In a CID experiment, mass-selected clusters collide with molecules of a neutral, non-reactive gas, such as helium, in the ion trap or collision cell of a MS. Multiple collisions between the ion and the neutral gas molecules result in an increase in the internal energy of the ion, ultimately leading to the fragmentation of the cluster. Fragmentation leads to the formation of at least two species: an ion, lower in mass than the precursor ion, and at least one neutral molecule that corresponds to the difference in mass between the fragment ion and the precursor ion. The degree of fragmentation depends on the internal energy of the activated ion, with greater internal energies corresponding to greater fragmentation.