Concretely: QST · 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.
How should I read this, and where are the traps?
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.
Trifluoroacetate has documented biological effects at higher concentrations in cell culture, which is the technical reason acetate exchange exists rather than a marketing one.
| Δ mass (Da) | Most likely cause | Distinguishing feature |
|---|---|---|
| +1 | Deamidation (Asn or Gln) | New peak, slightly earlier retention |
| −17 | Loss of ammonia | Often with deamidation |
| −18 | Dehydration / succinimide | pH-dependent, reversible |
| +16 | Oxidation (Met, Trp) | Earlier retention, light-related |
| −128 | Missing Gln or Lys | Deletion sequence from synthesis |
| 0 | Isomer: racemisation or scrambling | Same mass, shifted retention |
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.
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.
edited 30 Oct 2025 by cap_the_luer — added the method parameters
Aggregated, published test results and vendor ratings built from submitted batches. Methodology stated, dataset browsable, no listing fees.
Browse resultsSpecifically, acetate is generally preferred to trifluoroacetate for biological work, which is why the exchange is done at all.
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.
The relevant detail is that 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.
The honest answer is that most certificates are silent on it and that the number can be large enough to matter.
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.
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.
Peptides from reverse-phase purification with trifluoroacetic acid mobile phases are isolated as trifluoroacetate salts, which is standard synthesis practice.
Water plus counter-ion is why gross mass and peptide mass differ.
Concretely, this is the other half of the reconciliation problem, alongside water content.
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.
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.
More usefully, a net-peptide-content figure already accounts for it, which is why net content is the number worth asking for.
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.
Net peptide content varies between lots as the process varies, so one figure does not characterise a supplier.
Ask for net peptide content. It folds water and counter-ion into one usable number.
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.