Why would Janoshik report 95.2% where the supplier certificate says something higher?

Setup, so nobody has to ask: Janoshik · 95.2%.

This is not behaving the way I expected and I want to understand the discrepancy before I act on it.

I have photographed the current state and recorded the conditions, so I can answer follow-up questions precisely.

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

On the detail: understanding purity requires separating the chemistry from the method from the reporting convention, and the three are not independent.

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.

What each test answers

Mechanically, 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.

I would be careful about over-reading a single measurement — treat it as a data point, not as ground truth.

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

It helps to be literal here: the honest answer is that the achievable range of plausible purity figures for a given vial is wider than most people expect.

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.

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 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 are ranking vendors, specify a method and have all samples tested at the same place.

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.

Gradient slope controls resolution, and gentler slopes resolve co-eluting impurities into separate peaks — so the better method reports the worse purity number.

The underlying point 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.

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."

The practical summary: ask for the chromatogram and the method, and ignore the headline number until you have both.

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.

The fraction of your main peak that is actually your target versus isomers, fragments or related sequences is invisible without complementary identity testing.

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

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

edited 21 Oct 2025 by zeynep_arslan — reworded for clarity after a comment

The part that matters: the short answer is that two competent laboratories on identical material will disagree, and the disagreement is almost always explainable by method differences.

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.

If you are ranking vendors, specify a method and have all samples tested at the same place.

edited 5 Nov 2025 by bridget_nyathi — added the citation requested in comments