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c/coa·posted 15 days ago by u/milos_trevino

acetate counterion is why your 10mg vial has 8.7mg of peptide

Lab

Posting this as a discussion rather than a claim: acetate counterion is why your 10mg vial has 8.7mg of peptide.

Numbers, in the order they matter: 10mg and 8.7mg.

UV at 214nm sees the peptide bond and picks up almost everything; at 280nm you are looking at aromatics only. Which wavelength the number came from is part of the number.

Related substances itemised individually is a much stronger document than a single total, because it tells you the shape of the impurity profile rather than its size.

Screenshot none of this. Read the whole thread, including the parts where I am told I am wrong.

0 up / 0 down43% upvoted7 commentsid 1w6gkr15 Jul 2026

7 comments

7 in this archive, depth 6

best — the order this archive was captured in

u/yusuf_nakamura3 points·14 days ago

Area percent is the peak area of your compound over the total integrated area, at one wavelength, on one gradient. It is a relative measure of what the detector saw, not a mass fraction of the vial.

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[deleted]2 points·14 days ago

[deleted]

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u/milos_trevinoOP1 point·14 days ago

Are the related substances itemised or is it a single total?

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u/spreadsheet_gremlindata1 point·14 days ago

area% is not mass%, and the difference is the whole game

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u/milos_trevinoOP1 point·14 days ago

Is water content on the document? It changes your concentration arithmetic.

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u/zaid_balogun1 point·13 days ago

LC-MS gives you identity; UV purity gives you a relative quantity. A document with the first and not the second, or the reverse, is answering half the question.

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u/teodor_jansen1 point·14 days ago

The PDF creation date being later than the analysis date is usually a re-issue or a re-export. It is worth one question and it is not evidence of anything on its own.

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About c/coa

How to read a certificate of analysis and what it does and does not tell you. Area% versus mass%, identity versus quantity, water content, related substances, acetate counterions, and the difference between a vendor-supplied COA and an independent test on the vial you actually received.

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