Stated plainly: Janoshik · 97.9%.
I noticed this today and I have not touched anything since, in case the state is diagnostic.
I have not discarded anything yet, so a test is still possible if that is the recommendation.
How do I distinguish the benign explanation from the one that matters?
A 97.9 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 Janoshik and the supplier before treating 97.9 per cent as a contradiction.
To be exact about it, 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.
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 underlying point is that 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.
One qualification: achieving purity above roughly 98 per cent on a 30-residue peptide is fighting the chemistry of synthesis, not the quality of the purification.
Compare purity within a single laboratory on the same method, never across laboratories.
Aggregated, published test results and vendor ratings built from submitted batches. Methodology stated, dataset browsable, no listing fees.
Browse resultsStart 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.
The relevant detail is that integration of the shoulder — whether you use perpendicular drop or tangent skim — determines what area gets assigned to the main peak versus the impurity table.
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.
If you are ranking vendors, specify a method and have all samples tested at the same place.
The short answer is that two competent laboratories on identical material will disagree, and the disagreement is almost always explainable by method differences.
The fraction of your main peak that is actually your target versus isomers, fragments or related sequences is invisible without complementary identity testing.
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.
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.
The practical summary: ask for the chromatogram and the method, and ignore the headline number until you have both.
edited 9 Aug 2024 by sinead_gaffney — removed a claim I could not source
Understanding purity requires separating the chemistry from the method from the reporting convention, and the three are not independent.
Tailing factor measures peak shape, and a badly tailing peak spreads into the region where small impurities live, forcing tangent-skim integration that assigns tail area to the main peak.
I would be careful about over-reading a single measurement — treat it as a data point, not as ground truth.
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.
edited 8 Oct 2024 by area_percent — updated for the 2026 guidance change
The method matters more than the vial, which is why specifying a method buys you far more than changing suppliers does.
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.
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.