US2762927A - Mass spectrometer - Google Patents

Description

Sept. 1956 w. C. WILEY ET AL 2,762,927

MASS SPECTROMETER Filed Aug. 24, 1953 2 Sheets-Sheet 1 POL/[R JUPPL Y INVEN'TOR.

m H. MCLARE/V y lM/LZ/AM c. MLEY A T'TORNEV Sept. 11, 1 w. WILEY ET AL 2,762,927

MASS SPECTROMETER ill IN VEN TOR.

[4 H. M LARE/V BY MLL/AM C- MLEV United States MASS srncrnomnrun William C. Wiley, Detroit, and Ian H. McLaren, Dearborn, Mich, assignors to Bendix Aviation Corporation, Detroit, Mich, a corporation of Delaware Application August 24, 1953, Serial No. 376,(i62

6 Claims. (Cl. 250-413) This invention relates to mass spectrometers and more particularly to mass spectrometers for providing a relatively sharp resolution between ions of difierent mass. The invention also relates to a method of delineating between ions of different mass.

In certain types of mass spectrometers, ions are produced from the molecules of gases and vapors in an unknown mixture and are collected in a relatively confined region for a period of time. A force is then applied to the ions to produce a movement of the ions through a relatively great distance towards a detector. The force applied to the ions causes a greater velocity to be imparted to the ions of light mass than to the ions of heavy mass. By measuring the relative times at which the ions reach the detector, the masses of the ions can be determined.

In the time-of-flight mass spectrometers now in use, difiiculties have been experienced because of thermal and other energies in the ions. These energies have caused random motions to be imparted to the ions and have prevented all of the ions of each mass from being detected at substantially the same instant. Certain improvements have been made in such spectrometers to reduce the problem resulting from the thermal and other energies in the ions. These improvements have considerably, but not completely, alleviated the problem.

This invention provides a mass spectrometer which includes features for further reducing any adverse effects resulting from the random motion imparted to the ions by the thermal and other energy in the ions. The spectrometer reduces such adverse effects by eliminating from each ion pulse many of the ions having a random motion in a direction opposite to the direction of their movement towards the detector. Since these ions are most troublesome in obtaining sharp output signals, their elimination causes the resolution between the ions of different mass to be enhanced.

An object of this invention is to provide a spectrometer for producing pulses of ions and for determining the masses of the different ions in each pulse by measuring the time required for the ions to travel through a given distance.

Another object is to provide a spectrometer of the above character for providing a compensation for the random motion of individual ions resulting from the thermal and other energy in the ions.

A further object is to provide a spectrometer of the above character for eliminating many of the ions traveling in a random motion in a direction opposite to the direction of their movement through the given distance.

Still another object is to provide a spectrometer of the above character for enhancing the resolution between ions of adjacent mass units and for especially enhancing the resolution between ions of relatively large mass.

A still further object is to provide a method of producing a relatively sharp delineation between ions of different mass.

a 2,762,927 Patented Sept. 11, 1956 Other objects and advantages will be apparent from a detailed description of the invention and from the ap pended drawings and claims.

In the drawings:

Figure 1 is a somewhat schematic view, partly in block form and partly in perspective, which illustrates one embodiment of the invention; and

Figure 2 is a circuit diagram of a circuit shown in block form in Figure 1.

In one embodiment of the invention, a wedge-shaped filament 10 made from a suitable material such as tungsten is provided. An electrode 12 is disposed at a relatively short distance, such as millimeter, from the filament iii and is provided with a slot 14, the median position of which is at approximately the same vertical level as the filament 10. An electrode 16 is positioned in substantially parallel relationship with the electrode 12 and at a relatively short distance, such as l millimeter, from the electrode 12. The electrode 16 has a slot 18 which corresponds substantially in shape and position to the slot 14. A collector 20 is substantially parallel 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 l6 and the collector 20 in substantially perpendicular relationship to these members. The backing plate 22 is positioned slightly in back of an imaginary line extending from the tip of the filament 10 through the slots 14 and 13 to the collector 20. An electrode 24 having a horizontal slot 26 is substantially parallel to the backing plate 22. The electrode 24 is positioned a relatively short distance, such as 2 millimeters, from the backing plate 22 and slightly in front of the imaginary line disclosed above.

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 34 is provided in the bottom slat 28 in parallel relationship to, and 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, which holds the molecules of different gases and vapors in an unknown mixture.

An electrode 36 is disposed in substantially parallel relationship to the electrode 24 and at a relatively short distance, such as 2 millimeters, in front of the electrode. The electrode 36 has a horizontal slot 38 corresponding substantially in shape and position to the slot 26. A detector such as a collector 40 is positioned at a relatively great distance such as 40 centimeters in front of the electrode 36. A time indicator such as an oscilloscope 42 is connected to the collector 46 to indicate the signals produced by the ions of diiterent mass in the unknown mixture.

In the steady state operation, the electrode 12 is adapted to receive a positive voltage through a resistance 46 from a suitable power supply 48. The collectors 20 and 40 also receive slightly positive voltages through resistances 50 and 52, respectively, from the power supply 48. The collectors receive slightly positive voltages to attract back to them the electrons secondarily emitted from them upon the impingement of charged particles. A slightly positive voltage is also applied to the electrode 24 through a resistance 54 for reasons which will be disclosed in detail hereafter. The filament 10 and the back ing plate 22 are connected to grounded resistances 56. and 58, and the electrodes 16 and 36 are directly grounded.

Connections are made to the filament 10 and the electrode 12 through coupling capacitances 60 and 62, respectively, from a pulse forming circuit 64. The backing plate 22 and the electrode 24 are also coupled to the pulse forming circuit 64 through suitable coupling capacitances 66 and 68, respectively. The oscilloscope 42 is also connected to the pulse forming circuit 64 through a suitable coupling capacitance 70. The pulse forming circuit 64 is fully shown in Figure 2 and will be disclosed in detail hereinafter.

The electrons emitted by the filament 19 are attracted towards the electrode 12 because of the positive voltage on the electrode relative to the voltage on the filament. The electrons are decelerated in the region between the electrodes 12 and 16 since the electrode to is at substantially the same potential as the filament it) in the steady state operation. Because of this deceleration, such electrons as may reach the region between the backing plate 22 and the electrode 24 do not have suificient energy to ionize the molecules of gas and vapor introduced into the region from the receptacle 34.

At periodic times, negative pulses of voltage are applied from the pulse forming circuit 64 to the filament 10 and the electrode 12. These pulses may have a dura tion in the order of 1' microsecondas a median figure but they may also have longer or shorter durations. Upon the imposition of such voltage pulses, the voltage on the electrode 12 becomes negative with respect to the grounded potential on the electrode 16. This causes the electrons passing through the slot 14 to be attracted towards the electrode 16. Since the electrons have additional energy imparted to them in the region between the electrodes 12 and 16, they pass into the region between the backing plate 22 and the electrode 24 with suificient energy to ionize the molecules of gas and vapor introduced into the region. The electrons then move to the collector 20.

Most of the ions produced by the electron stream have a unitary positive charge. Since the ions have a charge opposite to the charge of theelectron stream, they are retained Within the stream. The ions are retained in a relatively confined region 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). A relatively large number of ions can be retained in the electron stream before the negative charge produced by the electron stream becomes neutralized.

When a relatively large number of ions have been produced for retention in the electron stream, the voltage pulses on the filament 10 and electrode 12 are cut oil? so as to interrupt the electron stream. By interrupting the electron stream, the ions retained in the stream are made available for easy withdrawal by the imposition of voltage pulses on the backing plate 22 and the electrode 24. These pulses may have a duration in the order of 3 microseconds. For reasons which will be disclosed in detail hereafter, these pulses are applied from 0.1 microsecond up to 2 or more microseconds after the electron stream is interrupted.

The voltage pulses applied to the backing plate 22 and the electrode 24 may be in the order of +400 and +380 volts, respectively. This causes an electric field of moderate intensity to be produced in the region between the backing plate 22 and the electrode 24 and an electric field of considerably increased intensity to be produced between the electrodes 24 and 36. These fields have a polarity to produce a movement of the ions from their place of retention towards the collector 40.

The force imposed on the ions in the region between the backing plate 22 and the electrode 24 and between the electrodes 24 and 36 causes the ions of relatively light mass to have a greater velocity imparted to them than the ions of heavy mass. Because of these differences in velocity, the ions of light mass travel through the region between the electrode 36 and the collector 40 before the ions of. heavy mass.

Since the beam on the oscilloscope 42 is triggered at substantially the same instant as the imposition of voltage pulses on the backing plate 22 and the electrode 24, the oscilloscope provides an indication of the relative times at which the ions of different mass are collected. By measuring the relative times at which the ions of dilierent mass are collected, the masses of the ions can a be asily determined.

While the ions are retained in the electron stream, they have a random motion which results from thermal and other energy in the ions. As a result of this random motion, some of the ions are traveling towards the backing plate 22 at the instant that the electron stream is cut off. Other ions are stationary at this instant and still other ions are traveling towards the electrode 24. The random motion imparted to individual ions by the thermal and other energy in the ions prevents all of the ions of a given mass from being collected at the same instant of time. In this way, the measurements which are obtained are somewhat clouded.

The ions which are moving towards the backing plate 22 upon the interruption of the electron stream have been found to be most troublesome. These ions are most troublesome because they are always separated at the collector 40 by a relatively great distance from the majority of ions of their particular mass.

As previously disclosed, voltage pulses are applied to the backing plate 22 and the electrode 24 a period of time-for example, 2 microseconds-after the electron stream is interrupted. The voltage pulses are applied on the backing plate 22 and the electrode 24 after this period of time so that many of the ions moving in a random motion towards the backing plate 22 will have an opportunity to reach the backing plate. When the ions impinge on. the backing plate, they receive an electron from the plate and become neutralized. Since many of the ions having a velocity component towards the backing plate 22 reach the plate and become neutralized the ions having no velocity component towards either the backing plate 22 or the electrode 24 and the ions having a velocity component towards the electrode 24 are more plentiful in the accelerated group of ions than are ions having a velocity component towards the electrode 22 and are mainly responsible for producing the output measurements..

Of course, some of the ions having a random motion towards-the backing plate 22 may not reach the backing plate during the period between the imposition of the voltage pulses on the filament 10 and the electrode 12 and the pulses on the backing plate 22 and the electrode 24. These ions are positioned closer to the backing plate than they would have been if the voltage pulses had been imposed on the backing plate 22. and the electrode 24. immediately after the interruption of the electron stream.

Because of their backward position, the ions receive an increased amount of energy during their travel through the electric field produced between the backing plate 22 and the electrode 24. This increased amount of energy in the backwardly positioned ions causes them to gain on the ions of the same mass positioned closer to the electrode24. In this way, the ions also receive a compensation for differences in their random motion.

During-the time that individual ions are moving in a random motion towards the backing plate 22, other ions are moving in a random motion towards the electrode 24. Just as some spread of the ions moving towards the backing plate 22 is desirable, some spread of the ions moving towards the electrode 24 is also desirable. However, too much spread of the ions moving in a random motion towards the electrode 24 is not desirable since the forward ions are then too close to the electrode 24. By imposing a direct voltage of positive polarity and relatively low magnitude on the electrode 24, the ions moving in a random motion towards the electrode 24 are prevented from moving too close to the electrode.

In addition to the compensation for thermal energy provided by the delay in the acceleration of the ions from their place of retention, compensation is provided for difierences in the random motion and positioning of individual ions by the imposition of the particular voltage pulses on the backing plate 22 and the electrode 24. The differences in the positioning of individual ions result in large part from the finite width of the electron stream. The particular pulses on the backing plate 22 and the electrode 24 cause an electric field of moderate intensity to be produced between the backing plate 22 and the electrode 24 and an electric field of considerably increased intensity to be produced between the electrodes 24 and 36. The compensating action provided on individual ions by the creation of the particular electric fields between the plate 22 and the electrode 24 and between the electrodes 24 and 36 has been disclosed in detail in copending application Serial No. 249,318, filed October 2, 1951, by William C. Wiley, now Patent No. 2,685,035.

One embodiment of the pulse-forming circuit 64 is shown in detail in Figure 2. The circuit includes a triggering source 100, the output from which is introduced through a coupling capacitance 102 to the Control grid of a normally non-conductive, gas-filled tube 104. The grid of the tube is negatively biased by a resistance 106 in series with a battery 108, the positive terminal of which is grounded.

The suppressor grid of the tube 104 is connected through a resistance 110 to the cathode of the tube. The plate receives a positive voltage through a resistance 112 from a suitable power supply 114. A pair of capacitances 116 and 118 are in series with each other between the plate and cathode of the tube 104. The common terminal between the capacitances 116 and 118 is grounded.

The voltage on the cathode of the tube 104 is introduced to an amplifier 120 the output from which is applied through the coupling capacitances 60 and 62 to the filament and the electrode 12, respectively, such components also being shown in Figure 1. The cathode of the tube 104 also has a common terminal with the plate of a normally non-conductive, gas-filled tube 122. The suppressor grid of the tube 122 is connected through a resistance 124 to the grounded cathode of the tube. The control grid of the tube 122 is coupled through a capacitance 126 to the plate of the tube 104 and is connected to a grounded resistance 128.

In addition to being introduced to the grid of the tube 104, the output from the source 100 is applied through a coupling capacitance 130 to a delay line, generally indicated at 132. The delay line 132 has characteristics which will be disclosed in detail hereinafter. The output from the delay line 132 is introduced to a delay line, generally indicated at 134. The delay line 134 has a plurality of sections each of which provides a delay of approximately 1 microsecond.

Connections are made from each of the sections of the delay line 134 to a different stationary contact in a switch 136. The movable contact of the switch 136 is connected to a delay line, generally indicated at 138, which is adapted to provide a delay capable of being varied between 0 microsecond and 1 microsecond. The variable delay in the line 138 is provided by the positioning of a movable contact 140.

The voltage on the contact 140 is coupled through a capacitance 142 to the grid of a normally non-conductive, gas-filled tube 144. The tube 144 and a gas-filled tube 146 are connected in a circuit similar to that disclosed above for the gas-filled tubes 104 and 122. The voltage on the cathode of the tube 144 is introduced to one terminal of a variable capacitance 148 and a stationary contact of a rheostat 150 and through the coupling capacitance 66 to the backing plate 22.

Connections are made from the other terminal of the variable capacitance 148 to the movable contact and a second stationary contact of the rheostat 150, to a grounded variable capacitance 152 and to a grounded resistance 154. The movable contact of the rheostat 150 is ganged to the capacitances 148 and 152 for reasons which will be disclosed in detail hereinafter. The voltageacross the resistance 154 is applied through the coupling capacitance 68 to the electrode 24, such components also being shown in Figure 1.

During the time that the tube 104 is cut 011, current flows through a circuit including the power supply 114, the resistance 112, and the capacitance 116. This current causes the capacitance 116 to be charged to a positive voltage. Current also flows through the power supply 114, the resistance 112, the capacitance 126 and the resistance 128 and charges the capacitance.

Upon the introduction of a positive triggering signal from the source 100, the tube 104 becomes conductive and provides a discharge path for the capacitance 116. Current then flows through a circuit including the capacitance 116, the tube 104 and the capacitance 118 and causes part of the charge in the capacitance 116 to be transferred to the capacitance 118.

When the capacitance 116 discharges through the tube 104, the voltage on the plate of the tube falls and causes the voltage on the upper plate of the capacitance 126 to fall correspondingly. This prevents the tube 122 from becoming conductive even though the voltage on the plate of the tube increases as a result of the charge of the capacitance 118. The capacitance 126 then discharges through a circuit including the capacitance, the resistance 128 and components in the power supply 114.

When the capacitance 126 has discharged sufficiently, its voltage rises to a level which causes the tube 122 to become conductive. The capacitance 116 then discharges through a circuit including the capacitance, the tube 104 and the tube 122. The capacitance 118 also discharges through a circuit including it and the tube 122. Since the tubes 104 and 122 have a relatively low impedance during the times that they are conductive, the capacitances 116 and 118 discharge in relatively sharp pulses. In this way, a voltage pulse indicated at 160 is produced on the plate of the tube 122. This voltage pulse is inverted by the amplifier and introduced as a negative pulse to the filament 10 and the electrode 12.

Each triggering signal from the source 100 is introduced to the delay line 132 as well as to the grid of the tube 104. The line 132 is designed to provide a delay provided by the line 134 is determined by the positioning of the movable contact of the switch 136. After passing through the delay line 134, the triggering signal from the source 100 is delayed by the line 138. The

particular delay provided by the line 138 is dependent upon the positioning of the contact 140. By varying the position of the movable contact of the switch 136 and the position of the contact 140, the delay can be adjusted to any value in a range of several microseconds.

The triggering signal passing through the line 133 is introduced to the grid of the tube 144. The tube 144 operates in conjunction with the tube 146 and with its associated components to produce an output pulse at its cathode. This pulse is indicated at 162 in Figure 2 and is applied to the backing plate 22. The amplitude of the pulse is reduced by the voltage divider network formed by the capacitances 148 and 152, the rheostat and the resistance 154 such that a voltage of decreased magnitude is produced across the resistance 154 for imposition on the electrode 24. The amplitude of the pulse applied to the electrode 24 can be varied by simulta- 7 l neously adjusting the values provided by the rheosta't IStland the capacitances 148 and 152. Simultaneous adjustment of these components is provided to make the voltage pulse across the resistance 154 independent of frequency.

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 stream of electrons, means for introducing a plurality of molecules into the electron stream for ionization by the electron stream and for retention of the ions within the stream, means for reducing the strength of the electron stream, means for applying a force on the molecules a period of time after the reduction of the electron stream for the acceleration mainly of the ions having a random motion with a velocity component in the direction of the force and with no velocity component in the direction of the force or in the opposite direction, means for detecting the ions after their travel through a particular distance past their place of withdrawal, and means for indicating the relative times at which the ions of different mass are detected.

2. A mass spectrometer, including, means for providing an electron stream, means for introducing a plurality of molecules of gas and vapor into the electron stream for ionization by the stream and for retention of the ions within the stream, an electrical circuit for cutting oil the electron stream and for withdrawing the ions in a pulse from their place of retention a particular time after the electron stream is cut off, means for detecting the ions after their travel through a given distance from their place of retention, and means for indicating the relative times at which the ions of different mass are detected.

3. A mass spectrometer, including, a backing plate, an electrode disposed in aligned relationship with the backing plate at a particular distance in front of the electrode, means for producing a stream of electrons between the backing plate and the electrode and in substantially aligned relationship to the backing plate and the electrode, means for introducing a plurality of molecules into the region between the backing plate and the electrode for ionization by the electron stream and for retention of the ions within the stream, an electrical circuit for reducing the strength of the electron stream and for subsequently producing an electrical field between the backing plate and the first grid at sufiicient period of time after the reduction of the electron stream for ions mov ing in a random motion towards the backing plate to reach the plate, a detector disposed to produce signals upon the movement of the ions a particular distance past the electrode, and means for indicating the relative times at which the ions of difierent mass are detected.

4. A mass spectrometer, including, a backing plate, electrode disposed in substantially parallel relationship to the backing plate, means for producing a stream of electrons between the backing plate and the electrode, means for introducing a plurality of molecules into the region between the backing plate and the electrode for ionization by' the electron stream and for retention of the ions within the stream, an electrical circuit for reducing the strength of the electron stream and at a particular time after the reduction of the stream for producing an electrical field between the backing plate and the electrode for accelerating the ions towards the electrode, a detector disposed to produce signals upon the movement of the ions a particular distance past the electrode, and means for indicating the relative times at which the ions of different mass are detected.

5. A mass spectrometer, including, a backing plate, an electrode disposed in substantially parallel relationship to the backing plate, means for producing a stream of electrons between the backing plate and the electrode and in substantially parallel relationship to the backing plate and the electrode, means for introducing a plurality of molecules into the region between the backing plate and the electrode for ionization by the electron stream and for retention of the ions within the stream, an electrical circuit for reducing the strength of the stream and for imposing a voltage pulse on the backing plate relative to the voltage on the electrode to accelerate the ions towards the electrode, the electrical circuit being operative to delay the imposition of the voltage pulse for a sufiicient period of time after the reduction of the electron stream for ions traveling in a random motion towards the backing plate to reach the plate, means for detecing the ions after their travel through a particular distance past the electrode, and means for indicating the relative times at which the ions of different mass are detected.

6. A mass spectrometer, including, a backing plate, an electrode disposed in substantially aligned relationship to the backing plate, means for producing a stream of electrons between the backing plate and the electrode and in substantially aligned relationship to the backing plate and the electrode, means for introducing a plurality of molecules into the region between the backing plate and the electrode for ionization by the electron stream and for retention of the ions within the stream, means for reducing the strength of the electron stream, means for imposing a force on the ions in a direction perpendicular to the backing plate and the electrode to accelerate the ions in a pulse towards the electrode and a sulficient period of time after the reduction of the electron stream for the movement to the backing plate of ions having a random motion with a velocity component in the direction of the backing plate, means for detecting the ions after their travel through a particular distance past the electrode, and means for indicating the relative times at which the ions of difierent mass are detected.

References Cited in the file of this patent

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