Selection of Chromatographic Solvent

Finding a chromatographic development solvent (or mixture of solvents) for the separation of an unknown mixture of additives is not always easy. In some cases a separation is not obtained with a single solvent or solvent combination but necessitates the preparation of several chromatograms using different solvents. 

An unknown mixture should be first chromatographed on 20 cm x 5 cm plates with solvents of different polarities to obtain an idea of the types of compounds present in the sample and to reduce the possibility of missing any of the sample components. Solvents of low polarity, such as n-hexane, tetrachloroethylene and carbon tetrachloride, cause polar sample constituents to migrate more readily. Solvents of intermediate polarity such as toluene, benzene, chloroform and methyl cellosolve have a greater elutive effect on polar sample components, whereas highly polar solvents such as dioxan, methylene dichloride, ethyl acetate, nitromethane, acetone, lower alcohols and water elute polar sample constituents towards the solvent front, i.e., Rf values near unity. Mixtures of 40/60 petroleum spirit and up to 10% v/v) ethyl acetate are very useful general solvents for the separation of unknown mixtures. 

Substances separated Stationary phase Mobile phase Detection Ref. 

Phenolic antioxidants Silica Gel G Methanol -cyclohexane (1 24) 30%Molybdophosphoric acid + ammonia vapour [4] 

Organotin stabilisers Not stated Acetic acid -isopropyl ether (1-5 98-5) 20% Molybdophosphoric acid + ammonia vapour [10] 

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