Aluminum oxide precoated plates

To achieve better separation efficiency and reproducibility compared with handmade layers also in the case of aluminum oxides, precoated plates are available for straight phase PLC. They are listed in Table 3.4. 

Aluminum oxides, similar to silica gels, are available as bulk materials and as precoated plates, to be used not only for straight phase adsorption chromatography, but also for partition PLC (see Table 3.3 and Table 3.4). In particular, the aluminum oxide type 150 (i.e., mean pore diameter 150 A [15 tun]) is suitable for partition chromatographic purposes. 

HPTLC separation material is available in the form of precoated layers supported by glass, plastic sheets or aluminum foil. Although precoated layers of aluminum oxide, cellulose, polyamide, ion exchange materials, reversed phase silica (alkyl bonded) have been commercially available for quite some time, the vast majority of TLC separations is carried out on normal phase silica gel, particularly in quantitative TLC analysis. Recently introduced plates modified with amino, cyano and diol functional groups bonded to the silica can be used as multimodal media. Depending on the developing solvent used, they extend normal phase or reversed phase chromatographic properties. These layers may affect the predominant role of normal phase silica gel in TLC to a certain extent. They are described in Chapter 4 of this book. Also chiral layers for the separation of optical isomers are available and cause increased interest (11). 

Pretreatment is necessary to achieve a clean surface on the piece to be coated. To remove fat, oxides, and other impurities from the surface, defatting, etching, and descaling operations are carried out, which are then generally followed by a precoating step. In the case of aluminum plating, a prelayer of nickel, cobalt or iron or alloys of these metals (0.5-2 pm) is applied. Each treatment step is followed by one or more rounds of washing and conducted in such a manner as to minimize loss and to recycle valuable material [28, 29, 33, 127]. 

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