Is a 95.2% result from VendorInvestigate comparable to 97.9% from Medutest?

Concretely: 95.2% · VendorInvestigate · 97.9% · Medutest.

I would like the axes of comparison first and the recommendation second.

I have tried the first option and it works; the question is whether the second is better rather than merely different.

Is there a defensible reason to prefer one, or is this a coin flip?

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

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.

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.

Proline conformer interconversion kinetics are well-characterised and the half-life is of the same order as the chromatographic peak width at room temperature.

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.

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

The underlying point 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.

The part that matters: 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.

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

The part that matters: 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.

Mass on column affects detector linearity and peak overlap — overloading broadens peaks and hides neighbours, while underloading improves resolution but loses sensitivity.

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

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.

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

edited 9 Sept 2025 by Dr_Rosalind_Achebe — tightened the wording; no substantive change

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.

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

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

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.

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