Overview

Thin-layer chromatography (TLC), like gas chromatography (GC), comes into its own when dealing with mixtures of substances. TLC using plates coated with 250 m absorbent is an excellent technique for separating quantities of up to 20 mg of additive mixtures into their individual components and provides enough of each to prepare a recognisable infrared (IR) or ultraviolet (UV) spectrum which can be compared with spectra of authentic known compounds. However, this technique does not conveniently handle larger quantities and in these cases separation on the 50 to 500 mg scale can be carried out with only a small loss of resolution by chromatography on a silica gel or alumina packed column. The separated bands are marked under UV light, removed from the plate and extracted with diethyl ether. Separations on this scale provide sufficient of each fraction for full characterisation by nuclear magnetic resonance spectroscopy (NMR) and mass spectroscopy (MS), as well as IR spectroscopic techniques.

Polymer additives - mould release agents, plasticisers, antioxidants and UV absorbers, with molecular weights extending beyond 1000 - are generally unsuitable for GC or liquid chromatographic (LC) analysis because of their low volatility, lack of chromophore or thermal instability.

The identification of mixtures of unknown additives in solvent extracts of polymers presents some difficult problems. The solvent extract is usually available in only fairly small quantities, often consists of a complex mixture requiring preliminary separation into pure components before identifications can be attempted, and is frequently a mixture of compounds of completely unknown...