For precursor ion quantification, a quantification channel is one of the different possible labeling states of a peptide corresponding to the different heavy amino acids used in the cell cultures.
For example, the SILAC 2plex methods are normally used with two quantification channels named light and heavy. The light quantification channel uses the natural isotopes of lysine (12C614N2) and arginine (12C614N4). In the heavy quantification channel, arginine 10 (13C615N4) replaces all arginines, and either lysine 6 (13C614N2) or lysine 8 (13C615N2) replaces all lysines.
You can use any chemical element and each of its available isotopes for precursor ion quantification. However, you must adhere to certain syntactical restrictions when you enter chemical compositions. The notation from unimod.org, which sets the nominal mass prefix in parentheses—for example, (13)C4 (15)N2—only applies to the elemental isotopes 2H, 13C, 15N, and 18O.