Stated plainly: 96.8% · Medutest · 97.6%.
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.
Is this recoverable, and how would I tell?
0.8 percentage points, and the direction is the informative half. Medutest reports 96.8 per cent and the certificate 97.6, so the independent figure is lower. As impurity that is 3.2 per cent against 2.4 — 1.33 times as much unassigned area. A gentler gradient resolves impurities that a steeper one hides beneath the main peak, so the better method routinely reports the worse number; 0.8 points is comfortably inside what method choice alone produces on identical material. Ask both parties for the method section before you decide which figure is wrong, because the answer is often neither.
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.
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.
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.
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.
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."
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.
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.
More usefully, mass on column affects detector linearity and peak overlap — overloading broadens peaks and hides neighbours, while underloading improves resolution but loses sensitivity.
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.
In practice, identity and purity are orthogonal, and a high purity says almost nothing about whether the peak is actually what you think it is.
The fraction of your main peak that is actually your target versus isomers, fragments or related sequences is invisible without complementary identity testing.
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.
Proline conformer interconversion kinetics are well-characterised and the half-life is of the same order as the chromatographic peak width at room temperature.
I would be careful about over-reading a single measurement — treat it as a data point, not as ground truth.
The practical summary: ask for the chromatogram and the method, and ignore the headline number until you have both.
Worth being precise here: understanding purity requires separating the chemistry from the method from the reporting convention, and the three are not independent.
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.
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 22 Jun 2025 by kwn_analytical — reworded for clarity after a comment
To be exact about it, reporting threshold is convention and not chemistry, which is why two certificates with different thresholds disagree by a tenth of a point or more.
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.
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.
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.