We are using a Butyl-NPR HIC-HPLC column to separate monoclonal antibodies with attachments at 0-8 of the 8 intradisulfide bonds. We generally come up with 7 (of 9 suspected) peaks. After 4 attachments, the antibodies become more difficult to separate. Since the attachment is slightly negatively charged, would an ion-pairing agent be a possiblity here? Any suggestions to one? Does anyone know of a place to buy HPLC grade (NH4)2SO4?