In trying to improve measurement of DNA concentration, I obtained a
spectrum of UV absorbance by scanning from 200 to 325 nm. In five
samples, there was only a very weak absorbance at 253 nm, and a stronger
absorbance at 217 nm. The absorbance at 253 nm is too weak to account
for the amount of DNA I see on an agarose gel, but the absorbance at 217
nm is about right.
I suspect that the DNA samples are contaminated with something. They
were purified from agarose using two different gel extraction methods.
What would cause the absorbance of a DNA sample to shift from the
expected maximum at 260 nm to 253 nm, or even 217 nm?