Is a 98.4% result from Janoshik comparable to 97.1% from PeptideMeter?

Numbers first: 98.4% · Janoshik · 97.1% · PeptideMeter.

I want to know what the trade-off actually is rather than which option is fashionable.

I would rather have a defensible reason than a marginal improvement.

Which axes does this decision turn on?

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.

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.

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

edited 14 Jan 2026 by u100_marks — removed a claim I could not source

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

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.

Stated carefully, 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 ICH Q3A impurity thresholds and the relevant pharmacopoeial chapters all specify method validation requirements that almost no research-grade certificate claims to meet.

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

Area percentage is not mass percentage, and conflating the two is the most common misreading of a purity figure.

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 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.

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.

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.

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

I would be careful about over-reading a single measurement — treat it as a data point, not as ground truth.

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.