For certain mixed mode resins having anionic character, a ligand is joined to a solid support via a linkage that includes a mercapto-, ether- or amino-containing moiety. A suitable ligand comprises an aromatic group, a heteroaromatic group, or a heterocyclic group, optionally fused, that is sulfate-, sulfonate-, phosphonate- or phosphate-substituted and that is linked to such a moiety. These resins possess an anionic character under conditions prescribed for their use. Separation of a biological substance, such as a peptide or protein, can be accomplished with a resin of this type via a change in the pH of eluants, thereby effecting adsorption and desorption.