Is a 97.1% result from PeptideMeter comparable to 96.8% from Medutest?

For reference: 97.1% · PeptideMeter · 96.8% · Medutest.

I am trying to choose between two options that are usually discussed as though only one exists.

I am not optimising for price, but I am not indifferent to it either.

So which one, and on what grounds?

97.1 and 96.8 are 0.3 percentage points apart, which sounds small until you restate it as impurity. PeptideMeter is leaving 2.9 per cent of the detected area unassigned and Medutest 3.2 per cent — a factor of 1.1 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, 97.1 and 96.8 are two measurements of slightly different quantities that happen to share a unit.

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

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.

What each test answers

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.

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.

The part that matters: identity and purity are orthogonal, and a high purity says almost nothing about whether the peak is actually what you think it is.

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.

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.

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.

Worth noting that method standardisation is poor in the research-grade space compared to pharmaceutical work, so identical-looking methods can produce different results.

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

edited 24 Apr 2026 by eoin_mcgarry — removed a claim I could not source

Mechanically, 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 relevant detail is that 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.

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

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.

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

The Arrhenius relationship for peptide degradation is the basis of accelerated stability testing and also governs how quickly methods drift with temperature.

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.

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.

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.