For reliable cross section data a focusing time-of-flight mass spectrometer has been presented which collects all the ions produced by the electron impact dissociative ionization of molecules. The focusing characteristic of the mass spectrometer generates the ions close to the axis and focuses the energetic ions back to the detector. By observing the deflection curves we can measure the cross sections conclusively. The complete collection allows one to extract the initial kinetic energy distribution of the ions from the time-of-flight profiles of the mass peaks. As a first example we measured the cross sections of electron impact dissociative ionization of CO2 for electron energies from threshold to 300 eV. The results agree very well with recent measurements of Straub et al. [J. Chem. Phys. 105, 4015 (1996)], although previous data are in very poor agreement with each other. We measured the initial translational energy distribution of the fragment ions from the dissociative ionization of CO2. With respect to the kinetic energy distribution of the ions we analyzed the techniques which make use of the quadrupole mass spectrometer. We discuss why the results from the quadrupole mass spectrometer underestimate the cross sections for the fragment ions. Finally we suggest to recheck the data from earlier quadrupole mass spectrometer measurements.