Nucleic acid applications
Spectrophotometry can be used to estimate DNA or RNA concentration and to analyze the
purity of the preparation. Typical wavelengths for measurement are 260 nm and 280 nm.
In addition measurements at 230 nm and 320 nm can provide further information.
Purines and pyrimidines in nucleic acids naturally absorb light at 260 nm. For pure samples
it is well documented that for a pathlength of 10 mm, an absorption of 1A unit is equal to a
concentration of 50 µg/ml DNA and 40 µg/ml for RNA. For oligonucleotides the concentration
is around 33 µg/ml but this may vary with length and base sequence.
So for DNA: Concentration (µg/ml) = Abs260 × 50.
These values are known as conversion factors.
A number of other substances which also absorb light at 260 nm could interfere with DNA values,
artificially increasing the result calculated from the absorption readings. To compensate for
this a selection of ratios and background corrections have been developed to help eliminate
false readings.
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
200 210 220 230 240 250 260 270 280 290 300 310 320 330 340 350 360 370 380 390 400
×
X: 300
Y: 0.063
Absorbance
Wavelength (nm)
Fig 4. A typical wavelength scan for a pure DNA sample.
7
There is a wide absorbance peak around 260 nm preceded by a ‘dip’ at 230 nm. Therefore
to measure the DNA absorption, the 260 nm DNA peak must be distinguishable from the
230 nm reading.
If the readings at 230 nm are too similar to those at 260 nm, DNA cannot be measured
accurately. Higher 230 nm readings can indicate contaminants in the sample. There should
also be a rapid tail-off from 260 nm down to 320 nm. For this reason, 320 nm is often used
to measure background (see background correction).