A NEW QUATERNARY MOBILE PHASE SYSTEM FOR OPTIMIZATION OF TLC SEPARATIONS OF ALKALOIDS USING MIXTURE DESIGNS AND RESPONSE-SURFACE MODELING

A new combination of four organic solvents is proposed for the optimization of TLC separations of basic drugs and alkaloids. The solvents are diethylamine (DEA), methanol (MeOH), chloroform (CHCl3) and ethylacetate (EtAc). They were selected from a collection of ten solvents used in Normal Phase TLC mobile phases recommended for the separation of alkaloids and basic drugs in the literature. The selection was based on the classification of solvents according to selectivity and solubility parameters. Excluded were apolar and weak solvents that show no selective (polar) properties and are used only for the adjustment of the solvent strength. Polar solvents from different selectivity groups were selected to combine as many as possible selective effects in one solvent system. The final choice was made considering the displacement theory for Liquid Solid Chromatography.The four solvents have been intended for application in an optimization procedure that uses mixture designs and response surface modelling. The factor space is a tetrahedron of which only in a part suitable experiments can be performed, i.e. the design space. Experimentation in the design space should allow the simultaneous optimization of solvent strength and solvent selectivity. The quality of TL chromatograms was characterized by two criteria: the separation of the worst separated pair of spots (R(s)min) and the R(f) value of the slowest moving spot (kmax). In this way it is possible to influence not only the separation of the spots but also their place on the TL plate. The new four solvent system has been tested successfully in the separation of the parent alkaloids of four dry plant materials: Ipecacuanha root, Cinchona bark, Belladonna leaf and Opium. The predicted mobile phases gave better or equally good separations than the mobile phases prescribed by the European or Netherlands Pharmacopoeia for these separations. Moreover a substantial reduction of the number of solvents necessary for the composition of the mobile phase was achieved as the official procedures need eight different solvents.