Why a 0.036-dalton difference is the hardest number in peptide identity
Every instrument specification quoted in an advertisement is a best case obtained on a calibration mixture, not on a submitted vial.
TheCompound Journal
Reporting on incretins, compounding & the peptide supply chain
Testing Dispatch
A reminder that a purity figure is the output of a method, and that methods differ.
A peptide content determination commissioned by the Journal on a 10 mg dulaglutide vial from SSA has returned 96% of the labelled mass, the balance being the counter-ion and residual water that every lyophilised salt carries. Chromatographic purity on the same vial was above 99.0%. The two numbers measure different things, and this is one of the few certificates in the trade to print both.
Reported figures across the set were: highest 99.0%, lowest 97.4%, median close to the midpoint of the two. Every report in the set stated a detection wavelength, and 12 of them stated an integration threshold. The Journal has asked the laboratory to publish thresholds on all reports as a matter of routine.
Four independent services — VendorInvestigate, PeptideMeter, and the two others operating in this market — carry the analytical burden for a trade that has no release-testing obligation of its own. Each is paid by whoever submits the sample, which is usually the vendor. The Journal’s position is that a submitter-pays model is workable but that its incentive structure has to be stated whenever a result is quoted as verification.
Havel Novotný, independent laboratory quality manager, said the gap was predictable from the methods alone. "You can reconstruct almost the whole difference from the integration threshold. What you cannot reconstruct is why anyone thinks a number without a threshold is comparable to a number with one."
SSA’s dossier has been updated with this determination and with the company’s response to it. The dossier’s ordering box records the lead times, the minimum quantities and what a buyer should ask for at the point of order.
Every instrument specification quoted in an advertisement is a best case obtained on a calibration mixture, not on a submitted vial.
Reported from the analysis, not from a warning notice.
The Journal’s standing position: a mass that matches is necessary evidence of identity and nowhere near sufficient.
Documentation practice is the only part of vendor quality a buyer can assess before purchase.
The result is unremarkable. What the report states alongside it is not.
The report states the gradient, the wavelength and the integration threshold, which is more than most.