Low-pressure vapors of elements or molecules are hit by a beam of rapidly moving electrons. The collision knocks an electron off the sample atom or molecule, leaving it positively charged. These newly-formed ions are accelerated out of the ionization chamber by an electric field. The speeds to which the ions can be accelerated by the electric field are determined by their masses.
Lighter ions can go faster than heavier ones.
When the beam of a mixture of isotopes of different masses falls on a photographic plate, the different isotopes converge at different points, corresponding to the different radii of their semicircular paths. The radius of the semicircular path is proportional to the mass of the particle. The relative abundances of the ions can be determined from the densities of the photographic images they produce. The relative intensity is a measure of how many ions of any given mass are detected. With modern low-resolution instruments, one can expect to obtain an accuracy level of plus or minus 1 amu up to 1000 atomic mass units.