The particulars: 98.4% · PeptideMeter · 95.2%.
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 is the differential here, and which test discriminates between the options?
3.2 percentage points, and the direction is the informative half. PeptideMeter reports 98.4 per cent and the certificate 95.2, so the independent figure is higher. As impurity that is 1.6 per cent against 4.8 — 0.33 times as much unassigned area, which is the unusual direction and worth a second look at the integration. Method differences usually make the independent number the lower one, so an independent result above the certificate points at integration limits, a different wavelength, or a different definition of the main peak rather than at a better vial. Ask both parties for the method section before you decide which figure is wrong, because the answer is often neither.
Worth being precise here: 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.
Gradient slope controls resolution, and gentler slopes resolve co-eluting impurities into separate peaks — so the better method reports the worse purity number.
| Δ mass (Da) | Most likely cause | Distinguishing feature |
|---|---|---|
| +1 | Deamidation (Asn or Gln) | New peak, slightly earlier retention |
| −17 | Loss of ammonia | Often with deamidation |
| −18 | Dehydration / succinimide | pH-dependent, reversible |
| +16 | Oxidation (Met, Trp) | Earlier retention, light-related |
| −128 | Missing Gln or Lys | Deletion sequence from synthesis |
| 0 | Isomer: racemisation or scrambling | Same mass, shifted retention |
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 practical summary: ask for the chromatogram and the method, and ignore the headline number until you have both.
edited 8 Apr 2025 by nine_point_nine — added a caveat about sampling
Aggregated, published test results and vendor ratings built from submitted batches. Methodology stated, dataset browsable, no listing fees.
Browse resultsThe method matters more than the vial, which is why specifying a method buys you far more than changing suppliers does.
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.
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.
Proline conformer interconversion kinetics are well-characterised and the half-life is of the same order as the chromatographic peak width at room temperature.
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.
The fraction of your main peak that is actually your target versus isomers, fragments or related sequences is invisible without complementary identity testing.
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.
If you are ranking vendors, specify a method and have all samples tested at the same place.
Area percentage is not mass percentage, and conflating the two is the most common misreading of a purity figure.
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.
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.
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.
Identity and purity are orthogonal, and a high purity says almost nothing about whether the peak is actually what you think it is.
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.
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.