An anion exchange separation article is provided that is useful for separation of complex samples that contain a mixture of materials having different ionic charges. The separation articles include a plurality of polymeric chains grafted to a porous polymeric substrate that is a solid. The plurality of polymeric chains extends away from a surface of the porous polymeric substrate and the side chains of the monomeric repeat units of the polymer chains are terminated with a guanidinium group or a salt thereof. The separation articles can be used, for example, for separation of biomaterials in a sample based on differences in their charges. Advantageously, the binding capacity of the anion separation articles is salt tolerant.