What a reversed-phase purity number actually is
Peaks that do not elute do not appear in the area percentage. Aggregates and strongly retained species can be entirely invisible to a standard method, which is a ceiling on what any purity figure can claim.
Post #10 and I disagree about the size of the effect, not about the direction.
Adding what did not work for me on reversed-phase purity number, since the failures never get written up and they are half the useful information.
Post #41 put the caveat in the right place and I want to underline it.
This is why a purity figure without the underlying chromatogram is weaker evidence than it appears. It is also why two competent laboratories can report different numbers on the same vial without either being wrong.
Not the answer, but possibly the question that gets there.
Trifluoroacetic acid at 0.1 per cent is the near-universal ion-pairing additive for this work, and it also raises the baseline at 214 nanometres. That is why the same sample looks noisier at low wavelength.
If the premise is wrong, everything after it is decoration.
Everything in post #65 holds. The case it does not cover is the one I have.
The version of reversed-phase purity number that I was taught turned out to be a teaching simplification. Useful, and not true in the way I had assumed it was.
Post #81 and I disagree about the size of the effect, not about the direction.
If two laboratories disagree by more than two or three percentage points, work through method, integration, sample handling, whether it was the same lot and the same vial, and whether suitability passed. After all five, a gap needs an explanation.
It is the kind of thing that is obvious once and never again.
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.
The reason reversed-phase purity number is hard to answer is that the obvious measurement and the relevant quantity are not the same thing, and substituting one for the other is silent.
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