What causes molecular ion peaks?

The removal of an electron from a molecule forms a molecular ion. Molecular ions are energetically unstable and will break up into smaller pieces. When a sample molecule passes through a mass spectrometer, it will be bombarded with ion beams knocking off one electron from the sample molecule creating one positively charged ion and one uncharged free radical. The positive ion will travel through the mass spectrometer and once detected, it will produce a line on the diagram. On the other hand, uncharged particles along with neutral fragments and negative ions will not be read by the mass spectrometer and will not produce a line on the diagram. Thus, molecular ion peaks are produced by the heaviest ions passing through the mass spectrometer.