I have homogenized rat plantar skin in lysis buffer and centrifuged at 13 500rpm for 50 minutes. I noticed that there seemed to be 2 phases in the supernatant so recentrifuged it. there are now definatley 2 phases - a white layer AT THE TOP and a clear (majority). I am doing this to probe for NGF by western blot. which phase should I take? what are they? could the top layer be salts?