I am performing DNA Stable Isotope Probing. I am currently only running samples that should have C12 DNA in it but have run into a problem I haven't seen before. All of my DNA is ending up in my last fraction at about a density of 1.645 g/cm^3. My gradient looks good starting with my heaviest fraction having a density of about 1.77 g/cm^3 and my lightest being around 1.645 g/cm^3. I have divided it into around 25 fractions so that it is spread out well and I should be able to catch my C12 DNA at around 1.7 g/cm^3 and my C13 at 1.72 g/cm^3. I am creating the gradient using a 70.1i fixed rotor running the ultracentrifuge for 40 hours at 4C at 32000 RPM. Why would all my DNA end up at 1.645 g/cm^3 instead of 1.7 g/cm^3?