Details up front: Medutest · 97.1%.
I have a result I cannot explain, and I would rather diagnose it than guess.
I have checked the obvious explanations and eliminated the two easiest ones.
What is the most likely explanation, and how would I confirm it?
A 97.1 per cent result against a higher certificate figure is usually a method difference, not a disagreement about the vial. Inter-laboratory spread on identical peptide material runs half a per cent to a full per cent by RP-HPLC. Gradient slope, detection wavelength and integration convention each move the number, and a gentler gradient resolves impurities that a steeper one hides under the main peak — so the better method reports the worse figure. Get the column, the gradient and the wavelength from both Medutest and the supplier before treating 97.1 per cent as a contradiction.
The underlying point is that the method matters more than the vial, which is why specifying a method buys you far more than changing suppliers does.
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.
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.
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 practical summary: ask for the chromatogram and the method, and ignore the headline number until you have both.
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 BiochemStart from what the detector sees, because that tells you what the number means.
Gradient slope controls resolution, and gentler slopes resolve co-eluting impurities into separate peaks — so the better method reports the worse purity number.
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.
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.
Read the chromatogram before you read the number, because the number without the trace is not a measurement, it is a claim.
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.
On the detail: 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 ICH Q3A impurity thresholds and the relevant pharmacopoeial chapters all specify method validation requirements that almost no research-grade certificate claims to meet.
If you are ranking vendors, specify a method and have all samples tested at the same place.
edited 22 Feb 2026 by bea_forsberg — added the placebo-arm figures
The part that matters: 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.
The limitation is that single-digit micro-impurities become invisible at typical reporting thresholds, so "no impurities detected" means "none above one in two thousand."
If you only pay for one test, pay for quantified content. Purity is the number everyone quotes and content is the number that changes what you do.
Specifically, 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.
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.
Published side-by-side method comparisons show that a two-point difference in purity on the same vial is easily explained by method choice alone.
I would be careful about over-reading a single measurement — treat it as a data point, not as ground truth.
In practice: ask for the chromatogram, check the method section, check the lot number against the vial, and set your accept threshold before you see the result rather than after.
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.