Methods for identifying a member of a mass-coded molecular library, which is a ligand for a biomolecule and binds to the biomolecule at the binding site of a known second ligand for the biomolecule are described. The methods includes contacting a mass-coded molecular library with a biomolecule; separating the biomolecule-ligand complexes from the unbound members of the mass-coded molecular library; contacting the biomolecule-ligand complexes with a second ligand to dissociate biomolecule-ligand complexes in which the ligand binds to the biomolecule at the binding site of the second ligand, thereby forming biomolecule-second ligand complexes and dissociated ligands; separating the dissociated ligands and biomolecule-second ligand complexes; and determining the molecular mass of each dissociated ligand.