47O JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS talc and starch, usually with the aid of 5% starch or 20% plaster of Paris added as a binder. Of the many possible materials, silica gel and alumina have since proved to be of widest applicability, with appreciable usage of kieselguhr and cellulose also. Other adsorbents of more recent introduction include dextrangel, DEAE-Sephadex and acetylated cellulose (22), microcrystalline cellulose (25), ion exchange materials (24), polyamides (25) and basic zinc carbonate (20). The choice of stationary phases for thin-layer chromatography has been discussed by Schorn (26), and reviewed by Mangold (27) and Russell (28). Reactivation of the layer Following the spreading of the adsorbent slurry over the carrier plates, these are usually left in a horizontal position at ambient temperature until the surface takes on a matt appearance (10-20 min). The manner and degree of reactivation then required will depend upon the adsorbent and the purpose for which the chromatoplates are required. The one important fact is that a definite procedure must be chosen and adhered to if consistent results are to be obtained. It has been shown (29) that during the heating of silica gel at 110øC the activity at first falls, being at a minimum after about 15 min, and then rises to a maximum value at about $0 rain, remain- ing constant thereafter. Alumina requires higher temperatures in order to reach its maximum adsorptive power, B•iumler and Rippstein (30) heated for 4 hr at 200-220øC to obtain an activity equivalent to Brock- man II. At these temperatures, however, the binding power of gypsum tends to fail, dehydration to the hemihydrate beginning at about 120øC while formation of the anhydrous salt occurs over the range 150ø-200øC. For these reasons it is preferable to activate at 120øC when using alumina G although consistency of activation is not as good as that given by silica gel at this temperature. Special applications Preparative-layer chromatography The use of chromatographic techniques for the preparation of pure specimens of organic compounds has until comparatively recently been confined to columnar separations. The introduction of gas-liquid chromato- graphy on a preparative scale has been invaluable in many cases but its use is limited to compounds that are stable at the temperatures required to ensure their volatility. Thin-layer chromatoplates offer an excellent

THIN-LAYER CHROMATOGRAPHIC TECHNI•)UES IN RESIDUE ANALYSIS 471 means of preparing pure specimens of substances available in limited quantity. Where larger samples are required the usual 250 • thick layer is inadequate as its acceptable load is limited. Increasing this thickness to 1 mm or more assists considerably (31), but many 20 x 20 cm chromatoplates must be developed to obtain reasonable quantities, accept- able loads being 5 to 25 mg/1 mm thickness of adsorbent. Equipment has now been described (32) which employs carrier plates 1 m long by 20 cm wide on which layers up to 4 mm thick can be spread, either by a moving spreader of increased capacity or by "casting" the layer in situ within a framework. Apparatus using carrier plates 40 x 20 cm has been advocated by Stahl (33). In practice 2 mm thick layers have been found to possess optimum properties in respect of ease of preparation, adhesion of the layer and uniformity of resolution. Samples are applied as a streak along one edge of the chromatoplate and up to 5 such plates may be developed at one time in a stainless-steel tank multiple runs are frequently necessary to obtain suitable resolution. Fluorescent indicators are used as visualization agents since they do not affect the separated compounds. The observed bands are marked under UV illumination, scraped from the carrier plate and extracted with a suitable solvent to recover the purified specimen. In this way very pure materials can be obtained, up to 1 g per chromatoplate being a practical amount provided that good resolution from associated impurities is obtained. When using thin-layer chromatoplates for pesticide preparative purposes on a micro-scale, with a view to subsequent electron-capture gas-chromato- graphic examination, it is preferable to pre-extract the adsorbent with ether to remove interfering impurities before spreading the layer. Several solvents may be used to elute the pesticide from the layer adsorbent after developing the chromatogram. In general, hexane is the solvent of choice for organo-chlorine pesticides, while for the elution of organophosphorus pesticides dichloromethane has been found preferable, recoveries ranging from 70 to 100% at levels from 10 to 80 •g (34). Wedge-layer chromatography The advantages of the greater acceptable load of thick preparative chromatoplates and the high R F values and greater resolution of thin (100 •) layers have been combined in the properties of wedge-layer chromatoplates which were designed for residue clean-up purposes (35). In this develop- ment the cross-section of the spread layer is not uniform but it is wedge- shaped, tapering from 2 mm to less than 100 •. Such layers are spread