Why would Janoshik report 97.6% where the supplier certificate says something higher?

Setup, so nobody has to ask: Janoshik · 97.6%.

I noticed this today and I have not touched anything since, in case the state is diagnostic.

I have not discarded anything yet, so a test is still possible if that is the recommendation.

How do I distinguish the benign explanation from the one that matters?

To be exact about it, gradient slope is the most powerful parameter and almost nobody mentions it, which is why two reports on the same material disagree by a point.

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.

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.

Compare purity within a single laboratory on the same method, never across laboratories.

edited 1 Dec 2024 by Dr_Rosalind_Achebe — expanded the table to cover the lower concentration

The method matters more than the vial, which is why specifying a method buys you far more than changing suppliers does.

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 part that matters: 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.

The ICH Q3A impurity thresholds and the relevant pharmacopoeial chapters all specify method validation requirements that almost no research-grade certificate claims to meet.

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.

Stated carefully, the honest answer is that the achievable range of plausible purity figures for a given vial is wider than most people expect.

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.

Mechanically, 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 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.

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.

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.

More usefully, understanding purity requires separating the chemistry from the method from the reporting convention, and the three are not independent.

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 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.