▁differences ▁at ▁ 2 8 ▁Da ▁since ▁b - ions ▁can ▁form ▁a - ions ▁by ▁loss ▁of ▁CO . ▁Look ▁for ▁b 2 - ions ▁at ▁low - mass ▁end ▁of ▁the ▁spectrum , ▁which ▁helps ▁to ▁identify ▁y n - 2 - ions ▁too . ▁Mass ▁of ▁b 2 - ions ▁are ▁listed ▁in ▁Table ▁ 2 , ▁as ▁well ▁as ▁single ▁am ino ▁ac ids ▁that ▁have ▁equal ▁mass ▁to ▁b 2 - ions . ▁The ▁mass ▁of ▁b 2 - ion ▁= ▁mass ▁of ▁two ▁am ino ▁acid ▁resid ues ▁+ ▁ 1 . ▁ ▁Ident ify ▁a ▁sequence ▁ion ▁series ▁by ▁the ▁same ▁mass ▁difference , ▁which ▁matches ▁one ▁of ▁the ▁am ino ▁acid ▁resid ue ▁masses ▁( see ▁Table ▁ 1 ). ▁For ▁example , ▁mass ▁differences ▁between ▁an ▁and ▁an - 1 , ▁b n ▁and ▁b n - 1 , ▁c n ▁and ▁c n - 1 ▁are ▁the ▁same . ▁Ident ify ▁y n - 1 - ion ▁at ▁the ▁high - mass ▁end ▁of ▁the ▁spectrum . ▁Then ▁continue ▁to ▁identify ▁y n - 2 , ▁y n - 3 … ▁ ions ▁by ▁matching ▁mass ▁differences ▁with ▁the ▁am ino ▁acid ▁resid ue ▁masses ▁( see ▁Table ▁ 1 ). ▁Look ▁for ▁the ▁corresponding ▁b - ions ▁of ▁the ▁identified ▁y - ions . ▁The ▁mass ▁of ▁b + y ▁ ions ▁is ▁the ▁mass ▁of ▁the ▁pe pt ide ▁+ 2 ▁Da . ▁After ▁ident ifying ▁the ▁y - ion ▁series