In an affinity-type purification, ligands dissociated from a packed bed that would otherwise leach into the solution containing the species being purified are captured by a second ligand that resides in a porous barrier downstream from the packed bed, the second ligand exhibiting an affinity-type interaction with the dissociated first ligand with sufficient specificity to avoid the undesired retention by the second ligand of species from the liquid sample or source liquid other than the species sought to be purified in the affinity column.