US3313934A

US3313934A - Field ion source for mass spectrometry with elongated emitter

Info

Publication number: US3313934A
Application number: US345407A
Authority: US
Inventors: Beckey Hans-Dieter
Original assignee: Atlas Meb- and Analysentechnik GmbH
Current assignee: Atlas Meb- and Analysentechnik GmbH; Finnigan MAT GmbH
Priority date: 1963-02-19
Filing date: 1964-02-17
Publication date: 1967-04-11
Anticipated expiration: 1984-04-11
Also published as: DE1283567B; FR1383384A; GB1052902A

Description

p il 11, 1967 HANS-DIETER BECKEY 3,

FIELD ION SOURCE FOR MASS SPECTROMETRY WITH ELONGATED EMITTER 2 Sheets-Sheet 1 Filed Feb. 17, 1964 7 MW 6 W W? l I [M w W PI M My 10 April 11, 1967 HANS-DIETER BECKEY 3,

FIELD ION SOURCE FOR MASS SPECTROMETRY WIT H ELONGATED EMITTER Filed Feb. 1'7, 1964 I 2 Sheets-Sheet 2 Fig. 3

lM/EI/TAR HANS -0/t' TE E EEC/(5y A TM J.

United States Fatent O 13 Claims. (Cl. 250-419 In previously known field ion sources for mass spectrometers, fine metal points were used exclusively for producing the field. In this connection it was assumed that the high field strengths necessary for field ionisation could be obtained only with the aid of metal points, Which in the emission zone have a very small radius of curvature, while a necessary condition was that the field strengths required must lie between 1X10 and x10 volts per cm.

The invention is based on the observation that field strength of 1 10 and 5 10 volts per cm. are necessary only for the rare gases, such as helium, neon and argon, and a few permanent gases, such as hydrogen, oxygen, carbon monoxide and methane, but that for the by far predominant majority of all molecules it is suflicient to apply field strengths of about 2x10 to 3 volts/ cm. to effect field ionisation. Based on this realisation, according to the invention the emitters for producing the field for the purpose of field ionisation are so constructed and disposed that the emission zone has an extent which is many times as great as corresponds to its small radius of curvature and preferably ha the form of an elongated curved surface.

The emission properties of an ion source with emitters of this type, for example in the form of thin wires or knife edges, differ considerably from those of previously known field ion sources and are distinguished from the latter by substantial advantages. The emission surface is about 10- times as great as that of field emission points. Referred to the same pressure in the ion source and to the same very low field strength, with a simple point having for example a radius of curvature r=5,000 A. a total emission of 10 amperes is obtained, and with a wire having a diameter d=2.5 and an effective emission length of 5 mm. the total emission amounts to 10" amperes. The increase of the field ion current reaching the mass spectrometer detector through the use of the new ion source to about 10 amperes enables a simple Faraday ion detector to be used instead of the secondary electron multiplier previously necessary. In addition the advantage is gained that the emission fluctuations are smaller and stabilisation of the ion current is also possible.

The fluctuations of that fraction of the field ion current which passes through the limiting outlet gap of the field ion source are about 10 times smaller with thin Wires than with emission points, in the event of the same low field strength prevailing at the emission point as on a 2.5 wire on application of a voltage of 14 kv. This field strength is known as threshold field strength.

At the threshold field strength the current fluctuations measured at the ion detector of a mass spectrometer amount to about i0.7% of the measured value when using emitting wires, Whereas the measured value of the ion current emitted from a point and measured at the ion detector fluctuates by -100% or more at the threshold strength. A simple emitting point can therefore not be used for analytical purposes at the threshold field strength, but the field strength must be increased apice proximately by the factor 2 in order to achieve con stancy, to within about i5%, of the ion current emitted by a single point and measured at the mass spectrometer detector. Even with .still higher field strengths, the accuracy of measurement of 20.7%; which can be achieved with wires, is not possible with individual points.

Electronic stabilisation of the ion current of the field emission of a point has hitherto not been possible because the fluctuations of the ion current at the mass spectrometer detector are due to considerable deviations from the mean value of the ion current emitted at the point. The ion detector current fluctuations are there'- fore not proportional to the fluctuations of the ion current emitted at the point. With the new ion source on the other hand this stabilisation is possible because the current fluctuations measured at the ion detector are substantially proportional to the fluctuations of the current emitted at the wire. I

The desired value of the current emitted at the wire can be adjusted by an R-C member in the circuit of the Wire emitter. The time constant amounts to about 10 seconds. Brief variations (time constant about 0 second) from the desired value can be reduced by a differential voltage regulator which controls the ion beam intensity through the voltage of a pair of deflector plates.

As an additional advantage it is to be observed that the molecule fragment formation in the case of field ionisation when using emitting wires is substantially smaller under normal operating conditions than when using emission points. For the purpose of characterising the molecule fragment formation, the c H -fragment (mass 29) can be used in the case of n-paraflins. the frequency of the C H +-fragment of n-hexane in the case of wire emission is only about 1% of the mother ions, but in the case of point emission is at least 10% ofthe mother ions.

For the application of the field ion source to mass spectrometer analysis it is essential that the mass spectra When using emission wires should be even substantially simpler than in the case of emission points.

In addition, the reproducibility of the mass spectra over long periods of time is substantially better in the case of wire emission than with point emission. In the case of point emission reproducibility depends greatly on the macroscopic shape of the point and hence on the angle distribution of the ion emission. The geometrical shape of the point varies in the course of long periods of time, for example by passing from a paraboloid to a hyperboloid or truncated conical shape; wire and knife edges on the other hand retain their circular sym-' metry even over longer periods of time.

The invention will now be explained with reefrence to one exemplary embodiment which is illustrated in the accompanying drawings, in which:

FIGURE 1 shows a field emission source according to the invention, in axial section,

FIGURE 2, a partial elevation corresponding to FIG- URE 1 and showing the emitter arrangement on a larger scale, turned through in relation to FIGURE 1,

FIGURE 3,'a front elevation corresponding to FIG- URE 2,

FIGURE 4, a form of construction of the emitter ar-' rangement with a group of parallel wires,

FIGURE 5, a front elevation corresponding to FIG- URE 4, p

FIGURE 6, a form of construction of the emitter arrangement with a group of parallel metal foils, I

FIGURE 7, a front elevation corresponding to FIG URE 6, and 7 FIGURE 8, a form of construction of the emitter ar rangement with a group or freely extended wires.

The drawings illustrate semi-diagrammatically the construction of a field ion source having an emitting Wire, the field ion source illustrated consists of the conventional manner of a two-part substantially cylindrical glass container 1a, 1b with flange connection 2, which is open at the bottom and by means of a flange 3 can be connected to the separating tube 4 of a mass spectrometer or other vacuum apparatus. For the production and maintenance of a high vacuum in the ion source and in the mass spectrometer tube, a passage 5 is provided in the wall of the spectrometer tube, this passage being connected to a high vacuum pump and leading to the vacuum space of the source of ions. In the vacuum space 6 of the ion source an ion chamber 7 is provided in which the ions are formed. A gas sup-ply pipe 8, consisting of Covar glass and a Vakon tube fused thereto, is connected by a screw connection to the ion chamber 7. Covar is a glass available from Corning Glass Works of New York suitable for being combined with Vakon metal available from Vakuum Schmelze of Hanau, West Germany, to provide an essentially vacuum-tight combination. The composition of Vakon is 29% nickel, 18% cobalt and 53% iron.

For the formation of ions, the ion chamber 7 contains an emitter 9 in the form of a Wollaston wire having a diameter of 2.5 and a free length of 5 mm., this wire being held at its ends by conductors 10. A Wollaston wire is a drawn silver wire of about 0.11 mm. diameter having a core, for example of platinum, of about l-5 microns (1-4 '10' cm). This wire is situated opposite a cathode 11 which contains a slot 12 the maximum dimensions of which are mm. length and 1 mm. width. The wire is at about +4 kv. in relation to earth and the cathode at about l0 kv. On the side of the emitter 9 which is associated with the cathode, an elongated emission zone corresponding to the effective length of the wire is thus formed, the ion formation occurring in this Zone. With the aid of an electrostatic lens, consisting of the cathode 11 and two

other lens electrodes

13 and 14, the ions formed are taken out of the ion chamber and the divergent ion beam thus formed is focused onto the admission slot of the mass spectrometer tube 4. The lens' electrode 13 is at a voltage between and a few hundred volts in relation to earth and the lens electrode 14 is at earth potential.

The ion beam is adjusted in two directions perpendicular to one another by pairs of

deflection plates

15 and 16. The pair of plates 16 also acts as a lens for focusing the ion beam in the direction in which the magnetic field of the mass spectrometer does not have a focusing action. For this purpose the mean potential at the pair of plates 16 is made a few hundred volts positive in relation to earth. The adjustment of this lens potential is very critical.

All gaseous, liquid, or solid compounds which have a vapour pressure of at least 10* torr between 20 and 150 C. can be passed into the ionisation zone in the vapour condition. The ion zone can be heated during operation to 150 C., or in special cases to 350 C. Solids or liquids having vapour pressures below 10- torr can be vapourised into the ionisation space in an electric furnaoe at the side of the electrode 11. Wollaston wires are suitable for forming field ions on wires. The platinum cores of the Wollaston wire should have a diameter of 2.5a. 5,u wires give a comparable emission only at a voltage of about kv. between wire and cathode, while discharges and electrical flashovers may give rise to disturbances. If 5 wires also emit at lower voltages, this is due to micropoints on the wire, which however do not lead to emission conditions which are very satisfactorily reproducible. 2.5;]. Wollaston wires are prepared by etching away the silver coating, for example in concentrated nitric acid. The remaining micropoints are etched away by very brief alternating current electrolysis (20 volts) in aqua regia, utilising a protective resistance of a few kiloohms. It has been found that Wollaston Wire is better etched by electrolysis in a potassium cyanide solution than by nitric acid. A l-molar KCn solution is made, the Wollaston wire is connected as anode, and a 1 mm. platinum wire used as cathode. The direct current voltage applied amounts to 3 volts, a l-kiloohm resistor being connected in series. After-treatment by alternating current electrolysis in aqua regia is then not necessary. In a microscope with an enlargement of 1,000 times the platinum wire appears smooth. The length of the emitting wire advantageously amounts to only about 38 mm., since with a greater length mechanical vibrations ofthe fine wire may be disturbing. Longer wires may be supported at intervals or supported over their entire length on a support of insulating material. Wires having diameter below 2.5; require lower voltages for field ionisation, but are more liable to destruction.

The emission zone may also be formed by the edge of a razor blade or similar emitter bodies or by the edge of a foil, the thickness of which corresponds to the desired radius of the emission zone to be formed. Sharp metal knife edges, particularly those of fine razor blades, generally require pulling tensions at least twice as great in order to achieve the same field strength as 2.5 1 wires. Exceptions are only a few selected examples which at a pulling voltage of 14 kv. supply the same ion current as 2.5 wires. Extremely thin metal foils are therefore to be preferred to razor blades in respect of field ion production.

In mass spectrometers the length of the freely extended wire is for example 8 mm. In that case additional supporting is not required. Such supporting would reduce the field strength. Since the width of the ion beam in a normal mass spectrometer is only of the .order of 8 mm., additional supporting need therefore not be considered for applications of the field ion source to mass spectrometers. Additional supporting would be required only in pressure measuring instruments in which the length of the emission wire could in certain circumstances be a multiple of 8 Amongst a number of possible applications of a field ion source with fine wires or knife edges for field production, mention will be made here only of the application to mass spectrometer analyses and to total and partial pressure measurement. Because of the abovementioned advantages of the field ion source having emission wires or knife edges, this field ion source is very suitable for chemical analysis with the aid of the mass spectrometer. At pressures below a few 10- torr in the ionization chamber, periodically condensed layers are built up and torn down on the emitted wires or knife edges, so that the ion current fluctuates periodically. This can be avoided if the total pressure in the ion source in an analysis always amounts to about 10* torr. At this pressure a thicker condensed layer forms on the emission wire or knife edge and leads to a stable ion current reproducible over a long period of time. If the total pressure in the ion source is kept constant, the ion currents are proportional to the partial pressure of the components, provided that the components are chemically not too dissimilar. If however this condition is not complied with, a large excess of an inert solvent is added to the mixture, this solvent greatly reducing the deviations from proportionality of the ion currents to the partial pressures.

Independently of the combination of the new field ion source with a mass spectrometer, the field ion emission on fine wires or knife edges may be used for measuring the total or partial pressure of vapours of inorganic or organic compounds in rare gases and permanent gases such as H 0 CO, CH The total ion current is exactly proportional to pressure at pressures 'below 10- torr. Field ionisation probability is at least a factor 1,000 smaller for the abovementioned rare gases and some permanent gases than for inorganic or organic vapours. A simple ion source, in which the total emission of a wire or knife edge is measured, may therefore be used for total or partial pressure measurement.

The emission zone may also be formed of a linear or areal group of a large number of apices (points or elongated apices), the radius of curvature of which, like that of the wire, is preferably equal to about 2.5 Such emission zones may be formed either, as illustrated in FIGURES 4 or 5, by fixing a plurality of thin wires 9 on an insulating body 17 in a parallel arrangement or, as illustrated in FIGURES 6 and 7, alternately superimposing thin metal foils 18 and insulating foils 19 and bounding them on one side by a sharp cut, so that that side becomes the emitter.

A lO-wire emitter can also for example be produced by freely extending wires. The distance between the etched wires is 0.2 mm. It is not possible first to weld the unetched Wollaston wires on the carriers and then etch them all at the same time. Each wire must therefore be prepared separately and finally all the wires must be joined together. This is done by welding a single Wollaston wire 9 on a U-shaped metal plate 20 of a thickness of 0.2 mm. corresponding to the spacing of the wires (V2A sheet)see FIGURE 8. In order to provide accurate guiding for the wire to be welded in place, a groove 21 of a depth of about 0.1 mm. is engraved close beneath the ends of the two arms 20a and 20b of the U-shaped plate. After the wire 9 (shown in chain-dotted lines) has been welded to the plate, the weld point is given a coat of Zapon varnish in order to avoid the detaching of the Wire ends from the plate during the etching operation. The plate with the Wollaston wire welded in position is then introduced into the etching solution. On completion of the etching operation the Zapon varnish is removed again by means of an organic solvent. The plates have bores 22 so that all 10 plates can. be pushed one after the other on to the guide bolts of a support of V2A steel. The plates are pressed together by tightening the screws of the support.

:Many modifications and other embodiments are also possible within the framework of the invention. In particular, the emission zone may be differently shaped from the examples given, if the ion optical system is constructed accordingly. The emitter must however always be so constructed and disposed that an emission zone is formed the total extent of which is many times as great as corresponds to its smaller radius of curvature. The emission zone may be a doubly curved surface having the radii of curvature r and r with the condition that r must be great in relation to R and in the limit case must be infinite, and that the extent of the emission zone in the direction in which it is only slightly curved or not curved at all must be great in relation to its extent in the direction in which it is greatly curved.

It has therefore also been found advantageous to produce emitters in wire form by applying to quartz threads of a diameter of a few n a thin metal or semiconductor coating by vaporisation, spraying, or chemical deposition.

I claim:

1. A field ion source for mass spectrometry comprising means defining an ion chamber,

inlet means coupled to said ion chamber for introducing matter into said ion chamber,

an emitter in said chamber for forming field ions from said matter and having an emission zone defining a small radius of curature,

said emitter defining a linear distance many times as large as the distance defined by said small radius of curvature,

means defining an inlet to mass spectrometric analyzing apparatus,

and means for directing said ions to the latter inlet for mass spectrometric analysis.

2. A field ion source in accordance with claim 1 wherein said emitter has a thin wire shape.

3. A field ion source in accordance with claim 1 wherein said emitter is a Wollaston wire.

4. A field ion source in accordance with claim 1 wherein said emitter comprises a body defining said emission zone having the shape of a knife edge.

5. A field ion source in accordance with claim 1 wherein said radius of curvature lies in the range of from about 1 to about 5n.

6. A field ion source in accordance with claim 1 wherein the maximum value of said radius of curvature is 2.5;.

7. Afield ion source in accordance with claim 1 and further comprising a plurality of emitter supports.

8. A field ion source in accordance with claim 1 wherein said emitter has the shape of a razor blade edge.

9. A field ion source in accordance with claim 4 wherein said emitter consists of a foil having a thickness corresponding to said radius of curvature formed by an edge of said foil.

10. A field ion source in accordance with claim 1 wherein said emitter is etched at said emission zone.

11. A field ion source in accordance with claim 1 wherein said emitter comprises a plurality of thin wires mounted on an insulator body parallel to one another.

12. A field ion source in accordance with claim 1 wherein said emitter comprises a plurality of thin metal foils and a plurality of insulator foils superimposed to one another.

13. A field ion source in accordance with claim 1 wherein said emitter is formed by a plurality of metal tips arranged close together over a small area.

References Cited by the Examiner UNITED STATES PATENTS 2,062,124 11/1936 Flaws 117231 2,764,707 9/1956 Crawford et al. 313-230 2,809,314 10/1957 'Herb 313-63 2,816,242 12/1957 Goodman 3 l3230 2,847,328 8/1958 Cline l17--201 2,930,917 3/1960 Nief 3l363 3,173,218 3/1965 Curtis et al. 3l3-231 JAMES W. LAWRENCE, Primary Examiner.

GEORGE N. WESTBY, Examiner.

S. SCHLOSSER, R. JUDD, Assistant Examiner.

Claims

1. A FIELD ION SOURCE FOR MASS SPECTROMETRY COMPRISING MEANS DEFINING AN ION CHAMBER, INLET MEANS COUPLED TO SAID ION CHAMBER FOR INTRODUCING MATTER INTO SAID ION CHAMBER, AN EMITTER IN SAID CHAMBER FOR FORMING FIELD IONS FROM SAID MATTER AND HAVING AN EMISSION ZONE DEFINING A SMALL RADIUS OF CURATUE, SAID EMITTER DEFINING A LINEAR DISTANCE MANY TIMES AS LARGE AS THE DISTANCE DEFINED BY SAID SMALL RADIUS OF CURVATURE, MEANS DEFINING AN INLET TO MASS SPECTROMETRIC ANALYZING APPARATUS, AND MEANS FOR DIRECTING SAID IONS TO THE LATTER INLET FOR MASS SPECTROMETRIC ANALYSIS.