Analytics · COA interpretation

Coming back to: "Greater than or equal to 99%" as a specification rather than a result

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

m.ilungaTL2 Moderator20 Nov 2025#1

Posting this under the heading it deserves: "Greater than or equal to 99%" as a specification rather than a result Everything below is what sits behind that.

Working through the identity arithmetic and I would like it checked.

semaglutide has a monoisotopic mass close to 4113.6 Da. On an electrospray instrument I would expect to see the multiply charged series rather than the intact singly charged ion, so for the doubly charged species I calculate (4113.6 + 2 x 1.00728) / 2, and for the triply charged the analogous expression.

The observed values in the report sit within a few ppm of those. My question is what that actually establishes, because I have seen people treat a mass match as a purity result and I do not think it is one.

0 likes 8mo

j.moreauTL2 Moderator21 Dec 2025#2

Having read the exchange above, I think I was wrong earlier in this topic and I want to say so plainly rather than quietly editing.

The correction was fair and I had been repeating something I had not checked carefully enough.

0 likes 7mo

k.otieno_statsTL3Statistician13 Jan 2026#3

Counter-ion form is crucial: peptides are isolated as salts, most often trifluoroacetate from reversed-phase purification or acetate after salt exchange. The counter-ion is part of the mass in the vial and is not part of the peptide. Two vials of the same purity, one as TFA and one as acetate, contain different amounts of peptide per milligram.

8 likes 6mo

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