The Collaborative International Dictionary of English v.0.48:

mass spectrometer \mass spectrometer\ n. (Physics, Chemistry)
   An analytical instrument which determines the mass of
   molecules of a substance or fragments of its molecules. It
   functions by injecting ionized moecules or molecular
   gragments into a vacuum chamber subjected to a strong
   magnetic field, in which charged particles move in a curved
   trajectory.

   Note: The mass is determined by observing the path of the
         molecule after being charged and accelerated in a
         magnetic field. The path of the ionic fragment depends
         on the charge, velocity, and mass. Several techniques
         of charging and detection are employed. The mass
         information is usually used to infer structure or
         chemical composition of the substance analyzed.
         Variants of mass spectrometer called high-resolution
         mass spectrometer permit determination of the mass of
         a molecule or molecular fragment with sufficient
         precision to unambiguously determine the atomic
         composition.
         [PJC]

WordNet (r) 3.0 (2006):

mass spectrometer
    n 1: spectroscope for obtaining a mass spectrum by deflecting
         ions into a thin slit and measuring the ion current with an
         electrometer [syn: mass spectrometer, spectrometer]