how to know RNA concentration? - (Jul/06/2008 )
I have done RNA isolation.I read in the protocol that an absorbance of 1 unit at 260 nm correspond to 40ug of RNA per ml. I don't understand with that statement. If I get absorbance score at 230nm = 0,090A, 260 nm = 0,184 A, 280nm=0,094A, 320nm=-0,005 ,concentration = 15,1, R 260/280 =1,909,and R 260/230= 1,989. which score refer concentration RNA per mL? 
this is quite easy.
first of all, we will convert the µg/ml to µg/µl because I think this is more convenient. 40µg/ml is the same like 0.04 µg/µl (just divided the 40 by 1000). now you calculate 0.04 * (your absorption at 260) * (your dilution factor) = µg/µl RNA in your sample tube.
in your example (lets say your dilution factor was 200 eg. 2 µl RNA in 398 µl water): 0.04 * 0.184 * 200 = 1.472 µg/µl
this is quite complicated but 0.04 * 200 = 8 so you can just calculate 0.184 * 8 = 1.472 µg/µl.
first of all, we will convert the µg/ml to µg/µl because I think this is more convenient. 40µg/ml is the same like 0.04 µg/µl (just divided the 40 by 1000). now you calculate 0.04 * (your absorption at 260) * (your dilution factor) = µg/µl RNA in your sample tube.
in your example (lets say your dilution factor was 200 eg. 2 µl RNA in 398 µl water): 0.04 * 0.184 * 200 = 1.472 µg/µl
this is quite complicated but 0.04 * 200 = 8 so you can just calculate 0.184 * 8 = 1.472 µg/µl.
OK Ned Land, thank's a lot for your answer
first of all, we will convert the µg/ml to µg/µl because I think this is more convenient. 40µg/ml is the same like 0.04 µg/µl (just divided the 40 by 1000). now you calculate 0.04 * (your absorption at 260) * (your dilution factor) = µg/µl RNA in your sample tube.
in your example (lets say your dilution factor was 200 eg. 2 µl RNA in 398 µl water): 0.04 * 0.184 * 200 = 1.472 µg/µl
this is quite complicated but 0.04 * 200 = 8 so you can just calculate 0.184 * 8 = 1.472 µg/µl.
OK Ned Land, thank's a lot for your answer
but I have one question more: Is the concentration displayed on spectrofotometer invaluable for assesment of purity of DNA/RNA? because, many protocol I read only mention ratio 260/280 or 260/230 to assess purity of DNA/RNA. So, what the purpose concentration displayed, and what's the mean?
first of all, we will convert the µg/ml to µg/µl because I think this is more convenient. 40µg/ml is the same like 0.04 µg/µl (just divided the 40 by 1000). now you calculate 0.04 * (your absorption at 260) * (your dilution factor) = µg/µl RNA in your sample tube.
in your example (lets say your dilution factor was 200 eg. 2 µl RNA in 398 µl water): 0.04 * 0.184 * 200 = 1.472 µg/µl
this is quite complicated but 0.04 * 200 = 8 so you can just calculate 0.184 * 8 = 1.472 µg/µl.
OK Ned Land, thank's a lot for your answer
but I have one question more: Is the concentration displayed on spectrofotometer invaluable for assesment of purity of DNA/RNA? because, many protocol I read only mention ratio 260/280 or 260/230 to assess purity of DNA/RNA. So, what the purpose concentration displayed, and what's the mean?
Because it is certain ammount/quantity of RNA that you will need downstream.