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c/coa·submitted 7 days ago by u/zeynep_zielinski

[Explainer] area% is not mass% and here is exactly why it matters

Explainerbranch of 6 comments

Confession thread, sort of. I went from 7.5mg to 2.4mg much faster than I should have because the scale had stalled and I got impatient. The stall broke about 6 weeks later, at which point I had no way of knowing whether the dose increase did anything or whether it would have broken anyway. So now I have reflux I did…

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6 comments, started 6 days ago
u/cato_achebe12 points·6 days ago

Inter-lab variance on this kind of assay is routinely 1–2 points. Different column, different gradient, different integration.

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u/zeynep_zielinskiOP8 points·6 days ago

Sent one vial to PeptideMeter and VendorInvestigate. 98.4% and 98.1%. Cost me more than the vial did and I would do it again.

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u/yara_antonsen7 points·5 days ago

Yeah, 96.8% area is not the problem, the missing impurity breakdown is the problem.

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u/ignacio_silva4 points·5 days ago

area% is not mass%, and here is exactly why it matters

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u/zeynep_zielinskiOP2 points·5 days ago

Water content was 3.8%, acetate was 6.2%, purity was 99%, but net peptide was only 90% of the label weight. Perfectly honest, just chemistry.

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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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