Separations by paper and thin layer chromatography General introduction

18 SEPARATIONS BY PAPER AND THIN LAYER CHROMATOGRAPHY. GENERAL INTRODUCTION Separations of certain cations (including some of the rarer elements, discussed in Chapter VII) and anions can be carried out effectively by either paper or thin layer chromatography or both. 

In paper chromatography we use filter paper, marketed for this purpose. It comes usually in the form of a 2-5 cm-wide tape, from which a strip of the necessary length can easily be cut. The more modern technique of thin layer chromatography (TLC), makes use of thin sheets of aluminium oxide, silica-gel, cellulose or some other material, supported by a metal sheet or a polymer. Chromatographic thin layers can be prepared in the laboratory from commercially available adsorbents. A thick suspension of these is made with water (usually a 2 1 w/w mixture of water adsorbent is made up) and this is then spread on a metal plate with a suitable spreader device. Techniques vary from device to device, and the instructions of the manufacturer should be followed whenever thin layer plates are to be prepared. Ready-made thin layer sheets are also available commercially. These contain the active material spread on a plastic support. Thin-layer chromatographic materials, especially ready-made plates, are much more expensive than chromatographic paper, but normally offer faster and sharper separations than the paper. The procedures described in Section VI.20 can be carried out both on a slow chromatographic paper (e.g. Whatman No. 1) or on a cellulose thin layer (e.g. Whatman cellulose). 

The position of AB (usually a pencilled line) is fixed by applying a drop of the solution from a capillary pipette or micro syringe along the line. The distance moved by the substance or ion is generally taken as the distance between the line AB and the centre of gravity of the band the resulting RF value does not take into account the width of the band and also some uncertainty may be introduced if the band has diffuse boundaries. 

The values of RF are reasonably constant providing close control of all variables is maintained. It is found, however, that the relative rates of movement are constant with less rigid control, thus enabling one band on a strip to be 

It is not proposed to deal with the theory of paper chromatography here, but it may be pointed out that an important factor is the distribution of the inorganic compound between an organic solvent and water. One way in which the distribution may be varied is by forming complexes of the cations with different organic solvents or mixtures of solvents. 

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