Micellar mobile phase capabilities

It was then recognized early in the development of the technique that there were possibilities for dramatic differences in the chromatographic performance of hydroorganic and micellar mobile phases. Since that review appeared there have in fact been several examples of micellar mobile phases providing solutions to inherent limitations of hydroorganic mobile phases allowing chromatographic capabilities that are not possible with traditional mobile phases. Yet in spite of these advances it was said in 1986 ( 3 ... 

It is always recommended to use the same column with the same type of surfactant. A column should be dedicated to the anionic surfactants, a second one to the cationic surfactants, etc. The reproducibility of the results in MLC depends on the column equilibration. The adsorbed layer of surfactant should be done correctly. It was shown that the time to reach the equilibrium between the stationary phase and the mobile phase could be very long in ion-pair chromatography with sub-micellar mobile phases. Two days at 1 mL/min were necessary to equilibrate a 15 cm x 4.6 mm i.d. column of Hypersil ODS with a mobile phase containing 0.0003 M CTAB [19]. These low surfactant concentration solutions do not contain micelles. So, they are not used in MLC. With a micellar phase, the equilibration time is reduced. It is possible to use the rapid gradient capability just mentioned above. Typically, a mobile phase containing a high surfactant concentration (10 to 100 cmc) can be used to quickly saturate the column with surfactant. Then 5 to 10 column volumes are used to rinse the column with the mobile phase containing the desired amount of surfactant. 

True micellar phases (surfactant + water) can be considered mobile phases with low elution strength. If organic modifiers, mainly alcohols, are added in small proportions to the micellar phases, a significant enhancement of the efficiency as well as elution strength is observed. Chapter 7 is dedicated to this topic. When micellar solutions and/or hybrid micellar phases with alcohol additions are used as mobile phases, it is possible to accurately predict the retention of each compound in a mixture. This capability is more decisive than in conventional RPLC because the MLC peaks are broader. The elaborated equations for modeling and optimization purposes given in Chapter 8 are easily applied with the aid of the MICHROM software included with this book. Appendix I is given as an aid to run the software. 

The many interactions that the solutes experience in a micellar chromatographic system enhances the differences among them. The possibility of using, simultaneously, the three most significant variables that affect the retention (j. e., pH, and concentrations of surfactant and modifier), will improve the capability of resolution of complex mixtures of ionic and nonionic compounds. The high accuracy in the prediction of retention factors in MLC permits the reliable and relatively rapid optimization of the composition of the mobile phase for the separation of a mixture of compounds, by using an interpretive method and a reduced number of mobile phases (at least two for one variable, four or five for two variables, and nine for three variables). 

Hinze was the first to investigate the capabilities of micellar bile salt mobile phases [11, 12]. He found that a significant amount ( 5% v/v) of a long chain n-alcohol (pentanol, hexanol or heptanol) was useful to minimize the bile salt adsorption on the C18 stationary plmse. A wide range of solutes could be separated by these phases, PAHs, quinones, steroids, indoles, polar and lipophilic vitamins. These phases were also able to resolve optically big enantiomers such as binaphthyl derivatives [12]. Such compounds are... 

We will first provide a very brief illustration of the governing equations for mass transport and the operating line for a two-phase continuous cocurrent separation system in a conventional chemical engineering context. This will be followed by a brief treatment of the multi-component separation capability of such a system. Cocurrent chromatographic separation in a two-phase system, where both phases are mobile and in cocurrent flow, will be introduced next. The systems of interest are micellar electrokinetic chromatography (MEKC) chromatography with two mobile phases, a gas phase and a liquid phase capillary electrochromatography, with mobile nanoparticles in the mobile liquid phase. Continuous separation of particles from a gas phase to a cocurrent liquid phase in a scrubber will then be illustrated. Finally, cocurrent membrane separators will be introduced. 

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