J. Soc. Cosmetic Chemists 17 467492 (1966) ¸ 1966 Society of Cosmetic Chemists of Great Britain Thin-layer chromatographic techniques in residue analysis J. THOMSON and D.C. ABBOTT* Presented at the Symposium on "Physical Methods," organised by the Society of Cosmetic Chemists of Great Britain, at Bristol on I6th November 1965. $ynOl•Sis--Thin-layer chromatography has rapidly developed into a precise technique of considerable use to the residue analyst. This paper attempts to deal succinctly with all aspects of thin-layer chromatography, from the choice of the adsorbent to the final docu- mentation of the developed plate. Recent advances in thin-layer chromatography, wedge- layer, multi-band layer and gradient-layer chromatography are also covered by the paper. INTRODUCTION The introduction of paper chromatography by Consden, Gordon and Martin (1) as an extension of the partition chromatographic technique of Martin and Synge {2) was followed by a very rapid expansion of the use of these procedures. The advantages of paper chromatography in partic- ular, e.g. small sample size, excellent resolution and short times involved, over earlier separatory methods were so great that the accompanying disadvantages tended to be overlooked. While being admirably suitable for amino-acids and hydrophilic compounds in general, many difficulties arose when lipophilic compounds were studied. These were only partially overcome by reversed-phase paper chromatography and although the classical adsorption chromatography worked very well for materials of this nature, no general analytical technique was available. The use by Izmailov and Shraiber (3) of a thin (2 mm) layer of powdered adsorbent, such as calcium oxide, magnesium oxide or alumina, to obtain *Laboratory of the Government Chemist, Ministry of Technology, London, S.E.1. 467

468 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS chromatographic zones from a single drop of a solution of a drug by drop- wise application of a mobile solvent, passed almost unnoticed. Thin-layer chromatography as it is recognized today can be said to have begun with the work of Meinhard and Hall (4) who used starch as a binding agent in order to endow the adsorbent layers with some mechanical stability. Application of this method to the preparation of "chromato-strips" (0.5 x 5.25") by Kirchner, Miller and Keller (5) demonstrated the usefulness of thin-layer chromatography for the separation of terpenoid compounds. They employed either starch or gypsum as binding agent and found the strips were easier to prepare and handle than the silica-impregnated filter paper they had previously used (6). Kirchner and Miller (7) also intro- duced the use of "chromatoplates" of larger size and these were used by Reitsama (8) in a study of essential oils. The thin-layer chromatography of pesticide residues also commenced at this time with the determination by Kirchner et al (9) of biphenyl in citrus fruits following a clean-up separation on starch-bound silica gel chromatostrips. With the publication of the series of papers by Stahl (10-14) the uses of thin-layer chromatography became more widely recognized and readily applied. The commercial availability of the materials and apparatus he described (14) for the production of gypsum-bound layers of silica gel, alumina and kieselguhr was a big factor in promoting this growth. Today thin-layer chromatography is regarded as a necessary adjunct to all fields of analytical chemistry, not solely for its diagnostic properties but also in the preparation of pure specimens, for clean-up purposes before the appli- cation of other analytical methods of determination, in the study of rates of reaction, etc. The number of publications describing various aspects of thin-layer chromatography has risen exponentially over the past few years while five books on the subject appeared within two years. PROCEDURES Preparation of chromatoplates Layering apparatus and techniques Thin-layer chromatography depends for its success on the ability to prepare reproducibly uniform layers of the desired thickness of a suitable adsorbent upon the chosen supporting plate. While glass carrier plates are by far the most popular for this purpose, aluminium (15), stainless steel (16), and plastic materials (17), have also been used for special pur- poses. The dimensions of the carrier plates are in general governed by