Phenols/polyphenols solid phase extraction

Phenolic acids are ionized at pH 7.0 and are un-ionized at pH 2.0. This property allows for solid-phase extraction of neutral polyphenolics at pH 7.0 and acidic polyphenolics at pH 2.0 to prevent interference. In this protocol, polyphenolics isolated as neutral and acidic fractions using pH adjustments are analyzed by reversed-phase HPLC in a thermostatically controlled environment. 

Retention times of the peaks are subject to the particular type of column. The acidic fraction from solid-phase extraction consists of phenolic acids such as cis-coutaric, trans-coutaric, and trans-caftaric acids. Isocratic elution is suitable because of the limited number of compounds found in the acidic fraction. Analysis of the acidic fraction is completed within 30 min. See Figure 11.3.1 for an HPLC chromatogram of the acidic polyphenolics isolated from Niagara grapes. 

Recently, Romani et al. [200] published a sophisticated solid phase extraction procedure for the qualitative analysis of grape skins. In a first extraction on Extrelut 20 (kieselguhr, Merck) polyphenols are separated from anthocyanins. The polyphenols are subsequently fractionated on a C-18 cartridge (Bond Elut , Varian) into phenolic acids and esters, procyanidins, flavonol glycosides and acylated anthocyanins. 

The extraction solvents are then evaporated in the rotary evaporator under vacuum condition to concentrate the extract, to which other solvents can be added for purification and separation. Solubihty of compounds in different solvents and pH are important criteria used for the separation and purification of polyphenolic compounds. Separation of phenolic compotmds from the mixtures of compounds can be done using solvent separation methods and various chromatographic methods (including solid-phase extraction). 

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