Concretely: SGN · dulaglutide.
I can parse the result. I am less sure what it licenses me to conclude.
I have deliberately not looked at anyone else’s interpretation yet.
What is the correct interpretation, and what is the common misreading?
Answer first: the counter-ion is part of the mass you paid for and none of the peptide you wanted, and it can run to several per cent or more.
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.
Worked reconciliation: a 10 mg vial at four per cent water and eight per cent trifluoroacetate contains 10.0 × 0.88 = 8.8 mg of peptide, a twelve per cent shortfall against label with a perfectly clean purity figure.
Peptides from reverse-phase purification with trifluoroacetic acid mobile phases are isolated as trifluoroacetate salts, which is standard synthesis practice.
Ask for net peptide content. It folds water and counter-ion into one usable number.
edited 30 Jun 2025 by tare_weight — clarified the distinction between purity and content
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Browse resultsThe 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.
Quantification differs by ion: trifluoroacetate by ion chromatography or by fluorine nuclear magnetic resonance, acetate by ion chromatography or by capillary electrophoresis. Neither shows up on a UV chromatogram.
Worth being precise here: 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.
Absence of a counter-ion figure is an absent measurement rather than a low one.
Count the basic residues and you can estimate the counter-ion load.
edited 30 Jun 2025 by nine_point_nine — added the citation requested in comments
Start with which counter-ion the process produced, because trifluoroacetate from reverse-phase purification and acetate from a salt exchange behave differently and are quantified differently.
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.
Trifluoroacetate has documented biological effects at higher concentrations in cell culture, which is the technical reason acetate exchange exists rather than a marketing one.
Counter-ion content scales with the number of basic residues, following directly from charge balance.
Net peptide content varies between lots as the process varies, so one figure does not characterise a supplier.
Water plus counter-ion is why gross mass and peptide mass differ.
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