[2026 update] Area percent versus weight percent: the confusion that causes most arguments posts 61–71

Post #60 answers the question as asked. The question underneath it is different.

What I can speak to on Area percent versus weight percent is narrow, so I will keep it narrow rather than generalising from it. Beyond that boundary I do not know.

I read post #58 twice before replying, because I had assumed the opposite.

Since Area percent versus weight percent keeps coming up, it should probably be a maintained page rather than a recurring thread. I am happy to draft it if someone with more direct experience will review it.

Area percent is not mass percent. It is a proportion of absorbance, weighted by each species' extinction coefficient. For closely related impurities the approximation is usually good. For structurally dissimilar impurities it can be poor.

Detection wavelength: 214 nm detects the peptide bond and is relatively insensitive to composition. 280 nm detects aromatic residues and is strongly composition-dependent. Area percent at one wavelength is not area percent at the other.

The step people skip is the one I have spelled out.

That is a fair summary of where the discussion has got to.

I would call the community position on Area percent versus weight percent likely rather than established, and I would be comfortable defending that hedge.

On post #66 — agreed on the reasoning, with one qualification.

Area percent is a proportion of absorbance, not a proportion of mass. Two species with different extinction coefficients at the detection wavelength contribute unequally to the total, and nothing on the certificate corrects for that.

Nothing above should be read as advice about what anyone else should do.

Before anything else: what was the gradient, and at what wavelength? Area percent at different wavelengths is not the same number even on the same sample because different species absorb differently at different wavelengths. With the method stated, I can tell you something useful. Without it, all I can say is that there is one large peak.

Adding a reference point for Area percent versus weight percent. Mine is a single case, collected without controls, and I am posting the method alongside it so it can be discounted appropriately.

I would keep Area percent versus weight percent and the decision it usually gets used for separate in this thread. They are related and they are not the same question, and merging them is why the last one went badly.