Radiant energy – Ionic separation or analysis – Static field-type ion path-bending selecting means
Patent
1990-11-29
1992-07-28
Anderson, Bruce C.
Radiant energy
Ionic separation or analysis
Static field-type ion path-bending selecting means
250284, H01J 4948
Patent
active
051342871
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a double focusing mass spectrometer particularly, though not exclusively; with variable dispersion, and is particularly useful in connection with a multi-channel detector.
Most conventional high resolution mass spectrometers are of the Nier-Johnson or Hintenberger-Konig geometry in which a double (i.e. both direction and velocity) focused image is formed at a narrow collector slit on the ion-optical axis of the last analyzer. In such an instrument a spectrum is obtained by scanning both the electrostatic and magnetic analyzers to successively focus ions of different mass-to-charge ratios on the collector slit. An ion detector, typically an electron multiplier, is disposed beyond the collector slit to receive the ions passing through the slit and produce an electrical signal therefrom.
Although such spectrometers are highly developed and often have high sensitivity and resolution, they are inefficient in so far as only a small proportion of the ions emitted from a sample are detected at any one instant during a scan. The efficiency may be improved by the use of a multichannel detector which is capable of recording a significant part of the spectrum simultaneously. Such detectors typically comprise one or more microchannel plate electron multipliers followed by a phosphor screen and either a photodiode array or vidicon television camera for detecting the position of the electron impacts on the screen. Usually a fibre optic coupling is provided between the phosphor screen and the array or camera.
Multichannel detectors have been fitted to several different kinds of spectrometers. Dukhanvov, Zelenkov et. al (Instrum. and Expt. Techniques, 1980 vol 23(3) pp 726-9), and Tuithof, Boerboom and Meuzelaar (Int. J. Mass Spectrom. and Ion Phys, 1975, vol 17, pp 299-307), describe single focusing magnetic sector instruments fitted with microchannel plate detectors, and Tuithof, Boerboom, Kistemaker and Meuzelaar (Adv. in Mass Spectrom, 1978, vol 7, pp 838-845) describe a more advanced single focusing spectrometer with a channelplate detector and variable mass dispersion. Hu, Chen, Boerboom and Matsuda (Int. J. Mass Spectrom and Ion Proc, 1986, vol. 71 pp 29-36) describe a single focusing spectrometer with an auxiliary magnet for improving performance, and several workers have described Mattauch-Herzog double-focusing instruments with such detectors (e.g., Murphy, Mauersberger Rev. Sci. Instrum., 1985 vol. 56 (2) pp. 220-226; and Boettger, Giffin and Norris, A.C.S. symp. ser. No. 102, 1979, pp 291-318) Ouwerkerk, Boerboom, Matsuo and Sakurai (Int. J. Mass Spectrom and Ion Proc, 1986, vol 70, pp 79-96) and Cottrell and Evans (Anal. Chem. 1987, vol 59(15) pp 1990-1995), report the fitting of multichannel detectors to double focusing mass spectrometers with Nier-Johnson geometry.
Serious limitations on performance arise when multichannel detectors are fitted to spectrometers with geometries designed for scanning. In general, the extent of the spectrum which can be imaged is limited, and resolution is often degraded because of the finite spacing of the individual channels in the channel electron multiplier or the resolution of the photodiode array or television camera. The limitations arise at least in part because the magnification and dispersion of the spectrometer have been selected without regard to the requirements of the multichannel detector. Even on the single-focusing instruments described by Boerboom and coworkers, the need to provide variable dispersion and magnification to gain full benefit from the multichannel detector was recognized in 1978, and the limited performance improvement reported for double focusing instruments which have fixed magnification and dispersion is apparent from the references cited.
In the case of double focusing instruments it is necessary that the velocity focal plane and the direction focal plane (also known as the energy and angular focal planes, respectively) are both coincident and substantially flat over the extent of the detector. These conditions are no
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Anderson Bruce C.
VG Instruments Group Limited
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