Objective: To establish a method for structural identification of the chemical constituents of alkaloids from Dendrobium nobile stem by using UPLC-ESI-Orbitrap-MS. Method: Thermo Q Exactive quadrupole rod-electrostatic field orbitrap high resolution mass spectrometry-MS system was used and the chromatography separation was performed on Hypersil Gold C18 column (2.1 mm×150 mm

1.9 μm) with 0.1% formic acid solution (A)-acetonitrile (B) as the mobile phase for gradient elution (0-2 min

5%B; 2-15 min

5%-95%B; 15-17 min

95%B).The flow rate was set at 0.3 mL · min-1; the column temperature was 40℃

and the sample size was 2 μL.Electrospray ionization (ESI) source was applied and operated in full scan/targeted-ddMS2 under positive ion mode. Result: A total of 8 sesquiterpene alkaloids were identified based on the reference compounds

high resolution mass number and comparison between experimental data and published data.They were dendrobine

mubironine B

dendramine

dendroxine

nobilonine

6-hydroxynobiline

N-isopentenyldendrobinium

and N-isopentenyl-6-hydroxydendroxinium. Conclusion: This study introduces a comprehensive analysis method for main chemical ingredients of alkaloids from D.nobilestem

and lays a foundation for the further analysis on the substance basis research

providing a significant guidance for identification and quality control of herbal D.nobile stem.