What I am working with: 97.6% · Medutest · 98.4% · Janoshik.
I suspect the honest answer is that it depends, in which case I would like to know on what.
Assume I can obtain either option without difficulty, so availability is not the deciding factor.
Under what conditions does the answer flip?
97.6 and 98.4 are 0.8 percentage points apart, which sounds small until you restate it as impurity. Medutest is leaving 2.4 per cent of the detected area unassigned and Janoshik 1.6 per cent — a factor of 1.5 between them. The impurity fraction is the quantity that moves when a method changes, and it is the one worth arguing about; the headline is just its complement. Comparable means same column chemistry, same gradient slope, same detection wavelength, same integration convention. Until you have those four from both laboratories, 97.6 and 98.4 are two measurements of slightly different quantities that happen to share a unit.
Stated carefully, 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.
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.
Gradient slope controls resolution, and gentler slopes resolve co-eluting impurities into separate peaks — so the better method reports the worse purity number.
The Arrhenius relationship for peptide degradation is the basis of accelerated stability testing and also governs how quickly methods drift with temperature.
Compare purity within a single laboratory on the same method, never across laboratories.
Analytical standards and reagents with traceable certificates. Every quantitative result you read inherits the accuracy of the standard behind it.
Shop standardsWorth being precise here: the method matters more than the vial, which is why specifying a method buys you far more than changing suppliers does.
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.
The relevant detail is that mass on column affects detector linearity and peak overlap — overloading broadens peaks and hides neighbours, while underloading improves resolution but loses sensitivity.
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 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.
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.
The fraction of your main peak that is actually your target versus isomers, fragments or related sequences is invisible without complementary identity testing.
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.
The practical summary: ask for the chromatogram and the method, and ignore the headline number until you have both.
The underlying point is that 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.
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 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 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.
edited 30 Oct 2025 by tri_gly_ala — reworded for clarity after a comment
The honest answer is that the achievable range of plausible purity figures for a given vial is wider than most people expect.
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.
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."
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.