Mass Spectroscopic Methods for the Characterization of Flavonoid Compounds

Flavonoids are a broad class of polyphenolic secondary metabolites widely dispersed throughout the plant kingdom and found in substantial levels in commonly consumed fruits, vegetables and beverages. These compounds have aroused considerable interest because of their potential beneficial biochemical and antioxidant effects on human health. The structures of flavonoids are characterized by a fifteen-carbon backbone consisting of two aromatic and one oxygenated rings. Flavonoids could be divided into various types of monomeric aglycones, bi-, tri-, and oligo-flavonoids including proanthocyanidins. Owing to the importance of flavonoids, isolation and structural elucidation of natural flavonoids play an important role in many areas of science. This is generally performed by a combination of chromatographic and spectroscopic methods, including ultraviolet, nuclear magnetic resonance and mass spectrometry. Among all these techniques, mass spectrometry remains an important tool for the identification and structural determination of flavonoids. Its advantages are high sensitivity and possibilities of hyphenation with liquid chromatographic methods for the analysis of mixtures of compounds. Different desorption ionization methods allow the analysis of underivatized glycosides. Advances in mass spectrometry methodology have been shown to be extremely valuable for flavonoid analysis especially the use of mild ionization techniques, which have improved the possibility of recording molecular ions and suppressed the detection limits by several orders of magnitude. Detailed structural information can be obtained by resorting to cone voltage fragmentation and use of various types of mass analyzers. In this chapter, examples of the usefulness of mass spectroscopy techniques to the structural characterization of various flavonoids will be presented. In particular analysis of flavonoid aglycones and mono and di-O-glycosides, oligomeric and polymeric procyanidins using electropsray (ESI), matrix-assisted laser desorption ionization (MALDI), and tandem (MS/MS) mass spectrometry will be also presented.