A method for purifying a desired substance by separating from each other a substance (I) from a substance (II), one of which is the desired substance, both of which have affinity for the same ligand structure, and wherein substance (I) is smaller than substance (II). The method comprising the steps of: (i) providing substances I and II in a liquid; (ii) contacting the liquid with an adsorbent which selectively adsorbs substance I; (iii) recovering the desired substance; The adsorbent has (a) an interior part which carries a ligand structure that is capable of binding to substances I and II, and is accessible to substance I, and (b) an outer surface layer that does not adsorb substance II, and is more easily penetrated by substance I than by substance II.