How do I reconcile 98.8% from Medutest with 98.4% from the supplier?

Stated plainly: 98.8% · 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.

How do I distinguish the benign explanation from the one that matters?

0.4 percentage points, and the direction is the informative half. Medutest reports 98.8 per cent and the certificate 98.4, so the independent figure is higher. As impurity that is 1.2 per cent against 1.6 — 0.75 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.

In practice, 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.

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.

Column pore size affects mass transfer — a 100 Angstrom packing on a 5 kDa peptide restricts diffusion, broadening the peak and potentially hiding small impurities in the shoulders.

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

Read the chromatogram before you read the number, because the number without the trace is not a measurement, it is a claim.

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.

In practice, 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 ICH Q3A impurity thresholds and the relevant pharmacopoeial chapters all specify method validation requirements that almost no research-grade certificate claims to meet.

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.

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

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.

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

The honest answer is that the achievable range of plausible purity figures for a given vial is wider than most people expect.

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.

One qualification: achieving purity above roughly 98 per cent on a 30-residue peptide is fighting the chemistry of synthesis, not the quality of the purification.

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.

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

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 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.

edited 3 Nov 2024 by tobias_reint — added the citation requested in comments