need: info on A260-A280 absorbance measurements

[Richard Van Frank]

In article <53tt16$9gc at info.service.rug.nl>,
   g.j.e.j.hooiveld at farm.rug.nl (Guido Hooiveld) wrote:
>Hi everybody!
>
>After performing a prep I estimate the amount and purity of my plasmid DNA 
sample by measuring 
>the A260 and A280. I use the ratio of A at 260 and 280 nm to get an indication 
of the purity of 
>my sample. 
>
>I was told an A280/A260 ratio of 1.8 to 1.9 indicates a highly purified DNA 
preparation and its 
>theoretically limit is 2.0. However, I often find a ratio larger than 2.0; like 
2.1 to 2.2. So, 
>is this (theoretically) value of 2.0 correct, if yes, does anybody have a 
suggestion why I find 
>values >2; if not, what is this value? 
>
>And what about the factor you have to use to determine the concentration of DNA 
present: 
>multiply A260 with 50 to obtain the concentration (ug/ml), or use 35? Which 
value is correct?
>
>Any comments would be appreciated!
>
>Thanks in advance,
>
>Guido
>You should look at the scan from 220 to 300nm to see if there are other peaks 
or shoulders. If there are your preps are not pure. A260/280 ratio is applicable 
only to preps that are pure.
RMVF