Resuspending a dried oligo: from nmol to µM

A synthesised primer arrives as a dried pellet labelled with an amount, not a concentration. To get a usable stock you add a volume of buffer — and that volume is a one-line calculation.

Amount, not concentration

The vendor spec sheet gives the yield in nanomoles (nmol) — a count of molecules — which varies from synthesis to synthesis. A concentration only exists once you dissolve that amount in a known volume. Concentration is amount ÷ volume, so the volume you need is amount ÷ target concentration.

The formula

With the amount in nmol and the target in µM (µmol/L), the buffer volume in microlitres is:

volume (µL) = (nmol ÷ µM) × 1000

A 10 nmol oligo to a 100 µM stock: (10 ÷ 100) × 1000 = 100 µL. Check it — 10 nmol in 100 µL is 100 µmol/L, i.e. 100 µM. A 25 nmol synthesis to the same 100 µM would take 250 µL.

The shortcut worth memorising

For a 100 µM stock the arithmetic collapses to µL = nmol × 10. That is why 100 µM is such a common resuspension target: the volume is just the yield with a zero on the end, no calculator needed.

Stock versus working concentration

100 µM is a concentrated stock for storage. Most PCR wants a 10 µM working dilution, which you make separately (a 1:10 dilution of the stock). Keep the concentrated stock frozen and dilute a small working aliquot to avoid repeated freeze–thaw of the whole tube.