How do I reconcile 95.2% from Medutest with 97.1% from the supplier?

Setup, so nobody has to ask: 95.2% · Medutest · 97.1%.

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?

1.9 percentage points, and the direction is the informative half. Medutest reports 95.2 per cent and the certificate 97.1, so the independent figure is lower. As impurity that is 4.8 per cent against 2.9 — 1.66 times as much unassigned area. A gentler gradient resolves impurities that a steeper one hides beneath the main peak, so the better method routinely reports the worse number; 1.9 points is comfortably inside what method choice alone produces on identical material. Ask both parties for the method section before you decide which figure is wrong, because the answer is often neither.

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.

Concretely, 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.

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

Start from what the detector sees, because that tells you what the number means.

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 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.

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

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

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.

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

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

edited 21 Jul 2026 by ruaidhri_o_shea — fixed an arithmetic slip in the third paragraph

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.

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

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 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.

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.

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.

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

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.

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.