Accepted answer
A 96.8 per cent result against a higher certificate figure is usually a method difference, not a disagreement about the vial. Inter-laboratory spread on identical peptide material runs half a per cent to a full per cent by RP-HPLC. Gradient slope, detection wavelength and integration convention each move the number, and a gentler gradient resolves impurities that a steeper one hides under the main peak — so the better method reports the worse figure. Get the column, the gradient and the wavelength from both PeptideMeter and the supplier before treating 96.8 per cent as a contradiction.
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.
Sample solvent strength affects peak shape — if you inject in strong solvent on a gradient starting in weak solvent, the solvent peak can distort your main peak or create a false shoulder.
Concretely, detection wavelength matters because 214 nm sees the peptide backbone while 280 nm sees only aromatic side chains — so truncation impurities lacking a tryptophan are invisible at 280 nm.
The ICH Q3A impurity thresholds and the relevant pharmacopoeial chapters all specify method validation requirements that almost no research-grade certificate claims to meet.
If you are ranking vendors, specify a method and have all samples tested at the same place.
3Does this hold for a longer chain length, where the deletion sequences accumulate? – sian_llewellyn 7 months ago add a comment