Ion Source For Vaporizing And Ionizing Solid Substances

Abstract

High vacuum ion source for use with a mass spectro-meter to analyze solid substances. A thermally conductive body having the substance to be analyzed supported on a surface thereof is disposed in the ionization chamber with the substance supporting surface facing the ionization region of the chamber. A heat source, such as an electron emitting device, is directed toward the opposite surface of the body to heat the body and cause the substance to vaporize. The heating of the body in turn heats the chamber surfaces to prevent condensation of the vaporized substance. The vaporized substance can be ionized by particle bombardment or field emission. The body may take the form of a flat plate or a cylindrical or other box-like structure.

Parent Case Text

This is a continuation, of U.S. Pat. application Ser. No. 888,320 filed Dec. 29, 1969, now abandoned.

Description

The invention relates to an ion source and a method for obtaining ions from solid substances comprising an oven for vaporization of said solid substances and means for ionization of the vaporized solid substances within an ionization zone of a high vaccuum chamber.

It is known to use an oven in the form of a heated crucible which is positioned adjacent a sample window of the ionization chamber. In order to obtain a high ionization efficiency the wall of the ionization chamber at the side of the crucible has been curved towards the ionization zone, so that the sample window for the inlet of the vapor is brought close to the ionization zone. However, even then the probability of ionization of the vaporized solid substances is yet unsatisfying because a considerable portion of the vaporized substances does not reach the ionization zone but fails to enter the ionization zone and condenses at the cooler walls of the ionization chamber and of the vaccuum chamber. It is an object of the present invention to improve the ion yield from solid samples with an ion source and method of the above mentioned art.

This problem has been solved by that using an ion source as defined above said ion source is provided with a heating surface which is turned towards said ionization zone and which is maintained at an even high temperature over its whole extend high enough to evaporize said solid substances to avoid condensation of said vaporized substances within the ionization chamber. Using such a heated surface forming the sole close by surface facing the ionization zone has the important advantage that optimum conditions for vaporization are obtained and that losses of material by condensation are avoided.

The heating surface preferably is made from a material of high thermal conductivity e.g. gold. Thereby an even high volatilization temperature is ensured all over the heating surface by using heating means of simple construction.

Preferably the heating surface is arranged parallel to said ionization zone and is made large compared to the extend of the ionization zone.

Advantageously the large heating surface is used as a support for an even layer of the substances to be vaporized.

The heating surface may be formed by one surface of a plate or tablet which is turned towards the ionization zone. Said tablet may have the shape of a shell arranged parallel to the ionization zone and with its concave surface concentrical to the ionization zones.

The heating surface may preferably be constituted by the inside of a box surrounding the ionization zone. By that means the inside wall of the entire ionization space is maintained at even high temperature and all substances which are captured within the ionization space have no chance to condense once having been vaporized. Thus a high ionization efficiency is ensured. If the ion source is provided with an elongated straight-lined ionization zone then the heating surface may consist of the inside of a hollow circular cylinder which is provided with an ion exit slit on a generatix and a window in one of its front sides as entrance for particle bombardment.

In a known manner the heating surface may be shiftable through a charging valve into and out of the vaccuum chamber.

The drawing shows several embodiments of the invention. In the drawing is

FIG. 1 an ion source with ionization by particle bombardment,

FIG. 2 a cross section along line II--II of FIG. 1,

FIG. 3 another embodiment of an ion source with ionization by particle bombardment like FIG. 1,

FIG. 4 an ion source with field ion emission,

FIG. 5 an ion source like FIG. 2 which is provided with a cooling device, and

FIG. 6 an ion source like FIG. 3 which also is provided with a cooling device.

The ion source as shown in FIG. 1 consists in a conventional manner of an ionization chamber 1 which is enclosed by a litlle metal box 2 connected by a joint 3 to a potential of e.g. + 3 kV. The ionization is effected by electron bombardment. To this end the metal box 2 is provided with an entry window 4 through which electrons from a cathode 6 by means of an accelerator electrode 7 are bombarded along an electron beam 5. The electron beam is concentrated on a band-shaped inization zone 8 by means of a magnet (not shown in the drawing), leaves the ionization chamber 1 through an exit window 9 and is collected by an anode electrode 10. The cathode 6 is connected to a joint 11 with a potential of 3,1 kV. The required inlet velocity of the electrons is effected by a potential difference of 100 V between the accelerator electrode 7 and the metal box 2.

The ions produced within the ionization zone 8 by electron bombardment from the gaseous or vaporous substances to be analyzed by means of a drain electrode 14 connected to a potential of e.g. 2,9 kV are extracted through an exit slit 12 arranged parallel to the ionization zone 8 and are emitted in the form of an ion beam 15 which in a conventional manner is subjected to a mass separation within a separator tube by a magnetic sector field. The separator tube 16 encloses part of the vaccuum space 17 including the ion source.

The substance to be analyzed is vaporized by an evaporator oven 18 which is arranged within the ionization chamber 1. The solid substance is applied in the form of a thin layer 19 upon a large metallic heating surface 20 which is turned towards the ionization zone 8. This heating surface 20 is formed by that side of a body made of material of a high thermal conductivity which is turned towards the ionization zone 8.

In the first embodiment shown in FIG. 1 and 2 the heating body exists of a tablet 21 in the form of a rectangular plane metal plate which is shiftable by a valve rod 23 of a conventional charging valve with channel 22 into or out of the vacuum space 17. For this purpose a window 24 is arranged in that side wall of the box 2 turned towards the charging valve is large enough to let through said tablet. The tablet 21 is maintained at the same potential as the box 2 by a flexible line or by a contact 25.

A suitable heating means is provieed to maintain the heating surface 20 at or above the required volatilization temperature. Conventional means may be used for this heating purpose. Heating is effected by electron bombardment in the embodiments shown in the drawing. The electron bombardment is effected by cathodes 26 in front of an electron entry slit 27 in the backside of the tablet. In order to obtain an essentially even volatilization temperature all over the heating surface an array of heating cathodes 26 may be provided as is shown in FIG. 2. The tablet 21 is made of gold in order to ensure said even volatilization temperature even with an uneven distribution of electron bombardment.

The heating surface 20 of the tablet 21 is arranged with a small distance d parallel to and at that side of the ionization zone 8 which is opposite the ion exit slit 12.

Preferably the ionization box 2 is maintained at a temperature which is higher than the volatilization temperature in order to avoid a condensation of vaporized substance at the inside of the box.

FIG. 3 shows an embodiment in which the ionization box 2 itself is formed as evaporator oven. This box has the shape of a circular cylinder 29 with the ionization zone 8 extending along the axis of the cylindrical inside surface whereas an ion exit slit 12 is arranged along a generatrix and an electron entry window 4 and electron exit window 9 are arranged in the opposite front sides. The box 29 is made of gold and is heated by electron bombardment from cathodes 26 through accelerator electrodes 30.

FIG. 4 shows the application of the invention in connection with means for field ion emission. A very thin Wolaston-wire as emitter 31 is connected to a voltage of e.g. + 3 kV and is arranged opposite a drain-electrode 32 with a potential of -10 kV which is followed by additional slit electrodes 33 destinated to form the required ion beam 15 for mass spectrometry.

In the shown embodiment the drain-electrode 32 forms part of the evaporator oven 18 and has the shape of a shell the concave inside of which is concentrical to the emitter 31 and which is completed by a flat back wall to a box thus enclosing completely the whole ionization chamber 1. The ions formed within the ionization source are extracted out of the box through an exit slit 12. A slit 34 is provided in the back wall to form a passage for the emitter 31 which is supported by an arm swingable around an axle 35.

Just as in the foregoing described embodiments the solid substance to be vaporized also in FIG. 4 is applied to the inside heating surface of the box 32 which is heated by electron bombardment from cathodes 26 through accelerator electrodes 30.

It is important for the utility of the described ion sources that after measurement the substance to be analyzed is rapidly cooled below the volatilization temperature. FIG. 5 and 6 are showing embodiments of the ion source like FIG. 2 and 3 which is provided with a special device for rapid cooling. In the embodiment of FIG. 5 the tablet 21 supporting the sample 19 is heated by electron bombardment from cathodes 26 and is connected to a cooling device at it both ends. This cooling device consists of two electric insulators 36,37 which consists of mzterial material high thermal conductivity and which are connected at one end to the tablet 21 and at the other end to cooling coils 38,39. By this arrangement the tablet 21 consisting of material of high thermal conductivity is rapidly brought to the desired volatilization temperature by the heating means and after switching out the heating means is very rapidly cooled below the volatilization temperature by leading of the heat through the insulators 36 and 37.

FIG. 6 is showing an embodiment in which a cylindrical box 29 at both ends is connected to ring shaped electrical insulators 40,41 made of material of high thermal conductivity which at their free outside are connected to cooling coils 42,43.

It will be understood that other applications of the invention are possible and the embodiments disclosed may be subjected to various changes, modifications and substitutions without necessarily departing from the spirit of the invention. Thus, in conventional ion sources with electron bombardment within an ionization box this box could be heated entirely to an even volatilization temperature. Of course, the substance to be vaporized must be brought into the ionization box e.g. by applying a layer of the substance on the inner surface of the box. If the ionization chamber is completely enclosed by the heating surface, then the substance could be brought within the ionization chamber in concentrated form e.g. in the shape of a little lump although the distribution of the substance all over the heating surface may be preferred on account of a more intensive vaporization and a higher ion yield.

The invention is not restricted to ion sources with electron bombardment but could be applied generally in ion sources in which ionization is effected by particle bombardment. Furthermore it may be applied to ion sources in which ionization is effected by exposure of substances to radiation e.g. to ultraviolet light.

Finally the substances to be ionized could be entered in vaporous or gaseous form from a primary oven outside the ionization chamber through a heated channel into the heated ion source.

In any case it is important that all close by surfaces are heated which are facing the ionization zone.

Claims

We claim:

1. In a high vacuum ion source having means defining an ionization chamber for containing therein a solid substance that is to be vaporized and means for ionization of the vaporized solid substances to form an ionization zone within the ionization chamber, the improvement comprising:

a thermally conductive body disposed within the ionization chamber having a heating surface facing said ionization zone,

said heating surface serving as a support means for a layer of the substance to be vaporized,

means for supporting said body within the ionization chamber, wherein the distance from the heating surface to the ionization zone is less than the distance from other surfaces defining the ionization chamber to the ionization zone,

said heating surface having a greater area than the facing area of the ionization zone,

and a heat source directed toward a surface of said body opposite the surface supporting the substance to be vaporized for maintaing the heating surface and in turn the surfaces defining the ionization chamber at a temperature sufficient to cause evaporation of the solid substance to avoid condensation of the vaporized substance at the surfaces defining the ionization chamber.

2. The apparatus of claim 1 wherein an even layer of the solid substance that is to be vaporized is disposed on the heating surface of the body.

3. The apparatus of claim 2 wherein said heating surface is essentially flat and serves as a support for the even layer of the substance to be vaporized.

4. The apparatus of claim 3 wherein said body is formed by a plate.

5. The apparatus of claim 2 wherein said body has the shape of a shell arranged parallel to said ionization zone and with its concave surface turned towards the ionization zone.

6. The apparatus of claim 1 wherein said ionization chamber is a box structure.

7. The apparatus of claim 1 wherein said body comprises a box surrounding the ionization zone.

8. The apparatus of claim 7 wherein the inside of said box is arranged concentrically to said ionization zone.

9. The apparatus of claim 1 wherein said heating surface is composed of gold.

10. The apparatus of claim 1 further comprising means for moving said body through a charging channel into and out of the vacuum chamber.

11. The apparatus of claim 1 wherein the ionization is effected by means of particle bombardment.

12. The apparatus of claim 1 wherein the ionization is effected by means of field ion emission.

13. The apparatus of claim 1 and further comprising a cooling device to cool the edges of said body and wherein said heat source is directed toward a central area of the body.

14. The apparatus of claim 13 wherein said cooling device is connected to said body by electrical insulators consisting of a material of high thermal conductivity.

15. The apparatus of claim 1 wherein said body comprises a plate having opposite facing surfaces of larger square area than any other surfaces defining the plate, said heat source being directed toward one of said opposite facing surfaces, the other of said opposite facing surfaces being directed toward and disposed in proximity to said ionization zone.

16. The apparatus of claim 15 comprising a cooling means coupled to said one of said opposite faces at the edges thereof.

17. The apparatus of claim 16 wherein said plate is heated by said heat source at the central area thereof and said cooling means includes a thermally conductive means disposed at the edge area of said plate.

18. The apparatus of claim 15 wherein said heating surface is arranged essentially parallel to said ionization zone.

19. In a high vacuum ion source having means defining an ionization chamber for containing a solid substance that is to be vaporized and means for ionization of the vaporized solid substances to form an ionization zone within the ionization chamber, the improvement comprising:

a thermally conductive body disposed within the ionization chamber having a heating surface facing said ionization zone,

said heating surface serving as a support means for a layer of the substance to be vaporized,

means for supporting said body within the ionization chamber,

said heating surface having a greater area than the facing area of the ionization zone,

and a heat source for maintaining the heating surface and in turn the surfaces defining the ionization chamber at a temperature sufficient to cause evaporation of the solid substance to avoid condensation of the vaporized substance at the surfaces defining the ionization chamber,

said body comprising a plate having opposite facing surfaces of larger square area than any other surfaces defining the plate,

said heat source being directed toward one of said opposite facing surfaces, the other of said opposite facing surfaces being directed toward and disposed in proximity to said ionization zone and supporting the substance to be vaporized.