US2765406A - Mass spectrometer - Google Patents

Description

W. C. WILEY MASS SPECTROMETER Oct. 2, 1956 3 Sheets-Sheet 1 Filed June 4, 1952 INVENTOR. I l V/L'L/AM 6. VV/LEV 2, 1956 w. c. WILEY MASS SPECTROMETER 3 Sheets-Sheet 2 Filed June 4, 1952 POWER JUPPA INVEN TOR. W/u/AM a, w/zz-y MK-m ATTORNE V Oct. 2, 1956 Filed June 4, 1952 W. C. WILEY MASS SPECTROMETER 3 Sheets-Sheet 3 TRIGGW/AG SOURCE POWER sup/ 4v 12 /Z&

ATTORNEY INVENTOR.

W/LL/AM C. W/Lfy BY United States Patent @fitice 2,765,465 Patented Oct. 2, 1956 MASS SPECTROMETER William C. Wiley, Detroit, Micln, assignor to Bendix Aviation Corporation, Detroit, Mich, a corporation of Delaware Application June 4, 1952, Serial No. 291,605

16 Claims. (Cl. 25041.9)

This invention relates to mass spectrometers and more particularly to a mass spectrometer for producing an increased resolution between ions of diflerent mass. The invention also relates to methods of producing such increased resolution.

In certain types of mass spectrometers, ions are produced from molecules of different gases and vapors and are stored for a period of time before being withdrawn in pulse form by an accelerating force. The ions formed from molecules of relatively light mass have a greater velocity imparted to them by the accelerating force than the ions of heavy mass. Because of their ditterent velocities, the ions of different mass separate into groups. Each group is positioned in accordance with the mass of the ions in the group provided that the ions have single positive charges. Since each group travels through a predetermined distance in a time dependent upon the mass of the ions in the group, the masses of the ions can be determined by measuring their travel times.

In the spectrometers now in use, it becomes increasingly difiicult to measure the masses of the ions as the masses increase. For example, it is much more difiicult to distinguish between ions having atomic mass units of 200 and 201 than between ions having atomic mass units of and 11. This difficulty occurs because the time required for the collection of all the ions in each group increases as the mass of the ions in the group increases and as the speed of the ions in the group correspondingly decreases. As a result of this difficulty, many mass spectrometers are not now being used for measuring the complete range of molecular weights that may be encountered but are utilized only to distinguish between ions formed from gas molecules having relatively low atomic mass units, such as 100 units or less. Such spectrometers as do have a relatively wide range of measurement are extremely expensive because of the considerable sensitivity that has to be imparted to them.

This invention provides a spectrometer having an operating range materially extended over that of spectrometers now in use. The spectrometer attains its increased range by effectively controlling the length of each group travelling to the collector in accordance with the mass of the ions in each group. For example, a group containing ions with an atomic mass unit of 100 may have a shorter length than a group containing ions with an atomic mass unit of 50 and a longer length than a group containing ions with an atomic mass unit of 200. In this way, the time required for each group of ions to be collected remains substantially constant even with changes in the mass of the ions in the group. This causes a corresponding enhancement of the resolution between ions of relatively large mass to be obtained as compared to the resolution inherent in time-of-flight 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 time requiring for the ions to travel through a predetermined distance.

Another object is to provide a spectrometer of the above character for enhancing the resolution between ions of different mass over that obtained in spectrometers now in use and for especially enhancing the resolution between ions of relatively large mass.

A further object is to provide a spectrometer of the above character for providing an accelerating force on a pulse of ions so as to produce a separation of the ions into groups, each composed of ions of a particular mass and positioned in accordance with the mass of the ions.

Still another object is to provide a spectrometer of the above character for adjusting the length of each ion group in accordance with the mass of the ions in the group so as to make the resolution between different ion masses substantially independent of the masses.

A still further object is to provide a method of increasing the range of satisfactory mass delineation by enhancing the 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.

In the drawings:

Figure l is a perspective view schematically illustrating the disposition of a mass spectrometer relative to a permanent magnet for controlling the travel path of ions in the spectrometer;

Figure 2 is an enlarged perspective view of the mass spectrometer shown in Figure l and schematically illus trates the relative disposition of an ion source and a collector assembly forming a part of the spectrometer;

Figure 3 is an enlarged view, partly in perspective and partly in block form, illustrating in further detail the ion source shown in Figure 2;

Figure 4 is an enlarged perspective view illustrating in further detail the collector assembly shown in Figure 2;

Figure 5 is an enlarged sectional view schematically illustrating the operation of some of the components shown in Figures 2 and 3; and

Figure 6 is a circuit diagram illustrating in some detail electrical components shown in block form in Figures 2 and 3.

In one embodiment of the invention, a permanent magnet generally indicated at it) (Figure l) is adapted to supply a magnetic field of substantially uniform intensity across a suitable air gap. For this purpose, the magnet is provided with a yoke 12 and a pair of oppositely disposed pole pieces 14 and 16 separated from each other by the gap. A cylindrical cover 13 for a mass spectrometer, generally indicated at 20, is positioned between the pole pieces 14 and 16, with the faces of the cover contiguous to the faces of the pole pieces. An ion source, generally indicated at 22 (Figures 2 and 3), and a collector assembly, generally indicated at 24 (Figures 2 and 4), are suitably disposed within the mass spectrometer 20, which is evacuated to a relatively low pressure.

, The ion source 22 includes a wedge-shaped cathode 26 (Figure 3) made from a suitable material, such as tungsten, for the emission of a large number of electrons when heated. An electrode 28 is disposed at a relatively close distance to the cathode 26 and is provided with a horizontal slot 3%, the median position of which is substantially vertically aligned with the cathode 26. An electrode 32 having a slot 34 corresponding substantial ly in shape and position to the slot 30 is provided in substantial alignment with the electrode 28 at a relatively close distance to the electrode. A collector 36 is disposed in substantial alignment with the electrodes 28 and 32 at a relatively great distance from the electrodes.

A backing plate 38 is provided between the electrode 32 and the collector 36 in perpendicular relationship to the electrode and to the collector and slightly to the right of an imaginary line passing from the tip of the cathode 26 through the slots 30 and 34. The plate 38 is in Substantial alignment with a pair of electrodes 40 and 42 having vertical slots 44 and 46, respectively, corresponding substantially in shape and position to each other. The electrode 40 is disposed between, and relatively close to, the plate 38 and the electrode 42 and is positioned slightly to the left of the imaginary line disclosed above. The electrode 40 and plate 38 form an enclosure with laterally disposed insulated slats 48, one of which has a vertical slot communicating with a conduit 52. The conduit 52 extends from a receptacle 54 adapted to hold molecules of the different gases constituting an unknown mixture.

The collector 24 is positioned relatively closely to the ion source 22 so that the ions will travel through substantially an integral number of revolutions before being collected. The collector 24 includes a receptacle 56 (Figure 4) positioned within a grounded shield 58 in insulated relationship to the shield. The receptacle 56 and the shield 58 are open at the side facing the flow of ions. The open side of the receptacle 56 is slightly recessed within the shield 58 so as to be covered by a screen 60 extending across the open side of the shield 58. A time indicator 62, such as an oscilloscope, is connected to the receptacle 56 to provide an indication of the relative times at which ions of difierent mass reach the collector.

The electrode 28 normally has a positive voltage applied to it through a resistance 64 from a suitable direct power supply 66 (Figures 3 and 4). The collector 36 and the receptacle 56 are maintained at slightly positive potentials by suitable connections through resistances 68 (Figure 3) and 70 (Figure 4) to the power supply 66, shown in both Figures 3 and 4. The collector 36 is maintained at a slightly positive potential to attract the electrons flowing from the cathode 26, and the receptacle 56 is at a slightly positive potential to attract back to it electrons secondarily emitted from it by the impingement of ions. The plate 38 is biased at a slightly positive potential by a series circuit including a resistance 72 and a battery 74 (Figure 3) the negative terminal of which is grounded, and the electrode 40 is biased at a slightly positive voltage by a resistance 75 in series with a battery 78 having its negative terminal grounded. The cathode 26 is grounded through a resistance 80; and the electrodes 32 and 42 are directly grounded.

The electrons emitted by the cathode 26 are attracted towards the electrode 28 because of the positive voltage on the electrode relative to the voltage on the cathode. In the steady state condition, the electrons are not further accelerated after they travel past the electrode 28, since the electrode 32 has a lower potential than the electrode 28. Therefore, the relatively few electrons that are able to reach the region between the backing plate 38 and the electrode 40 do not have sufficient energy to ionize molecules of gas introduced into the region.

Upon the application of negative pulses of voltage from a pulse forming circuit 82 through suitable coupling capacitances 84 and 86 to the cathode 26 and the electrode 28, respectively, the electrode 32 becomes more positive than the electrode 28 and provides an added acceleration on the electrons in the region between it and the electrode 28. This added acceleration causes electrons to travel into the region between the backing plate 38 and the electrode 40 with sufiicient energy to ionize molecules of gas that they may strike in the region. The ions which are produced from the molecules of gas are retained in the electron stream, since they have an opposite charge relative to that of the electron stream.

Because of the large charge produced by the electron stream, a relatively large number of ions can be retained in the potential well created by the electron stream before the stream becomes saturated. These ions are retained to a large extent in a space having a relatively narrow width because of the collimating action which is provided on the electrons by the slots 30 and 34. and by the magnetic field produced by the magnet 10. The operation of a source to store a large number of ions in an electron stream is disclosed in detail in co-pending application Serial No. 221,554 filed April 18, 1951, by Ian H. McLaren and myself, now Pat. No. 2,732,500.

As the electron stream approaches saturation by the retention of ions, it is cut oif by the removal of the voltage pulses on the cathode .26 and the electrode 28. Upon the removal of the negative potential well created by the electron stream, the ions retained within the stream become available for easy withdrawal by the imposition of an electric field of moderate magnitude between the backing plate 38 and the electrode 40 and an electric field of substantially equal magnitude between the electrodes 40 and 42. These substantially equal electric fields are produced by applying positive voltage pulses of predetermined magnitude on the plate 38 and the electrode 40 through suitable coupling capacitances 38. and 90, respectively, and by retaining the ground potential on the electrode 42. For example, when the electrode 40 is spaced approximately two millimeters from both the plate 38 and the electrode 42, voltage pulses of approximately +20O and volts may be respectively applied on the plate 38 and the electrode 40 from the pulse forming circuit 82.

Upon the application of the positive voltage pulses on the plate 38 and the electrode 40, the ions are repelled from their position between the plate 38 and the electrode 40 and are given an acceleration which is substantially inversely proportionate to their masses. Thus, the ions of relatively light mass have a greater velocity imparted to them by the electric field than the ions of heavy mass. Because of the diflferences in velocity, the ions become separated into groups as they travel towards the collector assembly 24. Each group is composed of ions of a different mass and is positioned relative to the other groups in accordance with the mass of the ions in the group. The length of each ion group may be considered as being substantially constant because of the retention of the ions in the region between the plate 38 and the electrode 40 before their acceleration towards the collector 24.

When the ions of relatively light mass have travelled a considerable distance into the region between the electrodes 40 and 42, the positive voltage pulses on the plate 38 and the electrode 40 are removed. The electrode 40 is then returned to ground potential, and a negative pulse of voltage is applied to the plate 38 to attract back to the plate the ions still in the region between the plate and the electrode 40. As illustrated schematically in Figure 5, a group formed from ions of relatively light mass may have reached a position 92 at the time that a negative voltage pulse is applied to the plate 38; a group formed from ionsv of moderate mass may have reached a position 94; and a group formed from ions of relatively heavy mass may have. reached a position 96. The ions in the different groups are shown separated from one another for purposes Of. illustration only.

Because of, its. position. 92, only a small part of the group formed from the ions of light mass are attracted back to the plate 38 when a negative potential is applied on the plate. However, an increased part of the group formed from ions of' medium mass in the position 94 are attracted back to the plate 38 and a still greater part of. the, group formed from the ions of a heavier mass in the position 96 are so attracted. The part of each group that is attracted back to'the plate 38 by the application of a negative voltage pulse on the plate is directly related to the mass of the ions in the group.

Since the electrodes 40 and 42 are both maintained at ground potential when a negative pulse is applied to the plate 38, substantially no electrical field exists between the electrodes. Therefore, the ions in the region between the electrodes continue to move towards the collector assembly 24 with the same velocity that they had at the instant that the positive voltage pulses on the plate 38 and the electrode 40 were removed. Because of the direct relationship between the part of each ion group attracted back to the plate 38 and the mass of the ions in the group, the number of ions of each mass travelling towards the collector 24 is substantially inversely proportional to the mass of the ions in the group.

The ions which continue their movement past the electrode 42 travel in a direction substantially perpendicular to the magnetic lines of flux produced between the pole pieces 14 and 16. Because of this perpendicular relationship, the magnetic flux acts upon the ions in a direction substantially perpendicular to both the initial movement of the ions and the magnetic field. This force causes the ions to move in a circular path, with the axis of the cylindrical cover 18 serving as substantially the center of the circular path. After the ions have moved through a predetermined number of revolutions, they are collected by the assembly 24. The predetermined number of revolutions is preferably greater than one so as to increase the length of the travel path and to produce a greater separation in time and space between the groups of ions representing the ions of difierent mass than the separation which would be produced if only one revolution were required. Because of the differences in atomic mass units, the ions of relatively light mass travel through the predetermined number of revolutions before the ions of heavy mass and reach the collector assembly 24 first. Since each ion group produces a signal when it impinges on the receptacle 56, the masses of the ions in the different groups can be determined by the relative times at which the output signals are produced.

As previously disclosed, each group of ions is reduced in length by an amount directly related to the mass of the ions in the group. The reduced length of the groups for ions of heavy mass compensates for the fact that the ions of heavymass travel more slowly than the ions of light mass. As a result, the total time between the collection of the first and last ions in each group remains substantially constant and independent of the mass of the ions in the group. It is this separation of the collection time from any dependence on the masses of the ions that causes the resolution of the spectrometer to be considerably increased over spectrometers now in use.

Resolution between ions of different mass in a timeof flight spectrometer may be defined as R=T/S (1) where R=the resolution between ions of different mass;

T=the time between the collection of groups of ions of adjacent mass units; and

S=the average time duration of the groups of ions of adjacent mass units.

Although the ions are concentrated to a large extent in the negative potential well created by the electron stream, they can be considered to be located throughout the region between the backing plate 38 and the electrode 40. At the time that the negative pulse is applied to the plate 38, the ions will have travelled through a distance where where F=the force producing the acceleration a on each ion;

M: the mass of each ion; and

E=the strength of the electric field imposed on each ion by the imposition of the positive voltage pulses on the plate 38 and the electrode 40;

Substituting Equation 3 in Equation 2,

where K=a constant.

As will be seen by Equation 4, the length of the ion group that has moved past the electrode 40 upon the imposition of a negative voltage on the plate 38 is inversely proportional to the mass of the ions in the group.

The velocity of the ions in each group is given by where V=the velocity of the ions at the instant that the voltages on the electrodes 40 and 42 become equal.

Substituting in Equation 5 the expression for a in Equation 3 But the time required between the collection of the first and last ions in each group can be expressed as For any given measurement, the time t between the imposition of the positive and negative voltages on the plate 38 is a constant regardless of the mass of the different ions. Furthermore, it is known that the time T between the collection of groups of ions of adjacent mass units is a constant. Therefore, Equation 8 can be written as where C=a constant.

This proves that the resolution for ions of different mass can be made substantially constant by utilizing the apparatus disclosed above.

One embodiment of the pulse forming circuit 82 is illustrated in Figure 6. This circuit includes a pair of gas filled tubes and 102 havingv a common connection between the cathode of the tube 100 and the plate of the tube 102. The plate of the tube 100 is connected through a resistance 104 to a suitable direct power supply 106 and is also connected to a resistance 108 in series with a grounded capacitance 110. The grid of the tube 100 biased by a resistance 112 in series with a battery 114, the positive terminal of which is grounded. Pulses are ap plied to the grid of the tube 100 from a suitable triggering source 116 through a coupling capacitance 18.

The triggering source 116 is also connected to a resistance 120 which in turn has two series branches connected to it. One of the branches includes a capacitance 122, a resistance 124 and a battery 126, the positive terminal of which is grounded. The other branch includes a rheostat 128 and a capacitance 130, the movable contact of the rheostat being connected to the resistance 120.

The cathode of the tube 102 is grounded, and the grid has a bias voltage applied to it from the common terminal between the capacitance 122 and the resistance 124. In addition to being connected to the cathode of the tube 100, the plate of the tube 102 is connected to a resistance 132 in series with a grounded capacitance 134 and to a resistance136: in series with a battery 138, the negative terminal of which is grounded. The battery 138 is adapt'ed to sup ply a: positive voltage which is somewhat lower than that applied from thepower supply 106. Connections are. also made from the plate of the tube 102 to the backing plate 38 through a coupling. capacitance 140 and to the electrode 40 through a resistance 142 in series with a coupling capacitance 144. A grounded resistance 146 is connected to the plate 38, and the cathode of a diode 148 having a grounded plate is connected to the electrode 140.

The grids of the tubes 100 and 102 are biased so that the tubes will not conduct during the period between pulses from the triggering source 116. Because of the non-conductance of the tube 100 during this period, the voltage on the plate of the tube approaches the same value as the voltage from the power supply 106" by a charge which is gradually applied on the capacitance 110 through a circuit including the power supply, the resistances 104 and 108 and the capacitance.

At some time after the capacitance 110 has become substantially fully charged, a pulse is applied from the source 116 to the grid of the tube 100. This pulse causes the tube 100 to conduct and current to flow in a sharp surge through a circuit including the capacitance 110, the resistance 108, the tube 100, the resistance 132 and the capacitance 134. The positive step of voltage produced across the resistance 132 and the capacitance 134 by the How of current is applied to the backing plate 38 and to the electrode 40 to produce a withdrawal of the ions from their place of retention.

The current flowing from the capacitance 110 through the tube 100 and the capacitance 134 causes the capac tance 134 to be charged to a voltage dependent upon the relative values of the capacitances 110 and 134. Since the voltage on the capacitance 134 is applied to the plate of the tube 102 and since the capacitance is charged to a voltage higher than that provided by the battery 138, the tube is put into condition for conductance upon the imposition of a triggering signal on its grid. This triggering signal is delayed slightly with respect to the triggering signal applied to the grid of the tube 100. The signal applied to the grid of the tube 102 is slightly delayed because of its relatively shallow rising characteristic, which is produced by the integrating action of the capacitance 122 and the resistance 124.

The shallow rising characteristic of the signal applied to the grid of the tube 102 causes the tube to conduct shortly after the conductance of the tube 100. The time between the flow of currents through the tubes 100 and 102 can be adjusted by varying the movable contact of the rheostat 128 and the values of the capacitance 122 and the resistance 124.

Upon the conductance of the tube 102, the capacitance 134 discharges in a sharp pulse through the tube. The

discharge of the capacitance 134 causes a negative pulse A of voltage to be produced at the plate of the tube 102 with respect to the positive bias normally applied by the battery 138, and this pulse of voltage is applied to the backing plate 38 through the capacitance 140. The negative pulse of voltage is not applied to the electrode 40 because of the action of the resistance 142 and the diode 148 in preventing the voltage on the cathode of the diode from falling below ground.

It should be appreciated that the ions may be attracted from their place of storage by the imposition of negative pulses of voltage on the electrodes 40 and 42. After a predetermined time, the electrodes 40 and 42 may be returned to ground and a negative pulse of voltage may be applied to the plate 38 to attract back to the plate the ions which are still within the region between the plate and the electrode. It should also be appreciated that difierent embodiments may be substituted for the collector assembly 24 as well as for the components which produce the electron stream. For example, since the collector assembly in effect serves as a detector, other types of detectors maybe used in which the ions may actually not be collected.

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 theappended claims.

What is claimed is:

l. A mass spectrometer, including, means for storing a plurality of ions, means for providing an. accelerating force on the ions to withdraw them in a pulse from their place of storage, means for acting upon the ions after their withdrawal to produce between ions of different mass a resolution independent of the ion mass, means for detecting the ions of difierent mass after their travel through a particular distance, and. means for determining the relative times at which the ions of different mass reach the detector.

2. A mass spectrometer, including, means for storing a plurality of ions, means for imposing an accelerating force upon the ions to withdraw the ions in a pulse from their place of storage and to separate the ions into groups dependent upon their mass, means for detecting the ions in each group after their travel through a particular distance, means for adjusting the length of each ion group in accordance with the times required for the ions in the group to travel to the detecting means, and means for determining the relative times at which the ions of different mass are detected.

3. A mass spectrometer, including, means for storing a plurality of ions, means for imposing an accelerating force upon the ions to withdraw them in a pulse from their place of storage and to separate them into groups positioned inaccordance with their mass, means for cutting oil the length of each ion group in accordance with the positioning of the group, means for detecting the ions after their travel through a particular distance, and means for determining the relative times at which the ions of different mass are detected.

4. A mass spectrometer, including, means for storing a plurality of ions, means for imposing an accelerating force upon the ions to withdraw them in a pulse from their place of storage and to separate them into groups having velocities dependent upon their mass, means for controlling the length of each group in accordance with the velocities of the ions in the group, means for detecting the ions after their travel through a particular distance, and means for providing an indication of the relative times at which the ions of different mass are detected.

5. A mass spectrometer, including, means for storing a plurality of ions, a detector disposed at a particular distance from the place of ion storage, means for withdrawing the ions in a pulse from their place of storage and for accelerating the ions towards the detector, means operative after a particular period of ion acceleration to impose a decelerating force on the ions that have failed to reach a position which is a particular distance from the detector, and means for indicating the relative times at which ions of different mass reach the detector.

6. A mass spectrometer, including, means for storing a plurality of ions, means for imposing an accelerating force on the ions through a first region, means operative. a particular time after the imposition of the accelerating force to cut off ions that are still within the first region, means for detecting the ions passing beyond the first region after their travel through a particular distance beyond the first region, and means for providing an indication of the relative times at which the ions of different mass are detected.

7. A mass spectrometer, including, means for storing a plurality of ions, means for withdrawing the ions in a pulse from their space of storage and for imposing a substantially constant accelerating force upon the ions in a sesame first region, means operative a particular time after the imposition of the accelerating force to cut off ions that are still travelling through the first region, means for detecting the ions after their travel through a particular distance past the first region, and means for determining the relative times at which the ions of different mass are detected.

8. A mass spectrometer, including, a backing plate, a first electrode disposed a particular distance from the backing plate, means for storing a plurality of ions in the region between the backing plate and the first electrode, a second electrode disposed a particular distance from the first electrode on the far side of the backing plate, a detector disposed at a particular distance from the second electrode, means for applying a substantially constant electric field between the backing plate and the first and second electrodes to accelerate the ions towards the electrodes, means operative at a particular time after the application of the substantially constant electric field to provide a negative electric field in the region between the plate and the first electrode to attract back to the plate ions still travelling through the region, and means for indicating the relative times at which the ions of different mass reach the detector.

9. A mass spectrometer, including, means for storing a plurality of ions, a backing plate, a first electrode disposed a relatively short distance from the backing plate, means for storing a plurality of ions in the region between the backing plate and the first electrode, a second electrode disposed a relatively short distance from the first electrode on the far side of the backing plate, a collector disposed at a relatively great distance from the second electrode, means for applying electric fields between the backing plate and the first and second electrodes to produce an acceleration of the ions towards the collector, means operative at a particular time after the application of the electric fields to produce a negative electric field in the region between the backing plate and the first electrode to attract back to the plate ions still within the region, means operative at substantially the same time as the application of the negative electric field to minimize the electric field between the first and second electrodes, and means for providing an indication of the relative times at which the ions of different mass reach the collector.

10. A mass spectrometer, including, a backing plate, a first electrode disposed at a particular distance from the plate, means for storing a plurality of ions in the region between the backing plate and the first electrode, a second electrode disposed at a particular distance from the first electrode, a detector disposed at a particular distance from the second electrode, means for applying pulses of voltage between the backing plate and the first electrode and between the first and second electrodes to provide accelerating forces on the ions, means operative at a particular time after the application of the accelerating forces on the ions to apply a pulse of voltage between the backing plate and the first electrode for the deceleration of the ions that are still travelling towards the electrode, and means for indicating the relative times at which the ions of different mass are detected.

11. A mass spectrometer, including, a backing plate, a first electrode disposed at a relatively small distance from the plate, means for storing a plurality of ions in the region between the backing plate and the first electrode, a second electrode disposed at a relatively small distance from the first electrode, a collector disposed at a relatively great distance from the second grid, means for applying on the ions a substantially constant accelerating force between the backing plate and the first electrode and between the first and second electrodes, means operative at a particular time after the imposition of the accelerating force to ap ply a decelerating force between the backing plate and the first electrode for preventing ions still moving towards the electrode from moving past the electrode, means operative at substantially the same time as the imposition of the decelerating force to minimize the electric force between the first and second electrodes, and means for determining the relative times at which the ions of difierent mass reach the collector.

12. A mass spectrometer, including, a backing plate, an electrode disposed at a particular distance from the plate, means for retaining a plurality of ions in the region between the backing plate and the electrode, an electrical circuit for initially applying a pulse of voltage between the backing plate and the electrode for the acceleration of the ions from their place of retention and for the separation of the ions on the basis of their mass and for subsequently applying a pulse of voltage between the backing plate and the electrode to prevent ions still in the region between the backing plate and the electrode from reaching the electrode, a detector positioned at a particular distance past the electrode to detect the ions moving past the electrode, and means for indicating the relative times at which the ions of different mass are detected.

13. A mass spectrometer, including, means for storing a plurality of ions, an electrical circuit for initially imposing an accelerating force on the ions in a first region to produce a separation of the ions on the basis of their mass and for subsequently imposed a decelerating force on the ions in the first region to prevent ions still in the region from travelling through the region, an ion detector disposed at a particular distance past the end of the first region to detect the ions travelling past the region, and means for indicating the relative times at which the ions of ditferent mass are detected.

14. A mass spectrometer, including, a backing plate, an electrode disposed at a particular distance from the plate, means for retaining a plurality of ions in the region between the backing plate and the electrode, an electrical circuit for initially producing an electrical field of a first polarity to produce a movement of the ions towards the detector from their place of retention and to produce a separation of the ions on the basis of their mass and, after the movement of some of the ions past the electrode, for subsequently producing an electrical field of an opposite polarity to prevent ions still moving towards the electrode from travelling past the electrode, a detector positioned at a particular distance past the electrode to detect the ions moving past the electrode, and means for indicating the relative times at which the ions of different mass are detected.

15. A mass spectrometer, including, a backing plate, a first electrode disposed at a particular distance from the plate, a second electrode disposed at a particular distance from the first electrode, means for providing a plurality of ions and for retaining them in the region between the backing plate and the first electrode, an electrical circuit for initially producing substantially equal electrical fields in the region between the backing plate and the first electrode and in the region between the first and second electrodes to withdraw the ions from their place of retention and to produce a separation of the ions on the basis of their mass and after the movement of some of the ions past the first electrode for producing an electrical field of the opposite polarity in the region between the backing plate and the first electrode to prevent ions still in the region from reaching the electrode, a detector positioned at a particular distance past the second electrode to detect the ions traveling past the electrode, and means for indicating the relative times at which the ions of different mass are detected.

16. A mass spectrometer, including, a backing plate, a first electrode disposed at a relatively small distance from the plate and in substantially parallel relationship with respect to the plate, a second electrode disposed at a relatively small distance from the first electrode and in substantially parallel relationship with respect to the first electrode, means for providing a plurality of ions and for retaining them in the region between the backing plate and the first electrode, an electrical circuit for initially applying voltages on the backing plate and the elec- 1 1 trodes to apply substantially equal accelerating forces on the ions between the backing plate and the first electrode and between the first and second electrodes for the withdrawal of the ions in a pulse from their place of retention and for a separation of the ions on the basis of their mass and after the movement of ions past the first electrode for applying voltages on the backing plate relative to the first electrode to produce a decelerating force on the ions Within the region between the backing plate and the electrode, a detector disposed at a relatively great distance past the second electrode to detect the ions moving past the electrode, and means for indicating the relative times at which the ions of different mass are detected.

References Cited in the file of this patent 5 UNITED STATES PATENTS 2,582,216 Koppius Jan. 15, 1952 OTHER REFERENCES

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