I am trying to purify monoclonal antibody from cell culture supernatent. Elution was done using 0.1M glycine pH 2.5. When checked by 12% SDS-PAGE, a thick prominent band was observed around 200 KDa along with specific bands at 50 KDa (Heavy chain) and 25 KDa (light chain). I have tried using elution buffer of various range (pH 2 to pH 5), the result remains same. The binding buffer used was Sodium Phosphate (pH 7). How can get rid of the non specific band ?