A method of capturing human immunoglobulin Fc domains in a biofluid sample is provided. The method includes providing an affinity capture device. The affinity capture device includes a surface having an aptamer that is at least 80% identical to SEQ ID NO 1 immobilized onto the surface of the affinity capture device. The biofluid sample is diluted with a binding buffer. The binding buffer includes (A) tris(hydroxymethyl)aminomethane (Tris), trimethylamine (TES), 2-ethanesulfonic acid (MES), or 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES); (B) a magnesium cation at a concentration between about 10 μM to about 20 mM; and (C) a total monovalent cation concentration from 0 to no greater than 100 mM. The human immunoglobulin Fc domains in the biofluid sample are adsorbed to the aptamer with the binding buffer.