Preparation of Thinlayer Plates

Most thin-layer work is done on layers prepared from water-based slurries of the adsorbents. Even with the same amount and type of binder, the amount of water used for a given slurry varies with kinds and brands of adsorbents. For example, in the case of cellulose the amount of powder to be mixed with water varies depending on the supplier: Serva, Camag and Whatman recommend the use of 60-80 mL, 65 mL and 25 mL water for 10 g of their cellulose powders, respectively. These slurries may be prepared by shaking a stoppered flask or by homogenizing for a few seconds with a mechanical mixer. On the other hand, for the preparation of an aluminium oxide slurry (acidic, basic or neutral), it is recommended that 35 g of aluminium oxide is used with 40 mL water for spreading equipment, and 6 g of adsorbent in 15 mL ethanol-water (9:1) mixture for pouring directly on to the plate without a spreading apparatus. A slurry of 120 g of alumina G in 110 mL of water has been used successfully to make 1 mm-thick layers for preparative TLC. In general, cellulose powders contain impurities that are soluble in water or organic solvents, and these should be removed by washing the cellulose several times with acetic acid (0.1 mol L-1), methanol and acetone and drying before use. The layer is made by turbo-mixing MN (Machery-Nagel) cellulose 300 (15 g) for 10 min in distilled water (90 mL) and then spreading it to give a 0.25 mm thick layer. The layers are left overnight to dry.

A slurry of silica gel G (50 g) in distilled water (100 mL) is prepared and spread with the help of a Stahl-type applicator on five glass plates of 20x20 cm to obtain 0.5 mm thick layers. The plates are allowed to set properly at room temperature and then dried (activated) in an oven at an appropriate temperature (60-90°C) for 6 h or overnight. The plates are cooled to room temperature before applying the samples.

The same method has been used successfully to prepare plates with silica gel, silica gel polyamide, cellulose and these adsorbents impregnated with a variety of reagents including di-(2-ethylhexyl) orthophosphoric acid (HDEHP), tri-octyl-phosphine oxide (TOPO), 8-hydroxyquinoline, dibenzoyl methane and several metal salts. Brucine and tartaric acid are also mixed in slurries of silica gel as impregnating reagents to resolve enantiomers of amino acids and their PTH derivatives. Mixtures of H2O-EtOH and other organic solvents can also be used, depending on the nature of the impregnating reagents. Citrate and phosphate buffers have also been used for slurrying silica gel in place of water. It is customary to use 0.25

or 0.50 mm thick layers in activated form, but for preparative purposes 1-2 mm layers are best.

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