Although h p t f f is still an emerging technology, a number of recent studies have clearly demonstrated the potential of this separation technology. Several of these results are summarized in Table 1. The separation of bovine serum albumin (BSA) from an antigen-binding fragment (Fab) could be achieved with either protein collected in the retentate depending upon the choice of solution pH and membrane charge. In each case, purification factors were > 800-fold using a single-stage h p t f f device operated at the optimal pH and with the appropriate membrane. These results are discussed in more detail in Section V. The results

for the BSA-hemoglobin system demonstrate that h p t f f can be used to separate proteins with essentially identical molecular weights by exploiting differences in protein charge. In this case, operation at pH 7 caused a strong electrostatic exclu­ sion of the negatively charged BSA from the negatively charged membrane. The separation of BSA monomer and dimer occurs primarily because of the difference in protein size, with the more permeable monomer collected in the filtrate. How­ ever, electrostatic interactions are also important in this system due to the com­ bined effects of size and charge on protein transmission and to possible differ­ ences in the charge-pH profiles for the monomer and dimer.