Support derivatized surface studies

In much of the definitive IR work on the silica surface researchers have chosen to work with fumed silica. This choice was mainly for experimental reasons (the ease of preparing the self-supporting disk), but also because it minimizes another important issue — the nature of porous silica surface. A major advance in the past decade has been in the controlled synthesis of many sUica polymorphs with variable pore size. Accordingly, the past decade has seen a renewed enthusiasm for the study of porous silicas, their reaction with chemical probes, and H2-D2 exchange reactions. An increasing body of evidence indicates that the basic silica structure is similar in both cases, but that accessibihty and derivatization of the porous silicas can stericaUy alter the process and the kinetics of the reactions. 

Countercurrent chromatography (CC) is a support-free, liquid-liquid partition system in which two immiscible liquids act as mobile and stationary phases. Absence of a solid adsorbent eliminates the possibility of analyte decomposition on an acitve surface, assures recovery of the sample material, and means that the capacity of a CC column is much greater than, e.g. a HPLC column of the same dimensions. CC has been used in glycoside research becasue it is ideally suited to separate such highly polar, water-soluble, and labile constituents without the need for their derivatization. The techniques of CC that have recently been applied in fruit flavour precursor studies, including those of grapes and wines, have been reviewed (Winterhalter 1993). 

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