Why would Medutest report 97.9% where the supplier certificate says something higher?

Details up front: Medutest · 97.9%.

I have a result I cannot explain, and I would rather diagnose it than guess.

I have checked the obvious explanations and eliminated the two easiest ones.

What is the differential here, and which test discriminates between the options?

A 97.9 per cent result against a higher certificate figure is usually a method difference, not a disagreement about the vial. Inter-laboratory spread on identical peptide material runs half a per cent to a full per cent by RP-HPLC. Gradient slope, detection wavelength and integration convention each move the number, and a gentler gradient resolves impurities that a steeper one hides under the main peak — so the better method reports the worse figure. Get the column, the gradient and the wavelength from both Medutest and the supplier before treating 97.9 per cent as a contradiction.

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.

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

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.

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

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

edited 27 Feb 2026 by Dr_Wren_Halliday — clarified the distinction between purity and content