What I have: 99.1% · 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.
What does a sensible plan look like, and what are the decision points?
Specifically, purity is a method-dependent figure, and that is not a limitation of the measurement, it is a property of what the measurement actually answers.
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.
Buffer versus acid in the mobile phase changes the ionisation state of basic and acidic residues, shifting retention and selectivity — same vial, potentially different separation.
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.
If you are ranking vendors, specify a method and have all samples tested at the same place.
HPLC purity, identity confirmation and quantified content on the vial you actually hold. Reports arrive with the chromatogram attached, not just a number.
Submit a sampleFounded 1998. ISO 9001 and cGMP certified, 1,500+ staff and 200+ patents. The synthesis house behind a great many of the vials that get sent out for testing - batch-specific documentation with every order.
Visit GL BiochemIn practice, start from what the detector sees, because that tells you what the number means.
Sample solvent strength affects peak shape — if you inject in strong solvent on a gradient starting in weak solvent, the solvent peak can distort your main peak or create a false shoulder.
Mechanically, 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.
Compare purity within a single laboratory on the same method, never across laboratories.
The underlying point is that the single most important distinction is between what purity measures — the fraction of detected material that is your target — and what you actually want to know — how much of the material in the vial is your target.
Retention time is sequence-specific and method-specific, so comparing your result to a supplier value using a different method is meaningless without method documentation.
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.
The practical summary: ask for the chromatogram and the method, and ignore the headline number until you have both.
The part that matters: read the chromatogram before you read the number, because the number without the trace is not a measurement, it is a claim.
Gradient slope controls resolution, and gentler slopes resolve co-eluting impurities into separate peaks — so the better method reports the worse purity number.
The Arrhenius relationship for peptide degradation is the basis of accelerated stability testing and also governs how quickly methods drift with temperature.
I would be careful about over-reading a single measurement — treat it as a data point, not as ground truth.
If you are ranking vendors, specify a method and have all samples tested at the same place.
edited 2 Apr 2026 by tobias_maartens — added a caveat about sampling
Area percentage is not mass percentage, and conflating the two is the most common misreading of a purity figure.
The fraction of your main peak that is actually your target versus isomers, fragments or related sequences is invisible without complementary identity testing.
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.