Comparing two chromatograms from different laboratories, properly

Posting this under the heading it deserves: Comparing two chromatograms from different laboratories, properly Everything below is what sits behind that.

I would like to understand what this number means before I repeat it anywhere.

A Medutest report on a retatrutide lot gives 96.6% purity. The supplier certificate for the same lot states 98.9%. Both documents name a reversed-phase method; neither states the same gradient.

My question is not "who is right". It is: given that those two figures were produced by different methods, what is the largest difference I should expect from method alone, and at what point does a gap stop being explainable that way?

On post #9 — agreed on the reasoning, with one qualification.

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.

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.

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.

post #31 answers the question as asked. The question underneath it is different.

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.