For determining the purity of a given DNA sample, what absorbance ratio is used?

The absorbance ratio used for determining the purity of a given DNA sample is the ratio of absorbance at 260 nm to absorbance at 280 nm, often represented as A260/A280.

Here are the steps to determine it:

1. Prepare the DNA sample and a blank (the solution without DNA).

2. Set the spectrophotometer to 260 nm.

3. Zero the spectrophotometer using the blank.

4. Measure the absorbance of the DNA sample at 260 nm.

5. Repeat the process at 280 nm: zero the spectrophotometer with the blank at 280 nm and then measure the absorbance of the DNA sample.

6. Calculate the ratio of the two absorbance values (A260/A280).

Pure DNA has an A260/A280 ratio of ~1.8. If the ratio is significantly lower, it may indicate protein contamination.