260/280 OD

"Debra Sellers ", CHE sellers at CHUMA.CAS.USF.EDU
Fri May 19 17:03:20 EST 1995

On 18 May 1995, Barry Moore wrote:

> I have been using a Beckman DU-64 spectrophotometer to quantify
> concentrations of DNA suspended in H2O or TE.  I expect and sometimes
> get a 260/280 ratio of 1.8 for pure DNA, and when the ration is 
> around 2.0 I suspect the presence of RNA.  However, I have not been
> able to find an explination for the occasional 260/280 readings of
> 2.5-3.0.  What type of contamination or situation would elevate the
> 260/280 ratio above 2.0?
> Thanks for any comments that you might have.
> Barry Moore
Are you using Phenol in your extraction of the DNA? If so, there may be 
residual phenol left in your sample. Since it has one lambda max at 
270nm, it can cause you to get a higher ratio, and an overestimation of 
your DNA concentration. This info is from a lab manual for recombinant 
DNA by Zyskind and Bernstein.

Hope this info helps

Debbie Sellers  

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