US3461285A

US3461285A - Mass spectrometer ion source with a two region ionization chamber to minimize energy spreading of the ions

Info

Publication number: US3461285A
Application number: US649544A
Authority: US
Inventors: Helmut Wilhelm Werner Werner
Original assignee: US Philips Corp
Current assignee: US Philips Corp
Priority date: 1966-07-02
Filing date: 1967-06-28
Publication date: 1969-08-12
Anticipated expiration: 1986-08-12
Also published as: DE1598884A1; NL6609292A; CH470757A; JPS5032638B1; GB1190451A; SE329216B

Description

Aug. 12, 1969 H. w.- w. WERNER 3,461,285- I MASS SPEG'I'ROMETER ION SOURCE WITH A TWO REGION IONIZATION CHAMBER 1'0 MINIMIZE ENERGY SPREADING OF THE IONS Filed June 28. 1967 2 Sheets-Sheet 1 v IIIIIII IIIIIIII 'IIIIIIIIIIIIIII;

. t y I F'IGCI INVENTOR.

HELMUT w. w. wsnnsn BY AGENT Aug. 12, 1969 v H. w'. w. WERNER 3.461285 --MASS SPECTROMETER ION SOURCE WITH A TWQ REGION *IONIZATION CHAMBER T0 MINIMIZE ENERGY SPREADING OF THE IONS Fil ed June 28. 1967 2 Sheets-Sheet 2 INVENTOR.

HELMUT w. w; wenusn BY AGEM United States Patent 3,461,285 MASS SPECTROMETER ION SOURCE WITH A TWO REGION IONIZATION CHAMBER TO MINIMIZE ENERGY SPREADING OF THE IONS Helmut Wilhelm Werner Werner, Emmasingel, Netherlands, assignor to US. Philips Corporation, New York, N.Y., a corporation of Delaware Filed June 28, 1967, Ser. No. 649,544 Claims priority, application Netherlands, July 2, 1966, 6609292 Int. Cl. B0711 59/44 US. Cl. 250-41.9 7 Claims ABSTRACT OF THE DISCLOSURE An ion source for a mass spectrometer in which ions are formed by ionization of a gas to be tested with an electron beam collimated by a magnetic field in the direc-;

tion of the beam, the ions formed being extracted from the ionization space through a small gap parallel to the direction of the electron beam by means of an electric field the direction of which extends at right angles to the direction of the electron beam, a fiat grid-like electrode being provided consisting of parallel wires which cross the direction of the electron beam, the latter electrode dividing the ionization space into two zones one of which contains the ionization gap, a potential being applied to the grid-like electrode which in combination with potentials applied to the other electrode produces a small voltage gradient in the first zone and a large voltage gradient in the other zone.

The invention relates to an ion source, in particular an ion source for a mass spectrometer.

In a known ion source ions are formed by ionization of a gas to be tested by means of an electron beam which is collimated by a magnetic field in the direction of the beam, the ions formed being extracted from the ionization space through a small gap parallel to the direction of the electron beam by means of an electric field the direction of which extends at right angles to the direction of the electron beam. In such a source the electrons emitted by a filament are confined into a narrow beam with a comparatively weak magnetic field having values up to a few hundred Gauss. Under the influence of a weak electric field which is at right angles to the beam the ions which are formed are extracted from the ionization space through a gap into an ion lens. The extracting field which is of the order of magnitude of a few volts per centimeter is supplied by the potential of the first electrode ot the ion lens, the potential of the wall of the ionization space, and the potential of a repeller electrode inside the ionization space. The repeller electrode is located so that the electron beam traverses between said electrode and the emanating gap. When the ions have traversed the ion lens they pass through a slit in the analyzer space of the spectrometer. The analysis takes place by means of a sector-like magnetic field.

In such an ion source there will always be mass dis- 3,461,285 Patented Aug. 12, 1969 crimination because of the collimating magnetic field in the ionization space. In fact, the ion paths in the crossed electric and magnetic fields are cycloids in a plane at right angles to the direction of the magnetic field in which the factor ME occurs as a parameter where M is the mass number of the eB /i0n, E is electric field strength of the extracting field, B the magnetic field in the ionization space, and e the charge of the ion. The particles having the smallest mass (M) will follow the most curved paths. Particles having a small mass (M) will not penetrate to the analyzer because they do not pass the gaps in the wall of the ionization space, or reach too far beyond the axis of the ion lens. This latter effect may be prevented by choosing the potentials of deflection electrodes in the ion lens to match the relative mass (M) value. However, this matching is very time consuming and is associated with loss of other mass (M) values. The mass discrimination could be checked by increasing the extracting field strength, i.e., by applying a larger voltage diiference to the extraction electrodes. However, this has an adverse influence on the electron beam and also increases the voltage gradient across the ionization area, i.e., the area where the electron beam is, which results in an increased energy spreading of the ions and consequently an undesired decrease of the resolving power of the spectrometen.

It is a principal object of the invention to provide an improvement of ion sources which are thus constructed.

It is a further object of the invention to reduce the mass discrimination as a result of the collimating magnetic field in an ion source for a mass spectrometer.

It is still further an object of the invention to increase the efficiency without increasing the energy spreading in the ionization area of an ion source for a mass spectrometer.

These and further objects of the invention will appear as the specification progresses.

According to the invention, in an ion source for a mass spectrometer of the above-described type a flat grid-like electrode consisting of parallel wires which cross the direction of the electron beam and that of the extracting field at right angles is arranged at a distance of a few tenths of a millimeter from the ionization area. This electrode divides the ionization space into two areas the first of which comprises the emanating gap. A potential is applied to that electrode which, in combination with the potentials of the extracting electrodes, produces a small potential gradient which is a maximum of a few volts per centimeter in the first area, and produces a large potential gradient which is at least some tens of volts per centimeter in the second area.

These differences in gradient are produced by the potentials of the electrodes having the following orders of magnitude.

The repeller electrode has a small positive potential with respect to the grid which is not more than a few volts. The flat front wall of the ionization space in which the emanating gap is located has a negative potential with respect to the grid of the order of tens of volts. Consequently, a strong field which is formed as a result of the grid and, possibly, also by the potentials of electrodes of the ion lens between the grid and the front wall does not penetrate too much into the area which is restricted by the grid and the housing of the ionization space which consists of a flat rear wall opposite to the emanating gap and a box-like part of the wall which has the rear wall as a base.

In one embodiment the repeller electrode is connected to the housing and the grid is insulated from the surrounding parts.

In another embodiment the grid is connected to the housing and the repeller electrode is insulated from the surrounding parts.

In another embodiment both the repeller electrode and the grid are connected to the housing. The ions are then extracted from the ionization area by the penetrating field of the front wall which has a negative potential with respect to the housing of a few hundred volts.

The grid is supported by a holder. Preferably a construction is used in which the wall of the said holder is,

not at right angles to the plane in which the grid is located but widens in the direction of the emanating gap. As a result of this a certain extent of focussing of the ions in the emanating gap can be obtained.

In each of the above-described embodiments in the area between the repeller electrode and the grid the ions need cover only a very short distance, namely at a maximum the thickness of the ionization area plus the distance from the ionization area to the grid. At the grid the deviation as a result of the collimating magnetic field is still small. In the area between the repeller electrode and the grid such a small potential gradient prevails that the energy spreading of the ions is not larger than in the conventional sources. In the area between the grid and the front wall a large potential gradient prevails, i.e., a strong field strength so that ions with different masses describe here substantially identical paths substantially parallel to the axis of the ion lens as a result of which this area does not contribute to mass discrimination and the efliciency is increased.

In each of the above-described embodiments laborious adjustment of the ion lens to efficiently pass particles the masses of which are located in a given range through the ion lens is not necessary. Potential differences between defiection electrodes on either side of the axis of the ion lens which, in the absence of the grid, must be of the order of 100 volts, need no longer be used when the grid is employed. These potentials need be a maximum of a few volts now and apparently serve only to remove field disturbances as a result of irregularities in the electrodes.

' It has furthermore been found that in the above-described embodiments the ion yields for all masses vary linearly over a large range as a function of pressure in the ionization chamber with one single adjustment of the ion lens.

The invention will now be described in greater detail with reference to the accompanying drawings which are drawn substantially entirely to scale.

FIGURE 1 is a cross-sectional view normal to the electron beam of a part of the ion source and the ion lens inside the vacuum envelope.

FIG. 2 shows part of FIG. 1 on an enlarged scale, equipotential lines also being shown.

In FIG. 1 the cross-section of the electron beam is denoted by the area 1 in which the ions are formed. In a direction along the axis of the electron beam which has a current strength of approximately 100 ,ua., the proportions of this area gradually increase from 2 mms. 0.5 mm. to 4 mms. 2 mms. The cross-section shown in FIG- URE l is chosen to be such that the upper limit of these dimensions 4 mms. 2 mms. is shown.

The collimating magnetic field has a strength of 150 Gauss and is denoted by B in the figure. The ionization space is limited by a flat front wall 2 which comprises an emanating gap 3, having a length of 10 mms. and a width of 1 mm., and a housing consisting of a flat rear wall 4 and a box-like part 5. The ionization space comprises a repeller electrode 6 and a grid-like electrode 7 which is located at a distance of 8 mms. from the rear wall 4, at a distance of 2.5 mms. from the front wall 2 and at a distance of 0.5 mm. from the area 1. The grid is secured to a molybdenum frame and consists of tungsten wires, 25,11. thickness, mutual distance approximately The wires are arranged at right angles to the direction of the electron beam in order to prevent the structure of the grid from appearing in the ion spectrum.

The grid is supported by a holder 8 which widens in the direction of the gap 3.

The ion lens is constituted by the

electrodes

9, 10, 11, 12, 13, 14 and 15.

The electrode 15 includes a slot 16 affording access to the analyzer space.

In FIG. 2 the numbers in brackets indicate the potentials with respect to the housing on the lines and electrodes at the applied voltages. The repeller electrode 6 is at a potential of 0 volts with respect to the housing of the ionization space. The grid 7 is at a potential of l.7 volts with respect to the housing. The front wall 2 is at 30 volts with respect to the housing. The

electrode pair

9 and 10 is at 500 volts with respect to the housing and the electrode 11 is at -2000 volts with respect to the housing. The potential of the housing with respect to ground is determined by the potential difference between the

electrodes

11 and 15 from FIG. 1, the latter of which is at ground potential.

In the area 1 the potential gradient at these voltages is only 3.5 volts per centimeter, but in the area between the grid 7 and the front wall 2 the potential gradient at these voltages is volt per centimeter.

Voltage differences exceeding 5 volts on oppositely located deflection electrodes (9 and 10; 13 and 14) need not exist in the ion lens, not even to cause particles having a mass of 1 mass unit to reach the analyzer.

While the invention has been described with reference to specific embodiments and applications thereof, other modifications will be readily apparent to those skilled in the art without departing from th espirit and scope of the invention as defined in the appended claims.

What is claimed is:

1. In an ion source for a mass spectrometer, the combination of a housing closed at one end by a wall having an aperture therein for the passage of ions, means within said housing for producing an electron beam in a given direction for ionizing a gas in said housing, means to produce a magnetic field in said given direction, means to produce an electric field in said housing in a direction perpendicular to said given direction for extracting ions through said aperture, a grid-like electrode within said housing having parallel wire-like elements intersecting the direction of the electron beam, said grid-like electrode being positioned between said wall having the apert-ure therein and said electron beam and being spaced further from said wall than from said electron beam, said grid-like electrode being spaced a distance from said electron beam of the order of tenths of a millimeter, said grid-like electrode dividing the interior of said housing into two zones one of which contains the electron beam, means to produce a potential gradient in a first zone containing the electron beam not exceeding the order of volts per centimeter, and means producing a potential gradient in the other zone of the order of tens of volts per centimeter.

2. An ion source as claimed in claim 1 in which a repeller electrode is positioned in the first zone on the opposite side of the electron beam from the grid-like electrode, a potential being applied between said electrodes for producing the potential gradient in said first zone.

3. An ion source as claimed in claim 2 in which the 5 repeller electrode is connected to the housing and the grid-like electrode is insulated therefrom.

4. An ion source as claimed in claim 2 in which the grod-like electrode is connected to the housing and the repeller electrode is insulated therefrom.

5. An ion source as claimed in claim 2 in which the repeller electrode and the grid-like electrode are connected to the housing.

6. An ion source as claimed in claim 2 in which the grid-like electrode is supported by a conductive holder 1 which widens in the direction of the emanating gap.

7. Anion source as claimed in claim 1 including an ion lens in said second zone in which potential differences exist between deflection electrodes on either side of the axis of the ion lens which are not more than a few volts.

References Cited UNITED STATES PATENTS 5/1960 Benson et al 250-419 OTHER REFERENCES 0 vol. 38, No. 5, May 1967, pp. 621-624.

RALPH G. NILSON, Primary Examiner C. CHURCH, Assistant Examiner