[Explainer] area% is not mass% and here is exactly why it matters to your vial
I have typed some version of this comment about four hundred times so it is going in one place.
Area% is the main peak as a fraction of total UV absorbance across all peaks the detector saw. It answers: of the UV-absorbing material here, how much is the thing I wanted?
Mass% / net peptide content is how much of the vial's contents by weight is your peptide. It answers: how much did I actually get?
They differ because a lot of things do not absorb at 214 or 220nm and are therefore invisible to the purity number:
- water — a few percent residual moisture is normal even in a well-dried lyophilisate
- the counterion, usually acetate or TFA, which is part of the mass on the scale
- most inorganic salts
Which is how a vial can be a perfectly honest 99% pure and contain noticeably less than the labelled milligrams. Nobody lied. You asked one question and assumed the answer to a different one.
best — the order this archive was captured in
Worked example so people can see the size of it.
| Field | Value |
|---|---|
| Label weight | 10 mg |
| Chromatographic purity | 99.0% |
| Water content (KF) | 3.8% |
| Acetate content | 6.2% |
| Net peptide, approx | ~9.0 mg |
Every number in that table can be true at once and the vial is still a tenth light on peptide. This is chemistry, not fraud — and it is exactly why reputable documentation states net content and the rest let you assume.
this is the table i should have put in the post. adding it to the wiki version.
and note what this does to your reconstitution arithmetic: if you calculate concentration off the label weight you are slightly over-estimating. it is usually within the noise of everything else, but it is not zero.
agreed, and i would still tell people to use the label weight, because the alternative is everyone inventing their own correction factor and getting it wrong in both directions.
completely agree actually. one consistent slight over-estimate beats forty inconsistent adjustments.
this is the most reassuring disagreement i have ever read
Stickying the c/hplc crosspost of this. One addition from the analytical side: area% is also blind to identity. A closely related impurity that co-elutes with the main peak is counted as product, which is the entire argument for LC-MS confirmation on anything unusual.
identity and quantity are two questions and most COAs answer one. that sentence should be on the front page.
i have been on this site four months and this is the first time the two numbers have made sense. edit: went back and re-read three of my own old posts and i was confusing them in all of them.
tl;dr — purity says how clean, net content says how much, identity says whether it is the right thing. your COA probably answers one of the three.
- 1Worked example so people can see the size of it. Field Value Label weight 10…6 comments in this branch · started by u/karl_fischer_kev