nucleic acid purity criteria - (Nov/30/2006 )

hello frnds!
One basic question I want to ask, what is the principle behind taking A260/A280 & A260/A230 values while checking the purity of nucleic acids?What should be their values for a good DNA or RNA prep?
bye!

OD260 - specific for nucleic acids
OD280 - specific for proteins
OD230 and OD320 - contamination from extraction buffers

A260/A280 - literaturely, around 1.8 to 2.1 but this may not hold in all circumstances (eg. whether the NA is buffered or not when the OD ratio is taken - the absorbance value changes with pH, and CO2 in air may lower the pH to less than 6)
A260/A230 - literaturely, around 2 but again this may not hold in all circumstances
Acceptable ratio values is to be determined by the lab and the downstream applications.

Run gel as well. If affordable, the BioAnalyzer will make your days easier.