Is a 95.2% result from Medutest comparable to 97.1% from PeptideMeter?

Stated plainly: 95.2% · Medutest · 97.1% · PeptideMeter.

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.

Under what conditions does the answer flip?

95.2 and 97.1 are 1.9 percentage points apart, which sounds small until you restate it as impurity. Medutest is leaving 4.8 per cent of the detected area unassigned and PeptideMeter 2.9 per cent — a factor of 1.66 between them. The impurity fraction is the quantity that moves when a method changes, and it is the one worth arguing about; the headline is just its complement. Comparable means same column chemistry, same gradient slope, same detection wavelength, same integration convention. Until you have those four from both laboratories, 95.2 and 97.1 are two measurements of slightly different quantities that happen to share a unit.

The relevant detail is that reporting threshold is convention and not chemistry, which is why two certificates with different thresholds disagree by a tenth of a point or more.

Temperature affects the dynamics of molecular conformation, and if a peptide has proline residues that interconvert on the chromatographic timescale, the peak will split or shoulder at low temperature and collapse at high temperature.

It helps to be literal here: 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.

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."

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

edited 19 May 2026 by tobias_reint — updated for the 2026 guidance change

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

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

In practice, 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.

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

To be exact about it, understanding purity requires separating the chemistry from the method from the reporting convention, and the three are not independent.

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.

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

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

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