Over-evaluation of total flavonoids in grape skin extracts containing sulfur dioxide

Sulphur dioxide (SO2) proved to increase absorbance at 280 nm of grape skin and seed extracts containing it, diluted with ethanol\u2013HCl to assess total flavonoids and anthocyanins in the same analysis. Additional absorbance at 280 nm was also observed in acetone:H2O extracts, if the acetone had not completely evap-orated before the extracts were diluted with a solvent. Flavonoids were correctly quantified in the extracts when SO2or acetone were removed by solid-phase extraction with a C18RP as sorbent and methanol as eluting solvent