Joost K. Strasters

Simultaneous optimization of variables influencing selectivity and elution strength in micellar liquid chromatography : Effect of organic modifier and micelle concentration

Previously, the simultaneous enhancement of separation selectivity with elution strength was reported in micellar liquid chromatography (MLC) using the hybrid eluents of water-organic solvent-micelles. The practical implication of this phenomenon is that better separations can be achieved in shorter analysis times by using the hybrid eluents. Since both micelle concentration and volume fraction of organic modifier influence selectivity and solvent strength, only an investigation of the effects of a simultaneous variation of these parameters will disclose the full separation capability of the method, i.e. the commonly used sequential solvent optimization approach of adjusting the solvent strength first and then improving selectivity in reversed-phase liquid chromatography is inefficient for the case of MLC with the hybrid eluents. This is illustrated in this paper with two examples: the optimization of the selectivity in the separation of a mixture of phenols and the optimization of a resolution-based criterion determined for the separation of a number of amino acids and small peptides. The large number of variables involved in the separation process in MLC necessitates a structured approach in the development of practical applications of this technique. A regular change in retention behavior is observed with the variation of the surfactant concentration and the concentration of organic modifier, which enables a successful prediction of retention times. Consequently interpretive optimization strategies such as the interative regression method are applicable.

Influence of operating parameters on reproducibility in capillary electrophoresis

The reproducibility of two migration parameters (retention time and mobility) of a seven-component test mixture was examined under various operating conditions using laboratory-built capillary electrophoresis systems. It was found that the frequency of rinsing the capillary and the solutions used for rinsing had the greatest effect on migration reproducibility. In addition, it was found that the migration behavior of solutes that interact with micelles is not repeatable unless the proper rinse protocol is applied. Inconsistent migration behavior is linked to inconsistent total current of the system. Preliminary investigations indicate that the fluctuation in total current were associated with non-equilibrium conditions between the buffer and the capillary wall.

Multiparameter optimizations in micellar liquid chromatography using the iterative regression optimization strategy

An extension of the iterative regression optimization strategy to multi-parameter optimizations is described and applied to the separation of ionic compounds (amino acids and peptides) by means of micellar liquid chromatography. The parameters examined are the concentration of surfactant, the concentration of 2-propanol and pH. Fairly regular (linear, weakly curved) retention behaviour of the compounds as a function of the parameters results in an efficient optimization using a relatively small number of initial experiments.

Gradient elution in micellar liquid chromatography II. Organic modifier gradients

The application of organic modifier gradients in micellar liquid chromatography (MLC) is discussed. The equation derived by Snyder and co-workers describing gradient elution in hydro-organic reversed-phase LC was verified for organic solvent gradients in the presence of micelles. It is also demonstrated that the use of these gradients require little re-equilibration time due to the limited range of organic modifier concentration used in the gradient. This would result in shorter analysis time. Lastly, a practical application of the use of propanol and acetonitrile gradients in MLC is described.

Comparative study of the determination of solute-micelle binding constants by micellar liquid chromatography and micellar electrokinetic capillary chromatography

Abstract Micellar liquid chromatography and micellar electrokinetic capillary chromatography were used to determine the solute-micelle binding constants (Kmw) of different compounds. A comparison of the results showed that MECC can be an alternative to MLC for the determination of Kmw. The two techniques are complementary for the determination of Kmw with respect to practical pH ranges.

Simultaneous optimization of reagent concentration and pH in reversed-phase ion-pairing chromatography

Abstract The procedure developed earlier by the authors for simultaneous two-parameter optimization in reversed-phase liquid chromatography has been adapted to ion-pair chromatography. From the many parameters controling reversed-phase ion-pair chromatography, the mobile phase concentration of the ion-pair reagent and the pH exert the largest effect on selectivity. Initial chromatograms are chosen to cover the parameter space such that a good initial estimate of the optimum can be obtained. The true retention behaviour is approximated iteratively, and the optimum is located in a few additional chromatograms. The procedure can be followed through appropriate visualization of the results obtained in each stepin the iterative procedure. Two samples were subjected to the procedure, one containing only anions, the other containing cations, anions, and neutral molecules. The ion-pair reagents were sodium octylsulfonate and tetrabutylammonium bromide. A citrate buffer was used to control the pH between 2.5 and 6.

Gradient elution in micellar liquid chromatography. I: Micelle concentration gradient

Gradient elution in micellar liquid chromatography (MLC) is discussed. On the basis of the gradient elution theory, first developed by Snyder, equations were derived for the prediction of gradient retention times in micelle concentration gradient from isocratic data. Likewise, partition coefficients into micelles and stationary phase, and subsequently isocratic retention at different micelle concentrations can be estimated from two gradient runs. However, more studies need to be done to achieve better agreement between isocratic and gradient data. The equations will be useful for efficient development of practical separations by MLC.

Simultaneous optimization of pH and micelle concentration in micellar liquid chromatography

A retention model for ionizable compounds in micellar liquid chromatography is derived and verified. The use of the model for the prediction of retention is illustrated and appropriate optimization strategies for the separation of ionizable compounds in Micellar Liquid Chromatography are discussed.