Studies on Negative Electrospray Ionization Mass Spectrometry on Saponins

Two kinds of saponins have been investigated by negative electrospray ionization (NESI) mass spectrometry. Under ESI conditions, the [M-H]- ions of saponins were observed which provide the molecular weights of saponins. The fragment pathways of [M-H]- ions of these two saponins depend on their structures. For steroidic saponins, [M-H]- ion only produces the fragment ions by the losses of sugar units. For oleanolic saponins, [M-H]- ion yields the cross-ring ions as well as the fragment ions by the losses of sugar units. Moreover, the abundance of the former is higher than that of the latter. The characteristic fragments are used to provide the sequence and some linkage information of sugar moieties of saponins. Especially, their fragment difference strongly depends on the linkage between the aglycone and the sugar moieties.