This paper describes a strategy for the development of chromatographic methods for drug candidates based upon the use of simple MS-compatible mobile phases and optimization of the chromatographic selectivity through variations of the stationary phase and mobile phase pH. The strategy employs an automated column selection system and a series of HPLC columns, varying in hydrophobicity and silanol activity, in combination with DryLab software to develop chromatographic methods for the separation of mixtures of bupivacaine and its metabolites, acidic, basic, and neutral compounds, and atenolol, nitrendipine, and their degradation products.

An optimization strategy for ternary solvent-strength gradient elution RP chromatography is described in which a 2-dimensional model of gradient time (2 levels) against ternary proportions of organic modifiers (4 levels) was constructed. Modelling was performed using Drylab. From the resolution surface the optimum ratio of organic modifiers could be selected. Excellent retention time and acceptable peak width and resolution simulations were obtained. The separation could be further optimized from the same input data by using a standard one-dimensional model in order to optimize for gradient slope, duration and shape. Excellent retention time and acceptable peak width and resolution simulations were obtained (< 1, 2 and 6% error respectively).

The suitability of three different retention models to predict the retention times of poly(ethylene glycol)s (PEGs) in gradient and isocratic chromatography was investigated. The models investigated were the linear (LSSM) and the quadratic solvent strength model (QSSM). In addition, a model describing the retention behaviour of polymers was extended to account for gradient elution (PM). It was found that all models are suited to properly predict gradient retention volumes provided the extraction of the analyte specific parameters is performed from gradient experiments as well. The LSSM and QSSM on principle cannot describe retention behaviour under critical or SEC conditions. Since the PM is designed to cover all three modes of polymer chromatography, it is therefore superior to the other models. However, the determination of the analyte specific parameters, which are needed to calibrate the retention behaviour, strongly depend on the suitable selection of initial experiments. A useful strategy for a purposeful selection of these calibration experiments is proposed.

Attempts to theoretically address the problems involved in transferring linear gradient elution methods have been somewhat ad hoc due to the simplifying assumptions usually made in conventional gradient elution theory. Until now, all equations based on the k* parameter of linear gradient elution theory used as the basis for predicting the separation selectivity have not explicitly included the effect of the dwell volume (VD). Using an exact equation for predicting k*, that is, one which fully accounts in an a priori fashion for VD, we find a set of simple yet exact equations which unequivocally must be satisfied to transfer an optimized linear gradient elution method from one system (column or instrument or both) to another. 

This paper is written to honor Csaba Horváth and to remember his work on Reversed Phase Chromatography, (RPC) a theoretical fundament of the mechanism of retention on nonpolar stationary phases, called the "Solvophobic Theory" from the subjective point of view of the author. The paper is trying to compile a few stations in the development of this important theory, which is valid more than ever and look out for its consequences in developing robust methods for routine work, especially in the daily applications of RPC in industrial settings worldwide. It was Horváth, who laid the fundaments of this valuable technique, which makes the application of RPC to a still growing use in scientific research and in pharmaceutical and chemical production. Although the Solvophobic Theory of RPC was reflecting only a part of Horváth's scientific work, the impact of RPC in life science is tremendous and the technique RPC is today one of the most popular, most widely used tools in analytical chemistry and will remain for long time in use due to its stability and to its robustness.