US2894136A - Ion source - Google Patents

Description

July 7, 1959 M. E. REINE-CKE 10N souRcE:

Filed oct. 7. 1954 annum INVEN TOR.

M. E. REINECKE :Inl-lll- A T TOR/v5 V3 lION SOURCE Marvin E. Reinecke, Bartlesville, Okla., assignor to Phillips Petroleum Company, a `corporation of Delaware Application "October 7, 1954, Serial No. 460,946

11 Claims. (Cl. Z50-413) This invention relates to the formation of ions by the bombardment of a gas sample with a stream of electrons. In one specific aspect it relates to an improved ion source for use in a mass spectrometer.

Mass spectrometry comprises, in general, ionizing a sample of material under investigation and separating the resulting ions according to their individual 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 the desired ions. The gas molecules may be ionized by the removal of electrons, by breaking up the molecules, or both. Although both positive and negative ions may be formed by such electron 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 which impart equal kinetic energies to ions having like charges such that ions of different masses have diiferent velocities after passing through the accelerating electrical field. The presently known mass spectrometers can be classied in one of two general groups: the momentum and the velocity selection types.` Themomentum selection instruments sort the ions into beams of different masses by a magnetic and/or electrical deiiecting eld. Ions of a selected mass are allowed to impinge upon a collector plate, to which is connected a suitable indicating meter. The velocity selection instruments sort the ions according to their velocities, and are referred to generally as time-of-ight mass spectrometers.

Inboth types of mass spectrometers it is common practice to produce the Iions by bombardment of gas molecules with a stream of electrons. The electrons are emitted from a heated cathode or filament and are accelerated by an` electrical iield into an ionization chamber which contains the gas molecules. It is of course important to maintain the electron beam of constant intensity because variations in the electron emission tend to introduce corresponding variations in the rate of ionization. Since the ion source is maintained at a relative low pressure, the probability of an electron striking a gas :molecule is decreased in proportion to the reduction of gas pressure. Thus, in order to produce the desired degree of ionization, a relatively large number of electrons must be provided or elsethe electrons must move through a relatively long path in the ionization chamber. Furthermore, it is desired that the ions be formed at substantially the same energy level inl order to obtain maximum resolution from the spectrometer.

In accordance with the present invention there is provided an ion source'which fuliills the above requirements. This source comprises an electron emitting lilament which is disposed along the axis of the spectrometer tube. The lament is surrounded by' a cylindrical grid which can be maintained at a negative potential of proper magnitude to ensure a constant ilow of electrons into the ionization chamber; The ionization chamber itself comprises an latent annular space surrounding the grid. A second cylindrical grid is mounted concentric with and outside the rst grid. This second grid is maintained at a negative potential to repel electrons back through the ionization chamber to increase the probability of a collision with the gas molecules. The gas molecules to be ionized are introduced into the ionization chamber from a housing having an annular opening therein. A plate is provided between the ilament and the remainder of the mass spectrometer tube to shield the lament from the ion separating means of the tube.

Accordingly, it is an object of this invention to provide an ion source capable of supplying a stream of electrons of constant intensity to a region containing the material to be analyzed.

Another object is to provide an ion source wherein an electron beam traverses the ionization chamber a plurality of times.

A further object is to provide an ion source wherein the ions are formed at substantially constant energy levels.

A still further object is to provide an ion source for a mass spectrometer wherein the electron emitting lament is shielded from the mass separating means of the spectrometer.

Other objects, advantages and features of this invention should become apparentfrom the following detailed description taken in conjunction with the accompanying drawing in which:

Figure l is a schematic view of a mass spectrometer having the ion source of this invention incorporated therein;

Figure 2 is a sectional View taken along line 2--2 in Figure 1;

Figure 3 is a sectional view taken along line 3--3 in Figure 1; e

Figure 4 is a sectional view taken along line 4-4 in Figure 1;

Figure 5 is a sectional view taken `along line 5-5 in Figure 1; and

Figure 6 is a schematic circuit diagram of a suitable electron emission regulator for use with the ion source of Figure 1.

Referring now to the drawing in detail and to Figures l, 2, 3, 4 and 5 in particular, there is shown a velocity selection type mass spectrometer tube lll. The interior of tube 10 is maintained at a reduced pressure by a vacuum pump, not shown, which is connected to tube lil by an outlet conduit 11. The gas sample to be analyzed is introduced through an inlet conduit 13 into an annular region 14 which is dened by a housing 15 having a front wail lo and a rear wall 17. The gas passes from housing 15 into an annular ionization chamber 17 through an annular opening 18 in wall 16. An annular plate Ztl, having a screen 21 across the opening thereof, is mounted in tube 10 in spaced relation with wall 16 of housing 15. Concentric inner and outer cylindrical screens 22 and 23 extend between Wall 16 and the assembly of plate 20 and screen 21 to define ionization chamber 17. Spaced annular plates 25 and' 26 are mounted in tube 10 between and electrically insulated from wall 16 and plate 20. A cylindrical screen- 27 extends between plates 25 and 26 in spaced relation With screen 23. An electron emitting tilament 30 is positioned within cylindrical screen 22 by means of supports 31 and 32 which connect the end ter minals of the filament to respective terminals 33 and 34 exterior of tube 10. A cylindrical screen 35 is positioned by a support 36 within screen 22 to enclose filament 30. Support 36 connects screen to a terminal 37 exterior of tube 10. An annular plate 40 is mounted in tube 10 in spaced relation with plate 20, and an annular plate 41 is mounted in tube 1'0 in spaced relation with plate 4G.

A cup shaped plate 42 is supported at the center of plate 40 by an annular screen 43.

Plate 20, screens 22 and 2,3 and housing 15 are electrically connected to one another and to a terminal 45 exterior of tube 10. Screen 27 and plates 25, 26 and. 41 are electrically connected to one another and to a terminal 46 exterior of tube 10.

The gas molecules to be analyzed pass from region 14 into the ionization chamber 17. Screens 22 and 23 are maintained at ground potential and filament 30 is maintained at a negative potential. A source of voltage is applied across terminals 33 and 34 to heat filament 30. The electrons emitted from filament 30 are thus accelerated into ionization chamber 17 by the ground potential applied to screens 22 and 23. Screen 35 is maintained at a negative potential intermediate the potential on lament 30 and screen 22 to control the electron flow into chamber 17. The electrons pass through chamber 17 in 4 The terminals of filament 30 are connected across the secondary winding 70 of a transformer 71, and a source of alternating voltage 72 is connected across the primary winding 73 of transformer 71. The center tap of transformer winding 70 is connected to the contactor of a potentiometer 76 through a resistor 75 and directly to the control grid of a triode 77. One end terminal of potentiometer 76 is connected to the cathode of triode 77, and the second end terminal thereof is connected to the negative terminal of a D.C. voltage source 78. The positive terminal of voltage source 78 is connected to ground. A voltage limiting diode 80 is connected between the control grid of triode 77 and ground. The anode of triode generally radial paths and collide with the gas` molecules, i

thereby forming ions. Screen 27 is maintained at a negative potential to repel the electrons back through chamber 17 toward screen 3S. The negative potential on screen 35, however, tends to repel the electrons back toward screen 27. In this manner the electrons tend to oscillate 'A Y in radial paths in chamber 17 and in so doing increase the probability of collisions with the gas molecules.

The positive ions formed in chamber 17 tend to drift out through screen 21 which is maintained at ground potential by being connected to screens 22 and 23. Screen 43 is maintained at a negative potential which accelerates the positive ions in a direction throughtube away from ionization chamber 17. Plates 40, 41 and 42 are maintained at a negative potential to trap those ions not moving in a direction parallel to the axis of tube 10. The ions emitted from the source thus pass through screen 43 in a direction substantially parallel to the longitudinal axis of tube 10. The grounded members deliningchamber 17 prevent the accumulation of an excess of electrons within the chamber.

While this ion source can be employed in nearly all types of mass spectrometers, ionization gages and ionic vacuum pumps, it is particularly well suited for use in the velocity selection type of mass spectrometer illustrated in Figure 1. A collector plate` 50 is mounted in the second end of tube 10 opposite the ion source. Plate 50 is connected to an output terminal 51 to which is connected a suitable indicating circuit, not shown. A plurality of spaced screens S2, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63 and 64 is disposed between screen 43 and collector plate 50. These screens are mounted within tube 10 by respective annular plates 52a, 53a, 54a, 55a, 56a, 57a, 58a, 59a, 60a, 61a, 62a, 63a and 64a. Plates 52a, 53a, 63a and 64a are connected to respective terminals 52b, 53b, 63b and 64b external of tube 10. Plates 54a, 56a, 57a, 59a, 60a and 62a are connected to one another and to a terminal 54b external of tube 10. Plates 55a, 58a and 61a are connected to one another and to a terminal 55b external of tube 10.

Terminals 52b and 53b are maintained at negative potentials to accelerate the positive ions toward collector plate 50. A radio frequency potential is applied between terminals 54b and 5517 to impart maximum energies to ions of a predetermined mass in the manner described in U.S. Patent 2,535,032. Terminal 63h is maintained at a positive potential of sucient magnitude for screen 63 to repel all ions except those having a maximum predetermined energy. Terminal 64b is maintained at a negative potential to repel secondary electrons resulting from the positive ions of predetermined energy impinging upon collector plate 50.

Grid 35 can be employed to maintain a constant emissionof electrons into ionization chamber 17 in addition to its function of causing electrons to oscillate through the chamber. The control circuit of Figure 6 can be employed in conjunction with screen 35 for this purpose.

77 is connected to ground and the cathode thereof is connected through a resistor 81 to the control grid of a triode 32. The control grid of triode 82 is connected to the negative terminal of potential source 78 through a resistor S4. One end terminal of a potentiometer 85 is connected to ground and the second end terminal thereof is connected to the negative terminal of potential source 78. The contactor of potentiometer 85 is connected to the control grid of a triode 86. The cathodes of triodes 82 and 86 are connected to one another and to the negative terminal of potential source 78 through a resistor 87. The anode of triode S2 is connected directly to screen 35 and to ground through a resistorSS. The anode of triode S6 is connected to ground.

The current ovv through cathode resistor 87 is divided between triodes 82 and 86 in accordance with the poten tials applied to the respective control grids and anodes thereof. The potential applied to screen 35 is a function of the current flow through triode S2 which results in a voltage drop across resistor 88 The potential on the control grid of triode 86 is maintained at a reference value by the setting of the contactor of potentiometer S The potential on the control grid of triode 82 is a function of the potential at the center tap of transformer winding 79, which in turn is a function of the current flow through resistor 75. If the electron emission from filament 30 should tend to increase, the potential applied to the control grid of triode 77 is increased because of the increased current flow through resistor 75. The increased potential applied to the control grid of triode 77 results in increased current flow through this tube and in a corresponding increase .in they potential at the cathode thereof. This increases the potential on the control grid of triode 82 so as to increase the current flow through triode 82 and anode resistor 88. The increased current flow through resistor 88 results in the potential on the anode of triode 82 and the potential on screen 35 being decreased. lf the electron emission from filament 30 should tend to decrease, the abovementioned poten-1 tial changes are reversed such that the potential on screen 35 is increased. The function of voltage limiting tube 8@ is to prevent an excess rise in potential on the control grid of triode 77 in the event of power failure in the lament heater circuit. The circuit of Figure 6 thus tends to maintain av constant ilow of electrons into ionization chamber 17, which results in ioniziation takingplace therein at a rate which is a function of only the number of ions present.

From the foregoing description it should be apparent that there is provided in accordance with this invention an improved ion source which is particularly well adapted to use in a mass spectrometer tube. The electron emission is in a radial path from filament 30 and thus tends to strike the molecule stream entering chamber 17 at right angles. The electrons tend to oscillate Within chamber 17 because of the negative potentials on screens Y27 and 35. This oscillation greatly increases the path of electron ow and thereby provides greater ionization for a given electron emission. The particular arrangement of annular housing 15 results in the gas molecules being introduced directly into the ionization chamber rather than. into the spectrometer tube itself. Plates 40, 41

and" 42 `formiorntraps such that the only ions `which enter the spectrometer tube proper are those traveling in a path substantially parallel to the axis ofthe tube. The ions are formed in a region maintained at ground potential and diffuse outwardly therefrom `into the accelerating region external of chamber 17. All of the ions formed in chamber 17 are thus at substantially the same energy level 'which greatly increases the resolving power of the spectrometer tube. Plate 42 serves an important function in that it shields the filament from the mass separating means of the spectrometer tube. This reduces the undesirable effects of electron space charge in the vicinity of filament 30. It should be noted that screen 21 is not essential to satisfactory operation of the ion source and can be omitted if desired. Although screens are shown in the ion source, other electrically conductive ion permeable walls canbe used for this purpose since the particular screen configuration is not essential.

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

What is claimed is:

l. An ion source comprising first and second concentric cylindrical metal screens defining an annular ionization chamber therebetween, an electron emitter disposed within the smaller diameter one of said screens, means to direct electrons from said emitter radially outwardly through the smaller diameter metal screen into said ionization chamber, means to introduce molecules to be ionized into said chamber, and a third cylindrical metal screen of smaller diameter than the smaller diameter one of said first and second screens, said third screen being concentric with said first and second screens and enclosing said electron emitter.

2. An ion source comprising an electron emitting filament, a first cylindrical screen enclosing said filament, a second cylindricalscreen concentric with'and enclosing said first screen in spaced relation, a `third cylindrical screen concentric with and enclosing said second screen in spaced relation thereby defining an annular ionization chamber between said second and third screens, a cylindrical wall concentric with and enclosing said third screen, means defining a second chamber adjacent one end of said ionization chamber, means to introduce mole cules to be ionized into said second chamber, the wall of said second chamber adjacent said ionization chamber having an annular opening therein, an ion impermeable generally circular plate spaced from the end of said filament on the side of said ionization chamber opposite said second chamber, and an annular screen surrounding said plate, the plane of said annular screen and said plate being substantially perpendicular to the axis of said first cylindrical screen.

3. The combination in accordance with claim 2 further comprising means to supply current to said filament, means to maintain said filament at a negative potential, means to maintain said second and third cylindrical screens at ground potential, means to maintain said first screen at a negative potential, means to maintain said cylindrical wall at a negative potential, and means to maintain said plate and said annular screen at a negative potential.

4. In a mass spectrometer including an elongated envelope, an ion source positioned in one end of said envelope, an ion collector positioned in the second end of said envelope, and means positioned within said envelope to direct ions of a predetermined mass-to-charge ratio from said source to collector; an improved ion source comprising first and second concentric cylindrical elec` tron permeable walls of electrically conductive material defining an annular ionization chamber therebetween, the axis of said chamber being the longitudinal axis of said elongated envelope, an electron emitter positioned along the axis of said chamber, means to direct electrons from said emitter radially outward through the smaller diamd eter wall into said ionization chamber, means' to introduce' molecules to be ionized into said chamber, and an ion impermeabe wall positioned between said electron emitter 'and said ion collector.

5. In amass spectrometer including an elongated envelope, an ion source positioned in one end of said envelope, 'an ion `collector positioned in the second end of said envelope,.and means positioned within said envelope to direct ions of a predetermined mass-to-charge ratio from said source to collector; an improved ion source comprising an electron emitting filament positioned along the `axisof said elongated tube, a first cylindrical electron permeable` wall of electrically conductive material enclosing said filament, a second cylindrical electron permeable wall 'of electrically conductive material concenn tric with and enclosing said first wall in spaced relation therewith, means to apply a potential between said filament and said walls whereby electrons emitted from said filament pass radially through the annular space between said first andsecond walls, means to direct molecules to be ionized into said annular space, a third annular electron permeable wall of electrically conductive material spaced from the end of said annular space toward said collector, the plane of said third wall being substantially perpendicular to said axis, a circular ion impermeable plate in the center of said third wall, the center of said plate being on said axis, and means to apply a potential between said third wall and said first and second walls to direct ions formedin said space in a predetermined path away from said space toward said collector.

6. An ion source comprising first and second concentric cylindrical electron permeable walls of electrically conductive material defining an annular ionization chamber therebetween; an electron emitter disposed within the smaller diameter one of said walls; means to direct electrous from said emitter radially outward through said ionization chamber; and means to introduce molecules to be ionized into said chamber comprising means defining a second chamber adjacent one end of said ionization chamber, the wall of said second chamber adjacent said ionization chamber being provided with an annular opening, and means to introduce molecules to be ionized into said second chamber, whereby the molecules enter said ionization chamber through said opening.

7. An ion source comprising an electron emitting filament, a first cylindrical electron permeable wall of electrically conductive material enclosing said filament, a second cyclindrical electron permeable wall of electrically conductive material concentric with and enclosing said first wall in spaced relation therewith, means to apply a potential between said filament and said walls whereby electrons emitted from said filament pass radially through the annular space between said first and second walls, means to direct molecules to be ionized into said annular space, a third electron permeable wall of electri cally conductive material spaced from one end of said annular space, said third wall being annular, the plane of said third wall being substantially perpendicular to the axis of said first and second walls, a circular ion impermeable plate in the center of said third wall, the center of said plate being on the axis of said first and second walls, and means to apply a potential between said third wall and said first and second walls to direct ions formed in said space in a predetermined path away from said space.

8. An ion source comprising an electron emitting filament, a first cylindrical metal screen enclosing said filament, a second cylindrical metal screen. concentric with and enclosing said first screen in spaced relation therewith, means to apply a potential between said filament and said screens so that electrons emitted from said filament pass radially through said first screen and the annular space between said first and second screens, means to direct molecules to be ionized into said annular space, a third cylindrical metal screen concentric with said first screen and positioned between said filament and saidrst screen, means to apply a negative potential to said third screen of magnitudeintermediate the potentials Ion said filament and said firstv screen, a cylindrical wall of electrically conductive material concentric with and positioned outside said second screen, means applying a negative potential to said wall, a fourth metal screen spaced from oneend of said annular space, and means toapply a potential between said fourth screen and said first Aand second screens to direct ions formed in said spalxceffirl` a predetermined path away from said space.

9. An ion source comprising an electron emitting filament; a first cylindrical electron permeable wall of clecf trically conductive material encolsing said filament; a second cylindrical electron permeable wall of electrically conductive material concentric with and enclosing said first wall in spaced relation therewith; means to apply a potential between said filament and said walls so that electrons emitted from said filament pass radially through the annular space between said first and second walls; means to direct molecules to be ionized into said annular space comprising means defining a chamber adjacent the second end of said space, the Wall of said lchamber adjacent said space being provided with an annular opening, and means to introduce molecules to be ionized into said chamber, whereby the molecules enter said ionization chamber through said opening; a third cylindrical electron permeable wall of electrically conductive material concentric with said first wall and positioned between said filament and said first wall, means to apply a negative potential to said third wall of magnitude intermediate the potentials on said filament and said first wall, a fourth cylindrical Wall of electrically conductive material concentric with and positioned outside said.' second wall; means applying a negative potential to said fourth wal-1; a fifth electron permeable wall of electrically conductive material spaced from one end of said annular space; and means to apply a potential between said fifth wall and said first and second walls to direct ions formed in said space in a predetermined path away from said space.

10. An ion source comprising an electron emitting filament, a first cylindrical electron permeable wall of electrically conductivematerial enclosing said filament, a second cylindrical electron permeable lwall ofvelectrically conductivermaterial concentriewith and enclosing `s aid first wall in'spaced `relation therewith, means to apply a potential between said filament and said walls so that electrons emitted from said filament pass radially through the annularv space between said first and second walls, means to direct molecules to be ionized into said annular space', 'a third cylindrical electron permeable wall of electrically conductive material concentric with said rst wall and positioned between said filament and said first wall, means to apply a negative potential to said third wall of magnitude intermediate the potentials on saidk filament and said first wall, means to vary the potential applied to said third wall responsive to the electron emission from said filament so that the electron fiow into said space remains constant, a fourth cylindrical wall of electrically conductive material concentric with and positioned outside said second wall, means applying a negative potential to said fourth wall, a fifth electron permeable Wall of electrically conductive material spaced from one end of said annular space, and-means yto apply a potential between said fifth wall and said first and second Walls to directions formed in said space in a predetermined path away from said space.

ll. The combination in accordance with `claim 10 wherein said means to vary the potential applied'tor said third lwall comprises a source of reference potential; means to establish a second potential responsive to Yelectron emission from said filament, means to compare said reference potential with said second potential, and means responsive to said last-mentioned meansV to vary the potential applied to said vthird wall.

References Cited in the leY of this patent UNITED STATES PATENTS

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