Usually, these are formed by a combination of b type and y type cleavage to produce the illustrated structure, an amino-acylium ion. Somethimes, internal cleavage ions can be formed by a combination of a type and y type cleavage, an amino-immonium ion. Internal fragments are labelled with their 1 letter amino acid code. An internal fragment with just a single side chain formed by a combination of a type and y type cleavage is called an immonium ion. These ions are labelled with the 1 letter code for the corresponding amino acid.
Bold face indicates strong signals, italic indicates weak.
In these structures, R' is the substituent, if any, at the beta carbon following the loss of the gamma carbon by side chain cleavage. Isoleucine and threonine are doubly substituted at the beta carbon, so that side chain loss can give rise to two different ion structures. These pairs are designated d and d' or w and w' . A useful feature of d and w ions is that they enable the isobaric residues leucine and isoleucine to be differentiated. Satellite ions from side chain cleavage are not observed.