The particulars: 96.8% · PeptideMeter · 99.1%.
Something has gone wrong and I would like to know how badly before I decide what to do.
Nothing else in the setup changed, which is what makes this puzzling.
What is the most likely explanation, and how would I confirm it?
2.3 percentage points, and the direction is the informative half. PeptideMeter reports 96.8 per cent and the certificate 99.1, so the independent figure is lower. As impurity that is 3.2 per cent against 0.9 — 3.56 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; 2.3 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.
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.
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.
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.
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.
Analytical standards and reagents with traceable certificates. Every quantitative result you read inherits the accuracy of the standard behind it.
Shop standardsRead the chromatogram before you read the number, because the number without the trace is not a measurement, it is a claim.
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.
Specifically, 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.
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.
Mechanically, the method matters more than the vial, which is why specifying a method buys you far more than changing suppliers does.
The fraction of your main peak that is actually your target versus isomers, fragments or related sequences is invisible without complementary identity testing.
Mass on column affects detector linearity and peak overlap — overloading broadens peaks and hides neighbours, while underloading improves resolution but loses sensitivity.
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.
If you are ranking vendors, specify a method and have all samples tested at the same place.
Identity and purity are orthogonal, and a high purity says almost nothing about whether the peak is actually what you think it is.
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.
Proline conformer interconversion kinetics are well-characterised and the half-life is of the same order as the chromatographic peak width at room 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.
Start from what the detector sees, because that tells you what the number means.
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.
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.
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.
edited 25 May 2025 by b_delacroix — updated for the 2026 guidance change
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.