Is a 96.4% result from Medutest comparable to 97.6% from Janoshik?

What I am working with: 96.4% · Medutest · 97.6% · Janoshik.

I suspect the honest answer is that it depends, in which case I would like to know on what.

Assume I can obtain either option without difficulty, so availability is not the deciding factor.

Under what conditions does the answer flip?

96.4 and 97.6 are 1.2 percentage points apart, which sounds small until you restate it as impurity. Medutest is leaving 3.6 per cent of the detected area unassigned and Janoshik 2.4 per cent — a factor of 1.5 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, 96.4 and 97.6 are two measurements of slightly different quantities that happen to share a unit.

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.

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.

Worth being precise here: 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.

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

edited 14 Mar 2025 by a_lindgren — added the citation requested in comments

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

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.

On the detail: mass on column affects detector linearity and peak overlap — overloading broadens peaks and hides neighbours, while underloading improves resolution but loses sensitivity.

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

edited 13 Apr 2025 by mz_4113 — clarified the distinction between purity and content

The short answer is that two competent laboratories on identical material will disagree, and the disagreement is almost always explainable by method differences.

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

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 limitation is that single-digit micro-impurities become invisible at typical reporting thresholds, so "no impurities detected" means "none above one in two thousand."

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

Put another way, gradient slope is the most powerful parameter and almost nobody mentions it, which is why two reports on the same material disagree by a point.

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

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 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.

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

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.

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.

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.