Paeoniflorin was studied by using Electrospray Ionization Tandem Mass Spectrometry(ESI-MS~n) both in positive and negative modes.Through the experimental method of hydrogen and deuterium exchange,the collision-induced dissociation fragmentation pathways were described and it proved that the precursor ion of paeoniflorin was clines to loss one molecule benzoic acid in both positive and negative MS~2 spectra,furthermore,the loss of glucose residue was observed in positive ion mode,while,only one formaldehyde ...