US2703843A - Mass spectrometry - Google Patents

Description

March 8, 1955 A. E. CAMERON MASS s am! 2 ShOOl-Shit! l F1106. April 14. 1954 INVENTOR. Anya: 6'. Cameron Mplif/erficcardcr ATTORNEY March 8, 1955 A. E. CAMERON 2,703,843

BASS SPECI'ROIETRY P1106 April 14. 1954 2 Shuts-Shoot 2 IN V EN TOR. Angus 6'. Cameron w drmm ATTOPNEY United States Patent Oihce Patented Mar. 8, 1955 MASS SPECTROMETRY Angus E. Cameron, Oak Ridge, Tenn asignor to the United States of America as represented by the United States Atomic Energy Commission Application April 14, 1954, Serial No. 423,263

6 Claims. (Cl. 250-413) The present invention relates to mass spectrometry, and more especially to an improved ion source for analyzing solid samples in a mass spectrometer.

Mass spectrometers of the prior art have generally been used in analysis of gaseous materials. The gas sample is ionized by electron bombardment and the resulting ions are accelerated through the successive apertures of an ion gun into an analyzer region. An electromagnetic field in the analyzer causes the ion beams to deflect by amounts depending upon the mass of the ions, so that at the end of the analyzer region the ions are sorted according to mass into distinct beams. By varying the energy of the ion gun or the strength of the deflecting field, ions of any given mass may be focused upon a collector electrode disposed near the exit of the analyzer.

Certain elements such as copper, rubidium, and potassium do not form volatile compounds which are readily prepared for mass spectrometer analysis. Such elements may be coated upon a filament and ions produced by thermal emission if the ionization potentials of the element lie below the work function of the filament surface. Such emission is difficult to stabilize for precision measurements, and an evaporation correction may be required to obtain an absolute ratio in isotope ratio analysis. A better arrangement for many purposes is the sublimation of a solid sample and ionization of the vapor by electron impact. However, this type source is attended with a serious disadvantage. When a second sample of different isotopic composition from the first is to be analyzed, the ion sources of the prior art must be removed and thoroughly cleaned to eliminate the memory effects resulting from deposition of particles of the first sample on the walls of the ion chamber. Moreover, changing the source geometry introduces some change in the mass discrimination effects and should be avoided where intercomparison of samples is to be made, or where altered samples are compared to a standard sample.

With the knowledge of the disadvantages of the ion sources in spectrometers of the prior art, I have as a primary object of my invention elimination of the memory elfect in analysis of samples of different isotopic composition. A further object of my invention is to eliminate changes in geometry in the ionization region of a mass spectrometer ion source. Yet another object of my invention is to permit rapid interchange of ion chamber and sample in a mass spectrometer. Another object of my invention is to remove as much as ible of the surface which has been in contact with the rst sample from the instrument before analysis of the second sample.

Other objects and improvements of my invention will become apparent from the following detailed description of a preferred embodiment thereof, when read in conjunction with the appended drawings, wherein Figures 1 illustrates schematically a mass spectrometer;

Figure 2 represents an exploded view of an ion source cogstructed according to the principles of my invention: an

Figure 3 represents a detailed sectional view of the ionization chamber-sample holder shown in Figure 2.

In accordance with my invention, 1 have provided a novel ion source for a mass spectrometer wherein a single removable, slotted cylindrical member serves as sample holder, volatilization chamber, and ionization chamber. The chamber is removably mounted upon a housing, in exact alignment with a filament, may be surrounded by a concentric heater for volatilizing the sample, and is readily removed for substitution of a second sample and chamber after each analysis.

Referring now to Figure l, a typical mass spectrometer may comprise a source chamber 1 in which is disposed the ion source, generally indicated by the numeral 2, which may be mounted on one end of the analyzer tube 4. Magnet 5 provides the field necessary to bend the ion beam and to separate the ions into discrete beams according to mass. Collector assembly 6 is disposed at the exit end of the analyzer 4, adjacent the exit slit 7. Vacuum conditions are maintained in the system by means of the fore vacuum pump 8, diffusion pump 9, and cold trap 10, connected in the conventional manner to the source chamber 1. Metal washer 11 may fit between glass flange 12 and metal end-cap 13, provided with O- ring 14, to form a vacuum-tight seal for the source chamber 1. Valve 15 may vent to the atmosphere. A vacuum lock, not shown may be provided at the upper end of the source chamber for insertion and removal of the novel sample container, which may, for example, be carried on a handle mounted to extend through the vacuum lock and into the source. Amplifying and recording apparatus 16 may be connected to the collector assembly to record the magnitude of the ion current collected during operation of the instrument in the conventional manner.

Referring now to Figure 2, ion-collimating plate 21 forms the first accelerating plate of the spectrometer ion gun, and may be provided with four apertures 33 for passage of support rods to mount the accelerating plates of the ion gun. A central aperture 22 is provided to allow exit of the ion beam into the ion gun. The plate also carries a pair of spaced apart, channeled brackets 25 adapted to receive the filament assembly 23 and to maintain the filament in direct alignment with the apertures 26 in the sample container 28. A standard electron trap 24 is provided on the opposite side of holder 27 from the brackets 25, the metal strip 34 being disposed in alignment with the filament and apertures 26, and being conductively coupled through leads 35 to an external source of heating current.

The upstanding tubular holder 27 is mounted in axial alignment over the aperture 22 and is provided with a pair of opposite, downwardly extending slots 30. Sample container 28 is a hollow tubular member provided with opposing slots 26 near the lower end and pierced with a pin 29. A tapered wedge 31 forms a cap for engaging the correspondingly tapered throat at the top of the container 28. The extremities of pin 29 protruding from the container 28 are adapted to slide in the slots 30 so as to position the container properly in place vertically and to keep it from rotating. when the filament assembly is held in place by the brackets 25 and the container 28 is inserted in the holder 27, the filament 45, slots 26, and trap 34 are in direct alignment, and the lower end of the container extends into the outlet aperture 22. Heater assembly 32 may comprise a metallic hollow, slightly concave spool about which is disposed a heater wire. The wire may be attached to a suitable source of heater current, not shown.

Figure 3 shows in greater detail the container 28 in cross section. The container comprises a body wall portion 40, taper reamed at the upper end to receive a tapered plug 31. A pin 29 pierces the walls 40 above the filament slots and is of such diameter that a glass capsule 42 inserted in the container will not drop past the pin. The capsule contains the solid sample 43 to be volatilized. oppositely disposed slots 26 are provided near the base of the container to allow electrons from the filament to pass through the chamber to the trap, while the lower extremity 44 of the container fits into the aperture 22. Effectively two chambers are formed within the container: the first chamber contains the capsule and sample, while the second chamber forms an ionization chamber through which electrons pass, and communicates directly with the ion gun.

in a preferred construction of my ion source, the top plate 21 may be non-magnetic stainless steel. The holder 27 may be made by copper brazing a non-magnetic stainless steel rod, ii; inch outer diameter, onto the plate 21, then boring axially and reaming the rod to 0.251 inch inrig-taste ner diameter. Ports opposite the filament mount and trap mount and the slots for receiving the pin 29 are provided by drilling. The container 28 may be non-magnetic stainless steel tubing, 0.250 inch in diameter, drilled to receive the pin 29, which may be 0.057 inch Nichrome V wire. The center of the pin may be 0.562 inch from the lower end of the container, which is 1.25 inches in length. The opposing slots may be 0.40 inch in width, extending inwardly 0.25 inch on either side, the base of the slot being 0.125 inch from the lower end of the container. The upper end of the container is taper reamed to receive :the tapered aluminum or stainless steel plug which may be driven in manually and cut to length. The slots in the holder 27 should be cut to approximate depth and then hand filed until with the container 28 in place, the electron slot 26 is inch above the surface of the plate 21. Rotation of the container through 180 should not alter the position of the slot. The bottom of the chamber should protrude slightly below the plate 21 as shown.

Various arrangements of the filament and electron trap are possible. In a preferred embodiment, a tungsten ribbon 45, .01 inch wide and .001 inch thick, is welded to two Kovar leads 46 from the glass press 47 which itself is sealed to the Kovar support 48. The support in turn is fastened with small screws to the shield 49 which is pierced with a rectangular slot 50 in rfront of she filament. Alignment of the ribbon filament relative to the slot is made as the mounting screws are tightened. The filament shield 49 slips into the slots in the brackets 25, and can be removed for replacement without disturbing the ion source. The trap 34 is a ribbon of thin plari-mrm or tungsten provided with two leads so that a current may be passed through it to heat it to incandescence, thus producing a cleaner surface. It is not normally operated while heated.

The heater 32 may conmnise a tantalum sheet formed into a spool 51 which slides down over the holder 27 andisstoppedbyapairofearsflatdieoogbentinas illustrated. On this spool is wrapped a layer of mica and a non-inductive heater winding 53 of the ten mil platinum wire. Another layer of mica is applied and a bandotthintantalumsheetiswrappedaroundamd fastened with a small screw. This furnace should be initially outgassed to remove water from the mica.

In operation of the source. samples of hydroscopic salts are conveniently prepared in glass tubes 5 mm. in diameter, the tubes are evacuated and sealed off. Before analysis, the bottom part of the tube is cut oil and dropped into the container 28. Alternatively, a sample may be deposited from solution in a scrap of inert porous material such as alundum, dried, and placed in the chamber. The sample size need not exceed five milligrams, and much less may frequently be used. The furnace 62 is energized with alternating current, preferably from the live vol-t secondary of the filament transformer having an autotransformer connected in the primary to allow temperature control. Temperatures up to 550 C. are readily attained in this manner. The filament and trap are energized from their respective sources, the trap being substantially 150 volts more positive than the filament in one operating condition successfully used. When the heater is energized, vapors 'from the sample flow out of the glass capsule and into the lower end of the container 28, where they are ionized by electrons from the filament passing through the slits 26. The positive ions formed by ionization of the vapors are drawn by the accelerating field established on electrode 17 outward through aperture 22 into the ion gun and accelerated into the spectrometer analyzer and to the collector. Electrode 17 may be maintained 300 volts negative with respect to the plate 21, for example.

When a second sample of different isotopic composition is to be run after the spectrometer has been used for first analysis, valve 15 may be opened to the atmosphere, breaking the vacuum seal and releasing O-ring I14 from metal ring 11. Then 28 may be removed by means of simple tongs extending into the source, and another container having sealed therein a capsule con taining the second sample may be immediately inserted into position in the holder 27. It has been repeatedly shown that less than one hour is required to replace a container and to bring the spgftrorr'igter system bachl:I to operatmg vacuum pressure. 'nce e ionization c mberand'thelamplerelaorthavebeenremovedfromthe system, virtually no memory effects are discernible with the new container in place.

It will be apparent to those skilled in the art that l have provided a new and novel ion source for mass spectrometer, simple in construction and easy to main rain. which includes therein a novel element serving as a sample holder for introducing a solid sample into the spectrometer, as a retort for volatilization of that sample, and as an ionization chamber in which vapors may be ionized by electron bombardment. The container is readily removable for complete replacement to avoid un desirable memory effects of the source, and the heater and filament are also quickly and easily replaceable in place in the spectrometer. While the preferred configuration of the container is cylindrical or tubular, it will be readily understood by those skilled in the art that rectangular, oval, or other known geometrical configurations may be utilized as well, the tubular chamber having been shown merely for purposes of illustration. Many advantages in case of operation and maintenance increased accuracy of the results accrue from the novel source which I have invented, the scope of which is not intended to be limited by the illustration herein provided, but only by the scope of the appended claims.

Having described my invention I claim as novel:

I. In a mass spectrometer ion source having a filament for emitting electrons, an electron trap, means for accelerating electrons toward said trap, and an ion gun assembly for focusing a beam of ions, the improvement comprising a tubular electrode communicating at one end with said ion gun and provided with removable closure member at the opposite end, means for supporting a sample of material within said tubular electrode, a sup port member mounted upon an electrode of said ion gun and adapted to detachably engage said tubular electrode, said electrode being provided with a pair of opposing slots near one end for passage of electrons between said filament and said trap for defining an electron beam, and a heater assembly removably disposable about said holder and said electrode to vaporize said sample, molecules from said sample vapors flowing through the electron beam to produce ions for said ion gun.

2. In a mass spectrometer ion source having a filament for emitting electrons, an electron trap, means for accclerating electrons toward said trap, and an ion gun assembly for focusing a beam of ions, the improvement comprising a tubular electrode communicating at one end with said ion gun and provided with removable closure member at the opposite end, a pin disposed through said tubular member normal to the axis thereof to support a sample therein and protruding beyond the walls thereof, a support member mounted upon said ion gun and provided with a pair of opposing slits to receive said protruding pin to position said tubular electrode, said electrode being provided with a pair of opposing slots near one end for passage of electrons between said filament and said trap for defining an electron beam, and a heater assembly removably disposable about said holder and said electrode to vaporize said sample, molecules from said sample vapors flowing through the electron beam to produce ions for said ion gun.

3. In a mass spectrometer ion source having a filament for emitting electrons, an electron trap, and an ion gun assembly for focusin a beam of ions, the improvement comprising a centralfy apertured plate forming the first electrode of said gun; an upstanding tubular holder provided with legs mounted on said plate and aligned with said aperture; a pair of upstanding channeled support brackets mounted on said plate; a shield member adapted to slidably engage said channeled brackets and provided with an aperture near one end; a filament holder adapted to support said filament in directive relationship with said shield aperture and trap; a tubular member provided with opposing slots near one end, a closure member at the other end, and a sample support and detachably disposed within said tubular support; and a tubular heater detachably disposed about said holder for vaporizing the sample in said tubular member.

4. in an ion source for a mass spectrometer, a tubular member provided with a pair of opposing slots near the open end thereof, a pin therethrough normal to the tube axis and protruding beyond the tube walls. and a closure member at the opposite end; a fixed tubular support of larger diameter than said member provided with slots to reccive opposite protruding ends of said pin; and a tubular heater structure of greater diameter than said support to fit concentrically therearound.

5. In a mass spectrometer ion source, an apertured accelerating electrode, a sleeve member mounted on said electrode, and an ionization chamber supported by said sleeve member and having one end provided with a passageway communicating with said electrode aperture, said end having a slot transverse to the passageway for passage of an electron beam. the other end of said chamber being adapted to receive and support a sample to be analyzed.

6. in a mass spectrometer ion source provided with a plurality of accelerating electrodes, a support sleeve mounted on a first electrode, a tubular container forming an ionization chamber removably supported within said sleeve, said container being rovided at one end with an ion exit passageway and with a slot transverse to said passageway, a filament and a trap aligned with said slot to establish an electron beam therethrough, means for receiving and supporting a sample to be analyzed within said chamber, means for vaporizing said sample in said chamber, means for moving the vapors through said electron beam to ionine the vapor molecules, an means for accelerating positive ions to remove them from said chamber, said support sleeve being cut away in the region adjacent said filament and trap to avoid contamination by vapors escaping through said slot.

No references cited.

Claims (1)

1. IN A MASS SPECTROMETER ION SOURCE HAVING A FILAMENT FOR EMITTING ELECTRONS, AN ELECTRON TRAP, MEANS FOR ACCELERATING ELECTRONS TOWARD SAID TRAP, AND AN ION GUN ASSEMBLY FOR FOCUSING A BEAM OF IONS, THE IMPROVEMENT COMPRISING A TUBULAR ELECTRODE COMMUNICATING AT ONE END WITH SAID ION GUN AND PROVIDED WITH REMOVABLE CLOSURE MEMBER AT THE OPPOSITE END, MEANS FOR SUPPORTING A SAMPLE OF MATERIAL WITHIN SAID TUBULAR ELECTRODE, A SUPPORT MEMBER MOUNTED UPON AN ELECTRODE OF SAID ION GUN AND ADAPTED TO DETACHABLY ENGAGE SAID TUBULAR ELECTRODE, SAID ELECTRODE BEING PROVIDED WITH A PAIR OF OPPOSING SLOTS NEAR ONE END FOR PASSAGE OF ELECTRONS BETWEEN SAID FILAMENT AND SAID TRAP FOR DEFINING AN ELECTRON BEAM, AND A AND SAID ELECTRODE TO VAPORIZE SAID SAMPLE, MOLECULES FROM SAID SAMPLE VAPORS FLOWING THROUGH THE ELECTRON BEAM TO PRODUCE IONS FOR SAID ION GUN.

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