US 3842269 A
The ion source of a mass spectrometer is designed to draw ions from a relatively large volume through a diaphragm with a relatively large opening into a magnetic field in a wedge shaped space between diverging plane pole faces, entering such space parallel to the line of intersection of the pole face planes near the wide end of said space. The ions are focused after 90 DEG deflection into an exit slit or diaphragm on the line of intersection of the pole faces. Certain parameters for the position of the exit slit are given.
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Description (OCR text may contain errors)
United States Patent [1 1 Liebl et al.
[451 Oct. 15, 1974 MASS SPECTROMETER OF HIGH DETECTION EFFICIENCY Inventors: Helmuth Liebl, Eching; Klaus sa s rr, M n h; 1101 1.
Germany Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V., Gottingen, Germany Filed: Dec. 26, 1973 Appl. N0.: 427,490
Related U.S. Application Data Continuation of Ser. No. 292,332, Sept. 26, 1972, abandoned.
Foreign Application Priority Data Oct. 5, 1971 Germany 2149716 U.S. Cl 250/298, 250/281, 250/427 Int. Cl. H0lj 39/34 Field of Search 250/282, 281, 290, 298,
 References Cited UNITED STATES PATENTS 3,286,187 11/1966 Gaber 250/427 3,421,035 1/1969 Brubaker... 250/41.95 B
3,445,650 5/1969 Liebl 250/49.5 B 3,705,998 12/1972 Jennings et a1. 250/427 3,761,707 9/1973 Liebl 250/298 Primary ExaminerJames W. Lawrence Assistant Examiner-B. C. Anderson Attorney, Agent, or FirmFlynn & Frishauf 5 7 ABSTRACT The ion source of a mass spectrometer is designed to draw ions from a relatively large volumethrough a diaphragm with a relatively large opening into a magnetic field in a wedge shaped space between diverging plane pole faces, entering such space parallel to the line of intersection of the'pole face planes near the wide end of said space. The ions are focused after 90 deflection into an exit slit or diaphragm on the line of intersection of the pole faces. Certain parameters for the position of the exit slit are given.
6 Claims, 2 Drawing Figures MASS SPECTROMETER OF HIGH DETECTION EFFICIENCY This is a continuation, of application Ser. No.
292,332, filed Sept. 26, 1912 now abandonegl The present invention relates to a mass spectrometer of high detection efficiency and more particularly to a I mass spectrometer of the type having an ion source in which a gas to be analyzed is ionized by electron bombardment, an apparatus for producing a substantially static magnetic field through which the ions produced in the ion source and projected therefrom by an electric field through a first diaphragm are deflected on the basis of their effective mass so that ions of different effective mass are focused along a focus line at different places and there enter an ion detection devie in which the ions to be detected enter through a diaphragm suitably positioned in the focus line.
In known types of mass spectrometers of high detection efficiency, particularly those used for analysis of residual gas or for detection of leaks in vacuum systems, the effective magnetic field provides the analysis by mass by projecting the intake slit ion-optically on the exit slit. The mass resolution thus depends primarily on the ratio of the deflection radius to the sum of the slit widths. Since in the case of residual gas analysis apparatus and'the like a compact form of construction is desired, the deflection radius and likewise the intake slit width cannot be made so large as would be desired with reference only to the mass resolution. And since, moreover, the ion jet may have a spreading angle of only a few degrees, the known mass spectrometers of this type can detect ions only from a small ionization volume in the ion source. For a given ionization density the obtainable detection efficiency is, however, proportional to the volume from which the ions can be detected. The detection efficiency of the known residual gas analyzers therefore leaves much to be desired.
The object of the present invention is accordingly to provide a mass spectrometer of high detection efficiency in which ions can be drawn from a large ionization volume in spite of a compact form of construction.
SUBJECT MATTER OF THE PRESENT INVENTION:
Briefly, the ion source in which the gas to be analyzed is ionized by electron bombardment includes an open metallic helix, such as a wire helix, a source of electrons external to the helix biased electro-negatively with respect to the helix and two open grids perpendicular to the axis of the helix respectively adjacent the two ends of the helix, one at the positive pole of the power supply and the other one slightly more negative. An acceleration voltage is provided between the less positive of these two grids and an adjacent diaphragm which also has an open grid across its opening and which serves as the intake diaphragm of the mass spectrometer proper. This diaphragm has an opening that is relatively large compared so that of the output diaphragm. The equipment for applying the magnetic field includes two pole pieces having mutually inclined pole faces lying in planes that intersect in a line parallel to the axis of thehelix of the ion source. The pole pieces also have two mutually perpendicular sides lying in the same mutually perpendicular planes. One of these planes intersects the throat of the diverging pole faces and is parallel to the line of intersection of the pole face planes and at a distance'A therefrom. The other of these two mutually perpendicular planes is perpendicular to the axis of the helix of the ion source and hence also of the incoming ion jet. The distance between that axis and the line of intersection of the pole face planes is substantially 2.10 A Finally, the exit diaphragm is bisected by the aforesaid line of intersection of the pole face planes and is offset from the plane extending the side of the pole faces which is adjacent to the intake diaphragm and perpendicular to its axis by a distance equal to 1.35 A,,.
By these arrangements the result is obtained that ions can be drawn from a large ionization volume in the ion source and the intake diaphragm or slit through which the ions proceed from the ion source into the magnetic field can be made very large in comparison to residual gas analyzers heretofore known, so that according to the invention a mass spectrometer can be provided which has a detection efficiency of about one order of magnitude better than the conventional residual gas analyzers. Y
The acceleration voltage, the magnetic field, or both may be made variable. The electron source is located outside the helix and is preferably a hairpin type thermionic cathode, biased substantially negative relative to the helix and the grids at its ends, but considerably less negative than the diaphragm adjacent one end of the helix which serves as an accelerating electrode. An electrostatic screen may enclose the electron source as well as the helix, including the back end of the helix, and this screen is preferably at about the potential of the electron source or slightly more negative. An illustrative embodiment of the invention is described below in further detail with reference to the accompanying drawing in which:
FIG. 1 is a greatly simplified plan view of a mass spectrometer according to the invention, and I FIG. 2 is a side view of a portion of the mass spectrometer shown in FIG. 1.
The embodiment shown in FIGS. 1 and 2 for illustration of the invention includes a mass analyzer 11 which works with a magnetic field that is produced between two plane pole faces that are inclined with respectto each other so as to form'a wedge shaped magnetic gap. The lines of force of the field, as shown in FIG. 2 by arrows, are circular arcs having as center the line of intersection 10 of the two planes containing the pole faces 11a and 11b respectively. The field strength between the pole pieces is inversely proportional to the distance from the intersection line 10. Charged particles describe cycloidal paths in such a field. It is known (H. Liebl, J. Appl. Phys., vol. 38 (1967) pages 5,277, 5283) that when a parallel jet of charged particles enters perpendicularly to the magnetic field at an offset distance A 2.10 A from the intersection line 10, with the momentum p, it will be focused after a deflection at a point on the intersection line if the field strength is so adjusted that at the distance A 2.10 A, from the intersection line it has the value B,, 1,35 p/Ae In the above expression: p particle momentum e particle charge A distance of the intersection line 10 from the pole piece wall surface 110, which is parallel to the intersection line 10.
The focusing effect follows a second approximation: the distance of the focus from the entrance edge of the pole pieces is substantially 1.35 A,,.
At the location of the focus an output diaphragm 12 in provided, for example in the form of a slit, behind which an ion detector, for example an ion capture device 13, is located for detecting the ions penetrating through the gap of the diaphragm l2 and for producing a corresponding electrical output signal at an output terminal 14 for processing in the usual way.
The ion source designated 20 as a whole in the drawing comprises a cylindrical wire helix 22 and an electron source 21 outside the helix and extending parallel to the helix axis (in the simplest case in the form of a hairpin filament connected to a suitable source of heating surrent 15). At the two ends of the-helix are parallel open wire grids or similar reticular structures 23 and 24, perpendicular to the axis 22a of the helix 22 and preferably connected with the adjacent ends of the helix. An electrostatic screen 26, which may be of wire screening in cup shape, surrounds the elements just named, the electron source 21 as well as the helix 22 and the grids 23 and 24.
Spaced somewhat away from the grid 24 is a first diaphragm 25, the opening of which is bridged by another open wire grid 25a parallel to the grid 24. The diaphragm 25 serves as an accelerating electrode for the ion source and as the intake diaphragm or slit for the magnetic field portion of the mass spectrometer.
v The opening of the intake diaphragm 25 is relatively large, its area being of the order of magnitude of the crosssectional area of the helix 22. Preferably the area of the opening is at least 50 percent on the area of the cross-section of the helix 22.
The electrodes of the ion source are connected during operation of the mass spectrometer with a voltage source 16, which is shown only diagrammatically in the drawing- It includes for example a voltage divider adapted to supply the voltages specified in FIG. 1.
The mass spectrometer above described operates as follows:
The electrons emitted by electron source 21 are accelerated by the positive bias voltage of helix 22 relative to electron source 21 and oscillate a few times through the helix until they finally land on it. In consequence the gas molecules inside the helix are ionized by electron bombardment. The oscillation of the-electrons just mentioned increases the effective path of the electrons, as is known in the particular context of the Bayard-Alpert ionizationmanometers. In contrast to the operation of the ionization manometers just mentioned, however, in the ion source of the present invention a small voltage (preferably less than percent and usually even smaller than 2 percent of the acceleration voltage U between grid 24 and diaphragm 25) is applied between the parallel open grids 23 and 24 at the ends of the spiral 22, so that a weak homogeneous electrostatic field is present on the inside of the helix. This field is so directed that the ions produced by electron bombardment are propelled towards grid 24. Between the latter and the diaphragm 25 the actual acceleration voltage U (preferably a variable voltage in the neighborhood of, for example, 500 volts) is applied to produce an electrostatic acceleration field between the grids 24 and 25a, which are arranged in parallel planes. The ion projected through the diaphragm 25 produce a parallel stream or jet that enters the magnetic field between the pole faces 11a and 11b at their edge 11d.
Typical potentials for the electrodes of the ion source are, with respect to ground potential 0 volts:
Diaphragm 25 0 V Grid 24 +500 V Grid 23 +5 I0 V Electron source 21 +300 V Screen 26 +290 V The theoretical mass resolution is AM/M AU/U S/A 1.4 0
In the above expression:
AU energy spread of the ions U acceleration voltage S width of the slit shaped opening of the exit diaphragm 12 in the plane of deflection A distance of the exit diaphragm 12 from the field edge 0 angular spread of the parallel particle stream or jet at entrance into magnetic field.
The energy spread AU arises primarily from the small voltage applied between the ends of the helix and causes the ions to arise at somewhat different potential. The angle spread 0 comes into play in the calculation of the mass resolution in the place of the entrance slit width which comes up in the corresponding formula for conventional equipment. The angular spread 0 is composed primarily of alignment errors of the grids 24 and 25d and their deviation from parallel plane relation and also the penetration of fields through these grids.
In a practical example of construction of equipment according to the invention, a partial pressure detection efficiency of 10 A/Torr was obtained for a half value resolution of 30. This detection SensitivityiE aboiit one order of magnitude better than that of most conventional residual gas analyzers.
The embodiment from which the above results was obtained had the following physical parameters:
B,, variable, e.g.
730 Gauss for mass 2 (H 2,720 Gauss for mass 28 (N,+);
3,410 Gauss for mass 44 (CO angle between the pole faces 11a and 11b 8 diameter of helix 22 1.5 cm
distance between grids 23 and 24 1.5 cm
diameter of opening in diaphragm 25 1 cm The adjustment of the mass spectrometer for a desired effective ion mass (ion mass/ion charge) can be accomplished by variation of the magnetic field strength and/or variation of the acceleration voltage applied between grids 24 and 25a. The acceleration voltage can readily be varied by an adjustable tap of a potentiometer in the voltage source 16. For variation of the magnetic field B A a magnet coil 28 may be used which is connected to a source 29 of variable current and is associated with a magnetic yoke llj which is only schematically represented in FIG. 2 as forming part of the magnetic circuit which includes the pole pieces 11 and the pole faces 11a and 11b.
Although the invention has been described with reference to a single specific embodiment, it is to be understood that modifications and variations may be made within the inventive concept without departing from the spirit of the invention. In particular, although the element 22 is described as a helix, it is clear that other cylindrical open cages of wire, rods, etc., would function in an equivalent fashion, and the use herein of the work helix" must be taken with that understandmg.
1. A mass spectrometer of high detection efficiency comprising:
an ion source (20) including a chamber in which a rarified gas to be analyzed is adapted to be ionized, a metallic open helix (22) surrounding a portion of said chamber, a source of electrons (21) outside said helix, open grids (23, 24) adjacent to and obstructing the two ends of said helix (22), a first diaphragm (25) spaced from one end of said helix (22) centered axially thereto, an open grid (25a) across the opening of said first diaphragm, each of the aforesaid grids (23, 24, 25a) perpendicular to the axis (22a) of said helix (22), and means for applying electric potential (16) so as to make said grid (23) adjacent the end of said helix (22) fatthest from said first diaphragm (25) most positive,
said grid (24) adjacent the other end of said helix (22) slightly less positive, said helix (22) at a potential intermediate that of said end grids (23, 24), said electron source (21) considerably less positive, and said first diaphragm (25) and its grid (25a), most negative, the voltage between said end grids (23, 24) of said helix being less than 5 percent of the acceleration voltage between the said open grid (24) at the end of said helix (22) nearer said first diaphragm (25 and the open grid (25a) across said first diaphragm;
means for producing a substantially steady magnetic field (11, 28, 29) directed at right angles to the axis (22a) of said helix (22) of said ion source (20) including two pole pieces (11) having diverging pole faces (11a, 11b) lying in planes intersecting in a line parallel to the axis (22a) of said helix (22) of said ion source (20) and each having also first and second mutually perpendicular sides (11c, 11d) forming edges with said pole faces (11a, 11b), said first sides (11d) being perpendicular to the axis (22a) of said helix (22) and said second sides (110), being parallel to the line of intersection (10) of said pole face planes and respectively intersecting said diverging pole faces (11a, 11b) in lines defining their edge of nearest approach and hence also defining the throat of the space enclosed by said diverging faces, and a second diaphragm (12) in the form of a slit bisected by said line of intersection (10) of said pole face planes, said second diaphragm (12) being at a distance A from a boundary plane comprising said second sides of said pole pieces which is parallel to said line of intersection (10) as aforesaid, being also offset by a distance of substantially 1.35 A from a plane comprising said first sides (11d) of said pole pieces, and being further offset by a distance of substantially 2.10 A from said axis (22a) of said helix (22). 2. A mass spectrometer as defined in claim 1 in which said second diaphragm (12) has an opening which is small compared'to the opening of said first diaphragm (25) and in which the voltage between the open grids (23, 24) at the ends of said helix is not more than 2 percent of said acceleration voltage.
3. A mass spectrometer as defined in claim 1 in which said helix (22), said source of electrons (21) and said open grids (23, 24) at the ends of said helix are surrounded by an electrostatic screen (26) which also encloses the end of said helix (22) farthest from said first diaphragm (25) and in which said electrostatic screen (26) is connected to said means for applying electric potential (16) so that it is at a potential in the neighborhood of that of said source of electrons (21).
4. A mass spectrometer as defined in claim 3 in which said source of electrons (21) is a thermionic cathode of substantially hairpin shape and in which said electrostatic screen (26) is connected to said means for applying of potential (16) so as to maintain it at a voltage slightly more negative than the potential at which said source of electrons (21) is maintained.
5. A mass spectrometer as defined in claim 1 in which said open grids (23, 24) adjacent to the extr'emeties of said helix (22) are electrically connected to the respectively adjacent ends of said helix (22).
6. A mass spectrometer as defined in claim 1 in which there are provided means for varying either the acceleration voltage or the magnetic field strength, or both, said acceleration voltage being defined as a voltage between said first diaphragm (25) and its grid (25a) and that one (24) of said open grids (23, 24) which is adjacent the end of said helix (22) nearer said first diaphragm (25). r