Why ng/µL and nM aren’t the same thing

A concentration in ng/µL tells you how much DNA you have by weight. It does not tell you how many molecules you have — and for library prep and loading, the molecule count is what actually matters.

Mass per volume vs molecules per volume

ng/µL is a mass concentration: nanograms of DNA per microlitre. nM (nanomolar) is a molar concentration: how many molecules are in that volume. They answer different questions — “how much stuff” versus “how many pieces” — and you cannot convert one to the other without one more piece of information.

The missing piece is fragment length

That information is the fragment length. A long molecule weighs more than a short one, so for the same mass you get far fewer long molecules than short ones. This is the whole point: the same ng/µL reading is a different molarity depending on how long your fragments are.

A worked example

Take 10 ng/µL of DNA. If the fragments are 200 bp, that works out to about 76.9 nM. If the fragments are 2000 bp — ten times longer — the very same 10 ng/µL is only about 7.69 nM. Identical mass concentration, but roughly a tenfold difference in molarity, purely because the longer fragments are ten times heavier each, so there are ten times fewer of them per nanogram.

Why it matters

Clustering on a sequencer, and most pooling and loading steps, depend on the number of molecules, not their combined weight. If you normalise libraries by ng/µL instead of by molarity, a library of long fragments will be loaded at a far lower molecule count than a library of short fragments at the same mass — leading to under- or over-clustering and skewed read shares. Converting to nM with the correct fragment length is what keeps loading concentrations honest.