Flash Chromatography

The traditional method for preparative separations of simple mixtures of organic compounds by means of column chromatography is giving way to flash chromatogra-phy (FC) or low-pressure liquid chromatography (LPLC) which was initially carried out by Still in 1978. FC is a simple, cheaper, and faster technique for routine purification of mixtures, allowing preparative separations from 10 mg to more than 25 g. The resolution associated with FC on a standard 5 in. high column of 40 ^m silica gel is as great as 200 theoretical plates. This amount of separating power effectively means that compounds having Rf values on analytical TLC as close as 0.1 may be reliably separated with sample recovery of at least 90%. FC is basically an air-pressure-driven hybrid of medium-pressure and short-column chromatography which has been optimized for rapid separations. Elution of the components is extremely rapid, usually taking about 5 to 10 min. As with other forms of chromatography, sample size is one of the most important variables. The amount of sample that can be separated on a given column is proportional to its cross-sectional area and the degree of separation of the components as indicated by TLC. Still et al. successfully separated a 1-g mixture of epimeric alcohols I and II with only a 65 mg mixed fraction in 7 min on a 40-mm diameter column using 500 mL of 5% ethyl acetate/petroleum ether.

Reversed-phase systems with nonpolar bonded C8 and C18 silica are used in the separation of sugars. A mixture of 200 mg of fructose and sucrose were completely separated in the 3/1 acetonitrile/water mobile phase incorporating a silylamine bonded phase with silica in less than 1 h.

This method has been used for the separation of various products of organic synthesis and small biomolecules.

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