Radiant energy – Ionic separation or analysis – Static field-type ion path-bending selecting means
Patent
1991-12-02
1993-03-16
Berman, Jack I.
Radiant energy
Ionic separation or analysis
Static field-type ion path-bending selecting means
250296, 250305, 250396R, H01J 4948, H01J 4932
Patent
active
051947323
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a charged-particle energy analyzer suitable for use in a double focusing mass spectrometer, and to a mass spectrometer incorporating such an analyzer.
The most common type of charged-particle energy analyzer incorporated in mass spectrometers is a cylindrical sector electrostatic analyzer. Such an analyzer provides energy dispersion and first-order focusing along only one axis and is therefore well suited to combination with a magnetic sector mass analyzer to make a double focusing (i.e., both direction and velocity focusing) mass spectrometer. Unfortunately, cylindrical sector analyzers comprise two curved electrodes which must be machined to very close tolerances and are therefore expensive to manufacture. Further, the use of a cylindrical sector analyzer in a mass spectrometer fitted with a multi-channel detector for the simultaneous detection of more than one mass-to-charge ratio imposes some serious limitations on its performance. Firstly, because of the limited spacing between the electrodes, the extent of the focal plane is limited, so that the range of masses that can be simultaneously imaged is also limited. Secondly, the focal plane of such a conventional analyzer is not usually perpendicular to the direction of travel of the ions leaving it, but inclined at a shallow angle. This further limits the maximum extent of the spectrum which can be simultaneously recorded and complicates the design of the detector system. Further, because a conventional analyzer comprises only 2 electrodes, the electrostatic analyzing field is determined entirely by the shape of the electrodes. This means that the homogeneity of the field cannot be varied and the number of aberrations (e.g. focal plane tilt and curvature) which can be corrected is very limited. Similarly, although a greater mass range can be transmitted by use of an analyzer with a wider gap, it is then necessary to increase the height of the plates to ensure that the field in the vicinity of the ion beam is sufficiently uniform, and this often results in a very large and prohibitively expensive analyzer.
Very few analyzers are known which do not rely on the field generated between two accurately shaped electrodes to define the energy dispersing field. Auxiliary electrodes are used in prior analyzers to compensate for the effect of fringing fields where the charged-particle beam enters and leaves the analyzer, but these do not define the main analyzing field. In these analyzers, one or more electrodes are provided at the entrance and exit of the analyzer and are maintained at potentials such that the field between the main electrodes is maintained as close as possible to the ideal field (e.g., a 1/r field in the case of a cylindrical sector analyzer). Similar fringing-field corrector electrodes may be provided around the edges of a parallel-plate analyzer, (see, for example, Stolterfoht in DE2648466 A1).
Matsuda (Rev. Sci. Instrum. 1961, vol 32(7), pp 850-852) has described a variable focal length cylindrical sector analyzer which comprises a pair of conventional sector electrodes and a pair of planar auxiliary electrodes, respectively disposed above and below the sector electrodes (i.e., displaced along the "z" axis). Application of a potential difference between these electrodes results in curvature of the equipotential surfaces along the "z" axis so that the analyzer exhibits some focusing in the "z" direction. A similar concept is disclosed in JP 61-161645 A1 (1986). Matsuda also suggests replacing each of the planar auxiliary electrodes with a number of wires disposed in concentric circular arcs and applying different potentials to each wire in order to correct aberrations, but does not give details as to how this might be achieved in practice. In a later paper (Int. J. Mass Spectrom Ion Phys., 1976, vol 22, pp 95-102), Matsuda suggests using the auxiliary electrodes in conjunction with shims on the main electrodes to reduce the height of the main electrodes needed to obtain adequate field homogeneity. In all these analyze
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Dymovich, Dorofeev, Petrov, "Triple Focusing Crossed Field Mass Spectrometer", published on Physical Electronics (Moscow) 1966 vol. 3, pp. 66-75, an English translation is enclosed. (Exhibit F).
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