Affinity chromatography
To prepare for affinity chromatography, scientists decide which molecule they want and find its ligand.  The ligand could be an antibody if the target molecule has an antigen; DNA if the target molecule is a polymerase, DNA-binding protein, or DNA-binding enzyme; aromatic molecules if the target is a different kind of enzyme; or a peptide if the target is another biomolecule.  Then they prepare a stationary phase and treat it with the ligand.  The stationary phase can be membranes, special beads, or supports with different-sized pores in them.