velocity of upon its movement past the electrode 24 and another ion may have a relative velocity of 13 at this position. Because of the considerable energies imparted to the ions in the region between the electrodes 24 and 36, the ions may have relative velocities of substantially 100 and 100.3 as they move past the electrode 36. As may be seen, the difference between velocities of 100 and 100.3 is considerably less than the difference between velocities of 10 and 13. Therefore, individual ions of each particular mass will attain substantially the same velocity upon movement past the electrode 36 and will reach the collector 40 at substantially the same time.

The imposition of a relatively great electrical field between the electrodes 24 and 36 to reduce the percentage difierences between the velocities of individual ions of a given mass has another advantage in that the position of optimum focus of the ions is projected. This projection causes the distance between the backing plate 22 and the collector 40 to increase since the collector is positioned at substantially the position of optimum focus of the ions. Because of this 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 42.

Since the electrical fields between the backing plate 22 and the electrode 24 and between the electrodes 24 and 36 cause differences in velocity to be imparted to ions of different mass, the ions of each mass will become materially separated from the other ions as the ions travel through the substantially field-free region between the electrode 36 and the collector 40. By measuring the relative times at which the ions of different mass reach the collector 40, the masses of the diiferent ions can be determined.

The mass spectrometer disclosed above has several important advantages. By the imposition of uniform voltages on the backing plate 22 and the electrode 24 during the time that the ions are being formed, a substantially field-free region is produced between the plate and the electrode. Because of the substantially fieldfree region between the backing plate 22 and the electrode 24, substantially no force is imposed on the ions to withdraw them from the electron stream and the ions are retained in a region having a relatively narrow width until a voltage pulse is applied to the electrode 24.

Since the backing plate 22 and the electrode 24 are both at relatively high positive potentials, a pulse of only moderate magnitude need be applied to the electrode 24 to produce an electrical field of moderate intensity between the backing plate 22 and the electrode 24 and an electrical field of considerable intensity between the electrodes 24 and 36. For example, a voltage pulse of moderate magnitude such as 20 volts may be applied to the electrode 24. Voltage pulses in the order of 20 6 volts can beprovided with a relatively fast rise time and with a flat top to provide a substantially rectangular pulse for application to the electrode 24. A rectangular pulse is advantageous in that the electrical fields provided are instantaneously produced and are thereafter maintained at a constant level until all of the ions are past the electrode 36. In this way, optimum focusing is obtained for individual ions of each particular mass.

Another advantage is that only a single pulse is required to produce an electrical field of moderate intensity vand an electrical field of considerable intensity. In the embodiment shown, a single pulse of moderate magnitude is applied to the electrode 24 to produce the desired electrical fields. It will be recognized that such fields may be produced in other ways. For example, a single positive pulse of moderate magnitude may be applied to the backing plate to produce a similar result. Because the mass spectrometer requires only a single voltage pulse of moderate magnitude to produce an enhanced resolution between ions of different mass, the mass spectrometer may be constructed with relatively few components and with relatively simple circuitry.

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:

1. 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, means for biasing the backing plate and the first electrode with voltages having substantially equal and relatively large magnitudes to produce a substantially field-free region between the plate and the electrode and to produce a field of considerable intensity between the first and second electrodes, means for providing a plurality of ions for retention in the field-free region between the backing plate and the first electrode, means for adjusting the bias on only the first electrode to provide a field of moderate intensity between the backing plate and the first electrode and to maintain a field of considerable intensity between the first and second electrodes for producing a movement of the ions pass the electrodes and a separation of the ions on the basis of their mass, a detector disposed at a particular distance from the second electrode, 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, means for providing a plurality of ions in the region between the backing plate and the first electrode, means for applying voltages of substantially equal and relatively great magnitude to the backing plate and the first electrode to produce a substantially field-free region between the plate and the electrode for retaining the ions in the region and to produce a field, of relatively great intensity between the first and second electrodes, an electrical circuit for adjusting the voltage on only the first electrode to provide a field of moderate intensity between the backing plate and the electrode for a movement of the ions past the second electrode and to maintain an electrical field of considerable intensity between the first and second electrodes to impart to the ions substantially constant increments of energy for a movement of the ions of a given mass past the second electrode with substantially constant velocities, means disposed at substantially the position of optimum focussing of the ions for detecting the ions, and means for indicating the relative times at which the ions of different mass are detected.

3. A mass spectrometer, including, a first electrode, a

v the first and second electrodes to produce a substantially field-free region between the electrodes for retaining the ions in the region and to produce an electrical field of relatively great intensity between the second and third electrodes to move any ions through the region between the electrodes, an electrical circuit for applying to only the second electrode a voltage pulse of moderate magnitude to, provide an electrical field of moderate intensity between the first and second electrodes for imparting a moderate amount of energy to the ions to produce a movement of ions past the second electrode, the voltage pulse applied to the second electrode being of a magnitude to maintain an electrical field of relatively great intensity between the second and third electrodes for imparting a relatively large amount of energy to the ions to produce a movement of the ions of any particular mass past the third electrode at substantially the same velocity, means disposed at substantially the position of optimum focusing of ions to detect the ions, and means for indicating the relative times at which the ions of different mass are detected.

4. A mass spectrometer, including, first, second and third electrodes disposed relative to one another to-provide a first region between the first and second electrodes and a second region between the second and third electrodes, means for applying substantially equal voltages of relatively great magnitude to the first and second electrodes to provide an electric field of substantially zero magnitude in the first region and to provide an electric field of considerable magnitude in the second region, means for providing a plurality of ions in the first region, means for applying to only the second electrode a voltage pulse of relatively small magnitude to produce a moderate electric field in the first region for moving the ions past the second electrode, the voltage pulse being of a magnitude to maintain an electric field of considerable magnitude in the second region for moving the ions past the third electrode, and means disposed past the third electrode for detecting the ions.

5. A mass spectrometer, including, a backing plate, a first electrode disposed at a particular distance from the backing plate to provide a first region between the backing plate and the electrode, a second electrode disposed at a particular distance from the first electrode to provide a second region between the electrodes, means for bias ing the backing plate and the first electrode at substantially the same positive potential with respect to the potential on the second electrode to produce an electric field of substantially zero magnitude in the first region and an electric field of considerable magnitude in the second region, means for providing a plurality of ions in the first region, means for applying a voltage pulse of relatively small magnitude between the backing plate and the first electrode to produce an electric field of moderate magnitude in the first region for moving the ions past the first electrode and to maintain an electric field of considerable magnitude in the second region for moving the ions past the second electrode, and a detector disposed past the second electrode to detect the ions.

6. A mass spectrometer, including, first, second and third electrodes disposed relative to one another to provide a first region between the first and second electrodes and a second region between second and third electrodes, means for normally biasing the electrodes relative to one another to provide an electric field of substantially zero magnitude in the first region for the retention of ions and an electric field of considerable magnitude in the second region for moving through the region any ions introduced to the region, means for providing a plurality of ions in the first region, means for applying a relatively small voltage pulse between the first and second electrodes to produce an electric field of moderate magnitude in the first region for moving the ions past the second electrode into the second region, and means disposed to receive any ions moving through the second region.

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

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