Accepted answer
96.4 and 98.4 are 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 PeptideMeter 1.6 per cent — a factor of 2.25 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 98.4 are two measurements of slightly different quantities that happen to share a unit.
The honest answer is that the achievable range of plausible purity figures for a given vial is wider than most people expect.
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.
Put another way, gradient slope controls resolution, and gentler slopes resolve co-eluting impurities into separate peaks — so the better method reports the worse purity number.
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."
If you are ranking vendors, specify a method and have all samples tested at the same place.
6The distinction between purity and content cannot be repeated often enough here. – priya_menon 5 months ago 5For what it is worth, my own independent result was within half a per cent of this. – e_dziedzic 4 months ago add a comment