Radiant energy – Ionic separation or analysis – Static field-type ion path-bending selecting means
Reexamination Certificate
1999-10-19
2002-12-31
Lee, John R. (Department: 2881)
Radiant energy
Ionic separation or analysis
Static field-type ion path-bending selecting means
C250S281000, C250S288000, C250S290000, C250S293000
Reexamination Certificate
active
06501074
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to mass spectrometers, e.g., compact miniature mass spectrometers, for use in chemical analysis of samples. More particularly, the present invention pertains to mass spectrometers employing superimposed magnetic and electric fields.
BACKGROUND OF THE INVENTION
Various types of mass spectrometers are being used in the field of chemical analysis and related fields. For example, such mass spectrometers may include deflecting type mass spectrometers in which the ions constituting the ion beam are separated by a magnetic field according to their mass to charge (m/e) ratio. Deflecting-type mass spectrometers can broadly be classified into two categories, single-focusing and double-focusing mass spectrometers, according to the type of ion beam optical system employed. In the single focus category, directional focusing but not velocity focusing is possible, and in the double focus category, both directional and velocity focusing are possible.
In the past, generally, in conventional types of double-focusing mass spectrometers, the electric field and magnetic field used for deflection were arranged separately. However, in U.S. Pat. No. 3,984,682 to Matsuda, entitled “Mass Spectrometer With Superimposed Electric and Magnetic Fields,” issued Oct. 5, 1976, a mass spectrometer employing superimposed electric and magnetic fields arranged substantially at right angles to one another is described. As described in Matsuda, if the electric field of the superimposed fields is swept and the magnetic field of the superimposed fields is kept fixed, ions having different m/e ratios will satisfy the double focusing condition, and therefore, the ions can be collected at a detector. In such a case, the ratio of the voltage applied for establishing the electric field strength and the accelerating voltage must be kept constant during the sweeping of the electric field. Matsuda arranges cylindrical electrodes between magnetic poles such that the electric field of the device is perpendicular to the magnetic field to provide suitable x-y focusing. Further, in addition, auxiliary electrodes are arranged symmetrically above and below the cylindrical electrodes. Voltages corresponding to those applied to the cylindrical electrodes are applied to the auxiliary electrodes to control the shape of the electric field, particularly for controlling z-direction focusing. It is recognized in Matsuda that in the case of superimposed fields with the cylindrical electrodes arranged between the magnetic poles, that consideration must be given to the size of the cylindrical electrodes. In Matsuda, the ideal ratio of the distance between the cylindrical electrodes and their height is 1:2. However, having cylindrical electrodes of such a height adds to the size of the overall device and further decreases a desirably high magnetic field between the poles, i.e., the magnetic field decreasing as the height of cylindrical electrodes increases and the gap between the magnetic poles is increased.
Another mass spectrometer employing superimposed magnetic and electric fields is described in U.S. Pat. No. 4,054,796 to Naito, entitled “Mass Spectrometer With Superimposed Electric And Magnetic Fields,” issued Oct. 18, 1977. U.S. Pat. No. 4,054,796 describes the use of superimposed electric and magnetic fields arranged substantially at right angles. In U.S. Pat. No. 4,054,796, electrodes having concentric cylindrically shaped curved surfaces are used to form the electric field of the superimposed magnetic and electric fields, while magnetic pole pieces form a magnetic field perpendicular to this electric field. The superimposed fields are provided in an ion path in an airtight throughway. At one end of the ion path is an ionization chamber for providing an ionized specimen to the ion path. Electrodes producing a constant accelerating voltage (as opposed to a varying accelerating voltage as described in Matsuda) draw the ionized specimen into the ionization chamber in an ion path in which the superimposed fields are formed. At the other end of the ion path is an ion collector. The ions introduced into the superimposed fields in the ion path are deflected according to their mass to charge (m/e) ratios with such deflected ions being detected by the ion collector. In this mass spectrometer, the central orbit of the ion beam in the ion path (that is, the orbit of the ions of mass to charge ratio being detected) is located on an equipotential surface of the electric field. The intensity of the electric field in the ion path is swept (i.e., voltage on the cylindrical electrodes is varied) to change the mass to charge ratio of the ions traveling the central orbit. However, such sweeping of the electric field when the accelerating voltage is kept constant, is accompanied by an undesirable shift in the focusing position of the ion beam. The change in focusing position is compensated by a variety of techniques. For example, such compensation may be achieved by auxiliary electrodes placed above and below the curved surface electrodes. The voltage on the auxiliary electrode is varied as a function of the voltage applied to the curved surface electrodes.
Such existing mass spectrometers as described above are generally constructed in whole or in part with discrete metal electrodes and insulators assembled inside of a metal vacuum envelope. Due to such construction, the size of such existing mass spectrometers is generally large and the cost of such mass spectrometers prohibits their use for various functions, e.g., environmental monitoring, bedside patient care in hospitals, battlefield chemical and biological agent detection, chemical plant process control, etc.
Further, generally, as described above, one way of obtaining superimposed electric and magnetic fields is with the use of two cylindrical electrodes having the same cylinder axis, but different radii. If the axial height of these electrodes is much larger than the spacing between them, then the field near the center of the electrodes will have suitable geometry. However, many mass spectrometers require that there be a high magnetic field along the direction of the cylindrical axis which limits the spacing between the poles, i.e., pole gap, for creating such a high magnetic field. Therefore, the axial height of the cylindrical electrodes is limited by the small spacing between the poles of the permanent magnet for generating such a high magnetic field. Due to, at least in part, the limited axial height of such cylindrical electrodes, the electric field geometry near the top and bottom edges of the cylindrical electrodes is generally not the correct and desirable geometry. This is particularly the case when the electric field must be maintained at a particular ratio with the magnetic field in such mass spectrometers employing superimposed magnetic and electric fields.
In addition, mass spectrometers which employ superimposed magnetic and electric fields in an ion path assume that the magnetic field is perfectly homogenous within the boundaries of the magnet poles and zero outside those boundaries. However, it is virtually impossible to construct a magnet with this ideal field. In practice, there is always a fringing field which exists beyond the pole boundaries. Further, there is usually a significant inhomogeneity of the magnetic field inside the pole boundaries, unless a substantial cost is outlaid for producing a magnet which is substantially homogenous. For example, the field may vary within the pole boundaries in the range of about 10% of the average field therein. Due to such magnetic field inhomogeneity, resolution of mass spectrometers having superimposed magnetic and electric fields is significantly affected as the ratio of magnetic and electric fields is not consistently correct along the ion path.
SUMMARY OF THE INVENTION
There is a need in the art for improved mass spectrometer apparatus having superimposed magnetic and electric fields and methods regarding such spectrometer apparatus which reduce the effects of the problems descr
Diaz-Diaz Jorge A.
Gentry W. Ronald
Giese Clayton F.
Fernandez Kalimah
Lee John R.
Mueting Raasch & Gebhardt, P.A.
Regents of the University of Minnesota
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