1 Introduction

Flavonoids comprise one of the largest groups of secondary plant metabolites occurring in plants. They share a common C ₆-C ₃-C ₆ skeleton, with two benzene rings (A and B) linked through a heterocyclic pyrane or a pyrone ring (C) (Figure 1). Several thousands of flavonoids have been described, the larger number corresponding to flavones, flavonols and their O-glycosides. Comprehensive revisions on flavonoid types (structures, characteristics, biological properties and analysis) can be found in various regularly updated reviews. ^{1 3} Flavonoids are found in nearly all plants. Major subclasses present in food correspond to anthocyanins, flavanols, flavones, flavanones, flavonols and isoflavones. Flavanones and flavones are often present in the same plant ( e.g. citrus); however, flavones and flavonols are usually not found together, nor are flavanones and anthocyanins.4 With the exception of flavanols, dietary flavonoids mostly occur in O-glycosidic forms.

Figure 1: Basic structure of flavonoids and spectrum. Numbering scheme for flavonoids other than chalcones and aurones

Since its introduction in the 1970s for flavonoid analysis, HPLC has become the technique most commonly used for their separation, screening, chemotaxonomical surveys or quantitative determination in plants and foodstuffs. Further introduction in the 1980s of diode array spectrophotometers as detectors associated to HPLC (HPLC-DAD) has led to great improvement for flavonoid identification, as not only the retention characteristics but also the UV-visible spectra can be used for this purpose. In HPLC-DAD the chromatographic eluent is scanned...