Chromatogram sandwich configuration

Valepotriates are detected by placing the chromatogram 0.3 mm from a TLC plate sprayed with cone, ammonia solution (sandwich configuration layer to layer), fastening with clips and heating to 110°C in a drying cupboard for 10 min [12]. 

Solvents for which pre-equilibration of the dry layer is undesirable. These are neat solvents and mixtures behaving like a one-component solvent. Examples are hexane-acetone 85 15, chloroform-ethyl acetate 1 1, ethyl acetate-methanol 95 5. With these solvents chromatograms are developed best in the sandwich configuration. Development in an unsaturated tank is an acceptable compromise. 

Solvents requiring pre-equilibration of the dry layer with the vapors of the solvent. These are solvents containing a volatile acid or base as well as solvent mixtures containing a comparatively large amount of a polar component such as methanol and/or acetonitrile. For these the sandwich configuration is not suitable. Development in a paper lined tank is an acceptable compromise. It is preferable, however, to develop these chromatograms after controlled (standardized) pre-equilibration of the dry layer. 

---