Preparative HPLC is used in reverse phase (water/solvent) or normal phase (solvent/solvent) to separate the main product fraction from the contaminating molecules. The waste stream is spent solvent that must be discarded. While the main fraction contains the product that is to be concentrated.

A hydrophilic solvent stable membrane is used for reverse phase separations. In this example, the product stream from the HPLC contains a low molecular weight pharmaceutical compound dissolved in a methanol: water solution (90:10). The compound concentration is 0.5% leaving the HPLC. The product is concentration (0.025%) in the same solvent composition and is processed in the solvent recovery system. The overall solvent recovery is 96%.