Is a 95.2% result from Medutest comparable to 98.4% from Janoshik?

Setup, so nobody has to ask: 95.2% · Medutest · 98.4% · Janoshik.

Both of these get recommended confidently by different people, which suggests neither is obviously right.

My constraints are cost, measurement resolution and how much handling I am prepared to do — in roughly that order.

What is the actual trade-off, and does it matter at the scale I am working at?

It helps to be literal here: reporting threshold is convention and not chemistry, which is why two certificates with different thresholds disagree by a tenth of a point or more.

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 part that matters: 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.

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.

edited 20 Jul 2024 by Dr_Priya_Raghunathan — clarified the distinction between purity and content

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

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

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

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.

Concretely, 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.

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

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 limitation is that single-digit micro-impurities become invisible at typical reporting thresholds, so "no impurities detected" means "none above one in two thousand."

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.