US2987618A - Mass spectrometer - Google Patents

Description

June 6, 1961 w, LONG 2,987,618

MASS SPECTROMETER Filed Sept. 12, 1957 2 Sheets-Sheet 1 fiaberf Warren LO/Vy INVENTOR.

BY MJMM) ATTORNEYS June 6, 1961 R. w. LONG MASS SPECTROMETER 2 Sheets-Sheet 2 Filed Sept. 12, 1957 \fioberz Warren L any i INVENTOR.

al-mo aanc/MM ATTORNEYS United States PatentfO 2,987,618 MASS SPECTROMETER Robert Warren Long, Dunedin, Fla. (Indian Bluff Island, Rte. 1, Box 801, Palm Harbor, Fla.) Filed Sept. 12, 1957, Ser. No. 683,509 4 Claims. (Cl. 250-413) This invention relates to mass spectrometers and more particularly to spectrometers for measuring the relative numbers of known ions in a gas or vapor after separating the component ions in accordance with their respective charge to mass ratios by projecting them along arcuate paths.

Conventional mass spectrometers function to separate ions produced from a sample to be analyzed in accordance with their individual charge to mass ratios and selectively collecting the separated ions. It is known that ions traveling transversely through a magnetic field so as to cross the lines of flux thereof tend to traverse a circular path in a plane normal to the magnetic field due to the force exerted on the ions by the magnetic field. If there is also imparted to the ions a force to move them in a direction parallel to the magnetic field the resultant path will describe a helix. Most mass spectrometers make use of the separating property of a magnetic field which cause ions of difierent mass to describe difierent arcuate paths in passing through the field. One or more collector electrodes may be disposed in space so that under certain controlled conditions only ions of the desired charge to mass ratio will impinge upon and discharge at the collector electrode.

According to most previous methods of measuring the mass of ions, the ions are projected in a direction perpendicular to the magnetic field lines of flux by an electrostatic force. Thereafter, a constant stream of the separated ions of different mass are received at point collectors and translated through relatively inefiicient direct current amplifiers. The probability of ionic collision in a concentrated beam are high, and interpretations from the collector electrodes have been distorted by the reception of extraneous events.

It is therefore an object of this invention to provide a mass spectrometer for determining the relative proportion of ions of different masses by subjecting a beam of ions to a predetermined transverse force to separate the ions according to the charge to mass ratio and detecting the relative occurrence of each.

It is a further object of this invention to provide means for spacing ions of the same charge to mass ratio in order to reduce the density at defining apertures and at collector electrodes, alleviate interference and reduce the occurrence of undesirable events distorted through ion collision.

It is a further object of my invention to provide a mass spectrometer wherein the events can be directly translated through alternating current amplifiers.

It is a further object of my invention to provide a mass spectrometer that is suitable for maintaining a quality check on a gas to be analyzed by measuring the relative presence of its ion components.

As a further object of my invention to provide a mass spectrometer that is simple and relatively inexpensive to construct and that is capable of analyzing gases or vapors from fluids or solids with a comparative degree of resolution.

In carrying out my invention, I propose projecting the ion mixture in a small radius beam parallel to a magnetic field and then revolving the source of a deflecting electrostatic force about an axis defined by the narrow beam of ions whereby the ions will be projected sequentially in all radial directions transverse to the magnetic field. Each successively deflected ion will travel a helical path and in Patented June 6, 1961 ice that way the helical paths of ions within each charge to mass group will be spaced angularly, with the result that the initial ion density will be spatially diluted to alleviate interference at the defining apertures.

Further, since the rotating electrostatic field deflects all ions of the same mass (and charge) to the same radial extent and since each successive ion path is displaced angularly, ions of the same mass to charge ratio will form a thin sheet diverging from the point of deflection on their original axis outwardly to the maximum radial deflection and thence inwardly to converge on the axis and then outwardly to repeat the cycle if uninterrupted. If this sheet of ions were cut by a plane perpendicular to the axis and between the points of intersection with the axis, there would be defined a circle of a predetermined radius dependent upon the location of the plane along the axis. Consequently, I propose interposing a sheet having annular apertures therein, such apertures being of such radius and width as to permit passage of only the desirable sheets of ions. I also propose annular collector electrodes disposed beyond the shields, such collector electrodes to be of a novel type particularly adapted for my apparatus to discharge the sheet of ions in an alternating current which is more easily amplified and used in the control circuits.

While my invention may take diiferent specific forms, I have shown in the accompanying drawings a typical embodiment in which the details of my invention will be better understood when viewed in connection with the description that follows:

In the drawings:

FIG. 1 is a view in elevation of a mass spectrometer embodying the principles of my invention;

FIG. 2 is a broken isometric view illustrating diagrammatically the characteristic paths traversed by mass-tocharge ion groups; and

FIG. 3 is a schematic diagram of my mass spectrometer with the electrical circuitry involved.

Referring now to the drawings the instrument there shown comprises an envelope 1 of metal or glass which constitutes the analyzer chamber. An exhaust port 1a leads from the chamber 1 for evacuation thereof. At opposite ends of the envelope are magnetic pole pieces 2 of opposite polarity which, for purposes to be hereinafter defined, are disposed so as to create a magnetic field parallel to an axis 4 passing through the envelope 1. This compact, unitary arrangement can be achieved by use of two permanent magnets supported on and interconnected by a base 3 of high magnetic permeability to form a horseshoe type magnetic circuit.

Within the envelope or analyzer chamber 1 is a gas box 5 into which the gas or vapor of the sample to be tested is introduced through inlet 6. Small apertures 7 and 8 concentric with axis 4 are cut through opposite sides of the gas box, the inlet aperture 7 providing an entrance for a stream of bombarding electrons collimated along axis 4 and the exit aperture 8 providing the outlet for a collimated beam of ions also hereinafter described. T o the left of the gas box 5 in the drawings is an electron source, shown here as a filament 9, the coils of the filament also being concentric to axis 4. The filament 9 is heated to the proper incandescence by a transformer 10 maintained at the proper level by filament control 11. As is well known, a cloud of electrons having its greatest concentration on the axis 4 is formed around the coils of the filament 9.

The electrons evaporating from filament 9 are propelled to the gas box by means of a positive potential thereon relative to the filament. This positive potential is maintained by an accelerating voltage and control element 12 which is connected by conductor 13 to the gas box, and by conductors 14 to the filament 9. The ac-,

celerating voltage and control element may derive its power from source 15 from a capacitor-filter 16 which is charged through rectifier 17 from an alternating voltage source of suitable magnitude. Such-a source could be one side of a transformer winding 18 which is also delivers a voltage to other elements of my apparatus to be hereinafter described. If desired meter 19 may be interposed in each of the parallel circuits and y in order to maintain a visual check on the system.

Supplementing the electron attracting force of the gas box 5 to propel electrons through aperture 7 is a repelling force on the opposite side of the filament 9 provided by electrode 20 which is slightly negative relative to the filament. If electrons manage to pass completely through. the gas box without effective collision with molecules of the gas therein and emerge through aperture 8, they then encounter a repelling field as a consequence of the electrode 21, through conductor 22, being essentially at the same potential as the filament. Consequently, the positive potential of the gas box attracts electrons on either side thereof and causes the electrons to oscillate back and forth until they accomplish their purpose of ionizing the gas introduced into the gas box or finally drift by various collisions to the gas box itself. As a consequence of this arrangement, the expectancy of collision with gas molecules in the gas box is enhanced considerably. Moreover, the continuous attraction of the gas box for electrons resists any tendency of the electrons to return and impinge upon the filament. I have achieved maximum elficiency of ionization for the electron current utilized.

As is well known, the collision of the electrons with the molecules of the sample gas forms the ions thereof, most of which are positively charged. 'Ihen, ions of the same polarity for example, positive ions, are accelerated and propelled through aperture 8 of the gas box and thence through an aperture 23 in the accelerator electrode 21 previously recited as carrying a negative charge relative to the gas box through conductor 22. The ions formed in the gas box are accelerated therefrom by the electric field between the electrodes 21, 20 and the gas box which field extends partially through and inside the holes 7 and 8. A voltmeter 24 maybe interposed between conductors 13 and 22 to enable adjustable setting of the voltage control 12.

As hereinbefore stated, the magnetic poles 2 and 3 develop a magnetic field having lines of flux parallel to the axis 4. The collimating action of the magnetic field helps to delimit the electron beam and, subsequently, the stream ofions, to very small radial dimensions. Should any electron or ion particles tend to traverse a path at variance with the axis 4, i.e., include a velocity vector in a direction transverse to the axis 4, the magnetic field will force it through a tight helical path back to the axis, sothat the desired collimation is achieved as the beam of ions passes through the accelerator electrode 21. Moreover, the geometrical disposition of the filament and the successive apertures tends to restrict the path of the electrons, and subsequently the ions, to a narrow beam; Positive ions created by the electron bombardment are, of course, accelerated in opposite directions to the electrons. Consequently, some of the positive ions formed nearer the filament side of the gas box will be accelerated toward the filament. However, the ions are also collimated by the magnetic field and the helix shape of the filament will allow most of them to passunhinder'e'd'. The'filament 9 was particularly designed to order to reduce the deleterious efiect of ion bombardment on filament life. The positive ions are further accelerated to ward electrode 20 maintained at a negative potential relative to the filament. These ions are discharged at electrode 20, the resulting current passing through meter 25, which measures the current at electrode 20. Under operating conditions, the number of positive ions reaching electrode 20'per unit of time is a measure of the gas In either event,

pressure in the mass spectrometer chamber 1. The gas pressure will be the results of natural evaporation, leaks and, of course, predominately, the gas sample being analyzed. The mass spectrometer will function best in certain pressure ranges. Consequently, the metering circuit 25 offers a check on the proper gas flow and vacuum conditions of the system. In this connection, it is contem plated that an alarm responsive to unusual conditions may be provided.

As stated, the ions emerging from aperture 8 of the gas box 5 are collimated by the magnetic field and for all practical purposes, there is immediately beyond the accelerator electrode 21 only one vector of velocity, i.e., along axis 4, for each ion propelled therethrough. The magnetic field thus far functions only to maintain this condition and, therefore, has substantially no effect upon the path of the ions.

The stream of ions is then passed through and influenced by deflector electrodes 26 and 27, which are shown in plan view at right angles to their actual plane which is perpendicular to the paper. As explained above, a single force as an electrostatic field acting on a stream of moving ions in a directionperpendicular to a magneticfield and also to the stream, will deflect the ions into helical paths, the extent of deflection of each ion from the stream being dependent upon the magnitude of the deflecting electric field and the magnetic field and the charge-to-mass ratio of the ion. Under the influence of such a unidirectional deflecting force, all ions of the same charge-to-mass group follow the identical path. How ever, I propose rotating the deflecting electrostatic force about the axis 4 so that the direction of the deflecting: force, and hence the initial direction of deflection velocity will also be rotated. The rotation of the electrostatic field can be accomplished by any suitable electrical or mechanical means, but in the arrangement shown, it is achieved by pairs of mutually perpendicular deflection electrode plates 26 and 27 supplied with alternating potentials of a suitable frequency, the mid-point of the transformers 18 and 1811 being essentially at the potentialof the electrode 21. 'If phases are chosen properly, the rotating electric vector is produced. As previously stated, the positive potential of the gas box 5 is derived, through the accelerating voltage and control element 12, from one side of the transformer winding 18. Therefore, since both the accelerating voltage and the deflecting voltage is derived from the same source, variations from extraneous sources, such as the power line supply transformers 27, would appear proportionately in both elements and thus tend to preserve the same ratio of accelerating to deflecting voltages.

Due to the rotating deflecting electrostatic field with the resultant rotating deflecting force, each ion is deflected in the direction of the electric vector of that instant and the ions are deflected sequentially in varying angu-= lar directions, with the result that the sequential series of helical paths followed by ions of the same charge-'to-mass ratio will define a sheet or envelope 29 gradually increasing in cross-section radius to the radius of maximum dc flection and then gradually converging on the original axis to continue the process if uninterrupted. The helical paths can be calculated for each ion group so that their distance from the axis in any plane perpendicular to the axis can be determined, the distance for all paths of the same mass-to-charge ion group being equal in that plane. Consequently, a circle having a radius equal to the cal, culated deviation from the axis will define the disposition of the sheet of ions at the selected point along'the axis. If" there are mixtures of various ions of certain discrete charge-to-mass ratios, then there will be a comixing of the ion paths or envelopes 29, with the maximum radius of each being different both in magnitude and location along the axis 4.

To facilitate separation of the desired mass-to-charge ratio ions and to eliminate stray ions andthose not deassists sired, I provide a shield 30 having annular-apertures 3 1 and 32 of widths and radii designed to permit the passage of only the particular charge to mass ratio ions to be measured. Preferably the shield 30 is disposed in a plane perpendicular to and along the axis 4 which intersects the sheets of ions at or near their maximum deflection from the axis. At this dimension the ions will be at relatively low density since the helical paths, being sequentially spaced angularly inherently become more spaced with radial distance. With my deflector electrodes 26 and 27 which produce a rotating deflecting force and achieve this curved sheet of ions with a resultant low density at the outer radii, there is less chance for collision between ions and less distortion due to the interference of extraneous events.

If the mass spectrometer is to be used as an analytical instrument, a single annular aperture placed at or near the maximum radius for a certain mass to charge ratio will limit the emerging beams to essentially one family up to the limit of resolution. By a proper adjustment of the various parameters such as the deflecting or accelcrating fields, or the magnetic field, or any combination thereof successive ion families can be passed through the single aperture and caused to impinge upon collector electrodes. Current resulting therefrom could be measured to determine the ratios of the numbers of the different families present as an analysis of the specimen.

However, in its preferred use as a quality control instrument, wherein it is desired to determine the relative numbers of each of certain chosen ion families, two or more suitably spaced concentric annular apertures 31 and 32 of proper dimensions are provided. When so used the instrument can provide a constant check on a desired ratio of the ion families and, particularly if the chosen ion families are key to a certain process, then the device would essentially monitor the process for deviation and indicate the nature of steps necessary to restore proper operation of the process.

Spaced beyond the ion shield 30 to intercept the sheetlike paths of the ions passing through the annular apertures are split annular collector electrodes 33 and 34, one for each aperture. Obviously the collector electrodes are also of a radius derived by calculation to conform to the cross-section radius of the sheet of ions at their particular plane along the axis. As another feature of my invention the collector electrode rings 33 and 34 are split into multiple sections, each split ring having at least one pair of gaps 35 and 36 therein. Since the deflector electrodes 26 and 27-and their potentials create a rotating transverse vector of velocity and hence a rotating direction of initial deflection, the arrival of the ions at the collector electrodes will also be angularly spaced in sequence. As a result I achieve a revolving point of impingement on each collector 33 and 34, the effect being that of a one-arm lawn sprinkler. Consequently, by providing a series of gaps on the collection rings and connecting conductors 37 and 38 to each adjacent arcuate portion of the collector rings the ions will discharge alternately to one portion and then to the other inherently to discharge through conductors 37 and 38 in alternating current. Consequently, it is possible to use alternating current amplifiers 39 and 40 instead of direct current amplifiers with the resultant advantages. The collected current is passed through resistors 41 and 42 and amplified through conventional A.C. amplifiers 39 and 40. The amplified current may then be directed by conductors 43 and 44 to a servo-amplifier 45 and then to a control element, which might include a recording means, to detect relative reception of the desired ion groups at the collector electrodes. A resistor 45a is provided between conductors 43 and 44 to balance the signals from the separate collector amplifiers 39 and 40 for increased efficiency in amplification. The degree of imbalance from initially set conditions will give a signal to the servoamplifier which then may be used to reset the initial con ditions by appropriate change of the controlled process feeding the original gas into the mass spectrometer. In the case of two section collector electrodes, as shown, the input to a balanced push-pull A.C. amplifier is proper for balanced conditions. The relative output to conductors 43 and 44 will depend upon the relative numbers to ions in the chosen families impinging on the collector electrodes. Deviations from this particular condition could be used to operate the servo-amplifier that would properly change a control of the process.

As a further control, the amplified voltage is directed through conductor 46 to accelerating voltage control 12 which maintains the potential at the gas box 5 at the desired level relative to the output signal. Thus my entire system is in balance and unwanted variations in the amplified output automatically compensate the system controls.

While the preferred embodiments of my invention have been shown and described, it is obvious that many modifications thereof can be made by one skilled in the art without departing from the spirit of my invention. It is therefore desired to protect by Letters Patent all forms of the invention falling within the scope of the following claims.

Having described my invention, I claim:

1. In a mass spectrometer having an analyzer chamber, means for ionizing a gas introduced into said chamber and means for establishing a magnetic field across said chamber, the combination therewith of means for projecting the ions of said gas along a narrow axis parallel to said magnetic field, means for establishing a rotating electric field across said axis in a plane perpendicular to said axis whereby said ions are deflected from said axis to traverse angularly spaced helical paths varying in radial displacement from said axis in accordance with their mass-to-charge ratios, an ion impervious shield across said chamber transverse to said axis, said shield having at least one elongate aperture therein, each said aperture being disposed along the line defined by the intersection with the surface of said shield of the paths traversed by desired ions of a predetermined massto-charge ratio and being of a width sufliciently narrow so as to permit passage through said shield of only said desired ions and a pair of electrically separated collector electrodes disposed beyond each said aperture to intercept the paths of said desired ions.

2. The combination defined in claim 1 wherein the collector electrodes of each pair thereof is separated electrically and each is connected electrically to a common alternating current amplifier.

3. A mass spectrometer comprising an analyzer chamber, means for ionizing a gas introduced into said chamber, means for projecting the ions of said gas along a narrow axis, a rotating electric field across said axis in a plane transverse thereto for deflecting said ions from said axis, a magnetic field across said chamber parallel to said axis, said rotating electric field and said magnetic field causing said ions successively to traverse angularly spaced helical paths deviating from said axis to a maximum deflection and then returning to said axis, the paths of only those ions of the same mass to charge ratio being of equal radial displacement from said axis in any plane perpendicular to said axis, an ion impervious shield across said chamber in a plane perpendicular to said axis, said shield having at least one annular aperture therein, each said aperture being of a particular radius and narrow width to permit the passage through said shield of only the ions of a predetermined mass-to-charge ratio near the maximum displacement thereof, and at least one collector electrode disposed in a plane perpendicular to said axis spaced from said shield to intercept ions that pass through said shield.

4. In a mass spectrometer having an analyzer chamber and means for ionizing a gas introduced into said chamher,- means for determining relative amounts or a plurality successively to traverse angularly spaced helical paths deviating from said axis to maximum deflection therefrom and then returning thereto, all of the paths ofonly ions of the same mass to charge ratio being of equal radial displacement in a plane perpendicular to said axis where by said paths define concentriccircles in any of said,

perpendicular planes, an ion impervious shield across said chamber in a given one of said perpendicular planes substantially at the point along said axis wherein the paths of ions of different mass-to-charge ratio are inmaximum radialseparation; said shield having aplurality of annular apertures therein each ofsaidapertures being of a radius equal to the radial deflection of ions of aselected mass-to-charge ratio insaid given perpendicular plane and being of a narrow width to pass only said selected ions, and a plurality of collectors each disposed in a plane perpendicular to said'axis spaced from said shield and of a size and shape to intercept only those ions that pass through one of said apertures.

References Cited in the file of this patent- UNITED STATES PATENTS Poole 2 oer. 15,1959

Images