US2764691A - Analysis by imparting unequal energies to ions - Google Patents

Description

Sept. 25', 1956 J. A. HIPPLE, JR

ANALYSIS BY IMPARTING UNEQUAL ENERGIES TO IONS Filed Aug. 3. 1955 GENERATOR VOLTAGE GENERATOR V l lmmuto 2,764,691 Patented Sept. 25, 1956 ANALYSIS BY I'MPARTING UNEQUAL ENERGIES TO IONS John A. Hippie, Jr., State College, Pa. Application August 3, 1953, Serial No. 371,859

Claims. (Cl. 250-413) My invention aims at the simplification of the art of mass spectrometry. I have in mind, particularly, the elimination of cumbersome and expensive equipment, such as heavy magnets in use in conventional equipment. In its place, I provide simple electronic devices.

The full nature of my invention will be understood from the following description of one embodiment which is for purposes of illustration only. The embodiment is shown schematically in the Fig. 1 of the accompanying drawing, Fig. 2 of which illustrates wave-forms charactistic of electric potentials created in the equipment.

In a common form of spectrometer, analysis is accomplished by creating ions and imparting a constant kinetic energy to all of the ions. Selected ions are then detected by an instrument which distinguishes between the ions on the basis of their different momenta. Usually, this is the heavy magnet previously referred to. In the present invention, I do the converse; I analyze by imparting the same or approximately the same momentum to all of the ions under analysis and then detect selected ions by apparatus which distinguishes between ions on the basis of their different energies or velocities with simple electrostatic field arrangements. By this expedient, I am able considerably to simplify the apparatus, principally in the elimination of the magnet previously referred to.

I am aware that the pulse technique which I employ to impart equal momenta has been utilized in certain types of analyzers dependent upon the measurement of the time of flight of the ions. However, I believe that the advantageous possibility of using the unique properties of pulsed sources (in imparting equal momenta) in combination with simple electrostatic field techniques for detection, as I propose, has been overlooked heretofore.

Ions of equal momentum and, consequently, different energy can be created by applying a voltage impulse to a field-free region in which the ions are at rest or in negligible motion and terminating the impulse before any of the ions have an opportunity to leave the region. Of course, additional ions must not enter the region during the impulse. This will be better understood by reference to the left-hand half of the Fig. 1 which illustrates a device for producing ions of the same or approximately the same momenta, the right-hand half illustrating a detector apparatus for detecting selected ions on the basis of their difierent energies or different velocities with respect to all other ions in an ionized gaseous mixture.

A suitable envelope 1, of glass, metal or other suitable material, encloses the various electrodes which are to operate in the atmosphere of the gas which is to be analyzed. The gas may be introduced through a suitable inlet 2 and may escape through the outlet 3 of the envelope 1. That portion of the gas within the region enclosed by the box-like electrode 4 is subject to ionization and analysis by the principles of my invention. The electrode 4 may be considered as though a box with rectangular sides, one side being open except for the fact that it is partially closed by a rectangular electrode 5 which is insulated from the electrode 4 by the indicated gaps. The side of the electrode 4 which is opposite the electrode 5 is provided with a plurality of apertures 6 through which impulsed ions may escape from the region within electrode 4 so that they may be directed toward a. collecting electrode 7.

The ion impulsing fields to be established between electrodes 4 and 5 (region of apertures 6) must be very uniform. This means that the surface of electrode 5 should be much larger than the region of apertures 6 in electrode 4, and the opposing surfaces of 4 and 5 should be accurately parallel. For high resolution more elaborate means will probably be required to insure field uniformity in order that all ions be subjected to essentially the same accelerating field between 4 and 5, but those means will be known to those skilled in the art. For example, apertures 6 may disturb field uniformity, but such effect can be minimized by cutting off the accelerating field before the ions get too close to apertures 6, or by more elaborate electrode configurations.

In order to produce ionization within the electrode 4, a filament 8 heated by suitable means, such as battery 9, is provided, and electrons emitted thermionically from filament 8 are accelerated during those periods of time when the voltage V3 applied to accelerating electrode 16 by the voltage generator 16 is sufficiently positive with respect to the filament voltage VF. Such electrons pass through an aperture 11 in the electrode it) and thence through a similar aperture 12 in the adjacent side of the electrode 4 into the region within the elecrode 4. The electron beam passes on through the electrode 4 and out the aperture 13 in the opposite side to be collected on the collector electrode 14. Such electrons are prevented from passing onward through the apertures 11 and 12 when V3 is sufi'iciently negative relative to VF- The variation of the voltage V3 applied to It? rela tive to the filament voltage VF is shown in Fig. 2. During the time interval T0-T1 electrons are accelerated through apertures 11 and 12. During the time interval T1T2, V3 is negative with respect to Jr and the electron beam cannot enter electrode 4-.

As indicated, the battery 15 applies a voltage ditference V1VF to the collector electrode 14 and electrode 4 relative to the filament. The voltage generator 16 applies to the electrode 5 the square wave voltage V2 as indicated in Fig. 2. During the time interval To-T 1 V2=V1 and there is no accelerating field between electrode 5 and electrode 4. It is during this period that the electron beam is creating ions in this region by ionizing the gas or vapor. At time T1 when the electron beam has been cut off, the voltage difference V2-V1 is applied between 4 and 5 and the ions created in the preceding time interval ToT1 are accelerated toward apertures 6 until time T2 when again V2=V1.

The voltage V2 on the electrode 5 .is, therefore, alternately equal to and greater than the voltage V1, and the alternations occur in time intervals T, the intervals between alternations of the wave forms in Fig. 2. In operation, then, the ions are created by the electron beam within the region of electrode 4 only during those intervals when no electric field exists between 5 and 4 and likewise no ions are created during those time intervals when the electric field is accelerating the ions towards the apertures 6 in the electrode 4. As an illustration of the operation, we may consider that the ions formed are the positive ions of nitrogen at mass 28 and oxygen at mass 32 (Nz-land 02+).

At time T1 the ions within the electrode 4 are essentially in a state of rest, but during the following time interval T, they are subjected to a voltage or field impulse due to the voltage between the electrodes 4 and 5. The voltage wave is so timed that the nitrogen and oxygen ions do not have enough time to escape to the outside of the electrode 4 during this time interval T. Therefore, after the expiration of the impulse, at time T2, when the voltage on the electrode 5* is again equal to that on electrode 4, the heavier 02+ ions are now at a distance inversely proportionate to their mass (distance indicated by length of the arrow 17 from: the axisof the electron beam, and the lighter N2+ ions are correspondingly at a somewhat greater distance inversely proportionate to their mass (distance indicated by length of the arrow 18). Both types of ions. have, however, the same momentum because they have been subjected to the same force or same field for the same period of time. Their energies are, however, diiferent' because they have fallen through different potential differences.

During the next time interval T2T3, when there is no field applied in' the region within the electrode 4, these 02-, and Na+ ions wili pass out of the electrode 4 through the apertures 6 and they will move on toward the electrode 7. Ions passing out through the apertures 6 will have the same momentum provided proper precautions are taken to insure that the electric field is uniform in the region of their acceleration. Moreover, accuracy of performance may be promoted by more elaborate means for collimating the electron beam which means will be readily within the knowledge of those skilled in the art, for example, a magnetic field in the arrangement conventionally used in existing analytical mass spectrometers of the sectored field type which collimate the electron beam at the source.

While a square wave shape of the type shown is convenient, it is not essential. Other wave shapes may be used since it is only necessary that the time integral of the field strength applied to each particle be the same for each particle. In thatevent, all ions will acquire the same momentum during the time interval T1-T2, regardless of the manner: in' which the field strength varies with time. it is only necessary that no ions be created within electrode 4 during the time that V2 is applying an accelerating field between electrodes 4 and 5 to produce the ions of equal momentum, and also that such field be terminated before the ions of the particular mass to be selected by the detector apparatus on the right-hand side of the Fig. 1 have approached too closely to the apertures 6. In other words, during the time interval when the voltage impulse is being applied to produce ions of equal momentum, no new ions must be created and preferably none must escape the region within electrode 4- through apertures 6. It is also important that each successive pulse of .ions acquires the same momentum, which means essentially that the shape of the wave remains constant. The pulse blocking the electron beam could be applied during a slightly longer interval" than the ion-accelerating pulse within electrode 4 to insure that no ions be created during the period when the ions are being accelerated in the region within electrode 4.

The assumption that the system so far described produces ions having the same momentum is not exactly true in all cases, but it does not matter. The ions of different mass will acquire the same momentum only if they are subjected to a single, complete pulse from the generator 16. Those lighter ions which emerge from the apertures 6 before the completion of the pulse will have less momentum than those heavier ones which follow but which do not reach the aperturesfi during the pulse; but since, unlike the heavier ones, they have fallen through more potential drop, theywill have more energy and, thus, can still be segregated from the heavier ones following by the detector apparatus on the right-hand side of Fig. 1. Those heavy ions which move so slowly that they are subjected to more than one accelerating pulse within electrode 4 will acquire mo're' momentum because of the multiple pulses, but they will still have less energy than the lighter ones which precede them because they will be coasting in between pulses, i. e., they will not be receiving energy in between pulses when there is no accelerating field moving the ions toward apertures 6. In other words, by suitable adjustment of amplitude and duration of the pulse moving the ions toward the apertures 6, it will always be possible to give to a selected kind of ion, or groups of ions, kinetic energy which difiers from the kinetic energy given to the ions of all other kinds, i. e., all other ions of different charge-to-mass ratio than those of the selected kind.

As previously indicated, since all ions emerging from the apertures 6 will have different energies depending upon their mass-to-charge ratio, each mass-to-charge ratio having its own characteristic energy, the ions so emerging can be segregated from each other on a basis of energy or velocities. On the right-hand side of the Fig. l, I have shown asystem for segregating or detecting selected ions, that is, ions of selected mass-to charge ratio, on the basis of energy differences. Briefly, it is a' matter of imposing a potential on the electrode 7 such that all ions of less than a given kinetic energy are repelled by electrode 7, and all of greater energy are collected by electrode 7. This simple arrangement is satisfactory where only two charge-to-mass ratios are involved or where one desires only to detect the highest energy ions.

However, for the general proposition, the problem is to detect or to distinguish ions of a particular energy range within a beam composed of ditferent types having larger and smaller energies. This can be accomplished by the detector or selector arrangement which I show in the right-hand side of Fig. 1 without physically sorting out the desired energy band. This arrangement is as follows.

By means of the variable uni-directional voltage source 24? (potentiomenter energized by a battery 19) which has a polarity, as indicated, when dealing with positive ions, the ions emerging from the apertures 6 are subjected to a retarding potential between the apertures 6 and the electrode 7, that retardingpotential being of sufiicient magnitude that the desired ions have just enough energy to reach the electrode 7, and those of'less' energy are repelled and not collected. In other words, the ions of charge-to-mass ratio which it is desired to select or detect are collected on electrode 7 along with all others of higher energy, but allof lower energy are rejected and sent back toward the apertures 6. The problem, therefore, is to distinguish or detect the selected mass-to-charge ratios from those of higher energy which are also collected on electrode 7.

This may be accomplished by modulating the voltage of electrode 4 with respect to the electrode 7 (preferably at ground potential) with a small alternating voltage which will periodically also reject the desired or selected charge-to-mass ratio. In other words, during one-half of the cycle of such modulating voltage, the desired selected ions will be received by electrode 7; and during the other half-cycle of the modulating voltage, they will be repelled. This modulating voltage may be of sufficiently small magnitude that the same periodic repulsion and reception will not also occur for ions of other chargeto-mass ratios. Such a modulating voltage is illustrated as being applied by a square wave voltage of relatively low frequency generated by a source 21. This is applied between the electrodes 4 and 7 in series with the unidirectional voltage from source 20. It will be apparent, therefore, that the ion current received by electrode 7 will be modulated at the frequency of the source 21 which may, for example, be 100 cycles per second and the modulation current will be a measure of the ions which his desired to select. The lighter, higher-energy ions will continue to be received by the electrode 7'; but they can easily be distinguishedfrom the modulation component, corresponding to the selected ions, because they will comprise a steadycurrent by comparison, i. e., they will be a uni-directional current except for the high-frequency interruption caused by the generator '16 in the momentum or impulse section of the analyzer. of generator 16 can be substantially different from that of source 21, there will be no interference in this respect.

It is, therefore, only necessary to measure the modulated current to the electrode 7, while rejecting the unmodulated signal; and methods of doing that are wellknown in the art of electronics. For example, in the Fig. 1, the amplified signal is fed to a synchronous detector 23 which is in synchronism with the frequency of the source 21, so that its output is proportional to the modulated current component on the electrode 7. It can, of course, be indicated by any suitable means such as the meter 24.

As previously suggested, other methods of detecting or segregating desired mass-to-charge ratios on an energy or velocity selective basis may be used in the place of the detector arrangement shown in the right-hand half of Fig. 1 since the invention in its broader aspects lies in the combination of pulse techniques imparting equal or nearly equal momenta with simple electrostatic field detectors. For example, there may also be used the type in which the ions are deflected through an angle by means of a cylindrical condenser.

With the foregoing method, a Well-defined ion beam is not required; and, for high-intensity signals, a very broad beam could be used. Since the modulation voltage from the source 21 may be made very small, compared to v the voltage provided by the potentiometer 20, when the desired ions to be selected have a small energy distribution among themselves, high resolution is attainable without the necessity of employing fine slits and preciselypositioned electrodes, as in the conventional type of mass spectrometer. The amplifier 22 should provide feed-back as indicated at 25 to minimize the voltage swing on the electrode 7 as the ion current varies.

It may be desirable to shield the electrode 7 from the induced voltage induced by the source 21, as well as possibly small, undesirable variations in the source 20. This could be done by interposing a system of grids between these sources and the electrode 7 and applying the retarding potential in this grid system, thereby permitting the field in front of electrode 7 to remain fixed. Such a grid system would also eliminate the necessity of varying the potential of the entire source with the lowfrequency source 21.

Of course, it will be desirable that means be provided to scan from one mass-to-charge ratio to another in analyzing, for example, a gaseous mixture comprised of many different types of ions, i. e., it is desirable to shift the apparatus readily so that one may select or detect quickly any one of the types of ions found present. This, of course, can be done by many means. For example, either the length or the magnitude of the voltage pulse from the source 16 may be varied, to control what chargeto-mass ratios will reach the electrode '7. Alternatively, the magnitude of the de-accelerating field imposed upon electrode 7 may be varied through the potentiometer 20.

In order to facilitate design and to show that the parameters are within practical ranges, the following formulas for the time and flight of the ions in the region within electrode 4 are noted. We have:

MA is the mass of a singly-charged ion in atomic-weight units, and t is the time in seconds. In terms of time, we

As an illustration, take X2 as equal to one centimeter, MA as equal to 28, and E as 56 volts per centimeter. Then, t will equal seconds. Even for this case of an Since the frequency accelerating field of only 56 volts per centimeter, the resolution should be sufficiently high for many analytical purposes since the mean thermal energy is a fraction of a volt.

Although the preceding discussion was in terms of positive-ion analysis, the same principle may be used for sorting out and detecting negative ions, and the necessary circuit modifications will be readily apparent to those skilled in the electronic arts. Moreover, the general method may be extended to sources of other types, such as those producing ions at surfaces.

I claim:

1. Apparatus for determining the relative numbers of charged particles in a selected energy band in a mixture of charged particles having a plurality of diiferent energy bands comprising a collector and means for applying to the collector a potential approximately equal to the selected energy band, means for modulating that potential so as alternately to collect and repel the said charged particles in the selected energy band on and from the collector, and means for determining the modulation current of the collector due to such collection and repulsion.

2. A system for analyzing selected gaseous particles of a mixture of particles comprising means for ionizing the mixture, means for applying to the ionized particles impulses which impart to the selected particles energies differing from the energies imparted to other particles in the mixture, and means for thereafter detecting the selected particles on the basis of the difierence between their energies and those of the other particles, including means for imparting a distinctive modulation to the flow of the selected particles.

3. Means for analyzing from a mixture of gaseous particles those of a selected mass-to-charge ratio comprising means for ionizing a sample of the mixture, means for subjecting all the ionized particles to an electric field for a predetermined time whereby to impart to the particles energies inversely proportional to their mass-t0- charge ratios, and means for thereafter detecting the selected particles on the basis of the difference between their energies and those of the other particles, including means for imparting a distinctive modulation to the flow of the selected particles.

4. A system as in claim 3 in which the last two means comprise a collecting electrode and means for applying thereto a potential such that it repels all particles of energies less than approximately that of the selected particles and collects those of greater energy, and means for modulating the energy with which the particles approach the collecting electrode so that the selected particles are periodically repelled from and collected by the collecting electrode.

5. A system as in claim 3 in which the last two means comprise a source of the particles and a collecting electrode and means for applying a potential diiference between the source and collecting electrode so that the collecting electrode repels all particles of energies less than approximately that of the selected particles and collects those of greater energy, and means for modulating the latter potential difierence so as periodically to repel and collect the selected particles on the collecting electrode, and means for measuring the modulated current due to the periodic collection and repulsion of the selected particles.

, 6. A system for analyzing by detecting selected gaseous particles of a mixture of gaseous particles comprising electrodes defining between them a region into Which a sample of the mixture may be introduced, means for subjecting the sample in the region to periodic ionizing pulses, means for applying between the electrodes and in between the ionizing pulses voltage impulses which impulse the particles toward the hereinafter mentioned collecting electrode and which terminate at least before the ionized selected particles leave the region, the voltage impulses imparting to the ions energies in inverse ratio to their mass-to-charge ratio, and means for detecting the selected gaseous particles on the basis of the difference between their energies and those of the other particles, comprising a collecting electrode and means for adjusting its potential so that it repels all particles of energies approximately that of the selected particles, means for modulating the energy with which the particles approach the collecting electrode so as periodically to repel and collect the selected particles on the collecting electrode, and means for measuring the modulated current due to the periodic collection and repulsion of the selected particles.

7. A system as in claim 6 including means for adjusting the same so that the mass-to-charge ratio of the particles selected maybe chosen at will.

8. A system for analyzing by detecting selected, gaseous particles of a mixture of gaseous particles comprising means defining a region into which a sample of the mixture may be introduced, means for subjecting the sample in the region to periodic ionizing pulses, means for subjecting the sample in the region in between the ionizing pulses to pulses of electric field terminating at least before the ionized, selected particles can leave the region to impart to the ions energies in inverse ratio to their massto-charge ratios, and means for detecting the selected gaseous particles on the basis of the difference between their energy and that of the other particles, the means for detecting comprising a collecting electrode and means for adjusting its potential so that it repels all particles of energy approximately that of the selected particles, and means for modulating the energy with which the selected particles approach the collecting. electrode so as periodically to repel and collect the selected particles on the collecting electrode.

9'. A system as in claim 8 including means for adjusting the same so that the mass-to-charge ratio of the particles selected may be chosen at will.

10. Apparatus for determining the relative numbers of charged particles in a selected energy band in a mixture of charged particles having a plurality of different energy bands comprising a source of the particles and a collecting electrode therefor, means for applying between the source and collecting electrode a potential difference approximately equal to the selected energy band, and means for modulating that potential diiference so as alternately to collect and repel the said charged particles in the selected energy band on and from the collecting electrode.

References Cited in the file of this patent UNITED STATES PATENTS Weiss Feb. 26, 1952 Schroeder June 16, 1953 OTHER REFERENCES

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