Deamidation and the close-eluting pair it produces posts 31–38

This follows post #28 rather than contradicting it.

Racemisation produces a diastereomer that is mass-identical and chromatographically resolvable only on a method chosen for the purpose. Standard reversed-phase frequently will not separate it.

I would want to see it done twice before believing it once.

Quietly grateful for the plain phrasing. Not every thread gets that.

Freeze-drying itself does not generate impurities in any significant way. Handling before and after it does, which is why transit conditions belong in the discussion.

What I would check first on deamidation is whether the thing being measured moved or whether the way of measuring it moved. Those look identical in a graph.

Practical experience of deamidation, offered as one case with the conditions stated, not as a general finding. Conditions first, because they are what make it interpretable.

An impurity profile that changes between lots is more informative than the total. A stable profile suggests a controlled process; a shifting one suggests something is varying.

The reason deamidation is hard to answer is that the obvious measurement and the relevant quantity are not the same thing, and substituting one for the other is silent.

Coming back to post #34, because the follow-up matters more than the original answer.

Deamidation at asparagine or glutamine adds approximately one dalton and frequently produces a close-eluting pair. It is the impurity most likely to be integrated into the main peak by accident.

Reading it back, the second half matters more than the first.