How to Resuspend Primers and Oligos

The formula

Oligos arrive dried, with the amount in nmol on the tube or the data sheet. For a stock in µM:

Volume to add (µL) = nmol × 1,000 ÷ concentration (µM)

For the usual 100 µM stock this is simply nmol × 10: a tube with 25.4 nmol takes 254 µL. If the sheet gives µg instead, nmol = µg × 1,000 ÷ molecular weight.

Step by step

1. Spin the closed tube briefly so the dried pellet is at the bottom, not on the lid.
2. Add the calculated volume of low-EDTA TE (10 mM Tris, 0.1 mM EDTA, pH 8) or nuclease-free water.
3. Vortex, leave for a few minutes at room temperature, vortex again and spin down.

Making a working solution

PCR usually uses primers at 10 µM. From a 100 µM stock that is a 1:10 dilution: 10 µL of stock plus 90 µL of water or TE makes 100 µL at 10 µM. Using a working tube keeps the stock away from repeated pipetting.

Water or TE?

TE holds the pH steady, and slightly acidic water can slowly damage DNA, so TE stocks keep longer. The small amount of EDTA in low-EDTA TE does not bind enough magnesium to upset a PCR at working dilutions. Some applications, such as certain sequencing reactions, ask for water instead.

Storage

Keep stocks at −20 °C. Split them into a few aliquots so a single contaminated or over-thawed tube does not cost you the whole order; dried oligos keep for years, and stocks in TE keep for many months frozen.

Calculators

The Oligo Resuspension Calculator gives the volume for any stock concentration and the mix for a working solution. PCR Primer Stats gives a primer's molecular weight, and the Dilution Calculator handles any other dilution.