Is a 95.2% result from Janoshik comparable to 97.1% from Medutest?

Details up front: 95.2% · Janoshik · 97.1% · Medutest.

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.

So which one, and on what grounds?

95.2 and 97.1 are 1.9 percentage points apart, which sounds small until you restate it as impurity. Janoshik is leaving 4.8 per cent of the detected area unassigned and Medutest 2.9 per cent — a factor of 1.66 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, 95.2 and 97.1 are two measurements of slightly different quantities that happen to share a unit.

Put another way, 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.

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.

Reconciling gross mass to label claim

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

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.

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

Start from what the detector sees, because that tells you what the number means.

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

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.

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

edited 5 Nov 2025 by assay_blank — corrected a unit error in the worked example

Specifically, reporting threshold is convention and not chemistry, which is why two certificates with different thresholds disagree by a tenth of a point or more.

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

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.

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

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

To be exact about it, area percentage is not mass percentage, and conflating the two is the most common misreading of a purity figure.

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 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.

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.

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.