How do I reconcile 99.4% from Medutest with 96.8% from the supplier?

Stated plainly: 99.4% · Medutest · 96.8%.

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?

2.6 percentage points, and the direction is the informative half. Medutest reports 99.4 per cent and the certificate 96.8, so the independent figure is higher. As impurity that is 0.6 per cent against 3.2 — 0.19 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.

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.

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.

Reconciling gross mass to label claim

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

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.

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.

The relevant detail is that 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.

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

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

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.

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.

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

edited 21 Mar 2025 by kwn_analytical — corrected a unit error in the worked example

The underlying point is that area percentage is not mass percentage, and conflating the two is the most common misreading of a purity figure.

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 Arrhenius relationship for peptide degradation is the basis of accelerated stability testing and also governs how quickly methods drift with temperature.

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 5 Apr 2025 by rune_thoresen — expanded the table to cover the lower concentration

The relevant detail is that 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.

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.

Inter-laboratory studies on identical peptide material routinely find half-a-per-cent to a full-per-cent spreads in reported purity on the same sample.

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.