48O JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS quantity required to produce the desired effect should be used. In order to retain the compactness of the located spot, and hence conserve sensi- tivity, the compound sought should not be soluble in the solvent used for the spray and should not form soluble complexes with the reagent. It is usual to remove as much as possible of the mobile solvent before spraying the developed chromatoplate with the visualization reagent. However, it is preferable to spray loose-layer chromatograms while they are still wet with mobile phase which acts as a binding agent and prevents the dispersal of the layer materials by the mechanical force of the spray. Some visuali- zation agents may be incorporated in the layer-mix when spreading the plates, fluorescent compounds in particular may be used in this way. Sometimes, however, such "built-in" indicators interfere with the normal development of the chromatogram. Sulphuric acid may be used either alone (67) or in combination with other compounds, such as nitric acid (5), sodium dichromate (68), potassium permanganate (69) or vanillin (12). Following suctx acid sprays it is usual to heat the treated plate to 100-200øC, various colour changes frequently being observed {27) which aid in the identification of the compounds under examination. Beroza (70) sprayed with sulphuric acid containing either chromotropic acid or furfural during a study of pyrethroids synergists. Although it is not usual to use starch-bound chromatoplates when sulphuric acid is included in the spray reagent, Yamamura and Niwaguchi (71) made use of such a combination when investigating the separation of aldrin, dieldrin, endrin and endosulfan. Alkaline potassium permanganate was found to be preferable to sulphuric acid for the location of pesticides of the aldrin-dieldrin type (72). Other reagents of general applicabihty include antimony trichloride or pentachloride (11), iodine (60) and various fluorescent compounds, the latter being frequently used together with a bromine vapour treatment. In the pesticide field the above-mentioned antimony chlorides have been used to locate pyrethroids (61) and chlorophenoxyacid esters (63). Iodine vapour has been used as a general visualization indicator by Conkin (73) the appearance of the brown spots is reversible and the located compound is unaffected by the reagent, a useful attribute if the material is required for further study. Walker and Beroza (74) list the limits of detection for 62 pesticides and associated compounds located by exposure to iodine vapour for five minutes the values quoted range from 0.1 to 5 [zg with an average of about 1.5 [zg. Iodine vapour has also been used by Stammbach et al (75) for phenkapton and its commercial impurities, by

THIN-LAYER CHROMATOGRAPHIC TECHNIQUES IN RESIDUE ANALYSIS 481 Chiba and Morley (76) in a study of carbaryl, and by Katz (77) for locating DDE and dichlorobenzophenone. Fluorescent compounds have enjoyed wider popularity among pesticide residue analysts, the spots usually being located and marked by observing the quenching effect of the compound on the background fluorescence when viewed under UV light. Kirchner et al (9) used zinc-cadmium sulphide as a built-in phosphor to indicate the presence of biphenyl on their chromatostrips. Fluorescein, either built-in or spray-applied, has been used by Spickett (78) for pyrethroids and by Salo et al (79) for organo- phosphorus pesticides, exposure to bromine vapour showing the yellow spots on a red background, Walker and Beroza (74) added silver nitrate to this fluorescein-bromine combination and give limits of detection ranging from 0.5 to 10 •g for 59 pesticidal compounds for the organochlorine com- pounds 7 min irradiation with UV light was also required. Dichloro- fluorescein has been found useful for organo-phosphorus pesticides (34). Some esters of MCPA and mecoprop have been located by means of Rhodamine B (63). Among the more specific visualization agents two have been widely used. Palladous chloride has been found preferable for organo-phosphorus compounds by B/iumler and Rippstein (30), Blinn (80) and Steller and Curry (81). Silver nitrate, with UV irradiation, has been most popular for chlorine-containing pesticides. The "chromogenic reagent" of Mitchell (82), i.e. silver nitrate with 2-phenoxyethanol, has been universally used for visualizing paper chromatograms (18,74) and has been widely adopted for similar purposes on thin-layers. On silica gel or alumina plates, how- ever, it is not very satisfactory because dark backgrounds are obtained which limit its sensitivity. A simple 0.5% ethanolic solution of silver nitrate has been found to be more satisfactory (83). Other variations on this theme include ammoniacal silver nitrate (39,84), ethanolamine or potassium hydroxide with silver nitrate (72), silver nitrate-formaldehyde- potassium hydroxide (79), and silver nitrate-nitric acid (63). Abbott, Egan and Thomson (85) have described spray reagents con- sisting of a combination of silver nitrate with one of several pH indicator compounds. Bromophenol blue gave yellow spots on a blue background with organo-chlorine pesticides without irradiation with UV light. Bromo- cresol green also showed promise but required irradiation and further heating for the best results. Plates containing built-in silver nitrate are suitable for the detection of organo-chlorine pesticides, although a rather "grainy" background may be obtained. For organo-phosphorus com-