260:280 DNA Ratio is higher than it should be

Question

Every time I measure 260/280 ratios, I get a value higher than 2 - typically 2.5-3. Other people using the same spec get 1.9ish values. What could I be doing wrong? What do high ratios mean?

Answer 1

You probably aren't doing anything wrong from a measurement standpoint but to check for operator error you could run a blank or a standard with a known pure amount of DNA.  If you get readings from your blank then you have contamination; if your A260/A280 ratio for the standard is not between 1.8 and 2 then you have operator error as well.

When you have A206/A280 ratios that fall outside of the 1.8 to 2.0 range this is an indication of protein contamination in your DNA sample.  You need to cleanup your sample using a purification kit!

Answer 2

If the ratio is above 2, then it is NOT protein contamination since that would reduce the ratio. It is more likely due to traces of phenol, alcohols or even some salts still in the sample