US3465142A

US3465142A - Mass spectrometer magnetic analyzer having pole pieces composed of spaced paramagnetic laminae

Info

Publication number: US3465142A
Application number: US571463A
Authority: US
Inventors: Thomas Oliver Merren
Original assignee: Associated Electrical Industries Ltd
Current assignee: Associated Electrical Industries Ltd
Priority date: 1966-08-10
Filing date: 1966-08-10
Publication date: 1969-09-02
Anticipated expiration: 1986-09-02

Description

Sept. 2, 1969 M R v 3,465,142

MAss SPECTROMETER MAGNETIC ANALYZER HAVING POLE I PIECES COMPOSED OF SPACE!) PARAMAGNETTC LAMINAE A Filed Aug. 10, .1966

w 2N 61 5 a .4 9

United States Patent 3,465,142 MASS SPECTROMETER MAGNETIC ANALYZER HAVING POLE PIECES COMPOSED OF SPACED PARAMAGNETIC LAMINAE Thomas Oliver Merren, Mawdesley, England, assignor to Associated Electrical Industries Limited, London, England, a British company Filed Aug. 10, 1966, Ser. No. 571,463 Int. Cl. H01j 39/36; B01d 59/48 US. Cl. 250-413 9 Claims ABSTRACT OF THE DISCLOSURE In a mass spectrometer apparatus including a magnetic analyzer establishing a path of ion travel from an ion source to a detector the magnetic pole pieces are composed of a plurality of spaced paramagnetic laminae having insulating elements therebetween. The laminae are stacked along an axis paalleling the beam path with the laminae positioned transversely with respect to the beam path.

This invention relates to improvements in mass spectrometers and mass spectrometry, and an object of the invention is the provision of apparatus which improves the resolution of a mass spectrometer at high scanning speeds.

According to the present invention a mass spectrometer includes means adapted to produce a beam of ions of a material to be analyzed, a magnetic analyzer in which a magnet produces a magnetic field which is transverse to the ion beam and differentially deflects the ions of that beam in a direction which is transverse to the beam in accordance with their mass/charge ratios, means for producing scanning of the mass/charge ratio spectrum of the ion beam, and output means providing an indication and/ or record of the scanned part of the spectrum of the deflected beam, wherein the magnet poles of the magnetic analyzer are built up of discrete magnetic elements electrically insulated from each other to inhibit eddy currents which would produce irregularities in the magnetic field, thereby to provide optimum focusing of the beam during scanning and thus to improve the resolution in the output from the output means between the various peaks in that output.

Preferably the magnetic elements are in the form of laminae ararnged with their planes parallel to the lines of force between the magnet poles of the magnetic analyzer and transverse to the path of the ion beam, but they could take the form of rods or bars arranged with their longitudinal axes parallel to the lines of force between the magnet poles, or could take the form of regular shaped parts, such as cubes, or even relatively small particles providing that the hysteresis loop of the resulting structure is not too broad.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which:

FIGURE 1 is a diagrammatic sectional side elevation of a mass spectrometer; and

FIGURE 2 is a perspective drawing of one magnet pole of the magnetic analyzer shown in FIGURE 1.

Referring first to FIGURE 1, the mass spectrometer includes an ion source chamber 1 into which a specimen carrying probe 3 can be inserted and in which ions can be liberated from that specimen. An electrode 5 to which an accelerating voltage is applied serves to repel these ions as a beam which passes first through an electrostatic analyzer 7 including opposed conductive plates 7P between which a potential difference is maintained, then through an intermediate slit 9 into a magnetic analyzer 11. In the magnetic analyzer 11 an electromagnetic coil 11C establishes a strong magnetic field directed in a direction transverse to the path of the ions, and since the ions are charged particles their paths will be curved in the magnetic field. The deflected ions, or certain of them if different groups of ions are deflected to different degrees, pass through an adjustable slit in a member 13 and are picked up by a collector electrode 15 associated with an electron multiplier 17.

Mass spectrometers as described above are well known in the art, and the output from the electron multiplier 17 is used, after amplification, to provide a record of the ions passing through the adjustable slit in member 13. The angular deflection of an ion in passing through the magnetic analyzer 11 will depend upon the accelerating voltage, since that determines the speed of the ions, the intensity of the field in the analyzer 11, and the mass of the ion. One method of scanning a large range of a mass spectrum is to maintain the voltages used in the electrostatic analyzer 7 and on the accelerating electrode 5 constant, and to scan by varying the current used in the electromagnet coil of the magnetic analyzer 11. This progressively changes the deflections of all the ions passing through the magnetic analyzer, so that the output from the electron multiplier 17 indicates the number of ions passing through the slitted member 13, and when presented on a cathode ray tube as the vertical deflection with a horizontal scanning speed corresponding to the decay or growth of the magnetic field in the magnetic analyzer 11, the trace shows peaks where ions having such a mass number that they are deflected to pass through the slitted member 13 are present.

It has been found that an improvement in the resolution can be obtained if certain irregularities in the field of the magnetic analyzer which are produced by eddy currents can be overcome. The eifect of eddy currents induced within the magnet poles during the scan is to alter the mean magnetic field through which the ions travel and thus alter the focal length of the magnetic field of the magnetic analyzer. Thus, for maximum resolution, the magnet poles of the magnetic analyzer 11 are built up of discrete magnetic elements which are electrically insulated from each other to inhibit these eddy currents. Referring now to FIGURE 2, which shows a preferred form of these magnetic elements, a yoke 31 of the magnetic analyzer has secured thereto an arcuate pole piece 33 built up of laminae 35 of magnetically soft material which are electrically insulated from each other. Typically, the laminae 35 which are arranged with their planes substantially normal to the path of the ion beam indicated by dotted line 37, have a length x of four inches, :1 width y of at least one inch and are about 0.014 inch in thickness. The laminae are separated from each other by a layer of insulating material 0.001 of an inch thick. In order to closely approximate to the arcuate shape of the pole piece 33, using substantially flat lamina, wedge shaped magnetic members 39 may interspace groups of the laminae 35. The magnetic scanning is produced by a control unit 51 arranged to progressively reduce the energizing current applied to the electromagnet coil 11C. The laminae inhibit eddy currents which would produce irregularities in the magnetic field during the scan and thereby improve resolution in the output from the output device between various peaks in the output.

The maximum speed at which the magnetic field can be scanned may also be limited by eddy currents flowing in the yoke 31 of the magnetic analyzer and these eddy currents may be inhibited by constructing the yoke 31 from laminae 41 of magnetically soft material electrically insulated from each other.

The invention is also applicable to mass spectrometers which do not have an electrostatic analyzer.

What I claim is:

1. A mass spectrometer comprising:

(a) an ion source for generating a beam of ions;

(b) a detector for receiving, measuring, and recording ions from said source;

(c) structure including a magnetic analyzer establishing a path of ion travel from the source to the detector; and

((1) said analyzer including a paramagnetic assembly having a plurality of paramagnetic elements, adjacent ones of said elements being spaced to provide insulating gaps therebetween with each of said gaps having elongated dimensions in a plane transverse to said path and a short dimension generally parallel to said path such that the elements are arranged to parallel magnetic lines of force of the analyzer.

2. A mass spectrometer as claimed in claim 1, wherein the paramagnetic elements comprise rods or bars arranged with their longitudinal axes parallel to the lines of force between the magnet poles of the magnetic analyzer.

3. A mass spectrometer as claimed in claim 1, wherein the paramagnetic elements comprise regular shaped parts.

4. A mass spectrometer as claimed in claim 1, wherein the paramagnetic elements comprise relatively small particles.

5. A mass spectrometer as claimed in claim 1, wherein the analyzer includes a yoke of the magnetic analyzer built up of discrete paramagnetic elements electrically insulated from each other to inhibit eddy currents which would produce irregularities in the magnetic field.

6. A mass spectrometer as claimed in claim 5, wherein the paramagnetic elements forming the yoke comprise laminae.

7. A mass spectrometer comprising:

(a) an ion source for generating a beam of ions;

(b) a detector for receiving, measuring, and recording ions from said source;

(c) structure including a magnetic analyzer establishing a path of ion travel from the source to the detector;

((1) said structure including a laminated pole piece composed of a plurality of spaced paramagnetic laminae and insulating elements therebetween; and

(e) said laminae being stacked along an axis paralleling said beam path with the laminae and elements positioned transversely with respect to said path.

8. A mass spectrometer as claimed in claim 7 wherein the laminae include wedge-shaped paramagnetic members interspaced with groups of substantially flat members to provide magnet poles of arcuate shape.

9. The mass spectrometer of claim 7 wherein each of said laminae is rectangular in cross section in a plane of cross section transverse to said ion path.

References Cited UNITED STATES PATENTS 2,541,656 2/1951 Long. 2,279,014 4/1942 Sawyer 336-219 X 3,226,587 12/1965 Chin 335210 X RALPH G. NlLSON, Primary Examiner A. L. BIRCH, Assistant Examiner U.S. Cl. X.R.