Differentiation and identification of ginsenoside isomers by electrospray ionization tandem mass spectrometry

Three pairs of ginsenoside isomers (Rg2 and Rg3, Rg1 and F11 as well as Rd and Re) were differentiated and identified through accurate mass measurement of mass spectrometry (MS) and MS-MS. [M + Li]+ and [M − H]− ions were detected in full-scan MS analyses and selected for the MS-MS experiments using positive and negative ion electrospray ionizations (ESI), respectively. The structures of aglycone and α- and β-saccharide sugars in various ginsenosides were determined from the spectrum interpretation and accurate mass measurement. Z and C type ions were predominantly observed in the MS-MS spectra of [M + Li]+ ions, while Y type ions were the most abundant ions in the spectra obtained from the negative ion mode analysis. Furthermore, X and A ions resulted from cross-ring cleavage on the sugar directly connected to aglycone were detected in both positive and negative ion spectra, which provided the site information of the saccharide chains. The obtained MS-MS profiles were used for the structural confirmation of ginsenoside Rg2 collected from column chromatography separation of a Chinese Panax ginseng extract. The ESI-MS data with accurate mass assignment suggested that a co-eluted ginsenoside also existed in the sample fraction. The interpretation of its MS-MS spectrum and fragmentation pathways allowed the detection of the ginsenoside Rf, differentiating from its isomers Rg1 and F11.