Nomenclature of Fragment Ions
The accepted nomenclature for naming fragment ions from carbohydrates is that introduced by Domon and Costello in 1988  (Figure 3.6). Ions that retain the charge at the reducing terminus are designated X (cross ring), Y, and Z (glycosidic), whereas ions with the charge at the nonreducing terminus are A (cross ring), B, and C (glycosidic). Sugar rings are numbered from the nonreducing end for A, B, and C ions and from the reducing end for the others. Ions are designated by a subscript number that follows the letter to show the bond that is broken. However, for discussion purposes, in order to avoid the A, B, and C subscripts changing for glycans that have different length chains, we have introduced a modification in which the subscript numbers have been replaced by the subscript R (for reducing-terminal GlcNAc), R-1 (penultimate GlcNAc), and so on. . Fragments from branched-chain glycans are distinguished by Greek letters following the subscript, with a representing the largest chain. The bonds that are broken in cross-ring cleavages are shown by superscripts preceding the letter.