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What a reversed-phase purity number actually is — the long version

Analytics › HPLC & UHPLC gradient method system suitability HPLC UV & DAD detection

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Solved by m.coelho in post #3

Gradient slope is the single biggest driver of apparent purity differences. A shallower gradient over a longer run resolves more impurities and gives a higher purity figure. A steep gradient produces a tidier-looking chromatogram with fewer visible peaks and gives a lower purity figure. Both are legitimate methods…

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MJayawardenaTL3Regular26 Jul 2025#1

What a reversed-phase purity number actually is — the long version I have a specific reason for asking rather than idle curiosity, and the context is below.

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

A Janoshik report on a semaglutide lot gives 96.3% purity. The supplier certificate for the same lot states 97.9%. 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?

49 likes 12mo

a.mwangiTL2 Moderator4 Oct 2025#2

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.

13 likes 10mo

m.coelhoTL2 Moderator Solution23 Nov 2025#3

Gradient slope is the single biggest driver of apparent purity differences. A shallower gradient over a longer run resolves more impurities and gives a higher purity figure. A steep gradient produces a tidier-looking chromatogram with fewer visible peaks and gives a lower purity figure. Both are legitimate methods and they will not produce the same number.

9 likes 8mo

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