The Mass Spectrometer

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The mass spectrometer is a device used to study isotopes of ions produced in a cathode ray tube. Isotopes of an element share the same chemical properties but have different masses and produce different values for their charge-to-mass ratio. An electron beam strikes vapor atoms that yield positive ions accelerated by a potential difference and deflected by a uniform magnetic field to hit a photographic plate. Mass spectrometry can also be used to identify molecules by analyzing the masses of its fragments.

Contributed by: Enrique Zeleny (March 2011)
Open content licensed under CC BY-NC-SA



The charge-to-mass ratio of an ion can be derived from Newton's second law and the Lorenz force, yielding


where is the charge, is the mass, is the potential difference, is the magnetic field intensity, and is the radius of gyration.

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