# Retatrutide Purity Testing: What Actually Needs to Be Checked
Retatrutide is a triple agonist targeting GLP-1, GIP, and glucagon receptors. It is a 39-residue peptide with a C20 fatty acid conjugate, similar in engineering philosophy to semaglutide and tirzepatide but structurally distinct enough that off-the-shelf HPLC methods do not always transfer cleanly. Testing retatrutide well requires method work.
Why retatrutide is analytically harder than it looks
The lipid modification changes chromatographic behavior. Retatrutide runs much later than an unmodified peptide of similar size on a standard C18 gradient. The main peak can broaden under generic conditions, which artificially lowers apparent purity and buries impurity peaks near the tail. An optimized method uses:
- A C18 column with 300 Å pore size for lipopeptides
- A shallow gradient across the elution window
- TFA at 0.1% as ion-pair modifier (formic acid for LC-MS compatibility)
- Elevated column temperature (40-60 °C)
Without those adjustments a genuine 98.5% retatrutide can read as 96% on a generic peptide method.
Identity confirmation
The theoretical monoisotopic mass of retatrutide is approximately 4731.62 Da. LC-MS with ESI gives multiply charged states; deconvolution should return observed mass within a few ppm of theoretical. Confirming identity by mass is non-negotiable — near-neighbor deletion sequences and des-lipid analogs pass a purity check that lacks mass confirmation.
Related substances to watch for
- Des-lipid backbone — no fatty acid attached, elutes earlier.
- Free C20 fatty acid — leftover conjugation reagent.
- Deletion sequences — single-residue omissions from SPPS.
- Aspartimide-related peaks — Asp-containing peptides are prone to aspartimide formation.
- Oxidation products — late shoulders on the main peak.
A COA that reports a single purity number with no related substances table is not doing the retatrutide-specific work.
