A type of secondary ion mass spectrometer (instrument used to disperse radiant energy or particles into a spectrum and measure properties such as wavelength, mass, energy, or index of refraction) in which primary ions are focused on a spot 1-2 micrometers in diameter, mass-charge separation of secondary ions is carried out by a double focusing mass spectrometer or spectrograph.

A magnified image of isotopic distributions on the sample surface is produced using synchronous scanning of the primary ion beam and an oscilloscope.