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

[Louis Geller]

I have found the same problem. The spec sometimes overestimates the 
DNA concentration; the agarose gel says it is lower.
	Routinely, after I run the Qiagen maxi prep, I let the sample stand for 2-3 
days in a microfuge tube at -20 C, then take the A260:A280 readings to 
calculate the plasmid DNA conc.
	
	I then run the sample on a gel and compare it with known concentrations 
of some other DNA sample (like lambda cut with HindIII).
	As far as the spec reading, we never measure a absorbance greater than 0.2.
Hope this helps,
Louis Geller...
Graduate Student - Cell & Molecular Biology
CSU Long Beach