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Testing Dispatch
Independent analysis puts a GGPeps tirzepatide lot at 95.4%, against 92.1% on the certificate
A reminder that a purity figure is the output of a method, and that methods differ.
PeptideMeter has published a purity determination of 95.4% on a 10 mg vial of tirzepatide supplied by GGPeps, against a figure of 92.1% on the certificate that accompanied the vial. The sample was submitted by the Journal in October 2025 as part of a continuing programme of arm’s-length purchasing, and the vendor was told before the order was placed that any result would be published.
Content by elemental nitrogen determination returned 98% of the labelled nominal mass, which on a 10 mg vial corresponds to roughly 13.15 mg of peptide rather than the labelled figure. The remainder is accounted for by counter-ion, residual water and residual solvent, none of which is visible chromatographically.
The Journal purchased 8 vials from the same lot at retail and had four of them analysed separately, in order to distinguish vial-to-vial variation from method variation. Vial-to-vial agreement was within 0.4 percentage points. Method-to-method disagreement was an order of magnitude larger.
Purity became this trade’s single figure of merit because it is the cheapest measurement to obtain and the easiest to compare. Peptide content, which determines how much drug is actually in a vial, costs several times as much and is quoted as a priced addition by most of the twenty companies in the Journal’s dossier programme rather than carried as a standard release test. Sterility and endotoxin are separate disciplines again, and are neither tested nor claimed.
"Ask for the suitability summary before you argue about the percentage," said Dr Sirichai Boonmee, peptide chemist, Chulalongkorn University. "If the tailing factor and the replicate precision are outside limits, you are arguing about arithmetic performed on an unfit separation."GGPeps’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.
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