Mobile phase optimization strategies

Finally, we will summarize here all the correlational equations and experimental solvent parameters required for predictions of solvent strength and selectivity in LSC, and discuss their significance in terms of mobile-phase optimization strategies. 

Several mobile-phase optimization strategies in TLC are based on the use of isoeluotropic solvents (i.e., solvent mixtures of identical strengths but different selectivities). Selecting mobile phase will be achieved based on the eluotropic series (Figs. 1 and 2). These considerations are... 

Retention mechanisms of adsorption chromatography have been extensively studied. There are two popular models for this process. The displacement model, originally proposed by Snyder, treats the distribution of solute between a surface phase, usually assumed to be a monolayer, and a mobile phase as a result of a competitive solute and solvent adsorption. A treatment of this model, including the significance of predictions of solvent strength and selectivity in terms of mobile-phase optimization strategies, has been published by Snyder (81). 

As in normal phase (see section 3.5.3), the first step in mobile-phase optimization is the determination of the solvent strength that will elute the analytes with a A value between 2 and 10 from the chosen stationary phase. It is not important which modifier is chosen to determine the initial conditions, and methanol-water (50 50, v/v) is a convenient starting place. Once the initial conditions have been established, a variety of techniques may be employed to obtain the optimum separation. Most optimization strategies involve the establishment of the isoelutropic concentrations of methanol-water, acetonitrile-water and tetrahydrofuran-water. The isoelutropic concentrations can be determined by experiment or from tables of isoelutropic mixtures (e.g. Table 3.5) (Wells, 1988). The binary solvent systems A, B, C (Table 3.5, Figure 3.7) define the isoelutropic plane, which is then explored to obtain the optimum combination of water, methanol, tetrahydrofuran, and acetonitrile required for the separation. 

For both TLC and HPLC many mobile phase optimization procedures and criteria have been described in the literature. Mainly, two strategies are followed ... 

Predictions of solvent elution strength e° and the retention parameter R/made with the help of Eqs. 54-56 cannot be regarded as error-free. The observed differences between the experimental and calculated e° and Revalues are in the first instance due to the simplicity of the assumed intermolecular interactions model in systems composed of solute, solvent, and mobile phase (see Eqs. 46, 46a, and 47). In fact, the model discussed fully ignores self-association of solute and solvent, as well as mixed intermolecular interactions simultaneously engaging the solute and the mobile phase. For the aforementioned reason the most successful optimization of the mobile phase can be attained for these solutes and solvents that are practically unable to interact intermolecularly (such as hydrocarbons). Still, the importance of Snyder s approach is undeniable as an easy-to-apply strategy for multi-component mobile-phase optimization. 

It has been stated that the global LSER equation (eq. 1.55) takes into consideration simultaneously the descriptors of the analyte and the composition of the binary mobile phase and it can be more easily employed than the traditional local LSER model [79], The prerequisite of the application of LSER calculations is the exact knowledge of the chemical structure and physicochemical characteristics of the analyses to be separated. Synthetic dyes as pollutants in waste water and sludge comply with these requirements, therefore in these cases LSER calculations can be used for the facilitation of the development of optimal separation strategy. 

D. Nurok Strategies for optimizing the Mobile Phase in Planar Chromatography. Chem. Rev., 89 (1989) 363-375. 

A review of the literature available that will help analysts to select alternative solvents, with particular emphasis on solvent extraction and liquid chromatography, has been done. Methods of classifying solvents are discussed and tables of solvent properties are given. Strategies for optimizing mobile phases for high-performance liquid chromatography are described in detail (Barwick, 1997). 

---