Mass spectrum characterization of five valepotriates by electrospray ionization tandem mass spectrometry Study on fragmentation of vitexin and isorhamnetin-3-O-β-D-rutinoside using electrospray quadrupole time of flight mass spectrometry Mechanisms of catalpol and jasminoidin by electrospray ionization mass spectrometry Fragmentation pathways of six aconitine alkaloids using HPLC-Q-TOF-MS technology