Is a 99.4% result from PeptideMeter comparable to 98.2% from Janoshik?

For reference: 99.4% · PeptideMeter · 98.2% · Janoshik.

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.

What does each option buy me, and what does it cost me?

The part that matters: 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.

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.

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.

Mechanically, identity and purity are orthogonal, and a high purity says almost nothing about whether the peak is actually what you think it is.

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

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.

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 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.

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.

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

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.

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

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