A polymer substrate functionalized with a functionality comprising at least one cyclic, metal ion coordinating ligand group which comprises at least 3 nitrogen donor atoms in the ring of the cyclic group, at least one of the nitrogen atoms having an optionally substituted carboxy(lower alkyl) or optionally substituted phosphono(lower alkyl) group covalently attached thereto, is well suited for use in conjunction with "hard" metal ions of low toxicity (such as Ca.sup.2+, Mg.sup.2+ or Fe.sup.3+) in the separation/purification of appropriately "tagged" polypeptides by Immobilized Metal ion Affinity Chromatography (IMAC).