picomole

The picomole is the everyday unit of DNA work, and it arrives with an identity that saves bench scientists arithmetic daily: 1 μM is exactly 1 pmol/μL, because the micro in the volume cancels the micro in the concentration. Primer stocks are kept at 100 μM, which is 100 pmol/μL, so a PCR wanting 10 pmol of each primer takes a microlitre of a tenfold dilution, and the whole calculation is done before the gloves are on.

The mass side has its own rule of thumb. A nucleotide contributes about 330 g/mol to a single strand, so a 20-base primer runs near 6600 g/mol and one picomole of it weighs about 6.6 ng. Synthesis houses sell oligos by nominal scale, 25 or 200 nmol, and that number names the starting synthesis scale, not the delivered yield, which is routinely several times smaller; the tube is quantified after delivery for exactly that reason.

One step down, proteomics counts the same way: a moderately abundant protein in one mammalian cell is femtomoles of material, and a strong stained gel band is low picomoles. The saving grace is the count underneath, six hundred billion molecules to the picomole, which is why this site's dilution solvers can treat vanishingly small amounts as continuous concentrations without apology.