Topic summary

Purity assay versus content assay: two different numbers

Analytics › COA interpretation documentation quality batch & lot numbers reference standards certificate of analysis amino acid analysis

This is a generated summary. It shows the 5 most-liked posts from a topic of 11, in their original order, with the accepted answer included where one exists. It is a reading aid and it will miss nuance — the full topic is the record.

s.silvaTL217 May 2026#1

On the subject in the title: Purity assay versus content assay: two different numbers Working notes rather than a conclusion.

Asking about purity assay versus content assay directly, because I have read four threads on it and each answered a slightly different question.

The version I want answered is the narrow one: given the method stated below, is the result within what anyone else has seen? I am not asking what it means yet.

Method, numbers and the two assumptions I am aware of making are below. If the assumptions are wrong that is more useful to me than agreement.

46 likes 2mo

m.ibarraTL25 Jun 2026#4

s.leclerc, post #3: The signature block matters more than it appears to. A named individual with a date is a different kind of document from an unsigned one, whatever the numbers say. I have deliberately not rounded that, because the rounding is where the argument starts. Go to post

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

The single most useful discipline: read the document for what it does not say. The omissions are more consistent between suppliers than the numbers are.

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

7 likes in reply to #3 2mo

endo_fellow_rkTL3Endocrinology fellow18 Jun 2026#7

m.ibarra, post #4: Coming back to the opening post, because the follow-up matters more than the original answer. The single most useful discipline: read the document for what it does not say. The omissions are more consistent between suppliers than the numbers are. That is the version I would defend. It is not the version I started with. Go to post

Water content (residual moisture) matters: lyophilised peptide is not dry. A few percent water is normal. The difference between a TFA salt and an acetate salt can be several percent. Content calculations need the water content to be accurate.

4 likes in reply to #4 1mo

r.ekstromTL222 Jun 2026#8

endo_fellow_rk, post #7: Water content (residual moisture) matters: lyophilised peptide is not dry. A few percent water is normal. The difference between a TFA salt and an acetate salt can be several percent. Content calculations need the water content to be accurate. Go to post

A document that puts the specification in the result column has told you the release criterion rather than the measurement. Those are different claims and the difference is the whole point of reading the document.

Adding this to the thread rather than to the wiki, because I am not confident enough for the wiki.

12 likes in reply to #7 1mo

system_suitabilityTL3Analytical chemist3 Jul 2026#11

Where two documents for the same lot disagree, the useful question is which was measured and which was transcribed. Most such disagreements are the second.

Noting that I have skin in this question and have tried to discount for it.

28 likes in reply to #7 25d

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