Detection wavelength and why 214 nm and 280 nm disagree

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.

I read post #11 twice before replying, because I had assumed the opposite.

System suitability testing: injections run before and during the sample run to establish whether the instrument, column and method were performing when the sample was analysed. If suitability did not pass, the sample results from that run are uninterpretable.

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.

On post #33 — agreed on the reasoning, with one qualification.

Column chemistry and particle size: smaller particles (1.7 μm) give better resolution and higher efficiency than larger particles (3.5 μm or 5 μm), at the cost of higher back pressure. Newer methods increasingly use smaller particles.

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.

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.

Column chemistry and particle size: smaller particles (1.7 μm) give better resolution and higher efficiency than larger particles (3.5 μm or 5 μm), at the cost of higher back pressure. Newer methods increasingly use smaller particles.

Coming back to post #77, because the follow-up matters more than the original answer.

Method validation is the demonstration that a method can separate the compound from its degradation products and impurities reliably. A method that cannot resolve an impurity from the parent peak will not detect that impurity.

post #82 is right about the mechanism and I think understates the practical bit.

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.