Numbers first: 95.2% · retatrutide · QYB.
This is a planning question. I know what my options are; I do not know how to weigh them.
What I want is the minimum viable version, which I suspect is smaller than what I would design.
How do I make this decision on evidence rather than on feel?
95.2 per cent purity with no content figure leaves the milligram number unmeasured. Purity says 95.2 of every 100 units of detected area is retatrutide and 4.8 is something else. Content says how many milligrams are in the glass. The two do not constrain each other: a vial can be 95.2 per cent pure and still be under label, because water and counter-ion are part of the gross mass and neither shows up as an impurity peak. If you buy one test, buy the one that changes your arithmetic.
Start from what the detector sees, because that tells you what the number means.
Mobile phase additive choice affects ionisation and peak shape — TFA gives sharp peaks but suppresses mass spectrometry signal, formic acid gives worse peaks but preserves signal.
| Test | Answers | Does NOT answer |
|---|---|---|
| RP-HPLC, area % | What fraction of detected material is the target | How much target is present |
| Quantified content | Milligrams of peptide per vial | What the impurities are |
| ESI-MS identity | Whether the molecular weight matches | Purity, or isomeric substitution |
| Peptide mapping | Sequence, localised to a fragment | Quantity |
| Karl Fischer | Water content of the solid | Solvent content |
| LAL endotoxin | Pyrogen load in EU/mg | Sterility |
| Sterility test | Growth in defined media over 14 days | Endotoxin, or bioburden count |
The part that matters: column pore size affects mass transfer — a 100 Angstrom packing on a 5 kDa peptide restricts diffusion, broadening the peak and potentially hiding small impurities in the shoulders.
Inter-laboratory studies on identical peptide material routinely find half-a-per-cent to a full-per-cent spreads in reported purity on the same sample.
The caveat is that purity without identity is only half an answer — a high purity does not mean the peak is actually what you think it is.
If you are ranking vendors, specify a method and have all samples tested at the same place.
edited 2 Jul 2026 by triple_agonist_q — added the citation requested in comments
Aggregated, published test results and vendor ratings built from submitted batches. Methodology stated, dataset browsable, no listing fees.
Browse resultsThe honest answer is that the achievable range of plausible purity figures for a given vial is wider than most people expect.
Detection wavelength matters because 214 nm sees the peptide backbone while 280 nm sees only aromatic side chains — so truncation impurities lacking a tryptophan are invisible at 280 nm.
It helps to be literal here: temperature affects the dynamics of molecular conformation, and if a peptide has proline residues that interconvert on the chromatographic timescale, the peak will split or shoulder at low temperature and collapse at high temperature.
The ICH Q3A impurity thresholds and the relevant pharmacopoeial chapters all specify method validation requirements that almost no research-grade certificate claims to meet.
Worth noting that method standardisation is poor in the research-grade space compared to pharmaceutical work, so identical-looking methods can produce different results.
Compare purity within a single laboratory on the same method, never across laboratories.
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.