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?

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

Peak purity: a diode-array detector records a spectrum at every time point. If a peak contains two co-eluting species with different spectra, the spectrum changes across the peak. A passing peak-purity result says the spectrum is constant; it is weak evidence of homogeneity if the impurities have similar spectra.

That is all I can say without guessing.

Nothing to add on the substance. Thank you for taking the question at face value.

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.

Taking post #29 at face value and following it one step further.

On integration: where the baseline is drawn matters more than most people realise. On a clean chromatogram with well-resolved peaks the choice is inconsequential. On a chromatogram with a trailing shoulder or a rising baseline it matters. Differences of one to two percentage points between defensible integrations are ordinary.

That is the version I would defend. It is not the version I started with.