Mass Spectrometry (MS) confirms the precise molecular identity and molecular weight of retatrutide. By measuring the mass-to-charge ratio ($m/z$) of ionized molecules, it verifies that the synthesized 39-amino-acid structure exactly matches retatrutide's theoretical molecular mass of 4731.33 Da.
Synthetic peptides are fragile macromolecules that would fragment under harsh thermal ionization. Electrospray Ionization (ESI) is a "soft ionization" technique that transforms dissolved peptide molecules into gas-phase ions without breaking covalent peptide bonds.
The dissolved retatrutide sample is sprayed through a high-voltage capillary needle (typically 2 to 4 kV), forming charged aerosol droplets. As solvent evaporates under a stream of warm nitrogen gas (desolvation), charge density increases until Coulombic fission ejects intact, positively charged peptide ions ($[M+zH]^{z+}$) into the high-vacuum mass analyzer (such as a Quadrupole or Time-of-Flight [TOF] detector).
Because retatrutide contains multiple basic amino acid functional groups (such as arginine, lysine, and the N-terminal amine), it acquires multiple protons ($H^+$) during positive-mode electrospray ionization. The mass spectrometer records the mass-to-charge ratio ($m/z$):
| Ion State | Charge ($z$) | Calculated $m/z$ Formula | Expected $m/z$ Value |
|---|---|---|---|
| [M + 3H]3+ | +3 | (4731.33 + 3.02) / 3 | 1578.12 m/z |
| [M + 4H]4+ | +4 | (4731.33 + 4.03) / 4 | 1183.84 m/z |
| [M + 5H]5+ | +5 | (4731.33 + 5.04) / 5 | 947.27 m/z |
Mass spectrometry software performs mathematical deconvolution across these charge-state peaks to reconstruct the uncharged parent monoisotopic molecular mass, confirming agreement within ± 1 Da of theoretical standards.
When reviewing the mass spectrometry section of a retatrutide Certificate of Analysis, verify: