Tips and notes on using NanoDrop for DNA measurements:
Unincorporated nucleotides and RNA absorbs at 260 nm. Low concentrations (<10 ng/uL) measured on NanoDrop may plausibly be caused entirely by impurities.
Proteins and phenolic compound absorb strongly at 280 nm.
NanoDrop may not be reliable for measuring oligomer concentrations? http://www.protocol-online.org/biology-forums/posts/41901.html
260/280 ratio should be ~ 1.8
260/230 ratio should be 1.8-2.2.