US3783278A

US3783278A - Single magnet tandem mass spectrometer

Info

Publication number: US3783278A
Application number: US00204336A
Authority: US
Inventors: L Cambey
Original assignee: Bell and Howell Co
Current assignee: Bell and Howell Co
Priority date: 1971-12-02
Filing date: 1971-12-02
Publication date: 1974-01-01
Anticipated expiration: 1991-01-01

Abstract

A mass analyzer utilizing a single magnet for providing two mass analyzing sectors in tandem. The analyzer is equipped with a magnet defining an ion trajectory subtending an angle of deflection between 180* and 360*. A slit is located within the magnet at a first point of focus for substantially limiting ions passing beyond the slit to those of a predetermined mass.

Description

United States Patent 11 1 11] 3,783,278 Cambey Jan. 1, 1974 [54] SINGLE MAGNET TANDEM MASS 2,680,812 6/1954 Berry 250 41.9 SPECTROMETER 3,492,480 1/1970 Vogel 250/419 X I 3,247,373 4/1966 Herzog et a1. 250/419 Inventor: Leslie y, a ,C 2,932,738 4/1960 Bruck 250 419 3,126,477 3/1964 Noda et al.. 250/419 [73] Assgnee' & Company 3,379,911 4/1968 Enge 250/419 x 3,500,042 3 1970 Castaing eta1.... 250 419 X [2 Filed: 1971 FOREIGN PATENTS OR APP CATIONS [21] Appl. No.: 204,336 141,559 12/1960 UAS.S.R 250/419 Application Primary ExaminerWilliam F. Lindquist [63] Continuation of Scr. No. 819,840, Apr11 28, 1969. Atl0mey Eugene F Friedman 52 us. 01 ..250/299, 250/397 511 Int. Cl. 110m 39/34 [57] -K 581 Field of Search 250/419 0, 41.9 D, A mass analyzer a 5 magnet for Pmvldm? 250/419 ME two mass analyzing sectors n tandem. The analyzer 1s equipped with a magnet defining an ion trajectory sub [56] References Cited tending an angle of deflection between 180 and 360. UNITED STATES PATENTS A slit is located within the magnet at a first point of focus for substantially limiting ions passing beyond the g z slit to those of a predetermined mass. y e a 3,231,735 1/1966 Peters 250/419 17 Claims, 6 Drawing Figures 1. SINGLE MAGNET TANDEM MASS SPECTROMETER This is a continuation of application Ser. No. 819,840, filed Apr. 28, 1969.

BACKGROUND OF THE INVENTION This invention relates to mass analyzers and in particular to analyzers providing tandem magnetic mass analyzing sectors between a source of particles to be analyzed and a collector for the charged particles.

Tandem mass spectrometers'utilizing two separate analyzers have been previously known. See, for example, U.S. Pat. No. 3,231,735 and a paper titled A Two Stage Magnetic Analyzer for Isotopic Ratio Determinations of to l or Greater, White and Collins, ASTM Committee, E-l4 on Mass Spectrometry, New Orleans, Louisiana, May 1954. As is indicated in the referenced patent and article, such tandem mass spectrometers are used in leak detection applications and in isotope ratio or abundance determinations. Mass spectrometers of this type are useful in such applications because of their inherently better ability to detect the presence of a mass or certain masses of predetermined values when only small traces of such masses are available or when the pressure of the sample being analyzed is at a relatively high value. Such analyzers are particularly effective in reducing the effect of gas scattering and ions formed by intermolecular processes.

As indicated, the present invention provides an in-,

strument which falls under the broad heading of tandem mass analyzer, but is an improvement over the conventional tandem instrument in that the two succesive analyses which are performed on a sample of ions in the course of their passage from an ion source to an ion collector are accomplished with a single magnet. As set forth in the '735 patent above, the passage of ions through a single stage ofdirectionalre s olutiofi allows a substantial quantity of unwanted ions to pass through the analyzer and arrive at the collector producing broad and poorly defined mass peaks at the analyzer recorder. Under such circumstances it is difficult and sometimes impossible to determinewhether ions of a predetermined mass are present in the sample, particularly when the sample includes 'only trace quantities of such ions. However, by subjecting the ions in the sampie to successive stages of single focusing resolution, elimination of a substantial portion of the unwanted ions is possible giving more nearly ideal peaks on the analyzer recording apparatus. Thus, a tandem instrument in comparison to a single analyzing sector instrument has increased sensitivity, is greatly improved in its capability of reducing spurious response and also more readily lends itself to use with secondary emission collectors.

SUMMARY OF THE INVENTION with respect to the exit boundary so as to receive particles from the sector emerging at a second predetermined angle to a normal to the exit boundary, the collector being spaced a predetermined distance from the exit boundary. The source, collector and analyzing sector define a charged particle trajectory having a first point of focus within the anaylzing sector for particles of a predetermined mass and a second point of focus for said particles at a point intermediate the exit boundary and the collector. Finally an aperture is located within the analyzing sector coinciding with the first point of focus.

The invention also contemplates a mass analyzing apparatus comprising a magnet defining a first and second analyzing magnetic field in tandem with the magnet having an entrance and an exit end. A source of charged particles is arranged to direct said particles into the entrance of the magnet and means for detecting the charged particles emerging from the exit of the magnet is also provided.

The invention further contemplates a magnetic analyzer for use in mass resolving systems comprising a pair of spatially separated magnetic pole means for creating an arcuate analyzing magnetic field therebetween having an entrance and an exit end. The field is arranged so as to have an arc in excess of whereby a first point of focus for charged particles of a predetermined mass is created interiorly of the magnetic field intermediate the entrance and exit ends of the field and a second point of focus for charged particles of said mass is created on the side of the exit end of the field opposite the first focus point.

A principal advantage of the present invention is that of economy of manufacture in that a tandem mass spectrometer can now be provided. which requires only a single magnet for creating the two mass analyzing fields. In a particular embodiment of the present invention a further advantage, that of stigmatic focusing, is also obtained, further enhancing the sensitivity of the instrument and preventing the loss or defocusing of ions in their passage along the analyzer axis. As with the priorart tandem mass spectrometers, it is contemplated that the present invention will have its primary application in leak detection and isotopic abundance work. Further refinements of a mass analyzer according to the present invention are also possible as will be shown in the following detailed description of the in vention.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be better understood by reference to the following figures in which:

FIG. 1 is a plan view in section of a tandem mass spectrometer according to the present invention;

FIG. 2 is a simplified sectional view taken along lines 22 of FIG. 1;

FIG. 3 is a circuit diagram illustrating the various electrical controls of the apparatus of FIG. 1;

FIGS. 4A and 4B are illustrations of alternate embodiments of the present invention; and

FIG. 5 is an enlarged sectional view of the exit end of the magnetic analyzing sector of the invention ineluding a pair of ion deflecting plates.

DESCRIPTION OF A SPECIFIC EMBODIMENT:

A mass analyzer 10 according to the present invention is shown in FIGS. 1 and 2. The analyzer includes an ion source 12, an ion collector 14 and an analyzing sector 16. Also shown is a connection 18 from the interior of the analyzer to a vacuum system (not shown) for lowering the pressure in the analyzer to the desired level. A feedthrough tube 20 provides electrical connections to the various electrodes of the analyzer from exteriorly located power sources.

The ion source 12 houses a source of electrons (see FIG. 3) and various electrodes associated with the ion source of a mass analyzer including a split focusing electrode 22 located between the ion source and an electrode 24 defining an object slit 25. Gas molecules to be ionized are introduced into the ion source through a port 26. Once in the ion source the molecules are ionized by electron bombardment and by suitable repelling potentials on the electrodes within the source are urged toward the focusing electrode 22 and the object slit 25.

The electrodes of the source, particularly focusing electrode 22, are electrostatically energized such that the ions emerging from the housing are focused at the center of slit 25 to provide that ions directed toward the analyzing sector emanate from a point source 28. The boundaries of sector 16 are defined by pole pieces which create a charged particle angle of deflection of 270. The particles entering the magnetic field created by pole pieces 30 experience a force tending to cause them to travel in an arcuately shaped path 32 toward an intermediate slit 34 located midway between the point of entrance 36 to the magnetic analyzing sector and the point of exit 38 from said sector. Due to the geometry of the magnetic analyzing sector as will be discussed ,in more detail below, the charged particles (ions) entering the entrance end 36 of the analyzing sector are swept through the curved path with ions of a predetermined mass coming to a first point of focus 40 located in the center of intermediate slit 34. Slit 34 (shown enlarged in FIG. 1 for purposes of illustration) is chosen of such a width dimension that ions of other than the predetermined mass to charge ratio are retarded from passing therethrough. In this manner the number of ions contributing to the general background and noise level are limited to a relatively low figure prior to the subjecting of ions of the predetermined mass to a second stage of analysis or resolution in the area of the magnetic field between the intermediate slit 34 and the exit end 38 from the magnetic analyzing sector. Ions emerging from slit 34 travel in an arcuately shaped path 42 and converge toward a point of focus 44 located in the center of an image slit (not shown) mounted in support 46 defining the entrance into the collector 14.

A retarding lens 48 is provided between the image point 44 and the collector 14 to establish a potential barrier between these two points which retards the passage of ions of undesired mass to charge ratio. The occurrence of such ions is due to a wide angle scattering effect experienced by some ions in their passage through the analyzer causing them to lose a certain amount of energy originally imparted thereto and leaving them with a momentum corresponding to the momentum of ions of the desired mass to charge ratio. Thus. witlmht the provision of retarding lens 48 such ions would pass to the collector. Ily crcnting n potential barrier only ions having a. sullicicntly high momentum. namely. those having the desired mass to charge ration are admitted into the collector portion of the analyzer.

An auxiliary collector 37 is positioned at a location interiorly removed a relatively small distance from the entrance to the magnetic sector 16. In leak detection applications, the primary tracer gas, e.g., helium follows the main ion trajectory 33, 42 to collector 14. An auxiliary tracer gas, e.g., hydrogen is collected by collector 37.

The configuration and operating parameters of the analyzer according to the present invention depend upon the selection of the angle of deflection through which the ions to be anaylzed are to be passed. Starting with the premise that two analyzing sections are to be provided, the total angle of deflection to be provided is in excess of 180. If two symmetrical analyzing sections are employed, a total single deflection to 270 has been found to be convenient. Thus, the angle of deflection for each of the two sections is Median focusing of the ions within the plane of FIG. 1 occurs at focus 44. Further, if Z-focusing is desired, i.e., provision against loss of ions due to drifting in a vertical direction above and below the optical axis of the analyzer, the object slit should be placed at the focal point of the Z lens. This means that the point of 2 focus is located at the same point that the image slit is spaced from the exit end of the magnet boundary. Relating this condition to the radius of curvature of the ions within the magnet, it can be shown that the angle between the ion trajectory at the exit or image side of the magnet and a normal to the pole boundary is 2635. This fixes the location of the object s litandfor tIiEradiHSBTc'urvature in the analyzer of FIGS. 1 and 2 means that the ratio of the distance of the object slit from the entrance magnetic boundary to the radius of curvature is 1.998. For the analyzer shown in FIGS. 1 and 2 the pertinent optical information is:

where 1' is the distance between the object slit and the entrance magnetic boundary, a,,, is the radius of curvature for ions of a predetermined mass in the magnetic field of the analyzing sector; e is the angle between the ion trajectory and a normal to the magnetic pole boundary; 1 is the angle of deflection for particles of a predetermined mass in each of the analyzing sectors of the analyzer; 1",, is the distance between the image slit and the exit magnetic boundary; e" is the angle between the ion trajectory and a normal to the exit pole boundary. Thus, for a given angle of deflection and a given requirement for Z-focusing, the angle which the ion path makes to the pole boundary and the spacing of the image and object slit from the pole boundaries for a given radius of curvature is specified. At a magnet angle of deflection of the image and object slit are located at infinity. At an angle of 360 the image and object slit coincide. Therefore, due to practical considerations inherent in providing a useful instrument, an analyzer providing an angle of deflection greater than 180 and less than 360 is contemplated by the present invention. Alternate embodiments of the analyzer shown in FIG. I are shown in FIGS. 4A nnd 413 having nngles of deflection of 240 and 300", respectively. Referring to these figures briefly, an object slit (not shown) is positioned at point source to direct ions into a magnetic analyzing sector 52 having an intermediate point of focus 54 and an image slit (not shown) located at a second point of focus 56 beyond the exit from analyzing sector 52. In FIG. 4B, the embodiment of the analyzer having an angle of deflection of 300, a point source of ions is lo cated at 58 and an image point is located at 60 with the ions being resolved by magnet 62 to a first intermediate point of focus 64 and thereafter to the second point of focus 60, the point of location of the image slit.

The electrical controls of a mass analyzer according to the present invention are shown in FIG. 3. A power supply 66 is connected to a potentiometer 68 having a plurality of taps connected to various parts of the analyzer. Tap 70 is connected to a repeller plate 72 located on the side of an electron generating filament 74 opposite the object slit 24. A second tap 76 is connected to a boundary electrode 78 which, together with repeller plate 72, define an ion chamber surrounding filament 74. Tap 80 is connected to an emission regulator 82 for regulating the production of electrons from filament 74 which bombard sample molecules admitted to the ion source. Tap 84 is connected to electron focus electrode 86 and provides means for controlling the focus of the electrons generated by filament 74 to maximize the ionization efficiency of the ion source. Taps 88 and 90 are connected to split ion focusing electrodes 92 and 94, respectively, for providing precise control of the ion focus to locate the point source ofions in the center of object slit 24.

Tap 96 is connected to retarding plate 48 for providing the potential barrier referred to in connection with FIGS. 1 and 2. Ions emerging from the analyzing section of the instrument are focused at a point 44 in the center of an image slit 98. An electron multiplier 100 is located so as to receive ions passing through the slit in retarding plate 48 and the slit in a grounded plate 102 located on the side of plate 44 opposite the image slit. Tap 104 is connected to the negative side of a high voltage supply to accelerate ions passing through the slit and plate 102 to a very high velocity thereby maximizing the efficiency at which the electron multiplier operates. An electrometer amplifier 106 is connected to the output of multiplier 100 for amplifying the signal detected at the collector for subsequent transmission to suitable recording apparatus.

As can be seen from FIG. 1, the entrant and emergent ion beam intersect at 108. However, it has been found that the probability of collision between the entrant and exit beam is low and does not constitute a problem in the operation of the instrument. To further reduce the probability of such collisions, an alternate embodiment of the exit portion of the analyzer is shown in FIG. 5. In that figure are shown the poles of magnet 30 and a pair of deflection plates 110 located on either side of the emergent beam. Connection of a potential according to the polarity shown in FIG. 5 produces a deflection of a resolved emergent beam of positively charged particles as shown and a deflection of the image point to position 1 12 as shown in FIG. 5. The undeflected position 114 of the image point (no voltage on deflection plates) is also shown in phantom, together with its relation to the cross section 116 of the entrant ion beam.

What is claimed is:

l. A mass analyzer comprising a curved magnetic analyzing sector for deflecting charged particles directed into said analyzing sector through a total angle of deflection between and 360, said analyzing sector having an entrance boundary and an exit boundary, the angular displacement of said exit boundary from said entrance boundary being less than the total angle of deflection of the charged particles;

an object aperture for defining a point source of charged particles, said object aperture being spaced a first predetermined distance from said entrance boundary and being positioned with respect to said entrance boundary so as to direct particles towards said analyzing sector at a first predetermined angle to a normal to said entrance boundary; an image aperture spaced a second predetermined distance from said exit boundary and positioned with respect to said exit boundary so as to receive emerging particles from the sector at a second predetermined angle to a normal to said exit boundy;

at least one of the said first and second predetermined angles being an acute angle;

said image aperture, object aperture and analyzing sector defining a charged particle trajectory having a first point of focus within said analyzing sector for particles of a predetermined mass and a second point of focus for such particles at said image aperture; and

means located within said analyzing sector, associated with the first point of focus, for selectively retarding the passage of charged particles having a mass different from such predetermined mass therebeyond;

said first and second predetermined distances, said first and second predetermined angles, the angular displacement of said exit boundary from said entrance boundary, and the position of said selectively retarding means being chosen so that both median and Z focusing of the charged particle trajectory occurs at said second point of focus.

2. A mass analyzer according to Claim 1 further comprising a source of charged particles provided with charged particle accelerating and focusing means for directing the charged particles emanating from said source toward a point offocus at said object aperture.

3. A mass analyzer according to claim 2 further including a collector of charged particles located after said image aperture and a retarding means located between said image aperture and said collector for preventing particles having less than a predetermined momentum from passing therebeyond.

4. An analyzer according to claim 3 wherein said selectively retarding means located within the analyzing sector defines a pair of symmetrical analyzing subsectors subtending equal angles of deflection between the entrance and exit boundaries.

5. A mass analyzer according to claim 4 wherein the magnetic analyzing sector is arranged so as to produce a deflection of 270 for charged particles introduced therein.

6. A mass analyzer according to claim 5 wherein the object aperture is spaced from the entrance boundary a distance such that the ratio of said distance to the radius of curvature in the analyzer of particles of the predetermined mass is 1.998.

7. A mass analyzer according to claim 6 wherein the image aperture is spaced a distance from the exit boundary such that the ratio of said distance to the radius of curvature in the analyzer of particles of the predetermined mass is 1.998 whereby the analyzer has stigmatic focusing for particles of the predetermined mass.

8. A mass analyzer according to claim 7 wherein the first predetermined angle to a normal to the entrance boundary is 26 35'.

9. A mass analyzer according to claim 8 wherein the second predetermined angle to a normal to the exit boundary is 2635.

10. A mass analyzer according to claim 9 wherein the I collector is an electron multiplier.

11. A mass analyzer according to claim 10 including electrostatic means for deflecting charged particles emerging from the exit boundary of the analyzer to prevent the intersection of said particles with particles passing from the source to the entrance boundary of the analyzer.

12. A mass analyzer according to claim 4 wherein the magnetic analyzing sector is arranged so as to produce a deflection of 240 for charged particles introduced therein.

13. A mass analyzer according to claim 4 wherein the magnetic analyzing sector is arranged so as to produce a deflection of 300 for charged particles introduced therein.

14. A tracer gas leak detector operating on a tandem mass analyzer principle comprising:

a magnet having a pair of spaced apart pole pieces arranged to produce an angle of deflection of 270 between entrance and exit pole faces for charged particles introduced into the space between the pole pieces, the pole pieces defining a charged particle analyzing sector therebetween;

a source for providing charged particles to be analyzed positioned with respect to the entrance pole faces so as to direct particles between the pole pieces at an angle of 26 35 to a normal to the entrance pole faces, the source including means for admitting a sample of particles including tracer gas particles to be analyzed into the source, means for producing charged particles from particles in the sample, and means for accelerating the charged particles from the source to a point of charged particle focus between the source and the entrance pole faces;

an object aperture located at said point of charged particle focus, the aperture being spaced a distance from said pole faces such that the ratio of said distance to the radius of curvature of the tracer gas charged particle trajectory is 1.998;

an image aperture spaced from the exit pole faces, the aperture being spaced a distance from said pole faces such that the ratio of said distance to the radius of curvature of the tracer gas charged particle trajectory is 1.998;

a secondary emission collector for tracer gas charged particles positioned with respect to the exit pole faces at an angle of 26 35' to a normal to the exit pole faces so as to receive particles emerging from between the pole pieces;

the source, collector and magnet defining a charged particle trajectory having a first point of focus within the analyzing sector for tracer gas charged particles at an angle of deflection of 135 and a second point of focus for tracer gas charged particles at the image aperture; and

an intermediate aperture located within the analyzing sector coinciding with the first point of focus for defining the boundary between a first and second analyzing sector within the magnet.

15. A leak detector according to claim 14 wherein the tracer gas is helium.

16. A leak detector according to claim 11 including an auxiliary collector for detecting the presence of an alternate tracer gas at said alternate tracer gass first point of focus.

17. A. leak detector according to claim 16 wherein

Claims

1. A mass analyzer comprising a curved magnetic analyzing sector for deflecting charged particles directed into said analyzing sector through a total angle of deflection between 180* and 360*, said analyzing sector having an entrance boundary and an exit boundary, the angular displacement of said exit boundary from said entrance boundary being less than the total angle of deflection of the charged particles; an object aperture for defining a point source of charged particles, said object aperture being spaced a first predetermined distance from said entrance boundary and being positioned with respect to said entrance boundary so as to direct particles towards said analyzing sector at a first predetermined angle to a normal to said entrance boundary; an image aperture spaced a second predetermined distance from said exit boundary and positioned with respect to said exit boundary so as to receive emerging particles from the sector at a second predetermined angle to a normal to said exit boundary; at least one of the said first and second predetermined angles being an acute angle; said image aperture, object aperture and analyzing sector defining a charged particle trajectory having a first point of focus within said analyzing sector for particles of a predetermined mass and a second point of focus for such particles at said image aperture; and means located within said analyzinG sector, associated with the first point of focus, for selectively retarding the passage of charged particles having a mass different from such predetermined mass therebeyond; said first and second predetermined distances, said first and second predetermined angles, the angular displacement of said exit boundary from said entrance boundary, and the position of said selectively retarding means being chosen so that both median and Z focusing of the charged particle trajectory occurs at said second point of focus.

2. A mass analyzer according to Claim 1 further comprising a source of charged particles provided with charged particle accelerating and focusing means for directing the charged particles emanating from said source toward a point of focus at said object aperture.

5. A mass analyzer according to claim 4 wherein the magnetic analyzing sector is arranged so as to produce a deflection of 270* for charged particles introduced therein.

8. A mass analyzer according to claim 7 wherein the first predetermined angle to a normal to the entrance boundary is 26* 35''.

9. A mass analyzer according to claim 8 wherein the second predetermined angle to a normal to the exit boundary is 26*35''.

10. A mass analyzer according to claim 9 wherein the collector is an electron multiplier.

12. A mass analyzer according to claim 4 wherein the magnetic analyzing sector is arranged so as to produce a deflection of 240* for charged particles introduced therein.

13. A mass analyzer according to claim 4 wherein the magnetic analyzing sector is arranged so as to produce a deflection of 300* for charged particles introduced therein.

14. A tracer gas leak detector operating on a tandem mass analyzer principle comprising: a magnet having a pair of spaced apart pole pieces arranged to produce an angle of deflection of 270* between entrance and exit pole faces for charged particles introduced into the space between the pole pieces, the pole pieces defining a charged particle analyzing sector therebetween; a source for providing charged particles to be analyzed positioned with respect to the entrance pole faces so as to direct particles between the pole pieces at an angle of 26* 35'' to a normal to the entrance pole faces, the source including means for admitting a sample of particles including tracer gas particles to be analyzed into the source, means for producing charged particles from particles in the sample, and means for accelerating the charged particles from the source to a point of cHarged particle focus between the source and the entrance pole faces; an object aperture located at said point of charged particle focus, the aperture being spaced a distance from said pole faces such that the ratio of said distance to the radius of curvature of the tracer gas charged particle trajectory is 1.998; an image aperture spaced from the exit pole faces, the aperture being spaced a distance from said pole faces such that the ratio of said distance to the radius of curvature of the tracer gas charged particle trajectory is 1.998; a secondary emission collector for tracer gas charged particles positioned with respect to the exit pole faces at an angle of 26* 35'' to a normal to the exit pole faces so as to receive particles emerging from between the pole pieces; the source, collector and magnet defining a charged particle trajectory having a first point of focus within the analyzing sector for tracer gas charged particles at an angle of deflection of 135* and a second point of focus for tracer gas charged particles at the image aperture; and an intermediate aperture located within the analyzing sector coinciding with the first point of focus for defining the boundary between a first and second analyzing sector within the magnet.

15. A leak detector according to claim 14 wherein the tracer gas is helium.

16. A leak detector according to claim 11 including an auxiliary collector for detecting the presence of an alternate tracer gas at said alternate tracer gas''s first point of focus.

17. A leak detector according to claim 16 wherein alternate tracer gas is hydrogen.