u/kavya_verhoeven

Right, and net peptide content is where the counterion and the water go. Both numbers can be honest and very different.

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.

Agreed — the itemised related substances table is the difference between a document and a receipt.

ask for the mass spectrum, not just the purity number

Not quite. Two labs with different gradients can resolve a close-eluting impurity differently and both be reporting honestly.

the related substances line is where the story is

Removed — cropped document.

Disagree on this specific point. Two honest labs can resolve that impurity differently and report different numbers.

What does the method line actually say? Not the technique — the column and the gradient.

Started keeping a folder of every certificate I have been sent. The ones that itemise impurities are a small group and they are mostly the same suppliers.

Bought small, tested identity first, and only then thought about anything else. That order felt right.

stability in solution is the practical question nobody asks

identity, quantity, stability — three separate questions

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.

the lot on the vial has to match the lot on the page

Was there identity confirmation, or only a purity figure?

That is the catalogue certificate rather than the batch-specific one. Same layout, completely different status.

Asked CPC for the batch-specific certificate and had the full document — batch, date, method line naming the column — inside six hours. That is now my baseline for everybody.

a certificate with no method line is decorative

This. Area percent answers "what fraction of what the detector saw was the peak I wanted", and people read it as "how much peptide is in the vial".

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.

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.

Area percent or net peptide content? They answer different questions.

ask for the storage condition the stability statement was written against

Did you get the chromatogram or only the summary line?

Batch number and date of analysis, are both on there?

What does the method line actually say? Not the technique — the column and the gradient.

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

It is a fifteen-residue peptide with a published sequence, which makes synthesis straightforward and makes identity verification the meaningful test rather than an optional extra.

Net peptide content is the fraction of the powder that is actually peptide once counterion, water and residual solvent are accounted for. A 99% pure acetate salt can still be about 85% peptide by mass.

Asked GGPeps for the theoretical mass alongside the certificate and they sent it without asking why.

Pushing back a little: a missing field is usually presentation, not concealment. Ask for the full document before drawing conclusions.

preclinical enthusiasm is not clinical evidence

Yes. The structure of the document matters at least as much as the number printed on it.