Is a 97.6% result from Janoshik comparable to 98.8% from Janoshik?

Numbers first: 97.6% · Janoshik · 98.8% · Janoshik.

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.

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

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.

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

Worth being precise here: 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.

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.

edited 9 Feb 2025 by Dr_Priya_Raghunathan — clarified the distinction between purity and content

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

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.

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.

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."

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

The method matters more than the vial, which is why specifying a method buys you far more than changing suppliers does.

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.

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 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.

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

It helps to be literal here: start from what the detector sees, because that tells you what the number means.

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 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.