is: a device that uses electric and magnetic fields to determine the masses of molecules and submolecular particles, including atoms, nuclei and (some) elementary particles. (Strictly speaking it is used to determine the mass of a related ion, rather than the electrically neutral particles themselves.) [P4.3, P8.1, P9.1]
is also used: to determine the relative abundances of various kinds of particles within a given sample.
occurs: in various types, using different arrangements of fields.
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