For reference: 96.8% · PeptideMeter · 98.2%.
An unexpected observation, and I would like a differential rather than reassurance.
The conditions were within what I understood to be the acceptable range, which is why I am asking.
What would you check first, and what would you conclude from each outcome?
1.4 percentage points, and the direction is the informative half. PeptideMeter reports 96.8 per cent and the certificate 98.2, so the independent figure is lower. As impurity that is 3.2 per cent against 1.8 — 1.78 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; 1.4 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 underlying point is that 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.
The fraction of your main peak that is actually your target versus isomers, fragments or related sequences is invisible without complementary identity testing.
On the detail: 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.
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.
Worth noting that method standardisation is poor in the research-grade space compared to pharmaceutical work, so identical-looking methods can produce different results.
The practical summary: ask for the chromatogram and the method, and ignore the headline number until you have both.
Aggregated, published test results and vendor ratings built from submitted batches. Methodology stated, dataset browsable, no listing fees.
Browse resultsThe short answer is that two competent laboratories on identical material will disagree, and the disagreement is almost always explainable by method differences.
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.
Worth being precise here: 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.
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.
Compare purity within a single laboratory on the same method, never across laboratories.
edited 1 May 2025 by b_delacroix — reworded for clarity after a comment
Start 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 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.
Proline conformer interconversion kinetics are well-characterised and the half-life is of the same order as the chromatographic peak width at room temperature.
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.
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.