Certificates issued by the seller versus by a laboratory

Posting this under the heading it deserves: Certificates issued by the seller versus by a laboratory Everything below is what sits behind that.

I would like to understand what this number means before I repeat it anywhere.

A Medutest report on a tirzepatide lot gives 96.5% purity. The supplier certificate for the same lot states 98.9%. Both documents name a reversed-phase method; neither states the same gradient.

My question is not "who is right". It is: given that those two figures were produced by different methods, what is the largest difference I should expect from method alone, and at what point does a gap stop being explainable that way?

Thank you — that answers what I came here to find out.

Where I part company with post #14, and it is a narrow parting.

A chromatogram supplied as a small image is legible for peak shape and not for baseline detail. That is enough to sanity-check an integration and not enough to reproduce it.

An itemised related-substances table with retention times says considerably more about the synthesis than a single total. A total tells you how much is not the main peak and nothing about what it is.

The distinction between release testing and characterisation is one industrial documents make and retail ones usually do not. It is the difference between "we tested every lot for this" and "we established this once".

Reading old certificates: if a certificate is dated years ago, the lot it describes might be old. That is not automatically a problem but it is worth noting. Fresh testing on the current lot is stronger than old testing on an older lot.

I have seen it go both ways, which is why I hedge.

An itemised related-substances table with retention times says considerably more about the synthesis than a single total. A total tells you how much is not the main peak and nothing about what it is.

That is a description of practice, not a recommendation of it.

Adding thanks rather than a view. I do not have a view worth the space.

Picking up post #58: that is the part I would want checked first.

Amino acid analysis gives content: it hydrolyses the peptide and quantifies the residues. It measures the peptide content independent of chromatographic purity. Very few supplier certificates report it.