US2838677A

US2838677A - Mass spectormeter

Info

Publication number: US2838677A
Application number: US546221A
Authority: US
Inventors: John G Skinner
Original assignee: Phillips Petroleum Co
Current assignee: Phillips Petroleum Co
Priority date: 1955-11-10
Filing date: 1955-11-10
Publication date: 1958-06-10
Anticipated expiration: 1975-06-10

Description

June 10, 1958 J. G. sKlNNER MASS sPEcTRoMETER 2 sheds-sheet 1 Filed NOV. lO, 1955 INVENTOR.

J. G. SKINNER HU@ f (fw/a/ ATTORNEYS United States Patent MASS SPECTROMETER John G. Skinner, Corvallis, Greg., assigner to Phillips Petroleum Company, a corporation of Delaware l Application November 10, 1955, Serial No. 546,221

' 8 Claims. (Cl. Z50-41.9)

This invention relates to sample inlet and evacuation systems for mass spectrometers. In another aspect it relates to electrode mounting means for vacuum tubes.

In recent years mass` spectrometers have been developed from highly specialized academic research instruments for measuring the relative abundance of isotopes into analytical tools of eXtreme sensitivity and accuracy. At the present time, applications are being found for the use ofimass spectrometers in process monitoring and control. Mass spectrometry comprises, in general, ionizing a sample of material under investigation and separating theresulting' ions accordingr to their masses to determine the relative abundance of ions of selected masses. The material to be analyzed usually is provided as a gas which is bombarded by a stream of electrons to produce thedesired ions. Although both positive and negative ions may be formed by such electrical bombardment, most mass spectrometers make use of only the positive ions.` These positive ions are accelerated out of the region of the electron beam by means of negative potentials. Suchrz'potentials impart equal kinetic energies to ions having like charges such that ions of different` masses have 'different velocities after passing throughl the electrica'llieldl and, consequently, have different momenta.

An important feature of the present inventionresides in :Q1-#combination sample inlet and evacuation system for a mass spectrometer which minimizes thenumlaerof` glassto-met'al seals in the tube. This is important to reduce leak' possibilities. The vacuum pump inlet is` connected by a metal tube to the glass spectrometer tube. sample inlet conduit lextends into the tube -through the evacuation outlet'. Another feature of thisA invention resides 'inV an electrode mounting structure which eliminate's microphonics. The collector plate is secured'l to a shield plate so` as to engage a plurality of spring biased spacer balls. This provides a flexible assembly which can accommodate temperature fluctuations.

Accordingly, it is an object of thisrinven'tion to provide an improved sample inlet and evacuation system for a mass spectrometer. 4 y

A further object is to lprovide improved electrode mounting meansfor use in vacuum tubes.

Other objects, advantages'and features 'of this inven- V tion should become apparent from the following detailed description taken in conjunctionrwith Vthe accompanying drawing in which:

Figure l is a 'schematic representation-of a mass spectrometer incorporating features of this invention;

Figure 2 is a detailedview of the collector plate mounting assembly; and

Figure 3 is a detailed view of the sample inlet and tube evacuating system.

Referring now to the Vdrawing in detail and to Figure linvparticul'ar, there is shown a mass spectrometer tube comprising a gas impermeable envelope, the interior of which is maintained at a reduced pressure by a vacuum pump, not shown, which communicates with the interior The 'ice

of tube 10 through a'conduit 11. A sample of gas to be analyzedA is directed into tube 10 through a conduit 12. An electron emitting filament 13 is disposed inr one end of tube 10 and an ion vcollector plate 14 is disposed in the secondend of the tube. The end terminals of filament i3 are connected to the respective end terminals of the secondary winding 15 of a transformer 16. The'end terminals of the primary winding 17 of transformer 16 are connected to an alternating current source 13. The center tap of the secondary Winding 15 of transformer i6 is connected to a negative potential terminal 19.

The gas sample supplied through conduit 12 diffuses into an ionization chamber 22 in tube 10 which is defined by a pair of spaced gridsfZS and 24 that are maintained at ground potential. Electrons emitted from filament 13 are accelerated into chamber 22 by the potential. difference between filament i3 and grids 23 and 24, A grid 26 is-positionedbetween filament 13 and grid 23. Grid 26 is connected to the output of an emission regulator circuit 27, while can be of the type disclosed in the copendingapplication of `M. C. Burk, Serial No. 412,790, filed February 26, 1954, now U .S. Patent No. 2,792,500. The input of emission regulatorV 27 is connected to the center tap of transformer Winding '15. This emission regulator is provided for the purpose of applying a potential to grid26-of magnitude such as to maintain a constant flow of electrons into ionization chamber 22 irrespective of minor fluctuations in the electron emission from filament 13. In thismanner the rate at which gas molecules are ionized in chamber 22 byA electron bombardment is afunction of only the gas pressure in the chamber.

Y'A first collimating electrode 28' is positioned on the second side of ionization chamber 22 and is connected to the contact'or of a potentiometerZil. One end terminal of potentiometer 2t) is connected to aA negative potential terminal 2l, the second end terminal of potentiometer 2li being" grounded. A secondcollimating electrode 23" is spaced from electrode 28. Electrode 28 is connected to the'conta'ctor of a'potentiometer 2li. The end' terminals of potentiometer 2li are connected to terminal' 21 and ground, respectively.

The positive ions produced in ionization chamber 22` are accelerated by the negative potentials'applied to electrodes 28and` 28 so as to travel through the'tube toward collector plate 14. A first set of three equally

spaced grids

35, 36 and 37 is positioned in tube l@ between grid 28 and collector plate i4; a second set of equally

Vspaced gridsy

38, 39 and 4l? is positioned in spaced` relation with the first set of grids; a third set of equally spaced

grids

42, 43 and 44 is positioned in spaced relation with the secondset of grids; a fourth set of equally spacedgrids 4S, 46 and-47 is positioned in spaced'rela` tion with the third set of grids; and a fifth set of equally spaced grids49, Sil and 5l is positioned in .spaced relation with the fourth setV of, grids. Gridy 35" is connected tothe contacter of a potentiometer 29. One end terminal of potentiometer 29 is connected to a negative potential terminal 3o, the second end terminal of potentiometer 29 being connected to ground. The contactor of potentiometer 29 can bey adjusted to apply selected. negative accelerating potentials to grid 3S.

Grids

36, 39, 43, 46 and 50 are connected to one another and to the output terminal of an electronic switch 55. The input terminal of switch 5S is connected to one output terminal of ay radio frequencyoscillator S6. 'Switch 55 is controlled by the output of a square wave generator 57, which in turn is energized by the output of an audio oscillator 5S. Switch 55 is thus turned on and off at the frequency of oscillator Sato apply the outputV of` oscillator 56 to grids 36, V39,. 43, 46 andvSti. Grid35 is connected to one output terminal of a step back detector 61, which in turn is connected to an output terminal of oscillator 56. The second output terminal of detector 61 is connected to grid 51. Grids 35 and 51 are connected to one another by a voltage dividing network 63 which comprises five

resistors

64, 65, 66, 67 and 68 that are connected in series relation.

Grids

37 and 38 are connected to one another and to the junction between

resistors

64 and 65; grids 40 and 42 are connected to one another and to the junction between

resistors

65 and 66;

grids

44 and 45 are connected to one another and to the junction between

resistors

66 and 67; and grids 47 and 49 are connected to one another and to the junction between

resistors

67 and 68.

A plurality of closely spaced stopping grids 70 is positioned between grid 51 and collector plate 14. Grids 70 are connected to one another and to the output of a stopping detector 71, which in turn is connected to an output of oscillator 56. A plurality of suppressor grids 72 is positioned between grids 79 and collector plate 14. Grids 72 are connected to a negative potential terminal 73 to suppress secondary electrons which may result from ions impinging upon metal parts of the tube. A grounded shield 75 is positioned adjacent collector plat 14.

Collector plate 14 is connected to one inputV terminal of an amplifier 76 which is tuned to pass signals of the same frequency as the frequency of oscillator 58. The second input terminal of amplifier 76 is connected to ground. The output terminals of amplifier 76 are connected to first input terminals of a phase detector 77. The second input terminals of phase detector 77 are connected to output terminals of oscillator 58. The output terminals of phase detector 77 are connected to a suitable indicating means such as a recorder 78.

The circuits illustrated schematically in Figure 1 are described in detail in the copending application'of M. C. Burk and F. W. Karasek, Serial No. 480,698, filed January 10, 1955, now U. S. Patent No. 2,761,974. i

Details of the collector plate assembly are illustrated in Figure 2. Plate 14 is attached by a hub 110 and a screw 111 to a support rod 112. Screen plate 75 has an elongated tube 113 integral therewith which fits on a tube 114 that is sealed to spectrometer tube end 104 at its outer end. Rod 112 is sealed in tube 114 by a plug 115. Plate 75 has a plurality of openings 117 therein. An annular spring 118 is welded toplate 75 so as to extend through openings 117. The portions of spring'118 which extend through the openings in plate 75 form seats to receive insulating balls 120. Balls 120 engage plate 14. Tubes 113 and 114 are held together by a clamp 121. 'In constructing the mass spectrometer tube, tube 114 is sealed into glass member 104. Rod 112 is sealed in plug 115. Tubes 113 and 114 are then pushed together so that pate 14 engages balls 120 and are secured in place by clamp 121.' It has been found that this particular structure is durable and eliminates mcrophonics. Glass end 104 is sealed to the remainder of the tube after the grid structure is inserted.

The sample inlet tube evacuation system is illustrated in Figure 2. A metal sleeve 125 is sealed to a glass nipple 126 of tube 10. There are only two glass-tometal seals in the system. This minimizes the danger of leaks. A third seal is required if the filament is removable, as illustrated. A rst flange 127 is welded to sleeve 125. A second fiange 128 is welded to an outlet pipe 11 which communicates with a vacuum pump, not shown. An annular plate 130 is positioned between

anges

127 and 128 and is secured in place by a plurality of bolts 131.

Gaskets

132 and 133 are employed to make a tight seal. Gas sample inlet tube 12 is welded to and extends through plate 130. The inner end of tube 12 extends into yspectrometer tube 10. The gas to be analyzed diffuses from tube 12 into the ionization chamber.

"From the foregoing description of a preferred embodiment of this invention, it should be apparent that there is provided a mass spectrometer tube having several improvements. The gas sample is introduced and the tube is evacuated through a single opening. This reduces the danger of leaks developing. The collector plate is positioned by a novel mounting structure which remains firm despite temperature changes in the tube and which eliminates microphonics.

While the invention has been described in conjunction with a present preferred embodiment, it should be evident that it is not limited thereto.

What is claimed is:

l. A mass spectrometer comprising a glass envelope having an opening therein, a first metal tube sealed to said envelope to communicate with said opening, a first flange on the end of said first tube remote from said envelope, a second tube having a flange at one end thereof, the second end of said second tube Vbeing adapted to be connected to a vacuum pump, a plate having an opening therein, means to seal said plate between said anges, a third tube extending from a region exterior of said envelope through said plate and said first tube to the interior of said envelope, said second tube being sealed in said plate and being adapted to introduce a sample of gas to be analyzed into said envelope, means toV ionize the gas sample, a collector plate in said envelope, and means to direct ions of predetermined mass to said collector plate.

2. A charged particle collector assembly comprising a support plate having a plurality of openings therein, a spring attached to said plate and extending through said openings, an ion collector plate, a spacer positioned adjacent each of said openings in engagement with said spring, and means securing said collector plate to said support plate so that said collector plate engages said spacers against the force of said spring.

3. Apparatus to introduce a gas sample into and to evacuate a mass spectrometer tube comprising conduit means sealed to the spectrometer tube to communicate at one end with the interior thereof, the second end of said conduit means being adapted to be connected to a vacuum pump, and a tube to supply a gas sample extending from a region exterior of said spectrometer tube to the interior thereof through at least a portion of said conduit means.

4. A charged particle collector assembly comprising a first tube, a rod extending through said first tube, sealing means mounting said rod in said first tube in spaced relationship therewith, a collector plate attached to one end of saidv rod, a second tube enclosing at least a portion of said first tube, a second plate secured to the end of Vsaid second tube adjacent said collector plate, said second plate having a plurality of openings therein, a spring attached to said second plate and extending through said openings toward said collector plate, a spacer positioned adjacent each of said openings in engagement with said spring, and means to secure said second tube to said first tube so that said collector plate engages said spacers against the force of said spring.

5. The combination in accordance with claim 4 wherein each of said spacers comprises a sphere of insulating material.

6.V A mass spectrometer comprising a gas impermeable envelope defining a spectrometer tube, conduit means sealed to the spectrometer tube to communicate at one end with the interior thereofthe second end of said conduit means being adapted'to be connected to a vacuum pump, a tube to supply a gas sample extending from a region exterior of said spectrometer tube to the interior thereof through at least a portion of said conduit means, means to ionize molecules of gas supplied to said spectrometer tube, a support plate in said spectrometer tube, having a plurality of openings therein,

' a spring attached to said plate and extending through said openings, an ion collector plate, a spacer positioned adjacent each of said openings in engagement with said spring, means securing said collector plate to said support plate so that said collecter plate engages said spacers against the force of said spring means to direct ions of predetermined mass to said collector plate, and means to measure ions impinging upon said collector plate.

7. A mass spectrometer comprising a glass envelope having an opening therein, a first metal tube sealed to said envelope to communicate with said opening, means to attach ,a vacuum pump to said iirst tube, a second tube extending through at least a portion of said rst tube into said envelope through said opening to introduce a sample of gas to be analyzed, means to ionize molecules of gas supplied to said spectrometer tube, a support plate in said spectrometer tube having a plurality of openings therein, a spring attached to said plate and extending through said openings, an ion collector plate, a spacer positioned adjacent each of said openings in engagement with said spring, means securing said collector plate to said support plate so that said collector plate engages said spacers against the force of said spring, means to direct ions of predetermined mass to said co1- lector plate, and means to measure ions impinging upon said collector plate.

8. A mass spectrometer comprising a glass envelope having an opening therein, a rst metal tube sealed to said envelope to communicate with said opening, a rst flange on the end of said iirst tube remote from said envelope, a second tube having a second flange at one end thereof, the second end of said second tube being 30 2,768,301

6 adapted to be connected to a vacuum pump, a plate having an opening therein, means to seal said plate between said flanges, a third tube extending from a region exterior of said envelope through said plate and said rst tube to the interior of said envelope, said second tube being sealed in said plate and being adapted to introducea sample of gas to be analyzed into said envelope, a third tube secured to said envelope and extending into said envelope, a rod extending through said third tube, sealing means mounting said rod in said third -tube in spaced relationship therewith, a collector plate attached to the inner end of said rod, a fourth tube enclosing at least a portion of said third tube, a second plate secured to the end of said fourth tube adjacent said collector plate, said second plate having a plurality of openings therein, a spring attached to said second plate and extending through said openingstoward said collector plate, a spacer positioned adjacent each of said openings in engagement with said spring, means to secure said fourth tube to said third tube so that said collector plate engages said spacers against the force of References Cited in the tile of this patent UNITED STATES PATENTS Bennett Oct. 28, 1956