During ionization, some electrons may transfer much more energy than required to ionize the analyte molecule. This excess amount of energy is converted intro vibrational excitation of the incipient molecular ion. Consequently, some of the chemical bonds may be stretched beyond the maximum acceptable bond length resulting in bond breaking, and thus, fragmentation of the molecular ion. Provided the molecular ion carried a single charge – essentially one elementary charge as the charge was introduced by emission of one electron – the process of fragmentation will result in the formation of one charged and one neutral fragment.
Example: The mass of a butanone molecule is 72 u. It becomes ionized by interaction with a 70 eV electron. The molecular ion formed has one electron less than the neutral, i.e., it represents a singly charged radical ion. Many of these ions carry enough internal energy to cause fragmentation. Due to the structure of this molecular ion, there are four main fragmentation pathways to undergo dissociation. Some of the molecular ions will take one route, some take another. Energetically, some fragmentation pathways are easier to access than others causing some fragment ions to be more abundant than others. In the end, this is reflected in the relative peak intensities of the mass spectrum. In case of butanone we mainly observe signals at m/z 15, 29, 43, 57, and 72.
Don’t worry if the scheme is not immediately clear to you. Just have a closer look. The butanone molecular ion, m/z 72, carries a charge (symbol +) and is a radical at the same time (•). Whatever fragmentation pathway it may take, the products will always be one ion and one neutral. Mass spectrometry exclusively detects ions while neutrals are recognized by their difference in mass between ions, e.g. when an ion of m/z 72 fragments to yield an ion of m/z 57, the corresponding neutral must have had 15 u.