US2975279A - Mass spectrometers - Google Patents

Description

March 14, 1961 R. D. CRAIG MASS SPECTROMETERS Filed June 22, 1959 ##arng United States PatentO MASS SPECTROMETERS Robert Derek Craig, Altrincham, England, assignor to Metropolitan-Vickers Electrical Company Limited, London, England, a British company Filed June 22, 1959, Ser. No. 821,885

Claims priority, application Great Britain June 23, 1958 2 Claims. (Cl. 250-413) The present invention relates to mass spectrometers and in particular to ion sources for mass spectrometers.

In known types of mass spectrometers the ion source comprises a region in which the ions are formed, for instance by bombardment of a gas with an electron beam, and a series of electrodes to accelerate the ions in one direction and for focussing the beam of ions so formed to pass through a slit. This slit is called the object slit of the ion source and the beam of ions passes therethrough into the body of the mass spectrometer where ions of different mass numbers are separated by passage of the beam through a magnetic field, as is well known in the art.

In the construction of ion sources of the type described a plate with a narrow slit has to be manufactured. This forms the object slit and it is not normally possible to vary the width of this slit mechanically. Also ions which do not pass through the object slit bombard the plate containing the slit and cause undesirable secondary'emission therefrom. i

The object of the present invention is to provide an improved type of ion source for a mass spectrometer.

According to the present invention an ion source for a mass spectrometer comprises an ion formation region from which ions are emitted in random directions, means for accelerating said ions so as to form an ion beam extending along an axis, and means for deflecting ions forming said beam so as to form an image of said ion formation region in a plane extending transversely to said axis.

Preferably the ion source comprises a plurality of of electric potential so as to deflect the ions forming said beam and forming said image.

In order that the invention may be more readily understood reference will now be made to the accompanying drawings, in which:

Fig. l is a diagrammatic elevational view in section of an ion source embodying the invention, and

Fig. 2 is a perspective view, partly in section, of a complete ion source.

With reference to the figures, the ion source comprises a block 1 of electrically conducting material, having a central chamber 2 and an aperture 3 in the face 4 at one end of the chamber. An ion formation region 5 is located within the chamber 2 and emits ions in random directions. The ion formation region may be, for example, a heated surface generating thermal ions, or

the space occupied by a collimated beam of electronswhich bombards a gas so as to produce ions.

Fig. 2 illustrates a ribbon filament 21 which is connected by two leads 22, 23 to a suitable source of potential. The filament is supported by an insulating disc 31 and is inserted into the chamber 2 from the end 2,9752% Patented Mar. 14, 196i ice opposite to the face 4. When a current is passed through the filament 21 ions are emitted therefrom.

In the embodiment of the invention illustrated in Fig. 2, the ion source is enclosed in a rectangular crosssection container 24, which may be evacuated. The end 20 of the container is adapted to be connected to the main body of the mass spectrometer. Thin plates 6, 7 and 8 of electrically conducting material extend across the axis of the ion source and are spaced axially along the container. The plates are supported by insulating rods 25, 26, 2.7 and 28 so that a rigid structure is formed. The plates 6, 7 and 8 are formed respectively with apertures 1t), 11 and 12, which are preferably rectangular in shape and are aligned along the axis of the container. The distance between the block 1 and the plate 6 and the distance between the plate 6 and the plate 7 are considerably less than the distance between the plate 7 and the plate 8.

The insulating rods 25, 26, 27 and 28 are formed with central conductors which are connected respectively to the block 1, the plate 6, the plate 7 and the plate 8 in order that these components may be connected to suitable sources of electrical potential, such as those provided by a source 29 with several tapping points. The conductors extend from the ends of the insulating rods through the end wall 32 of the container 24 and are insulated therefrom.

The block 1 and the plates 6 and 7 therefore produce an electrostatic field which acts upon the ions emitted from the region 5, is. the filament 21.

The electrostatic field accelerates the ions and forms them into a beam which passes through the apertures 3, 10 and 11. The field acts as an electrostatic lens and an image of the ion formation region is'formed in a plane transverse to the axis of the ion beam. The potentials on the block 1 and on the two plates 6 and 7 are adjusted so that the image of the ion formation region 5 is formed in the region between the face 4 and the plate 6 as indicated at 9 in the figure.

The size of the image can be varied by varying the potentials on the block 1 and on the two plates 6 and 7. If plate 7 is grounded and the block 1 is connected to a potential of V volts the ratio of the size of the ion formation region to the size of the image can be varied between 3 and 10 by varying the potential on the plate 6 between 0.8V volts and V volts. A suitable range of values for V has been found to be between 2 and 20 kv., a preferred value therefor being 6 kv.

The plate 6 may be formed in two halves with a central slit dividing the two halves and forming the aperture 10. This construction enable two different potentials to be applied to the two plates so as to correct the ion beam for alignment.

A beam of ions which passes through the aperture 11 has a very small natural divergence angle and appears to originate at the image of the ion formation region. This is therefore a virtual ion source. The ion beam passes along the axis towards the main body of the mass spectrometer. If it is desired to work with a still smaller divergence angle, this can be obtained by restricting the cross-section of the beam by an aperture 12 in plate 8. In this case, some of the ions forming the ion beam will impinge upon the surface of plate 8 and will cause secondary emission of electrons. However, since plate 8 is preferably at the same potential as plate 7, theseelectrons will not be accelerated towards plate 7 and will not have a harmful effect on the ion beam.

It should be appreciated that for certain applications the natural divergence angle of the beam emerging from aperture 11 may already be sufliciently small, in which cases aperture 12 may be made wider than the beam cross-section or, alternatively, plate 8 may be omitted altogether.

The ion beam appears to originate from the image of the ion formation region and this is therefore a virtual source. The size of this source and hence the size of the beam may be varied by varying the potentials on the block 1 and on the plates 6 and 7. Since the virtual source lies in a plane close to the surface of the plate 6, and the source is smaller than the apertures 10 and 11, there will be a considerable reduction in the number of ions which will strike the plates 6 and 7 and cause undesirable secondary emission eifects.

After passing through the aperture 12 in the plate 8 the ion beam enters the main body of the mass spectrometen The width and natural divergence angle of the ion beam are controlled by the potentials on the block 1 and on the two plates 6 and 7, or by the width of the aperture 12. An ion source of variable width is provided by the apparatus and undesirable secondary emission due to ion bombardment of the ion accelerating and focusing plates is considerably reduced. The image of the ion formation region takes the place of the object slit in known types of mass spectrometer ion sources and thus provides an effective object slit of variable width.

, What I claim is: V

-1. An ion source for a mass spectrometer comprising a chamber, an ion formation region within said chamber from which ions are emitted in random directions, an electrically conducting endwall to said chamber, surfaces defining a first aperture in said end wall, a pair of electrically conducting deflecting plates extending in planes parallel to said end wall and spaced apart from each other and from said end wall, edges to said plates defining second and third apertures which are aligned with said first aperture, means for connecting said end wall and said deflecting plates respectively to suitable variable sources of electric potential so that said ions are formed into a beam and are accelerated along an axis 7 through the three apertures, and so that said end wall and said deflecting plates constitute an electrostatic lens which forms an image of said ion formation region of a variable size and in a plane extending transversely to said axis. i

2. An ion source for a mass spectrometer comprising a chamber, an ion formation region within said chamber from which ions are emitted in random directions, an electrically conducting end wall to said chamber, surfaces defining a first aperture in said end wall, a pair of electrically conducting deflecting plates extending-in planes parallel to said end wall and spaced apart from each other and from said end wall, edges to said plates de fining second and third apertures which are aligned with said first aperture, means for connecting said end wall and said deflecting plates respectively to suitable variable sources of electric potential so that said ions are formed into a beam and are accelerated along an axis through the three apertures, and so that said end wall and said deflecting plates constitute an electrostatic lens which forms an image of said ion formation region of a variable size'and in a plane extending transversely to said axis, together with an electrically conducting limiting plate extending across said axis and spaced axially from said deflecting plates .on the side thereof remote from said ion formation region, edges to said limiting plate defining a fourth aperture through which said ion beam passes, and means for electrically connecting said limiting plate to the nearest deflecting plate so as to limit the size of the ion beam as it passes through said fourth aperture.

Inghram et a1. Jan. 31,1956 Backus June 9, 1959

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