UF–DF was performed with Millipore 0.1m² Pellicon 2, 5kDa regenerated cellulose membrane (Millipore, Billerica, MA) operated by the Millipore Proflux M12 system. Samples were withdrawn at various times during the diafiltration step.

Figure 3. Concentration profiles of various excipients during diafiltration. The pH level is measured as a surrogate for the concentrations of Specie 1. All other species are not as limiting.

Figure 3 presents concentration profiles of the various species through the diafiltration process. Transport across the UF–DF membrane is governed by the following expression:

where Y _p is the concentration of a species in the permeate, Y _o is the concentration in the feed solution, S is the sieving coefficient, and DV is the number of diavolumes. The data presented in Figure 3 were analyzed using Equation (1) to calculate the sieving coefficient for the various species and the results are listed in Table 1.

Table 1. Determination of the sieving coefficient by fitting Equation (1) to the data presented in Figure 3.

The data suggest that Specie 1 has the smallest sieving coefficient, i.e., it is the slowest specie to transfer across the membrane and hence, it was chosen as an indicator of the completion of the diafiltration process. Another interesting observation that can be made from Figure 3 is that the pH profile shows a good correlation with the Specie 1 concentration profile. The pH increases as Specie 1 concentration drops with both stabilizing at approximately five diavolumes.