Purity of DNA (Ratio of OD = 1.8-2.0) ?

[Michael CHENG]

  Hi! I need help with a question that one of the student asked me during lab
today:
  Why is a preparation of DNA considered to be pure if OD260/OD280 => 1.8?
  Actually, I've never thought of it myself so I went around asking my labmates.
Well, nobody really knows the answer. Then I heard that there's possibly a
theoretical formula that sets the above value range.
  If this is true, I would appreciate it if someone out there can point me to
the reference.
  Thanks in advance.

Michael CHENG
Dept. of Biological Chemistry
Faculty of Pharmaceutical Sciences
Kyoyo University
Japan