US3814936A

US3814936A - Mass spectrometers and mass spectrometry

Info

Publication number: US3814936A
Application number: US00186250A
Authority: US
Inventors: B Green
Original assignee: Associated Electrical Industries Ltd
Current assignee: Associated Electrical Industries Ltd
Priority date: 1965-08-12
Filing date: 1971-10-04
Publication date: 1974-06-04
Anticipated expiration: 1991-06-04

Abstract

In a mass spectrometer having a magnetic analyzer, auxiliary ion beam control means comprising electrode structures which behave as electrostatic lenses are located externally of the magnetic analyzer to adjust the focal length of the ion beam to compensate for irregularities in the magnetic field of the magnetic analyzer caused by eddy currents induced in the analyzer during high speed scanning.

Description

United States Patent 1191 Green June 4, 1974 MASS SPECTROMETERS AND MASS [56] References Cited SPECTROMETRY FOREIGN PATENTS OR APPLICATIONS [75] Inventor: Brian Noel Green, Sale, England 1.114.005 5/1968 Great Britain 250/419 ME 957,084 5/1964 G 1:13 1" 25041.9 ME [73] Ass1gnee: Associated Electrlcallndustrles rel "Am Limited, London, England [22] Filed: Oct. 4, 1971 Primary ExaminerJames W. Lawrence Assistant Examiner-C. E. Church 21 A l. N 186,250 1 pp 0 Attorney, Agent, or Firm-Watts, Hoffmann, F1sher &

Related US. Application Data 7 H i k [63] Continuation-in-part of Ser. No. 81,173. Oct. 15. I 1970, abandoned, which is a continuation-in-part of Ser. No. 767.565. Sept. 19. 19611, abandoned. which 1 -1 ABSTRACT is a continuation-in-part of Ser. No. 57!.04), June 8. r 1966' ahandmled' In a mass spectrometer having a magnetic analyzer,

auxiliary ion beam control means comprising elec- [30] Forelgn Apphcatlon Prmmy Data I trode structures which behave as electrostatic lenses 12,1965 Great Britain are located externall of the magnetic analyzer to ady I just the focal length of the lon'beam to compensate [52] US. 250/295, for irregularities in the magnetic field Of the magnetic int. Cl. analyzer caused by urrents induced in the ana.

Field of Search 250/4l.9 ME

lyzer during high speed scanning.

5 Claims, 4 Drawing Figures PATENTEDJun 41914 3L814L936 sum 1 or 2 1 MASS SPECTROMETERS AND MASS SPECTROMETRY CROSS-REFERENCE TO RELATED PATENT AND APPLICATIONS I British Patent No. 957,084 published May 6, 1964 on an application filed Oct. 9, 1962 by George Alan Errock, entitled IMPROVEMENTS RELATING TO MASS SPECTROMETERS.

This application is a continuation-in-part of application Ser. No. 81,173 filed Oct. 15, 1970 which, in turn, is a continuation of application Ser. No. 767,565 filed Sept. 19, 1968 which, in turn. is a continuation of application Ser. No. 571,049 filed'Aug. 8, 1966, all now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to improvements in mass spectrometers and mass spectrometry, and more particularly to the provision of an ion beam control means for a mass spectrometer which improves the resolution of a mass spectrometer at high scanning speeds by adjusting the focal length of the ion beam.

2. Prior Art In the analysis of a given compound'with a mass spectrometer, ions of the material to be studied are generated in a source and accelerated as a focused ion beam into an analyzer region where they are deflected. From the analyzerregion, some of the ions pass through a collector slit to a suitable detector.

The trajectory of an ion beam is often described in terms of X, Y and Z coordinates. The Y coordinate is along the beam axis. The X and Z coordinates are transverse to the ion beam axis in mutually perpendicular planes which intersect along the beam axis.

During a given analytical study, the magnetic analyzer is scanned by varying its magnetic field such that ions of different mass-charge ratios pass through the collector slit at different times during the scan. This scanning of the magnetic analyzer varies the amount of deflection of the ions passing through the analyzer. The type of deflection referred to is the deflection of the beam in the plane of the X -coordinate.

lon beams behave much like electron and light beams, and accordingly mass spectrometers utilize ion optics which are akin to electron and light optics. There are focal points, actually locii, along an ion beam which are referred to as crossover points.".At these crossover locations, the ion beam has its minimal crosssectional area.

The ions which pass through the collector slit have come to be referred to as focused on the slit. The

derivation of the term is from the fact that an ideally adjusted instrument has the focal length of the ion beam adjusted such that a crossover is at the collector slit. The term focused, however, has on occasion been used as a misnomer, to describe X coordinate deflection. It is a misnomer since deflection essentially affects the beam trajectory not its focal length.

When a mass spectrometer is scanned, particularly at high rates, irregularities in the magnetic field caused'by eddy currents or hysteresis affect the focal length of the ion beam. The progressive variation of the energizing current during scanning induces eddy currents in the magnet which tend 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. This means that the focal or crossover locations are caused to vary in position along the beam axis or Y coordinate. The ion beam can only be said to be focused on the collector slit when the focal length of the beam is adjusted such that a focal or crossover location is positioned within the collector slit.

Focusing of the ion beam on the collector slit is desirable in order to improve the resolution of the mass spectrometer. Since the beam has a minimum crosssectional area in the region of a crossover point, more of the ion beam will pass through the collector slit if a crossover point is positioned in the plane of the collector slit.

Such a focussing action whereby the focal length of the beam is adjusted along the Y axis, is entirely different and distinct from the action of X coordinate beam deflection. It is known, as is illustrated by the referenced British patent, to provide a means for adjusting the deflection trajectory of an ion beam to compensate for drift in the magnetic field induced by voltage and temperature variations. However, it must be realized that such trajectory compensations are X coordinate deflection compensations and do not serve to adjust the focal length of the beam along the Y coordinate to compensate for variations in focal length which occur due to eddy currents and hysteresis in fast scan conditrons.

' SUMMARY OF THE INVENTION According to the present invention, a mass spectrometer having a magnetic analyzer in which an electromagnet produces between poles thereof 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, and having means for producing scanning of the mass charge ratio spectrum of the ion beam by progressive variation of the electromagnetic current and thereby of the field, is provided with an auxiliary ion beam control means externally-located relative to the electromagnet poles. The ion beam control is arranged to produce on the ion beam, in dependence on the variation of the electromagnet current during deflection of the beam by the magnetic analyzer, an effect compensating for irregularities in the magnetic field caused by eddy currents induced in the magnet during scanning, thereby to improve the resolution of the spectrometer. The effect produced is that of adjusting the focal length of the ion beam so as to maintain a focal point or crossover point in the plane of the collec-' tor slit. The auxiliary ion beam control mean-s com-' prises one or more electrodes to which, or to each of which, a suitable varying voltage is applied during the scanning of the spectrum. The electrodes behave as an electrostatic lens to adjust the focal length of the beam along its axis-or Y coordinate. The control means functions so as to maintain a focal or crossover location in the plane of the collector slit so as to maximize resolution of the spectrometer.

Accordingly, it is a general object of the present invention to provide a mass spectrometer with an ion beam control means for adjusting ion beam focal length to compensate for hysteresis effects and maximize resolution during high speed scans.

Other objects and a fuller understanding of the invention may be had by referring to the following description and claims taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic sectional side elevation of a mass spectrometer embodying one form of the inventlOl'l;

FIG. 2 is a fragmentary view of an electrode structure shown in FIG. l'but drawn to a larger scale; and,

FIGS. 3 and 4 are diagrammatic sectional side elevations of mass spectrometers embodying further forms of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring first to FIGS. 1, 3 and 4, a 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 main- Mass spectrometers as thus far described are well known in the art, and the output from the electron multiplier I7 is used, after amplification, to provide a record of the ions passing through the adjustable alit in the'member 13. The angular deflection of an ion 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 scan by varying the current used in the electromagnet coil "C of the magnetic analyzer II. 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 I3, 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.

Referring now to FIG. I in particular, it has been found that under fast scanning conditions an improvement in the resolution can be obtained if certain irregularities in the field of the magnetic analyzer 11 which are produced by eddy currents in themagnet can be overcome. Thus, for maximum resolution, an auxiliary ion beam control means in the form of an electrode structure 21 is situated just before the electrostatic analyzer 7. The electrode structure 21 has applied thereto a control voltage which varies in-a predetermined manner in accordance with the current through the electromagnet coil 11C of the magnetic analyzer 11. As can be seen more clearly from FIG. 2, this electrode structure comprises three slit

plates

23, 25 and 27 disposed at spaced positions along the path of the beam with the minor axes of the slits parallel to the lines of force 29 of the electrostatic analyzer. The two

slit plates

23 and 27 are grounded and the slit plate 25 has applied thereto, by the control unit 51, a voltage which varies in accordance with the current through the electromagnet coil 11C so that during the scan of the mass charge ratio spectrum of thesubstance under analysis. the electrode structure behaves as an electrostatic lens which produces a compensating effect on the ion beam, and thereby improves resolution in the output from the output device between various peaks in the output.

The electrode structure need not be located as shown in FIG. 1 and could be located anywhere along the ion beam between the accelerating electrode 5 and the slitted member 13, except at the ion crossover points. For example as shown in FIG. 3, it could be located between the intermediate slit 9 and the electromagnetic analyzer 11, or as shown in FIG. 4 between the magnetic analyzer I1 and the slitted member 13. The operation of the arrangement is the same in principle whether the electrode structure is situated as shown in FIG. I, or as shown in FIG. 3, or as shown in FIG. 4, although the voltage waveforms required in the three cases will be different.

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

Although the invention has been described in its preferred form with a certain degree of particularity, it is understood that the present disclosure of the preferred form has been made only by way of example and that numerous changes in the details of construction and the combination and arrangement of parts may be resorted to without departing from the spirit and the scope of the invention as hereinafter claimed.

What is claimed is:

l. A mass spectrometer comprising:

a. an ion source for producing a beam of ions of a material to be analyzed;

b. a magnetic analyzer in which a main electromagnet produces between poles thereof 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;

c. scan means connected to the magnetic analyzer for producing scanning of the mass charge ratio spectrum of the ion beam by progressive variation of main electromatic current and therefore of said magnetic field;

d. output means for producing an indication of the scanned part of the spectrum'of the deflected beam;

e. auxiliary ion beam lens means externally located relative to the main electromagnet poles and comprising three slit plates defining apertures therein disposed transverse the ion beam with the apertures aligned with the ion beam to pass the ion beam therethrough, the first and third of said plates being grounded; and,

f. electric controlv means to electrically energize the second and central plates at varying potentials to produce on the ion beam, in dependence on the variation of the main electromagnet current during deflection of the beam by the magnetic analyzer, an adjustment to compensate for variations resulting from irregularities in the magnetic field caused by eddy currents induced in the main electromagnet during scanning, thereby to improve the resolution between peaks in the indication provided by the output means.

' 2. A mass spectrometer as claimed in claim 1, wherein the auxiliary ion beam focal length adjustment means is located along the path of the ion beam just before the magnetic analyzer.

3. A mass spectrometer as claimed in claim 1,

zer.

5. A mass spectrometer as claimed in claim I, including an electrostatic analyzer, wherein the auxiliary ion beam focal length adjustment means is located along the path of the ion beam before the electrostatic analy- ZGI'.

Claims

1. A mass spectrometer comprising: a. an ion source for producing a beam of ions of a material to be analyzed; b. a magnetic analyzer in which a main electromagnet produces between poles thereof 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; c. scan means connected to the magnetic analyzer for producing scanning of the mass charge ratio spectrum of the ion beam by progressive variation of main electromatic current and therefore of said magnetic field; d. output means for producing an indication of the scanned part of the spectrum of the deflected beam; e. auxiliary ion beam lens means externally located relative to the main electromagnet poles and comprising three slit plates defining apertures therein disposed transverse the ion beam with the apertures aligned with the ion beam to pass the ion beam therethrough, the first and third of said plates being grounded; and, f. electric control means to electrically energize the second and central plates at varying potentials to produce on the ion beam, in dependence on the variation of the main electromagnet current during deflection of the beam by the magnetic analyzer, an adjustment to compensate for variations resulting from irregularities in the magnetic field caused by eddy currents induced in the main electromagnet during scanning, thereby to improve the resolution between peaks in the indication provided by the output means.

2. A mass spectrometer as claimed in claim 1, wherein the auxiliary ion beam focal length adjustment means is located along the path of the ion beam just before the magnetic analyzer.

3. A mass spectrometer as claimed in claim 1, wherein the auxiliary ion beam focal length adjustment means is located along the path of the ion beam just after the magnetic analyzer.

4. A mass spectrometer as claimed in claim 1, including an electrostatic analyzer, wherein the auxiliary ion beam focal length adjustment means is located along the path of the ion beam after the electrostatic analyzer.

5. A mass spectrometer as claimed in claim 1, including an electrostatic analyzer, wherein the auxiliary ion beam focal length adjustment means is located along the path of the ion beam before the electrostatic analyzer.