Thin Layer Chromatography - Alumina
Alumina is a similar in character to silica as a stationary phase but is not nearly so popular. TLC plates coated with alumina do not, in general, provide the plate efficiencies that are realized from those coated with silica. Alumina is prepared by heating aluminum hydroxide precipitated from aluminum chloride solution to moderate temperatures. Initially, alkali metals are removed by washing with dilute aqueous acids, then with water, then dried with methanol and finally heated to about 800oC. There are three types of alumina that are available. The first type is acidic in character (pH 4.0) and has the following structure,
The second type of alumina is defined as neutral (pH 7.5), and is thought to have the following structure,
Finally the third type of alumina is defined as basic (pH 9.0) and thought to have the following structure
Alumina that is normally employed in TLC usually has a surface area of 100-250 m2/g and a particle size of about 20 mm. The material is mixed with an appropriate binder and spread on the plate (using the method that will be described later) and dried at 110oC.
The structure of alumina is complicated and somewhat uncertain and can be changed very significantly by heat treatment. Both the physical and chemical characteristics of alumina that is made available for use with TLC are usually consistent from a given supplier. The interactive groups on the surface of neutral alumina, in contact with aqueous solvent, are largely hydroxyl groups with similar interactive properties to the hydroxyl groups on the surface of silica gel.