US2958774A - Omegatron with orbit increment detection - Google Patents

Description

Nov. 1, 1960 1.. R. MONARRY EI'AL 2, ,7

OMEGATRON WITH ORBIT INCREMENT DETECTION Filed May '7, 1957 2 Sheets-Sheet 1 ORBITS OF NON-RESONT IONS I I i l l A AM'INVENTOR *LEONROBERTMc/VARRV JOH N PETER HOBSO/V AGENT Nov. 1, 1960 -1 R. McNARRY EI'AL OMEGATRON WITH ORBIT INCREMENT DETECTION r wmmm RESONANT ION ORBITS WW/U)! I AC FIELD NON RESONANT ION ORBITS IN VENTOR t 0/v ROBERT M: NARRY 4 kc/s JOHN PETER HOBSO/V SI NG LE COLLECTOR DUAL ELECTRODE OMEGATRON j!- 0 8 KC 7 mOPuwjOu O. .rZwEEDU (4) 25,4 kc/S (1.) 25,4 kc/s AGENT United States Patent C OMEGATRON WITH ORBIT INCREMENT DETECTION Leon Robert McNarry, Cumberland, Ontario, Canada,

and John P. Hobson, Cardinal Heights, Ontario, Canada, assignors to National Research Council, Ottawa, Ontario, Canada, a body corporate of Canada Filed May 7, 1957, Ser. No. 657,616

4 Claims. (Cl. 250-419) This invention is in improvements in apparatus for analytically separating ions on the basis of differences in their mass-to-charge ratio, and is particularly concerned to provide novel electrode dispositions in omegatron apparatus to improve resolution over a wider range of specific mass numbers of substances than has heretofore been possible.

Mass spectrometry is generally concerned with spatial separation of ions, the technique being based primarly on the inherent differences in motional behaviours of heterogeneous ions of a sample of a substance to be analyzed, when subjected to the influences of a magnetic or electric field or both. It has been found that particularly effective spatial separation as a function of the specific masses of ions is possible when the ions move initially from rest along a line source across a magnetic field while being subjected to a high frequency alternating electrical field in which the electric vector is normal to the magnetic vector. In such combined field the ions move in spiral paths about axes of gyration paralleling the magnetic vector, the gyratory movements being uniquely characteristic of each combination of field intensities, electric field frequency, and the mass-to-charge ratio of the ion. When a certain relationship of magnetic and electric intensities exists in the combined field and the alternating electric field frequency is non-varying, ions of a predetermined specific mass will be sped along a spiral path of steadily increasing radius. a manner closely approximating thereto are referred to as resonant ions. All other ions of different specific mass will travel in curved paths whose radius at first increases at a non-uniform rate to a maximum orbit diameter, thereafter decreasing at a non-uniform rate to return to the axis of origin. Such other ions are referred to as non-resonant ions. Accordingly, the mass spectrometer of the latter type can make a spatial separation by constraining all ions which are non-resonant under the effects of the applied fields to movements within a certain radius of the origin, while resonant ions are collected by a collector electrode placed at a larger radius from the axis.

The line source or origin of ions may be realized by propelling a slender ionizing beam of high speed electrons along the magnetic vector direction, or by injecting the sample as a slender ionized line beam along such direction, or by passing a slender rod-like beam of ionizing radiation along such axis through the chamber in which the rarefied sample is released. The axis of origin is ideally made to have as small a cross-section as possible in order to minimize deviations from theoretically perfect operation in which all ions start from rest along a straight line source.

The omegatron apparatus described may be adjusted in turn to energize ions of each mass number to resonance, by varying the frequency of the alternating electrical field while maintaining the magnetic field and the RMS voltage of the electric field constant. As ions of each mass number in turn are carried beyond their nonresonant orbit path diameter they are collected and de- Ions behaving in this manner or in' 2,958,774 Patented Nov. 1, 1960 2 tected by measuring the current delivered to the collector electrode.

The motional behaviour of non-resonant ions whose specific masses nearly correspond with the mass of a resonant ion for given steady conditions within the analyzer region is very closely similar to that of the resonant ions until a number of revolutions have been made by each about the axis of origin. It is only after the radius of the spiral paths has exceeded the value at which nonresonant ion orbits collapse that the separation is effected, and for large mass number ions the resolution in conventional omegatron mass spectrometers is poor while the required cross-section of an analyzer space is large.

In practical form, an analyzer apparatus employing a steady magnetic field of high intensity and large crosssectional area is very costly and presents difficulties in achieving uniformity of field and steady fiux density. A further disadvantage of conventional omegatron mass spectrometers as devices discriminating between ions of large mass number arises from the lengthening of the path with increase in the number of revolutions required to be made by the mingled resonant and non-resonant ions before separation is effected, during which space charge effects disturb the paths of resonant ions and impair resolution.

Heretofore a reduction in the area of the required magnetic field has been achieved either by intensifying the field, or by employing an orbit-distorting steady electrical field transversely of the line axis, as set out in the specification of United States Patent 2,718,595 to C. F. Robinson. A spectrometer as described in the aforesaid publication incorporates a scavenging electrode spaced diametrally opposite to a collector electrode to remove ions and diminish space charge, while resonant ions increase their orbit radius against the steady D.C. field to arrive at the collector electrode.

None of the prior art devices have good resolving power for discrimination between ions of adjacent large mass numbers, particularly for mass numbers of the order of and higher. Such prior art devices are relatively ineffectual in discriminating between ions of adjacent large mass numbers, such as heavier elements and molecular fractions of ionic character.

Applicants have realized a novel and greatly improved mass spectrometer device employing the combined magnetic and varying electric fields as mentioned above, which differentiates between ions according to a new principle, enabling resolution of a high order to be achieved in detecting and identifying ions over a very broad range of mass numbers, without any increase in the size of analyzer chamber or magnetic field intensity.

As compared with prior art devices, separation of ions of adjacent mass numbers which may be of the order of several hundred, is effected with a relatively reduced area and intensity of magnetic field and in fewer cycles of orbital travel around the line axis, by virtue of a novel collector system employing two electrodes.

Applicants have established a novel theory of spatial discrimination and have developed improved mass spectrometer apparatus of the omegatron type embodying the invention, employing an arrangement of dual electrodes between which the ions normally caught by a single collector are divided. The novel omegatron utilizes the property of the ion orbits that the resonating ions have a greater increment in radius in a single gyration than any non-resonant ions. Non-resonant ions and a certain proportion of resonant ions are caught upon an interceptor electrode at a lesser radial displacement from a source region than a collector electrode upon which only resonant ions are caught. In a mass spectrometer organization as described which is operated at predetermined magnetic and electric field intensities, theresolutionis optimum when the increment of radius between the inner edge of the interceptor electrode and the corresponding edge of the collector electrode is made not greater than or fractionally smaller than the increment of radius per revolution of those ions resonating at the applied field frequency 'The invention permits the use of much broader crosssections of ionized beam sources than have hitherto been operable, since the restriction to a fine line source imposed on conventional omegatrons is removed in the double collector omegatron.

According to the invention a mass spectrometer is real- 'ized as an evacuated envelope within which a specimen of substance to be analyzed is released in gaseous form, enclosing an electrode structure and supports for setting up a transverse electric field crossing a magnetic field applied externally, and a means to introduce an ionizing beam of electrons along an axis aligned with the magnetic field through the analyzer chamber, there being provided an interceptor electrode spaced from the beam axis and an adjacent collector electrode at a larger radius and disposed within the region pervaded by the crossed magnetic and alternating electric fields on that side of the interceptor impinged by accelerated ions during operation.

In carrying the invention into effect an analyzer chamber is disposed within a non-magnetic and gas impermeable vessel which may have separate spatial connections with an evacuating system and with a source of substance to be analyzed. Alternatively no special tube for introducing a sample need be provided where analysis of residual gases is to be made. The chamber is axially short and a steady magnetic field of uniform intensity is applied along the axis. A beam of ionizing electrons is produced by accelerating them along the chamber axis from a cathode source by an electron gun to pass through the chamber to an electrode spaced across the chamber and generally disposed centrally of the magnetic field. A system of electrodes is arranged to provide, when excited by applying alternating voltage from a source of adjustable intensity and frequency, an electric field transversely of the magnetic field and of substantially uniform strength throughout at any instant.

The invention may be the better understood by study of the description which follows, together with the accompanying drawing, wherein:

Figure 1 is a diagram representing half cycles of the theoretical movements of a resonant ion in a conventional omegatron type of mass spectrometer as projected .on a plane normal to the axes of gyration, and similarly the movements of a non-resonant ion of adjacent mass number.

Figure 2 is a diagram of a portion of the ion trajectories of Figure l in enlarged scale, showing the relative positions of an interceptor electrode and a collector electrode according to the invention, and the radial increments of ion orbits;

Figure 3 is a three dimensional sketch showing the arrangements of a complete omegatron including the electrode structure of the invention;

Figure 4 is a diagram describing the motions of resonant and non-resonant ions of adjacent mass numbers in a mass spectrometer having a steady field applied along the Z axis in addition to the combined magnetic and alternating electric fields, and the location of a pair of electrodes according to the invention.

Figure 5 is a diagram graphically comparing the resolving powers of an omegatron employing an interceptor electrode adjacent to and spaced radially inwardly from a collector electrode, for ions of mass number 200 in one case as a two-electrode system, and in the other as a conventional single electrode omegatron; and,

Figure 6 is another graph diagram to illustrate the dependence of resolving power on increment of radial distances between the inner edges. Qfi h interceptql and the collector electrodes.

with return of the ion to its source. be collected on an electrode 10 placed at'a radial distance Referring to Figure 1, the paths of ions in the X2 plane are shown as originating at an axis of origin Y of a system of three mutually orthogonal axes X, Y, Z, the Y axis being perpendicular to the plane of the drawing and drawn parallel with the direction of the magnetic vector. An alternating electrical field has its electric vector parallel to the Z axis throughout the orbital region designated in which the magnetic field is of uniform intensity. Half-cycles of the orbits of an ion of a given mass number resonant at the applied frequency w are traced above the 'X axis, these exhibiting a uniform and constant increase inorbit radius for each crossing of the X axis to the left of the origin. For purposes of comparison the path of an ion of adjacent mass number not resonating at the frequency of the applied alternating field is traced below the X axis; consecutive gyrations of these ions are in paths increasingly distant radially from the origin, the increase being at a non-uniform and diminishing rate, gradually becoming zero at an outer radial limit, then becoming negative as the orbit collapses Resonant ions may beyond the non-resonant ion maximum orbit. The dia gram is general and applies to ions of any mass number, with appropriately related dimensions and field parameters. It will be apparent that for the spatial separation of ions of mass numbers of the order of 200 and higher, very large magnets are required to sustain a field through a correspondingly large diameter analyzer region.

At Figure 2 the arrangement of a pair of electrodes 11 and 12 with respect to the arbitrary paths of a resonant ion and of a non-resonant ion may be examined. This figure shows in an enlarged scale a portion of consecutive half cycles tracks 13, 13 and 13 of a resonant ion drawn above the X axis, and similar portions of half cycles 14, 14 and 14" of non-resonant ions of adjacent mass number, below the X axis, the origin Y lying to the right of the figure. The omegatron combined fields are applied as has been described for the Figure I diagram.

Let it be assumed that a resonant ion expands its orbit from track 13 at radius r to trace the paths 13, 13 in its consecutive similar half cycles, with uniform increments Ar, and that path 13 clears the edge of interceptor electrode 11 by a distance 8. The radius distances from the axis of origin Y measured to the near edges of electrodes 11 and 12 are respectively R and R units, where In order for collision of an ion moving in path 13" to occur with electrode 12 there must be the relationship:

The quantity 6 is assumed completely random for any ion, depending in part on its point of origin and number of gyrations executed since it started. This stems from the fact that the source of ions never conforms to an ideally thin line axis of origin. When an intense, broadened source of ions is employed, as may be required for the quantitative separation of element isotopes, ions may just clear electrode 11 after having made widely different total numbers of gyrations around the axis.

The quantity Ar is controllable and for resonant ions it is preferably so adjusted that Ar-AR 2,0

It will be apparent that if ArAR 0 no ions would ever reach collector 12 and the device would function as a single electrode or conventional omegatron.

In the omegatron device employing the dual electrodes 11 and 12, all ions whether resonant or non-resonant whose orbits intersect electrode 11 are collected, thereby removing a population whose space charge effects would otherwise tend to disturb resonant ion orbits prior to collection.

A non-resonant ion gyrating at radius r and just 'clearing the near edge of electrode 11 in its track 14 so that will have a Ar which is always less than Ar for resonant 'ions. Consequently, for ion orbits at least a few cycles removed from their point of origin,

so that the ion collides with electrode 11 in its next consecutive orbit 14" and fails to reach collector electrode 12. It will be apparent that the swarms of ions streaming in the vicinity of electrode 11, with approach orbits of random 6 and distances will divide themselves between the near electrode 11 and the further collector electrode 12. A certain proportion of resonant ions will have 6 values such that in their next gyration they impinge on interceptor electrode 11 and fail to contribute to the measured ion current. However as will be directly apparent, only a resonant ion is capable of gyratory motion such that if it just misses being intercepted by electrode 11, in its next cycle it will impinge on electrode 12.

The starting point of ions need not be restricted to a line source since regardless of initial radius distances r or r, both resonant and non-resonant ions will approach the electrodes in their next-to-last orbits with random 6 andn 6' clearances. Therefore a greatly intensified 0r broadened source such as a tubular or columnar ionized zone may be employed with a correspondingly large increase in ion population. The electrode disposition moreover is not restricted within the analyzer region as to minimum radius from the source, since only a very few cycles of gyration need be executed by ions before they are sufficiently removed from the source to manifest different orbit increase distances.

A great reduction in magnet cross-section area is gained by this feature. Moreover the device operates to continuously intercept ions and remove them from circulation after a minimum history of gyration. As will be shown hereinafter, the required diameter may be only a few percent of that for a conventional omegatron of the same resolving power.

The breadth of the source may in some instances advantageously be made a considerable fraction of the analyzer space diameter, the electrode spacing AR being adjusted along a radius drawn through the center of the source zone. For resonant ions whose orbits are not substantially normal to this radius the effective AR is less than its measured value; as a limitation to the breadth of the source zone and the radial spacing of the electrodes therefrom, the effective AR must always be larger than the Ar of non-resonant ion orbits.

The motion of an ion in the crossed fields of an omegatron analyzer region is mathematically described in the analysis which follows, based on the diagrams of Figures 1 and 2. An electrical field E is assumed to be directed along the Z axis of Figures 1 and 2, produced as by applying an alternating voltage between parallel plates (not shown), so that at any point therebetween:

E=E0 Sin wt A magnetic field of intensity B is assumed to be applied along the Y axis of the diagrams, the vector direction being in such sense that ions move clockwise in these diagrams. Ions are formed along the Y axis, moving outwardly in the XZ plane from rest at time t=0. The initial conditions are: 7

Since there are no forces on the ion in the Y direction the motions in the XZ plane only need to be con- Sidered. 1 i

6 The equations of ion motion in the crossed electric and magnetic field is given by the expression:

=w ditldt+w Eo sin cot/B where (e/m) is the ratio of ion charge to its mass,

d x/dt is'the acceleration of the ion in the X direction,

d z/ d! is the acceleration of the ion in the Z direction,

dx/dt is the velocity of the ion in the X direction,

dZ/dt is the velocity of the ion in the Z direction, and w is the angular frequency of the applied voltage having a peak voltage E The integration of Equations 2 and 3 and the application of Condition 1' yield the solutions:

Z =F (w sin wtw sin w t) (5) as may be verified by substitution.

From a considerationof the displacement Functions 4 and 5 as expressed in the Cartesian co-ordinate system, it may be shown that the radial position of any ion at any time t is derived from the solution of components of a right-angle triangle "(0 t t is where r is the radius measured from the Y axis of origin.

Substituting the Solutions 4 and 5 into this expression and using the approximation that near ion resonance Where Am is the difi'erence between the resonant frequency of an ion and the alternating frequency of the applied electric field. The expression also may be written:

The rate of orbit increase is found by differentiation of Equation 6 with respect to t:

Applying Equation 7,

The time for one complete gyration of an ion about the Y axis is:

1 21r 21r m 7 7 7(?) At orbit radius R, the general expression for ion orbit increase may be defined by allowing Letting Aw 0, i.e. establishing resonance a resonant ion.

increases its orbit radius per cycle by the amount Ar 'H'Eg m From an examination of the form of the Equations 8- and 9, it will appear that the increment of orbit radius for a resonant ion is maximum and constant, increasing with the RMS field magnitude and increasing with the mass-to-charge ratio of the ion, while the increment diminishes as the square of the magnetic field intensity. Hence for resonant ions of any mass number to undergo uniform orbit expansions per cycle in the analyzer system, the applied voltage must be so related that where K'is a constant. Ions detuned from the frequency of the applied voltage have ion orbit increases which equal the resonant ion Ar only at the commencement of their gyrations very close to the Y axis where R is small; however with the expansion of orbits to increased R values a limiting radius is reached where i i The total spread or range of applied field frequencies for a single collector omegatron may thus be demonstrated to be ZAw which may be written 8 The resolving power of the apparatus may be defined as the ratio which is expressible in the standard form In Figure 6 a horizontal line is drawn through'the diagram at an arbitrary ordinate distance AR, correspond ing to a radial spacingof two electrodes within the analyzer region. In accordance with the discussion for Figure 2, no ions can reach collector 12 unless their orbit increase distances exceed AR. Equation 8 may be rewritten into the form expressing a circle having its center. at 0, and ordinate and abscissa values proportional to Ar and Aw respectively:

For each arbitrary AR and a corresponding maximum 5 for resonant ion orbits, it will directly appear that ions resonating at frequencies in the range Aw to +Aw will be collected on collector 12. The frequency limit A012 may be calculated by substitution into Equation 8:

AR 12 5mm ree 0 from which An expression may now be derived for the resolving power R of a two-electrode omegatron:

2E0 AR 1 res Substituting from Equation 10,

AR 2 l -(mt) e R B AT m2 DI: 1'85) 2 A 1 and substituting from Equation 9,

i Rx: res) 2 P Equation 12 defines the resolution capability of a twocollector omegatron in terms of location at radius R, magnitude of resonant ion orbit increment, and radial separation of electrodes. It shows that the resolving power increases as the square root of the difierence between the squares of Ar and AR, being controllable by adjustment of field intensities and by geometry of design. The ultimate discriminatory power of such omegatron device is indicated when Ar AR At AR=0, i.e. for the condition that a single collector confronts the moving ions,

The mass analyzing instrument shown in Figure 3 of.the. drawing. includes an analyzer chamber 15 having a housing 16 preferably of glass, enclosing an electrode structure and having plane parallel opposed walls disposed between poles 17 of a magnet. Suitable means are provided to admit a gaseous sample of substance to be analyzed, as by port 18, and a tubulation 19 communicating also with the chamber is adapted to connect the system to a device capable of evacuating the chamber to a high order of vacuum.

, An electron beam generating source 20 is contained in an electron gun, wherein an accelerating electrode 21 is at ground potential and the heated filament cathode 22 is at a high negative potential. A target electrode 23 spaced across the chamber from the cathode receives the beam and may be at a slight positive potential, suitable electrical current sources such as batteries 24, 25 being provided.

A pair of high-frequency electric field-forming plates 26, 27 are disposed across the chamber and are parallel with each other, being connected with the output terminals of a variable frequency oscillator 28 whose output voltage may be controlled and whose frequency is precisely determinable. An electrode structure comprising an interceptor electrode 11 and a collector electrode 12 are spaced a small distance apart and are generally aligned parallel and along a radial line drawn through the locus of the ionizing electron stream between the gun 20 and the target 23. Means are provided to bias the interceptor electrode suitably, and to measure the ion current delivered to the collector electrode 12 as by a galvanometer 29 capable of reading very low currents.

The operation of the mass analyzer may be described as follows: gaseous samples of substances to be analyzed and identified as to constituents are admitted by way of port 18 and allowed to diffuse into the chamber 15, to a predetermined low gas pressure. The application of the operating potentials to the gun 20 and the target 23 causes a beam of electrons to be formed as they traverse the chamber, colliding with gaseous molecules and ionizing them along a linearly extended region of small cross section coaxial with the beam. The electron motion is along the magnetic intensity vector of the field pervading the region between poles 17. Free ions formed by impacts are affected by the alternating electric field between the field-forming plates, being urged into spiral orbits around the beam, as described hereinbefore. Ions of mass number and charge corresponding to the resonance relationship as set out in relation are spiralled away from their point of origin, a certain proportion of these arriving upon collector electrode 12, while the remainder collide with and are removed by interceptor electrode 11. Near resonant and certain of the non-resonant ions also are removed by the interceptor electrode. A large number of non-resonant ions never reach either collector and fall back into the source. A current indicated by device 29 represents the effect of ions of a selected mass number discharging to the electrode 12.

For low mass numbers, the device may be operated as a conventional omegatron by connecting the electrodes 11 and 12 together as input to current meter 29. Above mass number 20 it will generally be found advantageous to resort to the dual electrode connection with outer electrode 12 serving as the collector. The operation of the device in analyzing mass numbers above 100 is substantially possible only with the electrode arrangement of the invention, and indeed the conventional omegatron would fail as a discriminator for higher mass numbers, as may be understood by comparing the current-versusfrequency measurements indicated by Figure for mass number 200. In this figure, the Aw values are measured on each graph between the flanks of the current spikes at the points where these rise above noise level, an improvement in resolution of five tim'es' being realized. The operating conditions in this system were as follows:

The applied frequency, 25.4 kilocycles was applied to the system plates in each test. The spread Aw in the caseof the single collector omegatron is 4 kc./sec., which is of the order of 16% of the resonance frequency, indicating that detection of ions having mass numbers in the: range from about to 220 are masked by the observed. current. In terms of the resolving powers of the instru-- ment, the measured resolution of the two-electrode omegatron is 63, whereas that of the single collector device is only 13.

The resolution of a dual electrode omegatron according to the invention may be increased by varying certain parameters, as may be appreciated by inspection of Equation 12, discussed above. By increasing the radial distance R of collector 12 from the origin, an improvement in resolution may be realized in direct proportion to increase in magnet diameter. A more efficient approach to the problem of improving resolution is to reduce the magnitude of the denominator of-the expression to as small a value as practicable. For a given AR, the resonant ion orbit increase Ar may be reduced and the denominator thereby made to approach a very small number, by the epedien-t of decreasing the magnitude of the applied alternating electric field. As a practical limit to the improvement possible by the latter means, the intensity of the ion current to collector 12 eventually falls to a value at which it is comparable to the level of circuit noise in the measuring system as a whole. It will be clearly evident to those skilled in the art that any improvement in number of ions per second liberated by the source will be reflected by an increase in signal current collected, permitting further adjustment of system parameters to gain an increase in resolution.

An increase in measured current, with corresponding improvement in the relative sharpness of a current spike between flanks at average noise level value, is practicable as set forth, by enlarging the cross-sectional area of the ionizing beam. Ideally, a ribbon beam having its Width aligned in the direction of the electrodes is preferable as minimizing the deviation of ion trajectories from normal incidence upon the electrode system.

It is possible to incorporate a number of the prior art teachings for the purpose of distorting the orbits for improved selection in certain zones of the analyzer, without diminishing the efiiciency of the dual electrode omegatron as described. A unidirectional electric field may be applied in addition to the alternating field, preferably the Z axis, to effect a drift of ions along the X axis as shown in Figure 4. An interceptor-collector electrode pair 11,, 12 is placed in the chamber as for Figure 3 embodiment. An increased AR radial separation is made possible due to the increased Ar and Ar increments of ion orbits resulting therefrom.

It has been shown earlier that for uniform Ar tobe achieved for ions of any mass-to-charge ratio whose resolution is within the capabilities of the apparatus, the applied alternating electric field strength B should be inversely proportional thereto. Accordingly, in order tomaintain a constant resolving power for the system over at least a practicable range of mass numbers the RMS value of the applied A.C. may be kept constant and the strength of a DC. field reduced in accordance with the reduction of A.C. frequency. By this procedure the Ar, values of resonant ions are kept substantially constant despite change in applied A.C. field, the adjustment of drift being; relatively simply effected by controlling a DC. voltage 11 component applied to the field-producing AECL. electrode system.

The electrode pair 11, 12 need not necessarily be located in the region of increased Ar orbit increments, and may be placed to the right of the source in Figure 4, with a suitably decreased AR radial separation.

From the foregoing-itcan'be understood that a new and improved mass' analyzinginstrument is provided by the practice of the invention, having very considerably improved resolving-power, particularly for high mass number ions. The device moreover provides for. reduced space charge effects despite operation withgreatly intensified ion generation means, with noreduction in the dis, criminatory capabilities of the instrument. Modification of the device to operate as a standard 'ornegratron at'low mass numbers is inherently simple and direct.

It can be appreciated that many variations and forms of'the invention may be realized in the practice of the teachings herein set out. It is to be understood therefore that modifications and changes may be made from What has been described within the broadest scope of the invention as defined by the appended claims.

We claim:

1. In a mass spectrometer of the ion-resonance type utilizing the combined action of cross magnetic and radio frequency electric fields for acceleration of ions into spiral orbits originating along an axial zone source of ions paralleling the magnetic field, the improvement comprising a discriminatory collector electrode structure disposed within said fields and spaced laterally from said zone, including a planar sensing electrode for collecting resonant ions and an adjacent planar interceptor electrode coextensive with said sensing electrode, said sensing electrode being radially spaced at a greater distance from said zone than said interceptor electrode. and lying in advance of said interceptor electrode with respect to moving ions, whereby said interceptor electrode. exposes a surface portion to be impinged both by non-resonant and resonant ions, said portion having a radial span measured in the range from a fraction of an orbit radius increment per gyration of a resonant ion to a distance substantially equal to but not exceeding one whole orbit radius increment.

2. In a mass spectrometer having an analyzer chamber, means for admitting a sample to be analyzed into the chamber, means for developing ions of the sample about the axis of a cylindric zone extending through saidchamber, means for developing a magnetic field across. the chamber paralleling the said axis, and means for establishing a high frequency alternating electricfield across the chamber transversely of'the magnetic field whereby to urge ions into gyratory movement, the improvement comprising a scavenging electrode and a collector electrode; said electrodes being axially coextensive and disposed within said fields and spaced laterally from said zone axis, said collector electrode lying adjacently ahead of said scavengingelectrode with respect to the direction of ion motion'and being laterally spaced a greater distance from saidvaxis than said scavenging electrode by a difference substantially equal to. but not exceeding the radius. increment. of theorbital pathof a resonant ionduring one complete'gyration about said axis, whereby said scavenging electrode removes non-resonant ions and a fraction of saidiresonant ions, andsaid collector electrode collects substantiallyzonly resonant ions.

3. In. a mass spectrometer, the combination comprising an analyzer chamber, means for developing ions of a sarnpleto be analyzed along an axial zone extending through said chamber, means for developing a magnetic field across the chamber paralleling said zone, means for establishing a high frequency alternating electric field transversely of the magnetic field for urging ions along spiralorbital paths about said zone, a first planar electrode spaced laterally from said zoneand a second planar electrode spaced radially outwardly from and disposed ahead ofsaid first electrode in the paths of gyrating ions, theinner marginal edges of said electrodes being parallel With said zone axis and the second electrode being spaced further from said zone by a distance not exceeding the radial increment of the orbit of a resonant ion for one complete gyration'about said zone, whereby said first electrode intercepts and removes both non-resonant and resonant ions, and said second electrode collects substantially only resonant ionswhose orbits do not interseotthe first electrode.

4. A high resolution mass spectrometer comprising an analyzer chamber, means for developing ions of a sample to be analyzed along an axial zone extending through said chamber, means for developing a magnetic field across the chamberparallel with the zone axis, means for establishing a high frequency alternating electric field transverselyof' the magnetic field for urging ions into gyratory motion, first and second electrodes spaced laterally outside saidzone within said fields and disposed adjacently of each other and having their inner marginal edges parallel. with said zone axis, said first electrode shielding .a portion of said second electrode from impingement bymoving ions, and being spaced a greater'distance from said zone axis to provide an unshielded ion scavenge ing area adjacent the inner margin of said second elec: trode, said area having a radial extent not exceedingthe radius increment of the orbit of a resonating ion, whereby said second electrode intercepts both non-resonant ions and a fraction of said resonant ions and said first electrode collects substantially only resonant ions.

References Cited in the file of this patent UNITED STATES PATENTS 2,627,034 Washburn Jan. 27, 1953 2,632,113 Berry Mar. 17, 1953 2,718,595 Robinson Sept. 20,1955

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