A comparison of C-glycosidic flavonoid isomers by electrospray ionization quadrupole time-of-flight tandem mass spectrometry in negative and positive ion mode

Flavonoid C-glycosides are important bioactive natural compounds. They often occur as 6-C and 8-C isomers, which makes the structural elucidation difficult. In this paper, the high resolution electrospray ionization quadrupole time-of-flight tandem mass spectrometry (ESI Q-TOF MS/MS) was employed to distinguish two pairs of isomeric C-glycosidic flavonoids, isoorientin (luteolin-6-C-glucoside), orientin (luteolin-8-C-glucoside), isovitexin (apigenin-6-C-glucoside), and vitexin (apigenin-8-C-glucoside), in negative and positive ion mode with different collision energy (collision-induced dissociation, CID). The two pairs of isomers can be distinguished via product ion mass spectra (MS/MS) of precursor ion [M−H]− and [M+H]+. The results demonstrated that these diagnostic characteristic ions used to identify the differences between 6-C and 8-C isomers were as follows: the product ions [M−H−H2O]−, [0,3X0−H]− and [0,3X0−H−H2O]− in product ion mass spectra (MS/MS) of precursor ion [M−H]− in negative ion mode, the product ions [M+H−4H2O]+, [0,3X0+3H−2H2O]+, [0,4X0+H−H2O]+, [M+H−H2O]+, [M+H−2H2O]+, [M+H−3H2O]+, [0,2X0+H]+, [0,1X0+H]+ and [0,4X0+H−2H2O]+ in product ion mass spectra (MS/MS) of precursor ion [M+H]+ in positive ion mode.

Graphical abstractFigure optionsDownload full-size imageDownload high-quality image (154 K)Download as PowerPoint slideHighlights► Two pairs of isomeric C-glycosidic flavonoids were distinguished by Q-TOF. ► The diagnostic ions in negative ion mode were [M−H−H2O]−, [0,3X0−H]− and [0,3X0−H−H2O]−. ► The diagnostic ions in positive ion mode were [M+H−4H2O]+, [0,3X0+3H−2H2O]+, [0,4X0+H−H2O]+, [M+H−H2O]+, [M+H−2H2O]+, [M+H−3H2O]+, [0,2X0+H]+, [0,1X0+H]+ and [0,4X0+H−2H2O]+.