What counter-ion should I expect on an ERP cagrilintide lot?

The specifics, since they change the answer: ERP · cagrilintide.

I have read the primary source rather than the summary, which has left me with more questions.

I understand the headline. I do not understand the footnotes, and the footnotes look important.

What can I legitimately conclude from this figure?

The short version: peptides are supplied as salts, the salt is real mass, and neither purity nor a chromatogram will tell you how much of it there is.

Net peptide content — the fraction of gross mass that is the peptide itself — is the figure that folds water and counter-ion together and is what you should ask for if you ask for anything.

A supplier who reports both water and counter-ion has given you everything needed to reconcile the vial, and on this site that is a very short list.

Net peptide content varies between lots as the process varies, so one figure does not characterise a supplier.

Count the basic residues and you can estimate the counter-ion load.

Answering this needs the purification method, since a trifluoroacetic acid mobile phase leaves trifluoroacetate unless a deliberate exchange was performed.

Acetate salts are produced by a deliberate ion exchange after purification. The exchange costs a processing step and is the reason acetate-form material is more expensive.

Put another way, peptides purified by reverse-phase chromatography with a trifluoroacetic acid mobile phase are isolated as trifluoroacetate salts. The trifluoroacetate content depends on the number of basic residues and can run from a few per cent to well over ten by mass on a heavily basic sequence.

The caveat is that a purity figure is completely blind to counter-ion content, and the two are routinely conflated.

Trifluoroacetate unless a deliberate exchange was done. Ask which.

edited 6 Jul 2026 by s_bhattacharya — added the method parameters