How do I reconcile 97.6% from Medutest with 95.2% from the supplier?

Setup, so nobody has to ask: 97.6% · Medutest · 95.2%.

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?

2.4 percentage points, and the direction is the informative half. Medutest reports 97.6 per cent and the certificate 95.2, so the independent figure is higher. As impurity that is 2.4 per cent against 4.8 — 0.5 times as much unassigned area, which is the unusual direction and worth a second look at the integration. Method differences usually make the independent number the lower one, so an independent result above the certificate points at integration limits, a different wavelength, or a different definition of the main peak rather than at a better vial. Ask both parties for the method section before you decide which figure is wrong, because the answer is often neither.

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

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.

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.

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."

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

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

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.

More usefully, 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.

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.

Read the chromatogram before you read the number, because the number without the trace is not a measurement, it is a claim.

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.

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.

Proline conformer interconversion kinetics are well-characterised and the half-life is of the same order as the chromatographic peak width at room 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.

edited 8 Apr 2026 by triple_agonist_q — added the method parameters