This thesis presents a description of a novel technique for the accurate experimental determination of absolute partial electron impact ionization cross sections of atoms and molecules by a calibrated time of flight mass spectrometer. The simple design of the present apparatus insures the total collection of the energetic fragment ions. Measurements for argon and nitrogen targets are presented over a range from threshold to 1 keV. Previously published electron impact data on argon, nitrogen, and oxygen are reviewed. Computer programs for the modeling of the data and fragment ions energy analysis are also presented.