Discrepancy between OD-value on spec and DNA on agarose gel

[Botma Visser]

Hi

I have a problem in determining the concentration of plasmid 
DNA on a spectrophotometer.  I extracted plasmied DNA using 
the Qiagen midiprep columns.  When I determined the 
concentration of the plasmid DNA at 260 nm, I got a very 
high reading, with the concentration going up to 1.1 mg/mL 
from a 100 mL prep.  I then ran 500 ng plasmid DNA 
(calculated using the determined concentration) on an 
agarose gel, and saw nothing.  The prep contained no RNA.  
Have you encountered similar problems, and if yes, what can 
I do about it?  It is critical for me to have accurate 
concentrations.

Thank you.

Botma Visser

pbv at rs.uovs.ac.za