I am trying to find interaction between two purified proteins .For that I am taking purified GST protein as a negative control,since GST should not bind to NI-NTA agarose beads.But after incubating GST with Ni NTA resins and washing the column with tris,NaCl and imidazole,i am still getting faint bands of GST in my elutions.(elution buffer has 500 mM imidazole). And I am getting clear bands of GST after boiling the beads in SDS sample buffer .Please suggest any troublshooting for removing this non specific binding.