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. 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 ]