Is a 98.8% result from Medutest comparable to 98.4% from Janoshik?

Setup, so nobody has to ask: 98.8% · Medutest · 98.4% · 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.

What is the actual trade-off, and does it matter at the scale I am working at?

98.8 and 98.4 are 0.4 percentage points apart, which sounds small until you restate it as impurity. Medutest is leaving 1.2 per cent of the detected area unassigned and Janoshik 1.6 per cent — a factor of 1.33 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, 98.8 and 98.4 are two measurements of slightly different quantities that happen to share a unit.

In practice, 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.

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

Specifically, 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.

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."

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

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

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.

Stated carefully, 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.

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.