482 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS pounds, however, such plates are quite unsuitable as considerable streak- ing of the pesticide occurs. One benefit given by the use of silver nitrate-UV irradiation as an indicator system is that Since only a very small amount (10%) of the pesticide is affected by the irradiation, the bulk may be extracted for examination by other means (34). These workers have also studied the properties of a number of other indicator compounds, belonging in the main to the azo and triphenylmethane classes of dyestuffs, when applied as visualization agents for pesticidal compounds. Brilliant green (colour index No. 42040) showed very useful reactions and good sensitivity. With the aid of this material it is possible to locate organo-chlorine, organo-phosphorus and triazine compounds and to dis- tinguish between them. On spraying the silica gel chromatoplate with an 0.5% solution of brilliant green in acetone, organo-chlorine pesticides are observed as pale yellow spots on a green background and may be readily marked. On placing the sprayed plate into an atmosphere of bromine vapour (the plate must still be damp with acetone) the green background and the spots of organo-chlorine pesticide disappear but triazine herbicides are located as semi-transient green spots on white and organo-phosphorus pesticides appear as permanent dark green or yellow spots. Among the specific visualization methods mention must be made of the use of esterase inhibition procedures for the detection of organo- phosphorus compounds and their metabolites. The sensitivity of this procedure varies very markedly with the compound studied, for example the anti-cholinesterase activity of phorate oxygen analogue sulphone is one thousand times that of the parent phorate and its limit of detection is correspondingly lower. Methods used on paper chromatograms (86,87) have been similarly applied to thin-layer separations (88). The developed paper or plate is briefly exposed to bromine vapour and placed in contact with a sheet of filter paper which has been impregnated with out-dated human plasma. After incubation for 10 to 20 min at 35 to 40øC the paper is removed and sprayed with acetylcholine bromide and bromo- thymol blue or bromophenol blue. After a short while (5-10 min) the transferred spots appear in blue on the yellow background. Using tech- niques of this nature sensitivities down to about 10ng have been obtained from developed thin-layer chromatograms. The documentation of chromatograms As thin-layer chromatography increased in importance and widened

THIN-LAYER CHROMATOGRAPHIC TECHNIQUES IN RESIDUE ANALYSIS 483 in scope, the need for a documentation system became recognized. The number and variety of suggested systems implies that this is not an easy matter. This is one field in which paper chromatography usually scores over thin-layer procedures the filter papers are readily marked, should the spots be transient, and can be stored almost indefinitely for future reference as required. The fragile nature of the layer of material, however, and the relative expense of carrier plates makes prolonged storage of thin- layer chromatoplates impractical and uneconomic. Published methods for preserving or recording thin-layer chromatograms can be divided into four main classes :--(i) Preservation of the layer (ii) simple tracing processes (iii) photographs or photocopies (iv) direct documentation on to sensitized papers. Meinard and Hall (4) preserved their chromatograms by applying a length of transparent adhesive tape to the layer in such a way that the two ends were free for notebook mounting while the layer surface bearing the located spots adhered to the tape. Complete chromatoplates were also protected by covering them with a film of transparent material. For the fixation and preservation of loose-layer chromatograms, Mottier and Potterat (89) employed impregnation with paraffin wax or cetyl alcohol at 104øC spraying with collodion (90) or polyvinylacetate was found to be successful only in fixing the surface of the layers. For bound-layers on plastic plates, Squibb (17) used a spray of Tuffilrn No. 545 which gave the surface an acetate-film finish and enabled the plates to be sliced into strips for radioactivity counting purposes. Lichtenberger (91) found that a spray of a polyvinylpropionate solution could be used to fix the layer material together in such a way that the entire chromatogram could be peeled off the plate or floated off after a short immersion in water. Similar sprays are available commercially (Neatan, E. Merck and Co.), but in our hands only limited success has been achieved, fragmentation readily occurring. The use of tracing paper or cloth (8,92) to document chromatograms has the benefit of simplicity, but manual processes of this nature can only yield an approximate representation and the rather soft nature of the layer surface makes the procedure somewhat indecisive. Similarly a copy- drawing (11), of the plate, while adequate for the reproduction of the observed colours of spots and their relative position, is unlikely to yield an accurate document in terms of R F value, size of spots, etc. Photography, in black and white or in colour, has been widely used for the production of copies or slides for projection (8,11). Brown and