GB2028575A

GB2028575A - Ion sources

Info

Publication number: GB2028575A
Application number: GB7906006A
Authority: GB
Original assignee: Osaka University NUC
Current assignee: Osaka University NUC
Priority date: 1978-08-12
Filing date: 1979-02-20
Publication date: 1980-03-05
Also published as: DE2906359A1; JPS5812703B2; DE2906359C2; GB2028575B; JPS5525943A

Abstract

An ion source for a mass spectrometer comprises a semiconductor emitter E having a number of semiconductor whiskers 1a grown around an opening of an electrode support 1 through which a fluid specimen is to be supplied and ionized. The specimen is supplied from a cylinder 14 having a heater 16 and fitted with a piston 15. A process for manufacturing such ion source includes steps of evaporating gold onto a part of the support on which the whiskers are to be grown, preheating the support and supplying a gas containing the semiconductor for growth of the whiskers on the gold coated part of the support. An apparatus for such process comprises a vacuum vessel for enclosing the support, means for controllably heating the support and means for controllably supplying a gas containing the semiconductor into the vacuum vessel. <IMAGE>

Description

SPECIFICATION Improvements in or relating to ion sources This invention relates to a field-desorption type ion source, and more particularly to an ion source suited for use with a mass spectrometer for highmolecular organic compounds.

Mass spectrometry is used widely in many fields, such as physics, chemistry, biology, medical science, pharmaceutics, agriculture, and engineering. Analysis of atoms, molecules and organic conpounds by mass spectrometry first calls for ionization.

Conveniently, ionization has been accomplished mainly by use of an electron-impact type ion source. Because it needs gasification of a specimen, however, this ion source is unfit for specimens with low vapour pressures.

Another method proposed is the direct insertion probe, which exposes a specimen, carried on the tip of a glass rod or similar means, to an electron beam in the ion source so that it is ionized by the striking electrons.

The impingement of electrons in this method, however, imparts many complex mass spectra to a specimen of organic compound as a result of fragmentation. Then it often becomes difficult to obtain a characteristic spectrum (especially for molecular ions) necessary for identification and structure analysis.

A solution proposed is ionization by field desorption. According to this method, a liquefied or suspended specimen is put on an emitter having conductive acicularwhiskers at one end. A high voltage of approximately 10 kv is applied between the emitter and an oppositely disposed cathode to produce a strong electric field at the tip of the conductive acicular whiskers.

By the tunnel effect, the electrons in the specimen passes through the potential barrier distorted by the strong electric field to the emitter, whereupon the positive ion of the specimen, taken out by the cathode, is introduced to the mass spectrometer.

This field desorption method enabled the observation of the molecular ion peaks of specimens with low vapour pressures and high polarities. But this method cannot obtain mass spectra continuously, because the emitter cannot carry very much specimen at a time.

The present invention is intended to solve the problem encounted with the conventional fielddesorption type ionizing method. Accordingly, the object of this invention is to provide a fielddesorption type ion source that permits continuously obtaining mass spectra of lowvapour-pressure or high-polarity substances (especially organic compounds).

Accordingly, the present invention provides an ion source comprising a semiconductor emitter, the emitter having a number of semiconductor whiskers grown around an opening in a support to serve as an electrode, and specimen supply means for continuously supplying a fluid specimen to be ionized through said opening.

One form of ion source embodying the invention comprises a semiconductor emitter having a number of semiconductor whiskers grown around an opening in a support to serve as an electrode, specimen supply means for continuously supplying a fluid specimen to be ionized through said opening, and means for heating the fluid specimen to be ionized.

A method of manufacturing an ion source according to the invention comprises the steps of evaporating gold onto a part of a support where whiskers of a semiconductor are to be grown in a vacuum atmosphere, preheating the support supplying a gas containing the semiconductor at,a regulated pressure so as to control the growth of the whiskers on the support, and heating the support to a regulated temperature.

An apparatus for manufacturing an ion source according to the invention comprises a vacuum vessel to enclose a support in a vacuum atmosphere, means for heating the support in the vacuum vessel, the heating means having a temperature control function, and means for supplying a gas containing a semiconductor into the vacuum vessel, the gas supplying means communicating with the vacuum vessel through a control valve.

In order that the invention may be readily understood, an embodiment thereof will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a perspective view that schematically shows general arrangement of the ion source; Figure 2 is a block diagram of apparatus for manufacturing the ion source: and Figure 3 is an enlarged, longitudinal cross section of the ion source.

As shown in Figures 1 and 3, a semiconductor emitter E comprises a small-diameter metal (stainless steel) pipe 1 serving as a support and a number of acicularwhiskers 1 a, of a semiconductor such as silicon, standing along the opening- at a tip end of the'support pipe 1.

As seen in Figure 3, specimen supply means S is connected to the pipe1, so that fluid specimen to be ionized can be continuously supplied through the opening at the tip end of the pipe 1.

More specifically, a cylinder 14 serving as a specimen storage chamber is fitted to a base end of the pipe 1, and a piston 1 5 is reciprocally fitted therein.

An electric heater 1 6 is buried in the circumferential wall of the cylinder 14. When energized with current, this heater 16 develops a thermal motion or pressure differential, which causes, in conjunction with the pushing action of the piston 1 5, the liquid or suspended specimen in the cylinder 14 to continuously move toward the opening at the tip end of the pipe 1 through the passage 1b.

The semiconductor emitter E is prepared as follows. As shown in Figure 2, the pipe 1 fitted with the specimen supply means S is evacuated in a vacuum box 6, and gold is evaporated to a thickness of several hundred angstroms where the aricular whiskers 1 a of silicon are to be grown.

Then the pipe 1 is preheated by supplying a predetermined current from a constant-voltage power supply 8 to the heater 1 6.

A leak valve 11 is opened to supply a silane gas (SiH4 5% + Ar 95%) from a gas cylinder 12 to the vacuum box 6 until a pressure of approximately 200 torr is established therein.

On heating the pipe 1 again by supplying a predetermined current from the constant-voltage power supply 8 to the heater 1 6, a number of thin, short, amorphous silicon whiskers 1 a grow in approximately 1 minute.

The time for the growth of the whiskers 1 a changes with the pressure of the silane gas. The length and diameter of the amorphous silicon whiskers 1 a can be varied freely by changing the temperature of the pipe 1 by supplying different currents to the heater 16.

The semiconductor emitter E thus prepared is fitted to a field-desorption type ion source as an electrode (anode), as shown in Figure 1.

In Figure 1, reference numeral 2 designates an electrode (cathode) disposed opposite the semiconductor emitter E. Reference numerals 3 and 4 denote lens electrodes, and 5 a main slit.

In Figure 2, reference numeral 9 designates a pressure gauge, 10 a rotary pump, and 13 an oil diffusion pump.

To ionize the liquid or suspended specimen continuously supplied to the opening at the tip end of the pipe 1 of the above-described ion source embodying this invention, a current is supplied to the heater 1 6 to heat the cylinder 14, piston 1 5 and pipe 1. At the same time, the piston 1 5 is gradually moved in the direction indicated by the arrow A in Figure 3.

This causes the fluid specimen to move from the cylinder 14 through the passage 1 b to the opening at the tip end of the pipe 1 carrying many whiskers 1 a, where molecular and other ions are continuously produced by the field-desorption type ionization process.

Because it can provide continuous ionization of the specimen, the ion source embodying the invention can increase the S/N ratio by accumulating signals. Now highly accurate mass spectrometry can be applied to such substances as high-molecular organic compounds with greater mass numbers than 1000 for which satisfactory mass spectrum has not been obtained conventionally.

It has been confirmed that the whiskers 1 a of such semiconductors as silicon and germanium can be grown on a surface of whatever shape.

Therefore, the whiskers 1 a can be formed on differently shaped surfaces of support material.

As the metal support, a porous material may be used instead of the small-diameter pipe 1 in the above-described embodiment.

An example of such porous material is a smaller-diameter stainless steel pipe that has many minute openings in its circumferential wáll formed by etching. By using a method similar to the above-described, a number of semiconductor whiskers can be grown around each minute opening to form a semiconductor emitter.

Also, a pipe or porous material of tungsten or tantalum may be used in place of a stainless steel support.

Further, the metal pipe and porous supports may be replaced by supports of such semiconductors as silicon and germanium and such nonmetallic materials as glass and synthetic resin covered with metal coating. The metalcoated nonmetallic base has an advantage of high workability.

The whiskers of such semiconductors as silicon and germanium can be grown on the surface of the semiconductor or nonmetallic supports according to procedures similar to the abovedescribed one. Their operations and results have been confirmed empirically.

With the above-described semiconductor emitter, comprising a number of semiconductor whiskers growing around the opening at the tip end or the support pipe to serve as an electrode, and specimen supply means to continuously supply the fluid specimen to be ionized through the support pipe opening, an ion source embodying this invention can continuously ionize the fluid specimen, which permits applying accurate mass spectrometry to such substances as high-molecular compounds whose mass number exceeds 1000.

The semiconductor whiskers can readily and quickly grow on a surface of whatever shape, so that they can be grown around the opening of a support pipe or the openings in a porous support material easily and securely.

Claims

1. An ion source comprising a semiconductor emitter, the emitter having a number of semiconductor whiskers grown around an opening in a support to serve as an electrode, and specimen supply means for continuously supplying a fluid specimen to be ionized through said opening.

2. An ion source according to claim 1, wherein the support is a pipe.

3. An ion source according to claim 1 wherein the support is a porous object.

4. An ion source according to any one of claims 1 to 3, wherein the support is made of a metal such as stainless steel, tungsten or tantalum.

5. An ion source according to any one of claims 1 to 3, wherein the support is made of a semiconductor such as silicon or germanium.

6. An ion source according to any one of claims 1 to 3, wherein the support is made of a nonmetallic material such as glass or synthetic resin covered with a metal coating.

7. An ion source according to any preceding claim, wherein the specimen supply means comprises a cylinder communicating with the opening in the support and a piston snugly fitted in the cylinder.

8. An ion source comprising a semiconductor emitter, the emitter having a number of semiconductor whiskers grown around an opening in a support to serve as an electrode, specimen supply means for continuously supplying a fluid specimen to be ionized through said opening, and means for heating the fluid specimen to be ionized.

9. An ion source according to claim 8, wherein the specimen supply means comprises a cylinder communicating with the opening in the support and a piston snugly fitted in the cylinder.

10. An ion source according to claim 9, wherein the heating means is an electric heater buried in the circumferential wall of the cylinder.

1 A method of manufacturing an ion source, comprising steps of evaporating gold onto a part of a support where whiskers of a semiconductor are to be grown in a vacuum atmosphere, preheating the support, supplying a gas containing the semiconductor at a regulated pressure so as to control the growth of the whiskers on the support, and heating the support to a regulated temperature.

12. A method according to claim 1 wherein the semiconductor is silicon and the semiconductor-bearing gas is a siiane gas.

1 3. An apparatus for manufacturing an ion source, comprising a vacuum vessel to enclose a support in a vacuum atmosphere, means for heating the support in the vacuum vessel, the heating means having a temperature control function, and means for supplying a gas containing a semiconductor into the vacuum vessel, the gas supplying means communicating with the vacuum vessel through a control valve.

14. An apparatus according to1 claim 13, wherein the heating means is an electric heater.

15. An apparatus according to claim 13, wherein the gas supplying means is a gas cylinder communicating with the vacuum vessel.

1 6. An ion source substantially as hereinbefore described with reference to the accompanying drawings.

17. A method of manufacturing an ion source, substantially as hereinbefore described with reference to the accompanying drawings.

18. Apparatus for manufacturing an ion source, substantially as hereinbefore described with reference to the accompanying drawings.

19. Any novel feature or combination of features herein described.