Effects of different organic modifiers in optimization of reversed-phase high-performance liquid chromatographic gradient elution of a mixture of natural secoiridoid compounds

Optimization of the linear gradient elution of a crude extract of Swertia herb (Gentianaceae) was performed by means of sequential methods (Fibonacci and simplex) using a reversed-phase partition system and three different organic modifiers. The optimum gradient elution separations obtained with methanol, acetonitrile and tetrahydrofuran as different organic modifiers are discussed and compared by using the linear solvent strength theory. The Poisson character of the optimized separations was checked and the Davis-Giddings statistical overlap theory was applied in order to calculate the number of components. The value obtained is independent of both the organic modifier and column type. The quality of the best separation obtained was close to the absolute optimum expected from the Davis-Giddings theory. The usefulness of dealing with two or more optimized gradient separations is discussed. Solvent strengths are reported for selected compounds (amarogentin, amaroswerin and gentiopicrosi...