The presently disclosed subject matter relates to compositions, systems and methods of screening one or more species of polypeptide in a complex mixture of polypeptides, e.g., multi-subunit proteins. For example, the subject matter relates to ligands used in connection with affinity capillary electrophoresis, as well as methods and systems for detecting polypeptides in a mixture of multimers that include multispecific antibodies, e.g., bispecific antibodies.