[2026 update] Why 99.2% and 97.8% on the same lot can both be correct

Why 99.2% and 97.8% on the same lot can both be correct — that is the question, and I have not found it answered plainly anywhere I have looked.

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

A Medutest report on a tirzepatide lot gives 98.4% purity. The supplier certificate for the same lot states 98.2%. 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?

Answering the question the opening post raises rather than the one it answers.

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.

That is the shape of it. The detail is where I would expect to be corrected.

Small methodological point on 99.2% and 97.8%: repeating a measurement is cheap and resolves most of what is being argued about here at no cost to anyone.

Narrowing post #4, because the general version has more than one answer.

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 anyone can point at the primary source I would be grateful.

Mobile-phase preparation is a genuine source of between-laboratory variation. Acid concentration and organic modifier both shift retention, and neither is usually specified to the precision that would matter.

None of the above is medical advice and I am not qualified to give any.