Extinction coefficient (e) is the absorbance of a 1M solution
in a cuvette of 1 cm path-length.
Oligonucleotide measured at A260
0.89 * ( #A * 15,480 + #C * 7,340
+ #G * 11,760 + #T * 8,850 ) (M-1
Protein measured at A280
#Trp * 5,500 + #Tyr * 1,490 + #Cys * 125 /2
Here # stands for number of.
To estimate sample concentration, use:
Molar concentration (M) = A / e
Concentration (mg/ml) =
A / e * Molecular Weight * 1000
For oligonucleotides, replacing # and Molecular Weight with averages, then
Concentration (mg/ml) ~
30 * A260
For more information and calculators, see BioToolKit.