How to resuspend primers and oligos to 100 µM

Updated October 30, 2026

Primers and oligos usually arrive dried in a tube, with the amount printed on the label in nanomoles. Resuspending them to a round concentration, almost always 100 µM, makes every later dilution simple.

The calculation

1 µM is 1 pmol per µL. So for a stock concentration in µM:

volume (µL) = nmol on the tube ÷ desired µM × 1000

For a 100 µM stock, this simplifies to nmol × 10 µL.

Step by step

1. Spin the tube briefly before opening. The dried oligo may be on the lid or walls.
2. Add the calculated volume of nuclease-free water or low-EDTA TE buffer (10 mM Tris, 0.1 mM EDTA, pH 8).
3. Vortex briefly, let it sit for a few minutes at room temperature, vortex again and spin down.
4. Label the tube with the name, concentration and date.

Make a working stock

Most PCR recipes call for a 10 µM primer working stock. Dilute the stock 1:10, for example 10 µL of 100 µM stock + 90 µL water. Use the working stock day to day and keep the concentrated stock in the freezer, so it goes through fewer freeze–thaw cycles and is less exposed to contamination.

Water or TE?

TE keeps the pH slightly alkaline and protects DNA from nucleases, so it is better for long-term storage. Water is fine for working stocks. Avoid regular TE (1 mM EDTA) for oligos you will use in reactions sensitive to Mg²⁺ chelation; low-EDTA TE avoids that problem.

Storage

• Stocks: −20 °C, stable for a year or more.
• Working stocks: −20 °C, or 4 °C for a few weeks of frequent use.
• Fluorescently labelled probes: protect from light and aliquot to limit freeze–thaw cycles.