What I have: CPC · tirzepatide.
The figures are clear enough; the question is what they mean and what they do not.
I can supply the numbers if the specifics change the answer.
What would I need in addition before this supported a decision?
The honest answer is that most certificates are silent on it and that the number can be large enough to matter.
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.
| Component | Typical share | Counted in purity? | Counted in content? |
|---|---|---|---|
| Target peptide | 88–94 % | Yes, as main peak | Yes |
| Related impurities | 1–3 % | Yes, as other peaks | No |
| Counter-ion (TFA or acetate) | 2–8 % | No | No |
| Residual water | 2–6 % | No | No |
| Bulking agent, if present | 0–40 % | No | No |
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.
Counter-ion content scales with the number of basic residues, following directly from charge balance.
Purity says nothing about counter-ion. They are unrelated measurements.
Aggregated, published test results and vendor ratings built from submitted batches. Methodology stated, dataset browsable, no listing fees.
Browse resultsAnswer 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.
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.
The relevant detail is that trifluoroacetate has documented biological effects at higher concentrations in cell culture, which is the technical reason acetate exchange exists rather than a marketing one.
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.
More usefully, this is the other half of the reconciliation problem, alongside water content.
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.
Trifluoroacetate effects in cell culture are documented and are the technical basis for salt exchange to acetate.
Trifluoroacetate unless a deliberate exchange was done. Ask which.
edited 4 May 2025 by n_takahashi — added a caveat about sampling
The underlying point is that a net-peptide-content figure already accounts for it, which is why net content is the number worth asking for.
The number of basic residues drives counter-ion load, because each protonated basic site pairs with one counter-ion. A sequence with several lysines and arginines carries proportionally more.
Nothing here is medical advice, and research-use compounds are not approved for human use.
Water plus counter-ion is why gross mass and peptide mass differ.
To be exact about it, acetate is generally preferred to trifluoroacetate for biological work, which is why the exchange is done at all.
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.
Ion chromatography and fluorine NMR are the accepted quantitative methods for trifluoroacetate, and neither is a chromatographic purity method.
Count the basic residues and you can estimate the counter-ion load.
Ask PeptideStack is a static archive. Posting is closed, but the norms are worth stating: answer the question that was asked, show your working, cite the trial or the certificate, and say plainly where the evidence runs out.