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Relationships between RFvalues and mobile phase composition have been determined for the last twenty of almost one hundred pesticides studied in this work. Chromatography was performed on silica gel (and other adsorbents) with mobile phases comprising a weakly polar diluent (heptane) and a polar modifier (ethyl acetate, tetrahydrofuran, or dioxane). The relationships obtained constitute a retention database. Group selectivity was compared by use of correlation between RFcoefficients, by analogy with two-dimensional TLC. The greatest spread of points, indicative of the highest selectivity, was obtained by use of non-aqueous mobile phases on silica and by use of the aqueous mobile phase methanol-water, 60 + 40 (v/v) on water wettable octadecyl silica (RP-18W). The most selective systems (normal- and reversed-phase) were chosen for separation of a mixture of nine pesticides; these were fully separated by two-dimensional development (2D TLC) on a TLC plate comprising coupled layers of octadecyl silica (reversed-phase, RP) and plain silica (normal-phase, NP).

Thin-layer chromatography can be used as a convenient small-scale model of preparative liquid chromatography in the same adsorbent-eluent system. Large volumes of sample solution should be introduced from the edge of the thin-layer and then eluted continuously; these conditions, similar to column chromatography, can be secured using a horizontal sandwich tank with a glass distributor. The formation and separation of zones can be frequently observed under UV light. Specific chromogenic reagents can also be used to obtain preliminary information about the qualitative and quantitative composition of the partly separated mixture. A modified thin-layer chromatographic technique for two-dimensional preparative separations is described.

Demixing effects in thin-layer chromatography have been investigated for frequently used binary solvents of various compositions, and methods of eliminating such effects are described. The simplest way of eliminating solvent demixing consists in presaturation of the plate in the vapours of the polar solvent component or pre-development with its more concentrated solution. The sample should be spotted some distance behind the front of the developing solvent to secure steady-state conditions analogous to column development with an equilibrated mobile phase. For more concentrated polar solvents (e.g., ⪢ 20%) it is sufficient to spot the sample several centimetres behind the solvent front; the RFvalues of solvent demixing are then high so that the presaturation or predevelopment step can be omitted. Procedures for the elimination of solvent demixing for typical solvent mixtures in the minimum time are given in terms of the distance of predevelopment.

A modified kemula-buchowskiequation for partition ratio in certain simple ternary solvent systems can be used for deriving a simple relationship between RMand RFvalues and volume composition of the mixed solvent phase. Experiments using mixtures of dimethylsulphoxide (DMS) and water as the polar phase have given results in fair accordance with theoretical anticipations. A method of interpretation of experimental data is described.