The slit system of a conventional mass spectrometer discriminates against ions having initial kinetic energies. Relations between the amount of the discrimination and the geometry of the system are developed and confirmed experimentally for ions having only thermal energy. Curves showing the discrimination against high energy fragments from a number of simple molecules are given. These curves show that measured relative abundances depend strongly on the geometry of the system and on the accelerating voltage.

In theory, the initial velocity distribution can be obtained from discrimination data. This method is shown to be impractical, and a better method is described, which involves electrostatic deflection of the ion beam in a direction parallel to the magnetic field. Curves of ion intensity vs. deflecting potential give directly the shape of the distribution for one component of initial velocity. Distribution curves are given for the same ions for which discrimination curves were obtained.

• Received 16 January 1950

DOI:https://doi.org/10.1103/PhysRev.78.597