Concretely: 98.4% · VendorInvestigate · 96.4%.
I have two candidate explanations and no way to distinguish them.
The same procedure has worked without incident several times previously, which argues against technique.
How do I distinguish the benign explanation from the one that matters?
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.
The fraction of your main peak that is actually your target versus isomers, fragments or related sequences is invisible without complementary identity testing.
| 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 |
Put another way, 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.
Proline conformer interconversion kinetics are well-characterised and the half-life is of the same order as the chromatographic peak width at room temperature.
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.
HPLC purity, identity confirmation and quantified content on the vial you actually hold. Reports arrive with the chromatogram attached, not just a number.
Submit a sampleFounded 1998. ISO 9001 and cGMP certified, 1,500+ staff and 200+ patents. The synthesis house behind a great many of the vials that get sent out for testing - batch-specific documentation with every order.
Visit GL BiochemMechanically, read the chromatogram before you read the number, because the number without the trace is not a measurement, it is a claim.
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.
The relevant detail is that 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.
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.
If you are ranking vendors, specify a method and have all samples tested at the same place.
Stated carefully, understanding purity requires separating the chemistry from the method from the reporting convention, and the three are not independent.
Mass on column affects detector linearity and peak overlap — overloading broadens peaks and hides neighbours, while underloading improves resolution but loses sensitivity.
Gradient slope controls resolution, and gentler slopes resolve co-eluting impurities into separate peaks — so the better method reports the worse purity number.
The ICH Q3A impurity thresholds and the relevant pharmacopoeial chapters all specify method validation requirements that almost no research-grade certificate claims to meet.
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.
It helps to be literal here: start from what the detector sees, because that tells you what the number means.
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.
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.
Compare purity within a single laboratory on the same method, never across laboratories.
Stated carefully, area percentage is not mass percentage, and conflating the two is the most common misreading of a purity figure.
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.
The Arrhenius relationship for peptide degradation is the basis of accelerated stability testing and also governs how quickly methods drift with temperature.
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.