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Layer pretreatments

المؤلف:  Sue Jickells , Adam Negrusz (Editors)

المصدر:  Clarkes Analytical Forensic Toxicology

الجزء والصفحة:  350

2026-11-02

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Layer pretreatments

Prior to chromatography it is common practice to prepare the stationary phase layer for use by any or all of the following steps: washing, conditioning and equilibration. TLC plates may also be cut to preferred sizes using scissors for plastic or aluminium-backed plates and diamond or carbide glass-cutting tools for glass-backed plates. Newly consigned precoated layers are invariably contaminated, or quickly become so, because of residual contaminants from the manufacturing process, contact with packaging materials and adsorption of materials from the atmosphere. To remove contaminants, single or double immersion in a polar solvent, such as methanol or propan-2-ol, for about 5 min is generally superior to predevelopment with the mobile phase. For trace analysis, sequential immersion and predevelopment may be required to obtain the best results.

For inorganic oxide adsorbents the absolute RF value and the reproducibility of RF values depend on the activity of the stationary phase. The latter is controlled by the adsorption of reagents, most notably water, through the gas phase. Physically adsorbed water can be removed from silica gel layers by heating at about 120C for 30 min. Afterwards, the plates should be stored in a grease-free desiccator over blue silica gel. Heat activation is not normally required for chemically bonded layers. Equilibration of activated layers by exposure to the atmosphere is extremely rapid and layer activation is at times an unnecessary step. In modern air-conditioned laboratories, layers achieve a consistent level of activity that should provide sufficient reproducibility for most separations. Inorganic oxide layers can be adjusted to a defined activity by exposure to a defined gas phase in an enclosed chamber. Since manipulation in the atmosphere almost certainly readjusts this activity, it is best performed after application of the sample zones in a developing chamber that allows both layer conditioning and development in the same chamber (e.g. a twin-trough chamber), or in a separate conditioning chamber immediately before development. Atmospheres of different constant relative humidity can be obtained by using solutions of concentrated sulfuric acid or saturated solutions of various salts (CRC Hand book of Chemistry and Physics, 2007). Acid or base deactivation can be carried out in a similar manner by exposure to, for example, ammonia or hydrochloric acid fumes.