yl ▁bond ▁( N H - CH R ). ▁ ▁D ifferent ▁types ▁of ▁fragment ▁ ions ▁ ▁When ▁the ▁back bone ▁b onds ▁cle ave , ▁six ▁different ▁types ▁of ▁sequence ▁ ions ▁are ▁formed ▁as ▁shown ▁in ▁Fig . ▁ 1 . ▁The ▁N - term inal ▁charged ▁fragment ▁ ions ▁are ▁class ed ▁as ▁a , ▁b ▁or ▁c , ▁while ▁the ▁C - term inal ▁charged ▁ones ▁are ▁class ed ▁as ▁x , ▁y ▁or ▁z . ▁The ▁sub script ▁n ▁is ▁the ▁number ▁of ▁am ino ▁acid ▁resid ues . ▁The ▁nom en cl ature ▁was ▁first ▁proposed ▁by ▁Ro ep stor ff ▁and ▁F ohl man , ▁then ▁Bi emann ▁modified ▁it ▁and ▁this ▁became ▁the ▁most ▁widely ▁accepted ▁version . ▁ ▁Among ▁these ▁sequence ▁ ions , ▁a , ▁b ▁and ▁y - ions ▁are ▁the ▁most ▁common ▁ion ▁types , ▁especially ▁in ▁the ▁low - energy ▁collision - indu ced ▁dis soci ation ▁( C ID ) ▁mass ▁spect rom eters , ▁since ▁the ▁pe pt ide ▁am ide ▁bond ▁( CO - N H ) ▁is ▁the ▁most ▁vulner able ▁and ▁the ▁loss ▁of ▁CO ▁from ▁b - ions . ▁ ▁Mass ▁of ▁b - ions ▁= ▁ ∑ ▁( res id ue ▁masses ) ▁+ ▁ 1 ▁( H +) ▁ ▁Mass ▁of ▁y - ions ▁= ▁ ∑ ▁( res id ue ▁masses ) ▁+ ▁ 1 9 ▁( H 2 O + H +) ▁ ▁Mass ▁of ▁a - ions ▁=