US2680812A

US2680812A - Mass spectrometry

Info

Publication number: US2680812A
Application number: US259584A
Authority: US
Inventors: Clifford E Berry
Original assignee: Consolidated Engineering Co Inc
Current assignee: Consolidated Engineering Co Inc
Priority date: 1951-12-03
Filing date: 1951-12-03
Publication date: 1954-06-08
Anticipated expiration: 1971-06-08

Description

June 8, 1954 c. E. BERRY MASS SPECTROMETRY 2 Sheets-Sheet 1 Filed Dec. 5, 1951 FIG.

ZERO LEVEL c 0 I/VCREAS/NG MASS FIG. 2.

T0 EVACUAT/NG SYSTEM /ON SOURCE C //?C U/ T INVENTOR. CLIFFORD E. BERRY ATTORNEY June 8, 1954 C. E. BERRY 2,630,312

MASS SPECTROMETRY Filed Dec. 3, 1951 2 Sheets-Sheet 2 I N V EN TOR. c4 IFFORD E. BERR r ATTORNEY SENS/NG CIRCUIT i 4 3 5 5 7 m n NC m 0 4 a 5 f 4 2 4 m GT F w/ a w u. 5 mm SC L n 0 mm 5 BMT 6 c P .w m F 5mm Patented June 8, 1954 MASS SPECTROMETRY Clifford E. Berry, Altadena, Califl, assignor to Consolidated Engineering Corporation, Pasadena, Caliil, a corporation of California Application December 3, 1951, Serial No. 259,584

Claims. 1

This invention relates to mass spectrometry and particularly to improvements in apparatus whereby undesirable effects or negative ions are minimized or eliminated.

In mass spectrometry a sample to be analyzed is first ionized as by bombardment with an electron beam. Positive ions are thus produced and are collimated into a heterogeneous ion beam which is propelled into the influence of suitable electric or magnetic fields. These fields act upon the beam to disperse it into a plurality of spatially separated homogeneous beams, each of which is composed of ions of a given mass-tocharge ratio, 1. e. specific mass. One or more of these individual beams is selectively focused on and discharged at a collector electrode, the resultant discharge current constituting a measure of the abundance of the ions of a given specific mass.

In the process of directing these ion beams from the point of origin tothe point of collection, positive ions, particularly those not in focus, are caused to strike and discharge upon confining metal surfaces. In this process negative ions of considerable energy are produced at the point of impact of the positive ions with the metal surface. The negative ions are emitted to travel in a circular path from the point of their origin until they strike a conductive surface and discharge. The radius of curvature of the path of travel of such ions is a function of their initial energy, and the direction of travel is invariably counter to the direction of curvilinear travel of the positive ions responsible for their formation. If such negative ions impinge upon a collector electrode oriented to receive positive ions they K will develop at the collector electrode a spurious signal which, if overlapping or superimposed on the signal produced by discharge of positive ions, will lead to erroneous interpretation of the net discharge signal.

In usual practice the beams of positive ions are selectively focused on the collector electrode through an aperture or resolving slit in a barrier electrode. Such a resolving slit provides a high degree of selectivity so that adjacent ion masses, i. e. ion beams not in direct focus on the resolving slit, will discharge at the barrier electrode or on the metal surfaces of the confining closure adjacent the barrier electrode. Because of the curvilinear path of travel of the negative ions they do not generally approach the resolving slit in a perpendicular plane but rather at an angle determined by the point of origin and the radius of the curvature of the ion. Accordingly,

I have provided improvements in mass spectromv,

eters adapted to take advantage of the above described characteristics of negative ions to selectively trap and collect these ions without interference with the travel of positive ion beams or with the collection of positive ions.

The invention contemplates in a mass spectrometer comprising an ion source, an analyzer tube in which ions are segregated into beams in accordance with their specific mass, a collector electrode upon which ion beams are selectively discharged and resolving means for selectively passing a given ion beam to the collector, the improvement comprising auxiliary electrode means interposed ahead of the collector electrode outside the path of positive ions and so arranged as to cause collection of negative ions and thereby prevent their access to the collector electrode.

The invention also contemplates the entirely new concept of removing negative ions as an infiuence in the measurement of positive ions and involves the method of selectively trapping the negative ions at a point or points outside the path of positive ion flow to a collector electrode, such removal being conveniently accomplished in terms of apparatus in the manner above described, but being not so limited. The provision of auxiliary collecting means is the presently preferred method of carrying out the selective removal of negative ions, but since removal thereof can also be accomplished by appropriate trapping fields the invention is not limited to the specific apparatus illustratedv and in general contemplates the method of selectively removing such negative ions regardless of the apparatus employed for such purpose.

The invention will be more clearly understood with reference to the following detailed description thereof taken in conjunction With the accompanying drawing in which:

Fig. 1 is a reproduction of a recording trace representing the discharge current at a collector electrode when negative ions are not selectively removed in accordance with the invention;.

Fig. 2 is a schematic diagram of a conventional mass spectrometer provided with selective negative ion collecting means in accordance with one embodiment of the invention;

Fig. 3 is a diagrammatic section view of the collection system of a mass spectrometer of the type shown in Fig. 2 illustrating an alternative embodiment of the invention;

Fig. 4 is a view similar to Fig. 3 illustrating yet another alternative embodiment of the invention; and

Fig. 5 is yet another view similar to Fig. 3 illustrating a fourth alternative embodiment of the invention.

Referring to Fig. l of the drawing the recorded trace ill there reproduced includes peaks A and B typical of the mass it and i9 peaks of a conventional analysis with negative peaks and D preceding respectively the positive peaks A and B. The negative peak 0 is produced as the result of negative ions developed as mass 18 ions strike the metal surfaces adjacent the instrument resolving slit prior to focus on the slit to form the peak A. The negative peak D is similarly formed as a consequence of discharge of ions of mass I9 prior to focusing these ions through the resolving slit to develop peak B. The trace Iii of the figure has been considerably simplified in the spatial separation of the peaks C and D from the positive peaks A and B. In

actual practice, however, the negative peaks often 1 overlap or are entirely obscured by the adjoining positive peaks so as to introduce an error into the peak reading. It is for this reason that it is desirable in accordance with the invention to remove negative ions as an innuence in the discharge current developed at a collector electrode.

A typical mass spectrometer is shown diagrammatically in Fig. 2. The instrument cornprises an ion source 12 including a sample inlet i3, electron beam M developed at an electron gun (not shown), a so-called repeller electrode 15, a first apertured accelerating electrode iii, a second apertured accelerating electrode H and a so-called ion source circuit 13 adapted to supply appropriate potentials to each of the ion source electrodes and to the electron gun. The second accelerating electrode I! gives access to an analyzer tube 2a which terminates at its opposite end in a barrier electrode 22. The particular instrument illustrated is an isotope measuring instru ment for which purpose the barrier electrode 22 is provided with two resolving

apertures

22A, 22B and two collector electrodes 2.4, 25 aligned respectively with the

apertures

22A, 22B. The two collector electrodes are in turn connected to a ratio measuring circuit 26. The details of the ion source circuit l8 and ratio measuring circuit 25 are well known and form no part of the present invention. The ion source, analyzer tube and collector system are enclosed within an i envelope 28 provided with an exhaust line 29 communicating with an evacuating system (not shown).

In operation, ions formed in the ion source by the electron beam M are propelled under the influence of potentials established between the several electrodes in the ion source as a heterogeneous beam into and through the analyzer tube 20. While passing through the tube 28 the ions are subjected to a transverse magnetic field by magnet means (not shown) under the influence of which the beam disperses into a plurality of separate beams each composed of ions of a different mass-to-charge ratio. Three such separate ion beams are identified as beams a, b and c in the figure. As illustrated, the beams a and b are focused through the resolving slits 22A, 2213 respectively and discharge at the collector electrodes 24, respectively, the ratio of the ion currents thereby produced being measured at the ratio circuit 26. Beam c, representative of ion beams not focused on the resolving slits, strikes the walls of the analyzer chamber adjacent the barrier electrode 22 and there discharges. In this process negative ions are given Iii:

4 off traveling in circular paths represented by the dotted lines it. The direction of rotation of these ions is counter to the direction of curvature of the path of the beam 0. Hence, as illustrated, the negative ions tend to approach the resolving

slits

22A, 22B.

In the embodiment shown in Fig. 2 I provide a pair of

conductive bafiles

32, 33 extending inwardly from the barrier electrode 22 and disposed adjacent the resolving slits 22A, 2213 respectively and on the side thereof toward the center of curvature of the analyzer tube. Since the negative ions travel in circular paths they do not approach the resolving slit in a perpendicular plane and hence can be trapped and discharged by bafiles as illustrated projecting into the analyzer tube and shielding the respective resolving slits.

The collection end of a conventional mass spectrometer is shown schematically in Fig. 3 including analyzer tube at, barrier electrode ii at the end of the analyzer tube, collector electrode 42, a shield electrode 33 substantially enclosing the collector electrode, all contained within an envelope A l. Disposed between the barrier electrode 4i and the collector electrode is a series of apertured electrodes id electrically connected to a suppressor grid control circuit 41 which supplies appropriate potentials to the electrode 16 to suppress passage of ions of less than a predetermined energy. The advantages of a suppressor grid system form no part of the present invention and have been previously described in the patent literature. In addition to having a suppressor grid system, the particular instrument shown in Fig. 3 differs from that shown in Fig. 2 in having only a single aperture 41A in the barrier electrode. Such a single resolving slit is conventional in all but special types of mass spectrometers, that is, in the ordinary analytical mass spectrometer not involving isotope ratio measurement or anticipation circuits.

In this instrument ion beams striking the barrier electrode 4! in the process of being brought into focus on the resolving slit MA will, in the same manner as above described, develop negative ions. Again the paths taken by these negative ions are circular and of a radius proportional to their energy. Thus some of the negative ions will follow a circular path from the point of inception on the inner surface of the barrier electrode which carries them through the resolving slit HA and into the influence of the field established by the suppressor grid. Since this field is such as to repel positive ions of low energy, it will at the same time attract negative ions and unless precautions are taken some of such negative ions will find their way through the suppressor grid system to the collector electrode 542. To avoid this undesirable result, and in accordance with the invention, I provide an auxiliary apertured electrode 48 interposed between the barrier electrode 4! and the suppressor grid system it and maintained at the same potential as the barrier electrode M. In many instances, and as illustrated, this is a ground potential. The auxiliary electrode serves to protect the barrier electrode from the influence of the field established by the suppressor grid system so that the negative ions passing through the resolving slit is. will follow the path represented by the dotted line it insensitive to the effects of the suppressor grid.

Another form of collection system is shown in the partial view of Fig. 4 which, like Fig. 3, is a we w schematic diagram of the collection portion of a mass spectrometer. In this particular instance the mass spectrometer is provided with

dual collectors

50, 52 to which positive ions gain access through a resolving slit 53A in a barrier electrode 53 forming the end of an analyzer tube 54. The first collector electrode 52 is apertured olT-center with respect to the barrier aperture 33A to give access to the collector electrode 52, different ion beams thereby being simultaneously discharged on the two collectors 5D and 52. Such simultaneous collection of different ion beams on different collector electrodes is used for isotope ratio measurement and also for anticipation purposes where it is desired to set the sensitivity of a recorder in accordance with the abundance of ions of a given mass prior to collection thereof on the ultimate collector electrode. One arrangement of dual collectors has been illustrated in Fig. 2, Fig. 4 merely showing a different arrangement of such collectors. In the case of the apparatus of Fig. 4 wherein the first collector electrode 58 is employed for anticipation purposes only, the effects of what negative ions may collect thereon are not sufficiently pronounced to interfere with the anticipation function. However, negative ions formed on the analyzer tube or formed at the collector 59 itself may very well pass through the aperture 56A in the collector to gain access to the collector 52. To prevent this I interpose an electrode trap 56 between the collector 5G and the collector 52, the trap comprising a rectangular electrode system apertured at opposite sides to give access from aperture 53A to the collector 52, but of such dimensions that ions following a circular path through the aperture 59A will strike upon and discharge on the electrode system 555 irrespective of their radius of curvature within the theoretical limits thereof. Collector electrodes 5i} and 52 are connected toa sensing circuit 53 which, in this case, takes the form of an anticipation circuit operable to adjust the sensitivity of the sensing and recording network responsive to signals from the collector 5i! and to thereupon record the corresponding signal from the ultimate collector 52.

A mass spectrometer substantially similar to that shown in Fig. 4 is shown in Fig. 5 and includes the

same collectors

50, 52, barrier electrode 53 in the end of analyzer tube 54 and having a resolving aperture 53A with ion flow being through the aperture 53A to strike the anticipatory collector 5% or to pass through the aperture 50A therein to strike the ultimate collector 52.

As in the foregoing instrument, the collector electrodes 59 and 52 are connected to a sensing circuit 58. In this particular embodiment the electrode trap 56, as shown in Fig. 4, is replaced by a single electrode 651 apertured at 60A and having a tubular extension 66B extending into the cup of collector electrode 52. The purpose of the extension 5GB is the same as that of the electrode trap 5! namely to collect negative ions whose paths carry them through the apertures 50A and fitA. The apparatus of Fig. 5 is not as highly emcient as that of Fig. 4 since certain negative ions are produced on the collector electrode 52 itself, and it is desirable that these'negative ions be discharged also on the collector electrode, for which reason the collector electrodes are frequently cup-shaped, as shown. If

' any of the negative ions produced at the collector electrode discharge elsewhere than on the collector electrode, a faulty discharge signal is thereby induced, and since the tubular extension 60B does collect a fraction of the negative ions developed at the collector electrode as well as those developed elsewhere, a small error will remain. However, this error is smaller than that produced by uncontrolled negative ions and the system of Fig. 5, although secondary in efficiency to that of Fig. 4, is still superior to any system in which negative ions are ignored.

Principally the invention involves selective oollection of negative ions at a point or points outside the path of positive ions focused on positive ion collection electrodes, the auxiliary selective collection means for the negative ions being oriented, bearing in mind the curvilinear paths of such negative ions and the maximum theoretical radius of such paths. With this consideration it is possible to arrange the geometry of the system such as to prevent access of any substantial number of negative ions to the principal collector electrode. Different orientation of such auxiliary selective collectors may be devised for various types of instruments without departing from the scope of this invention, and various refinements such as biasing such electrodes to attract the negative ions will occur to those skilled in the art.

I claim:

1. In a mass spectrometer comprising an ion source, an analyzer tube in which positive ions are segregated into beams in accordance with their specific mass, a collector electrode upon which ion beams are selectively discharged, and resolving means for selectively passing a given ion beam to the collector, the improvement comprising auxiliary electrode means interposed in front of the collector electrode outside the path of positive ions and so arranged as to cause collection of negative ions and thereby prevent their access to the collector electrode.

2. In a mass spectrometer comprising an ion source, an analyzer tube in which positive ions are segregated into beams in acordance with their specific mass, a resolving slit at an end of the analyzer tube through which an ion beam may be selectively focused, and a collector electrode disposed to collect positive ions passing through the resolving slit, the improvement which comprises at least one auxiliary electrode disposed adjacent the resolving slit and displaced from the path of the ion beam focused on the resolving slit, the auxiliary. electrode being adapted to induce collection of negative ions and thereby prevent access thereof to the collector electrode.

3. In a mass spectrometer comprising an ion source, an analyzer tube in which positive ions are segregated into beams in accordance with their specific mass, a barrier electrode at an end of the analyzer tube including a. resolving slit through which an ion beam may be selectively focused, a collector electrode disposed to collect positive ions passing through the resolving slit and a suppressor grid disposed between the barrier electrode and the collector electrode, the improvement which comprises an auxiliary apertured electrode disposed between the barrier electrode and suppressor grid, and means maintaining the auxiliary electrode at a potential to shield the barrier electrode from the influence of the suppressor grid so that negative ions will not be attracted to the suppressor grid.

4. Apparatus according to claim 3 wherein the barrier electrode and auxiliary electrode are at the same potential.

5. In a mass spectrometer comprising an ion source, an analyzer tube in which positive ions are segregated into beams in accordance with their specific mass, a barrier electrode disposed at an end of the analyzer tube and havin a resolving slit therein through which an ion beam may be selectively focused, and a collector electrode disposed to collect ions passing through the resolving slit, the improvement which comprises at least one auxiliary collection means disposed adjacent the resolving slit and displaced from the path of the ion beam focused on the resolving slit, the auxiliary electrode being adapted to collect negative ions and thereby prevent access thereof to the collector electrode.

6. Apparatus according to claim 5 wherein the auxiliary collection means comprising at least one bafile extending from the barrier electrode into the analyzer tube from a point adjacent the resolving slit.

'7. Apparatus according to claim 5 wherein the auxiliary collection means comprises an electrode trap, apertured to pass positive ions and disposed between the barrier electrode and the collector electrode whereby ions having paths diflering from the passed positive ions will, if passed through the resolving slit, impinge upon and discharge at the electron trap.

8. In a mass spectrometer comprising an ion source, an analyzer tube in which ions are segregated into beams in accordance with their specific mass, a barrier electrode disposed at an end or" the analyzer tube and having a pair of spaced resolving slits through which ion beams may be selectively focused, a pair of collector electrodes disposed respectively to collect ions passing through the resolving slits, the improvement which comprises conductive baiiies extending from the barrier electrode into the analyzer tube, one of such barriers being disposed adjacent each resolving slit, the conductive baffles being adapted to collect negative ions and thereby present access thereof to the collector electrode.

9. Apparatus according to claim 8 wherein the analyzer tube is curvilinear and barriers are disposed on the sides of said resolving slits towards the center of curvature of the tube.

1%. In a mass spectrometer comprising an ion source, an analyzer tube in which ions are segregated into beams in accordance with their specific mass, a barrier electrode disposed at an end of the analyzer tube and having a resolving slit through which ion beams may be selectively focused, a first collector electrode disposed to collect ions passing through the resolving slit, the first collector electrode having an aperture adapted to pass a part of the ions traversing the resolving slit, and a second collector electrode disposed to collect ions passing through said aperture, the improvement which comprises an auxiliary electrode disposed between the first and second collector electrodes and apertured to pass positive ions, the auxiliary electrode being adapted to collect negative ions and thereby prevent access thereof to the collector electrode.

11. Apparatus according to claim 10 wherein said auxiliary electrode comprises an apertured plate electrode and a tubular extension projecting from the plate coaxially with the aperture and toward the second collector.

12. Apparatus according to claim 10 wherein said auxiliary electrode comprises first and second apertured disks spaced apart in the direction of positive ion travel from the first to the second collector electrodes and electrically connected to each other.

13. In a mass spectrometer comprising an ion source, an analyzer tube in which ions of a given polarity are segregated into beams in accordance with their specific mass, a collector electrode upon which ion beams are selectively discharged, and resolving means for selectively passing a given ion beam to the collector, the improvement comprising auxiliary electrode means interposed in rout of the collector electrode outside the path of ions of said given polarity and so arranged as to cause collection of ions of opposite polarity than the ions of said iven polarity and thereby prevent access of said ions of opposite polarity to the collector electrode.

14. In a mass spectrometer comprising an ion source, an analyzer tube in which ions of given polarity are segregated into beams in accordance with their specific mass, a resolving slit at an end of the aria-year tube through which an ion beam may be selectively focused, and a collector electrode disposed to collect ions of said given polarity passing through the resolving slit, the improvement which comprises at least one auxiliary electrode disposed adjacent the resolving slit displaced from the path of an ion beam focused on the resolving slit, the auxiliary electrode bein adapted to induce collection of ions or" opposite polarity from the ions of said given polarity and thereby prevent access of said. ions of opposite polarity to the collector electrode.

1 In a mass spectrometer comprising ion source, an analyzer tube which ions are segregated into beams of ions or". given polarity and in accordance with their specific mass, a barrier electrode disposed at an end of the analyzer tube and having a pair of spaced resolving slits through which ion beams may be selectively focused, a pair of collector electrodes disposed respectively to collect ions passing through the resolving slits, the improvement which comprises conductive baflles extending from the barrier into the analyzer tube, one of such barriers being disposed adjacent each resolving slit, the conductive bailles being adapted to collect ions of opposite polarity from said given polarity and thereby prevent access of said ions of opposite polarity to the collector electrode.

No references cited.