Abstract: AimTo determine the molecular weight and first-order structure of somatostatin. MethodsThe molecular weight of somatostatin was determined by electrospray ionization mass spectrometry. Somatostatin was deoxidized by 2-mercaptoethanol. A series of typical fragment ions of deoxidized product were obtained by in-source collision-induced dissociation (CID). ResultsThe m/z of quasi-molecular ion ［M+H］⁺ of somatostatin was 1 637.8 and ［M+Na］⁺ was 1 659.5. The m/z of double-charge ion ［M+2H］²⁺ was 819.5 and ［M+H+Na］²⁺ was 830.3. It showed that the molecular weight of somatostatin was 1 636.7. The y and b series of fragment ions of deoxidized product were obtained by adjusting the fragmentor voltage. It was determined that the first-order structure of deoxidized product of somatostatin was A-G-C-K-N-F-F-W-K-T-F-T-S-C. ConclusionThe molecular weight and first-order structure of somatostatin were confirmed.