Analytics · Impurities & related substances

Deamidation and the close-eluting pair it produces — the long version

Analytics › Impurities & related substances counter-ion content endotoxin testing data table

k.marchandTL2 Moderator18 Jun 2026#1

Posting this under the heading it deserves: Deamidation and the close-eluting pair it produces — the long version 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 semaglutide lot gives 97.9% purity. The supplier certificate for the same lot states 98%. 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?

2 likes 1mo

m.nwosuTL2 Moderator25 Jun 2026#2

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

Aggregates: multiples of the monomer mass. May not elute at all under a standard reversed-phase method. A species that does not come off the column does not appear in the area percentage.

6 likes 1mo

o.abrahamsenTL3Regular29 Jun 2026#3

I disagree with the reply above, and I think the disagreement is substantive rather than terminological.

The distinction being drawn does not survive when you look at the published data for this specific question. I would be glad to be shown wrong on this, because the version I am arguing against is more convenient.

22 likes 28d

h.ramosTL2 Moderator4 Jul 2026#4

o.abrahamsen, post #3: I disagree with the reply above, and I think the disagreement is substantive rather than terminological. The distinction being drawn does not survive when you look at the published data for this specific question. I would be glad to be shown wrong on this, because the version I am arguing against is more convenient. Go to post

Deamidation at asparagine and glutamine: adds 1 approximately. Frequently appears as a close-eluting pair. It is a chemical modification that occurs during storage.

0 likes in reply to #3 24d

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