How do I reconcile 97.9% from Medutest with 98.4% from the supplier?

What I am working with: 97.9% · Medutest · 98.4%.

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.

Should I be treating this as a failure or as noise?

Stated carefully, 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.

Reconciling gross mass to label claim

Mechanically, 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 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.

Mechanically, 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.

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.

To be exact about it, 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 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.

In practice, 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.

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

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.

Area percentage is not mass percentage, and conflating the two is the most common misreading of a purity figure.

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

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.

edited 20 Jul 2026 by p_mkhize — reworded for clarity after a comment

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

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.

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.

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