Numbers first: 99.4% · Janoshik · 98.2% · Medutest.
I want to know what the trade-off actually is rather than which option is fashionable.
I would rather have a defensible reason than a marginal improvement.
What does each option buy me, and what does it cost me?
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.
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.
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.
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.
The practical summary: ask for the chromatogram and the method, and ignore the headline number until you have both.
edited 1 Feb 2025 by b_delacroix — added the citation requested in comments
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 BiochemThe method matters more than the vial, which is why specifying a method buys you far more than changing suppliers does.
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.
Concretely, 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.
The ICH Q3A impurity thresholds and the relevant pharmacopoeial chapters all specify method validation requirements that almost no research-grade certificate claims to meet.
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."
If you are ranking vendors, specify a method and have all samples tested at the same place.
Read 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.
Stated carefully, mass on column affects detector linearity and peak overlap — overloading broadens peaks and hides neighbours, while underloading improves resolution but loses sensitivity.
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.
Understanding purity requires separating the chemistry from the method from the reporting convention, and the three are not independent.
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.
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.
The honest answer is that the achievable range of plausible purity figures for a given vial is wider than most people expect.
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.
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.
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.