Chromatography, general stationary phases

In gas-solid chromatography, the stationary phase is an active solid. These solids may be inorganic materials, e.g. synthetic zeolite molecular sieve, carbon molecular sieve, silica gel, or graphitised carbon, or they may be oiganic polymers. They are generally used for the separation of low molecular weight materials, i.e. gases and liquids. 

In normal-phase liquid chromatography the stationary phase is a polar adsorbent and the mobile phase is generally a mixture of non-aqueous solvents. 

In general, the eluting strength of commonly used solvents for normal phase chromatography is stationary phase = silica gel neutral alumina increasing order proceeds as follows petroleum ethers < hexanes < cyclohexane < toluene < diethyl ether < dichloromethane < chloroform < ethyl acetate < acetone < ethanol < methanol < acetic acid. 

Normal and reverse phase. In normal phase chromatography the stationary phase (the column) is relatively polar and the mobile phase is non-polar i.e. hexane and chloroform). This is not generally utilised in modem analytical applications as only small changes in the mobile phase composition and the presence of small amounts of water can lead to marked changes in the retention times of the target... 

General structures of common stationary phases for gas chromatography. 

In general, the longer a chromatographic column, the better will be the separation of mixture components. In modem gas chromatography, columns are usually made from quartz and tend to be very long (coiled), often 10-50 m, and narrow (0.1-1.0 mm, internal diameter) — hence their common name of capillary columns. The stationary phase is coated very thinly on the whole length of the inside wall of the capillary column. Typically, the mobile gas phase flows over the stationary phase in the column at a rate of about 1-2 ml/min. 

Three general methods exist for the resolution of enantiomers by Hquid chromatography (qv) (47,48). Conversion of the enantiomers to diastereomers and subsequent column chromatography on an achiral stationary phase with an achiral eluant represents a classical method of resolution (49). Diastereomeric derivatization is problematic in that conversion back to the desired enantiomers can result in partial racemization. For example, (lR,23, 5R)-menthol (R)-mandelate (31) is readily separated from its diastereomer but ester hydrolysis under numerous reaction conditions produces (R)-(-)-mandehc acid (32) which is contaminated with (3)-(+)-mandehc acid (33). 

Liquid-solid chromatography (LSC). This process, often termed adsorption chromatography, is based on interactions between the solute and fixed active sites on a finely divided solid adsorbent used as the stationary phase. The adsorbent, which may be packed in a column or spread on a plate, is generally a high surface area, active solid such as alumina, charcoal or silica gel, the last... 

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