need: info on A260-A280 absorbance measurements

[Guido Hooiveld]

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

-- 
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* Guido Hooiveld                              |                           *
* Groningen University Center for Pharmacy    |phone: (+)31 50 3637565    *
* dep. of Pharmacokinetics and Drug Delivery  |       (+)31 50 3633272    *
* A. Deusinglaan 1                            |fax:   (+)31 50 3633247    *
* 9713 AV Groningen                           |                           *
* The Netherlands                             |                           *   
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