US2583121A - Mass spectrometer ion source - Google Patents

Description

1952 F. L. REYNOLDS MASS SPECTROMETER ION SOURCE 2 SHEETS-SHEET 1 Filed Dec. 29, 1949 IN V EN TOR. HQEDER/CK L. REY/v01. 05

KM/QM A TTOR NE K Jan. 22, 1952 F. L. REYNOLDS MASS SPECTROMETER ION SOURCE 2 SHEETSSHEET 2 Filed Dec. 29, 1949 POWER F JUPPL Y IN V EN TOR. FREDER/C/f L. REM/0L 05 KAN/dim A TTORNEK Patented Jan. 22, 1952 MASS SPECTROMETER ION SOURCE Frederick L. Reynolds, San Francisco, Calif., assignor to the United States of America as represented by the United States Atomic Energy Commission Application December 29, 1949, Serial No. 135,701

11 Claims. 1

The present invention relates to ion source mechanisms, and more particularly to an ion source mechanism adapted to eifectively ionize minute samples. More specifically the invention provides apparatus for the rapid insertion and ionization of samples, which is particularly advantageous, for example in the analysis of unstable elements and isotopes.

The principles of electromagnetic mass spectrometry have long been understood and diligently explored by many workers. Within recent years this art has found wide application in the analysis of gaseous mixtures and has also been found to be very advantageous in the analysis of minute samples which are not easily analyzed by chemical methods. In line with these advantages attempts have been made to employ electromagnetic mass spectrometry in the analysis of the end products of bombardment specimens and the like. Modern particle accelerators and like equipment have provided means to change the structure of atoms and molecules and thereby promise to afford an insight into atomic physics; however, difficulty has been encountered in the analysis of the materials produced by such ap paratus. Many of the materials produced by bombardment of samples with high energy particles have a very transitory nature and decay rapidly into other more common elements, and

in order to accurately determine and identify :7

" the art of electromagnetic mass spectrometry,

there is an appreciable delay from the time that a sample (other than gaseous) is inserted in a conventional mass spectrometer until an analysis is obtained. It is to overcome this limitation that the present invention was conceived, although it will, of course, be apparent that the applicability of the invention is in no wise limited to cooperation with an analyzing'mass spectrometer, but instead the scope of the invention includes all uses to which an ion source may be put. It is therefore an object ofthe present invention to provide a method and means of rapidly analyzinga sample in a mass spectrometer.

. Another object of the invention is to provide a method and means of rapidly inserting and removing the ion source mechanism of mass Cir 2 spectrometer without afiecting the vacuum thereof.

Another object of the invention is to provide an ion source mechanism capable of easy and rapid insertion in a mass spectrometer.

Still another object of the invention is to provide a method and means for rapidly and easily inserting a sample in an ion source mechanism.

Yet another object of the invention is to provide an ion source mechanism having a preadiusted removable filament for the rapid handling if ionizable samples.

A still further object of the invention is to provide a method and means of determining the eiiiciency of removal of ions from an ion source mechanism.

Further objects and advantages of the invention will become apparent from a consideration of the following description and accompanying drawings, of which Figure l is a vertical sectional view of the ion source proper and ion source box taken along the line I-l of Fig. 2;

Fig. 2 is a plan view of the ion source proper as viewed from theleft of Fig. 1;

Fig. 3 is a partially exploded, perspective view of the ion source proper; and

Fig. 4 is a schematic wiring diagram for the ion source.

Considering the invention in general, it will be noted from the drawings that the ion source proper 50 includes a thermal ion production means 5| and a plate assembly 52 comprising a plurality of parallel spaced plates having openings therethrough in line with the ion production means. The source proper is provided with a number of prongs which mate into cooperating apertures in an ion source support assembly in the manner of a socket thereby providing means for the rapid removal and insertion of the ion source proper so the ion source box it of an electromagnetic mass separator. Also the ion production means 5! is provided with preadjusting means and a socket connection to the ion source plate assembly 52 whereby the time required to initiate operation of the separator following insertion of the sample to be ionized minimized.

Referring more specifically to Figs. 1, 2, and 3, it may be noted that the plate assembly 52 comprises a plurality of plates secured together by suitable mounting studs I and spaced apart by insulating spacers 2 immediately surrounding the studs I. In order to prevent electrical connection between the plates of the plate assembly 52, the studs I may advantageously comprise a metallic central member threaded on each end and surrounded by an insulating material, as shown, or may comprise an insulating member having threads formed directly on each end. The studs pass through the plates of the plate assembly 52 and tie it together by means of nuts secured to each endof the studs I and bearing on a face plate 3 at one end of the studs and a collar or washer at the other end of the studs to hold the end spacers in position as shown The face plate 3 has a central circular opening l therein and four spaced prongs 6 normal to the. surface.-

of the plate and rigidly attached thereto. Arocusing plate I abuts the rear: wall of faceplateS and has a central slit 8 formed therein within the area of the opening 4 in face plate: Directly behind focusing plate I and separated therefrom is an accelerating plate 9 having a slit II provided therein in alignment with the slit 8 of plate 1. Behind the acceleratingv plate 9 and in close proximity therewith is situated a protection plate I2 having a large central opening. Spaced behind the protection plate I2 is a thermal plate I3 having a central slot I 4 in alignment with the slits 8 and I I of plates: 1 and 9', respectively.

The ion production means 5| is separably mounted on the rear of the plate assembly 52 by means of four prongs I5 which are rigidly attached to the thermal plate I3 andv extend outwardly from the plate assembly. Prongs I5 mate with suitable bores in the ends of spacing means such as the four adjusting screws I6 which have threaded mounting in a filament holder plate I1 and thereby determine the spacing between the latter and the thermal plate. I3. Also attached to thermal plate I3 is a pair of bolts E8 which pass through openings in the filament holder plate I! and maintain said plates in operative position by suitable means such as nuts threadably engaging the bolts I8 and bearing upon the. back of the filament holder plate I'll, as will be. understood from Fig. 1. A filament such. as wire 2I may be positioned in the slot I4 in thermal plate I3 through correct mounting on plate IlI which is accomplished preferably by the use: of a pair of metal-glass supporting insulator connectors I9 which extend through the filament holder plate Il. Insulator connectors I9 serveto support the filament 2| in a definite and fixed. po sition upon the filament holder plate IT and provide a means of connecting the filament to an external power source while maintaining an insulated relationship between the filament 2| and the filament holder plate I'I'.

As may be noted from Fig. l, the ion source proper 50 is adapted to be contained within an ion source box 30. The latter has an opening in one wall thereof which is closed by a vacuum pipe 3 I, the outer end of which may be connected to a vacuum system (not shown). An opening 32 is provided in the front wall of the source box 30 and about this opening and on the inner face of such wall is provided an annular socket plate 33 secured in any suitable manner thereto, as by the cap screws 35. For facilitating union with the ion source face plate 3 the socket plate 33 is provided with four spaced holes 35 which match the size andv position of the four prongs 6 of the former. As will be readily apparent. the ion source proper 50 is assembled with the ion source box 30 by inserting the prongs 6 in the holes 36, the outer rim of the face plate 3 resting upon an inwardly disposed shoulder of an adjusting nut 3'! which threadably engages the outer circumference of socket plate 33. The spacing between the face plate 3 and the front wall of the apertured source box is determined by the setting of adjusting nut 31. The ion source proper 50 is locked in adjusted position by means of av clamping nut 38 which surrounds; the ion source. proper, abuts the rear. face of the outer rim portion of the face plate 3, and threadably engages an inner circumference of adjusting nut 31. One wall 39 of the ion source box 30 is preferably removable, as shown. to permit easy accessto theion. source proper and such a wall is. maintained in vacuum tight contact with source. box 30' by, suitable means such as bolts 49 and suitable: sealing means, such as a ring gasket.

It will, of course; be appreciated that in addition to the above-noted means for facilitating rapid removal and insertion of the ion source, the mass. separator. apparatus. with which the ion source cooperates may advantageously be equipped with means to isolate the. remainder. of the. system under vacuum from. the ion source box. Such means. (not shown.) may comprise vacuum tight gates between. the vacuum. pump.- ing system and the source box, and between the mass separator chamber and the ion source: box. Also, it may be advantageouaalthough not necessary, to provide a special pumping arrangement to rapidly evacuate the ionsource box and at the same time maintain thev vacuum on. the re.- mainder of. the system. The above-noted provisions in the vacuum system serve. to further accentuate the advantages. of the present invention which permit a. rapid analysis of. the sample. following insertion of. the ion source, in.- asmuch as the time required to obtain proper vacuum conditions pre-requisite to. operation. of a mass. spectrometer is minimized.

Before; considering the operation of the invention, it is of importance to note the electrical connections and potentials associated. with the elements of. the source, and in this respect, attention isv invited to Fig. 4-. The filament 2.I which is shown withdrawn from its operating position for ease of illustration, isv connected across an appropriate. power supply 41. It will, of course, be appreciated that numerous types of supply r components would be suitable for supplying. the

potentials applied to the individual. plates of. the plate assembly, batteries only being illustrated for reasons of simplicity and convenience. By means of batteries 42 and 43,. respectively, the accelerating plate 9 and the: protection plate I2 are each maintained at a large negative. potential with respect to the filamentv 2! and the thermal plate I3 which may be grounded. The focusing plate I may be maintained at a lesser negative potential than the accelerating plate 9 by means of a battery 44. A current measuring device, such as a galvanometer G or similar instrument, is connected between the thermal plate I3 and the accelerating plate 9' to indicate the current flow therebetween for the reasons setv forth in detail below.

With the physical structure of the invention and the electrical connections of the individual elements thereof in mind, the operation of the invention may now be considered. Prior to an thermal plate I3.

analysis of a sample the filament is properly positioned in the assembly. This is accomplished by rotation of adjusting screws :6 until, with the ion production means 5| in place, the filament Following the above adjustments the ion source is ready for operation. The ion production means 5| is removed from the ion source proper 59 and the sample to be analyzed is placed on the filament wire 2|. In many cases the sample will be in solution and thus the filament is merely dipped inthe solution. The ion producing means is then placed in position on the ion source plate assembly 52 by slipping the prongs I5 into the bores in the enlarged ends of the four adjusting screws IS. The ion production means 5! is secured in this position by placing the nuts on the bolts I8 and tightening them until they bear on the filament holder plate I'I. Following this operation the filament is in proper position with respect to the thermal plate I3 because of the previous adjustment of the adjusting screws It. The ion source proper 59 is then plugged into the ion source box assembly by inserting prongs 6 into holes 36. The ion source is secured in this position by engaging the clamping nut 38 and tightening it onto the adjusting nut 3'! until the face plate 3 is tightly secured between the two.'

In this position the filament 2! is properly spaced for operation because of the previous adjustment of the adjusting nut 37. The cover 33 is then placed on the source box and secured in vacuum tight relation therewith by tightening the nuts on bolts 40. Following evacuation of the ion source box the apparatus is in proper condition to ionize the sample for analysis.

Ionization of the sample is produced by heating of the filament 2| as a result of current passing therethrough from the current source II. Ions produced by heating the sample are attracted toward the accelerating plate 9 as a result of the potential difference existing between the accelerating plate 9 and the filament 2| or large aperture in the protection plate I2 and by virtue of their induced velocity pass through the slot I I in the accelerating plate 9 and on through the aligned openings in the focusing plate 1 and the face plate 3 and through the openings in the socket plate 33 and ion source box wall into the spectrometer chamber (not shown). The focusing plate is maintained at a lesser negative potential and thus acts to constrict the ion beam passing through the slot 8 therein, thereby focusing the beam.

Upon completion of the analysis of a sample,

the ion source proper may be easily removed from The ions pass through the of the sample upon the filament, is to slip the ion production means 5| upon the ion source plate assembly 52 and secure it there; and the second is to plug in the ion source proper in the ion source box and tighten the clamping nut 38. The operation consumes at most a few seconds and the entire operation following produc- I tion of the sample to complete ionization and analysis requires but an extremely short period. The advantages of this rapid analysis will, of course, be appreciated in connection with the analysis of radioactive samples containing isotopes having very short half lives; however, advantage also lies in the rapid analysis of ordinary stable isotopes in that many experiments are facilitated and the time required for the compilation of extensive separation data materially decreased.

As may be noted from the above description, the sample ionized in the source is quite small and thus it is important for the separator to have a high efficiency; that is, the ratio of-ions impinging upon the receiver to ions formed at the source must be maximized. The losses in the source may be determined by the current measuring apparatus G connected between the thermal plate It and the accelerating plate 9 in series with power supply 42 as shown; the current flow between the thermal plate I3 and the accelerating plate 9 being an indication of the amount of ions impinging the accelerating plate. It is to be noted, however, that a large potential difference exists between the thermal plate and the accelerating plate, and thus a certain amount of leakage may occur across the surface of the insulating spacers 2 which would be recorded on the meter. In order to prevent this type of error the protection plate I2 is inserted in the ion source plate assembly. The

. protection plate I2 is maintained at approximately the same potential as the accelerating plate 9 and thus any leakage across the spacer surface would occur between the thermal plate and the protection plate. The acceleration plate 9 is thereby protected from current leakage, and thus the meter accurately indicates the quantity of ions impinging upon the accelerating plate.

Only one type of ion production means has been described; however, it will be apparent to those skilled'in the art that other types are adaptable to the same principles set forth above without departing from the spirit of the invention. A bombardment type source may be employed j and is advantageous in connection with the analysis of extremely small samples in that a stronger signal may be produced; however, this type of ion producing means provides a contaminated beam as a result of the complex ions produced by the bombardment. Thus the type of ion producing means employed in any particular analysis is dependent upon the circumstances of that analysis, with any one of a number of well-known types of ion producing means being adaptable for use in accordance with the principles of the present invention.

It will, of course, be appreciated by those skilled in the art that many variations and modifications are possible within the spirit and scope of the invention, and thus the invention is not to be limited to the exact details disclosed except as for accelerating and focusing ions, and ion producing means, said plate assembly and said ion producing means including cooperating socket elements whereby said ion producing means is adapted to plug into said plate assembly for ease and rapidity of assembly and disassembly.

2. In an ion source mechanism the combination comprising a plate assembly adapted to accelerate and focus ions, ion producing means, and adjustable spacing means connecting said plate assembly and said ion producing means, said spacing means comprising a socket assembly whereby said ion producing means and said plate assembly may be rapidly connected and disconnected.

3. In an ion source mechanism the combination comprising a plate assembly adapted to expel ions from the source, ion producing means, preadjusting means interconnecting said plate assembly and said ion producing means whereby the spacing therebetween is predeterminable, said preadjusting means slidably engaging said plate assembly whereby connection of said ion producing means and said plate assembly is facilitated, and locking means secured to said plate assembly and engaging said ion producing means whereby said predetermined spacing therebetween may be preserved.

4. In an ion source mechanism the combination comprising a plate assembly adapted to expel ions from said source and having one element of a socket connection secured thereto, ion producing means, a plurality of adjustable spacers connecting said plate assembly and said ion producing means, said spacers comprising one element of a socket connection mating with said plate assembly socketv element whereby said ion producing means and said plate assembly may be assembled and disassembled without readjustment of the spacing therebetween.

5. In an ion source mechanism the combination comprising a plate assembly adapted to expel ions from said source and having a male socket element secured to the back thereof, ion producing means including a plate having threaded apertures therethrough, a plurality of adjusting screws threadably engaging said plate and bearing upon said plate assembly whereby the spacing between said plate assembly and said ion producing means is adjustable, said adjusting screws comprising a socket element engaging said plate assembly socket element whereby said plate assembly and said ion producing means may be disconnected without disturbing the setting of said adjusting screws, and locking means securing said ion producing means in position as determined by said adjusting screws.

6. In an ion source mechanism the combination comprising a plate assembly having ion accelerating and focusing plates, ion producing means including a filament plate and a filament secured to said plate at a predetermined distance therefrom, and spacing means connecting said ion producing means and said plate assembly, said spacing means including adjusting screws threadably engaging said filament plate and slidably engaging said plate assembly, thereby providing means for adjusting the spacing between said filament and said plate assembly and disconnecting said ion producing means from said plate assembly without disturbing said adjustment.

7. In an ion source mechanism the combination comprising a plate assembly adapted to expel ions from said ion source and including a plurality of prongs secured thereto and extending from the back thereof, ion producing means including a filament holder plate and a filament Wire secured to said plate, adjusting means connecting said plate assembly and said ion producing means, said adjusting means comprising a plurality of adjusting screws threadably en gaging said filament holder plate and each of said screws having a hole in the end thereof slidably engaging the prongs on said plate assembly in the manner of a socket connection whereby said filament may be adjusted with respect to the plate assembly and removed from said plate assembly without afiecting said adjustment, and locking means comprising a plurality of bolts secured to said plate assembly and passing through said filament holder plate with cooperating nuts thereon whereby said filament may be firmly secured in position as determined by the setting of said adjusting screws.

8. In an ion source mechanism the combination comprising an ion source proper including a plate assembly adapted to expel ions from said ion source, and ion producing means removably spaced therefrom, said ion source proper having a male socket connection at one end thereof, an ion source box adapted to contain said ion source proper, said box enclosing a female socket connection slidably engaging the male socket connection on said ion source proper, an adjusting nut threadably engaging said female socket connection and bearing on said ion source proper thereby determining the position of said ion source proper within said box, and a locking nut threadably engaging said adjusting nut and bearing on said ion source proper whereby said ion source proper is secured in position as determined. by said adjusting nut.

,9. In an ion source mechanism the combination comprising a plate assembly adapted to expel ions from the ion source, ion producing means including a filament, adjusting means connecting said plate assembly and said ion producing means, said adjusting means defining the position of said filament with respect to said plate asesmbly and having a quick disconnect attachment whereby said filament may be removed from said plate assembly without affecting the setting of said adjusting means, a vacuum tight ion source box enclosing said plate assembly and ion producing means and having vacuum connections thereto, said box having an opening in one wall thereof and a hollow socket connection element disposed about said opening, saidplate assembly comprising a socket connection element at the end opposite the ion producing means connection, said socket elements mating together to properly position said plate assembly in said ion source box, an adjusting nut threadably engaging said ion source box socket element and bearing upon said plate assembly, and a locking nut threadably engaging said adjusting nut and bearing upon said plate assembly whereby said plate assembly is secured in the position determined by said adjusting nut.

10. An ion source mechanism including the combination comprising three spaced plates disposed in parallel relationship with insulators therebetween, each of said plates having a slotted aperture in the center thereof in alignment, the first of said plates having a filament disposed in the slot therein and spaced from the edges of the slot, current supply means connected across said filament whereby material disposed thereon is ionized, potential supply means impressing a large negative potential upon said center plate with respect to said first plate and filament whereby ions formed at said filament are attracted therefrom in a stream through the aperture in said center plate, said potential supply means also impressing a negative potential upon said third plate whereby said stream of ions passing through the slot in said third plate is focused into a beam of small cross section.

11. An ion source mechanism as defined in claim 10 further characterized by a protection plate interposed between said first and center plates in parallel relation thereto and adjacent said center plate, means impressing substantially 10 the same potential on said protection plate as is impressed on said center plate whereby electrical current leakage between said first and center plates is prevented, and current measuring means connected between said first plate and said center plate in series with the power supply means therefor and thereby, measuring the current flow therebetween as an indication of the effi ciency of said ion source.

FREDERICK L. REYNOLDS.

No references cited.

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