# Tirzepatide vs Semaglutide COA: How to Compare Them Side by Side
Tirzepatide and semaglutide are the two most-tested GLP-1 class peptides at independent labs today. Their COAs look superficially similar. The details differ in ways that matter.
Structural differences that drive testing choices
Semaglutide is a 31-residue GLP-1 analog with a C18 fatty di-acid conjugate at Lys26 via a γGlu-2xOEG linker. Tirzepatide is a 39-residue dual GIP/GLP-1 agonist with a C20 fatty di-acid at Lys20 via a γGlu-2xAEEA linker. Both are heavily engineered lipopeptides; both share analytical challenges around lipid-driven peak broadening. The methods overlap, but the impurity fingerprints are distinct.
Purity thresholds
Both peptides typically report ≥98% HPLC purity from a competent SPPS route. A COA reporting 95% for either should be treated with caution.
- Semaglutide — most common flagged impurities: des-lipid backbone, Asp8 aspartimide product, Met14 oxidation.
- Tirzepatide — most common: des-C20 lipid, N-terminal deletions, aminoisobutyric acid (Aib) coupling failures at positions 2 and 13.
A COA that reports only "total impurities" without naming these peaks is not doing peptide-specific work.
Identity confirmation
- Semaglutide theoretical mass: approximately 4113.58 Da monoisotopic
- Tirzepatide theoretical mass: approximately 4813.53 Da monoisotopic
Both should be confirmed by LC-MS deconvolution within a few ppm. Des-lipid analogs and near-neighbor deletions pass a purity check but fail a mass check.
