How do I reconcile 99.4% from Medutest with 96.4% from the supplier?

Stated plainly: 99.4% · Medutest · 96.4%.

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.

Is this recoverable, and how would I tell?

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

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

To be exact about it, 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.

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

edited 31 Jan 2025 by lane_transit — clarified the distinction between purity and content

Stated carefully, identity and purity are orthogonal, and a high purity says almost nothing about whether the peak is actually what you think it is.

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.

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.

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

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.

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.

More usefully, understanding purity requires separating the chemistry from the method from the reporting convention, and the three are not independent.

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.

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.