Reversed-phase chromatography micellar mobile phases

Immaculada Rapado-Martinez, M., Garcia-Alvarez-Coque, C., and Villanue-va-Camanas, R.M., Performance of micellar mobile phase in reversed-phase chromatography for the analysis of pharmaceuticals containing [i-blockers and other antihypertensive drugs, Analyst, 121,1677, 1996. 

One of the major differences between micellar chromatography and standard reversed-phase chromatography is the selectivity of the separation. As the micelle concentration is increased, solute retention decreases as a result of increased solute-micelle interactions in the mobile phase. The rate of decrease varies from solute to solute, however, since different solutes will have a different affinity for the micelles thus, inversions in retention orders are produced.34... 

MLC uses micellar mobile phases with classical RPLC columns. This chapter expands the field to include some mobile phases that can be considered close to micellar phases, such as normal and reverse microemulsions, bile salt solutions, and surfactant solutions in supercritical fluids. Also, this chapter rapidly surveys the use of micellar mobile phases with non-RPLC stationary phases such as size exclusion or gel permeation polymer phases. Allied techniques using micellar phases such as ion-exchange chromatography and capillary electrophoresis are also briefly presented. 

M.A. Hemandez-Torres, J.S. Landy and J.G. Dorsey, Reversed Micellar Mobile Phases for Normal Phase Chromatography, Anal. Chem., 58 744 (1986). 

The term MLC is usually given to the use of micellar mobile phases with RPLC columns. A few examples have been described in other chromatographic modes that use similar mobile phases, with normal or reverse microemulsions, bile salts, and surfactants in supercritical fluids. Also, studies using non-RPLC stationary phases and micellar mobile phases have been reported in size exclusion and gel permeation chromatography. These topics are beyond the scope of this article. 

Quinones-Torrelo et al. (1999 2001) have demonstrated a correlation of pharmacokinetic properties with results from micellar liquid chromatography. In this method micellar solutions of nonionic surfactants are used as the mobile phase in reverse-phase liquid chromatography. Interactions between the mobile and stationary phases are purported to correspond to the membrane/water interface of biological barriers as hydrophobic, steric, and electronic interactions are important for both. For a series of 18 antihistamines Quinones-Torrelo et al. (2001) showed that both volume of distribution and half-life values were better correlated with retention on these columns than with the classical log K, w descriptor. 

Enormous advances and growth in the use of ordered media (that is, surfactant normal and reversed micelles, surfactant vesicles, and cyclodextrins) have occurred in the past decade, particularly in their chromatographic applications. New techniques developed in this field include micellar liquid chromatography, micellar-enhanced ultrafiltration, micellar electrokinetic capillary chromatography, and extraction of bioproducts with reversed micelles techniques previously developed include cyclodextrins as stationary and mobile-phase components in chromatography. The symposium upon which this book was based was the first major symposium devoted to this topic and was organized to present the current state of the art in this rapidly expanding field. 

A major drawback in the early reports of micellar chromatography was a serious loss of efficiency when compared to traditional hydroorganic mobile phases. If micellar mobile phases are ever to be widely accepted as a viable chromatographic technique, the efficiency achieved must at least approach that of conventional reversed-phase LC. 

Micellar Liquid Chromatography (MLC) uses surfactant solutions as mobile phases for reversed phase liquid chromatography. The two main properties of surfactant molecules, as related to chromatography, are micelle formation and adsorption at interfaces. The micelles play the role of the organic modifier, so their influence on retention has been extensively studied (1). At surfactant concentrations above the critical micellar concentration (CMC), micelles are present and the amount of free surfactant is essentially... 

The first report on the analytical use of an aqueous solution of a surfactant, above its critical micellar concentration (CMC), as mobile phase in reversed-phase liquid chromatography (RPLC) was published in 1980. The technique, named micellar liquid chromatography (MLC), is an interesting example of the modification of the chromatographic behavior taking advantage of secondary equilibria to vary both retention and selectivity. 

Pseudo-Phase or Micellar LC is obtained by using an aqueous micellar solution which contains a svuTactant as the mobile phase. The stationary phase is bonded and non-polar, hence reverse-phase separation occurs. The mobile phase is conferred special properties and allows unique selectivity. The technique is inexpensive and produces low amounts of toxics since small volumes of organic solvents are required. The amounts of solvent are small enough that miceUar chromatography can be coupled with Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) for the detection of organotin compounds [31]. Further information is available in an earlier review on this specific topic [32]. 

Inositol phosphates isomers have also been separated by micellar mobile high-performance liquid chromatography (Brando et ah, 1990). This involved addition of the surfactant hexadecyltrimethylammonium hydroxide (HDTMA+OH ) to the mobile phase to form micellar ion association complexes that were separated on a reversed-phase column. Enhanced sensitivity and selectivity can be achieved by use of high-performance liquid chromatography in con-... 

Brando, C., Hoffman, T. and Bonvini, E. (1 990) High-performance liquid chromatographic separation of inositol phosphate isomers employing a reversed-phase column and a micellar mobile phase. Journal of Chromatography B 529, 65-80. 

To overcome this problem, our laboratories have initiated a program of study in the area of micellar liquid chromatography (MLC). The mobile phase in a MLC experiment consists of a surfactant that is at a concentration above the critical micellization concentration (cmc). We have learned that the addition of a co-surfactant to a micellar mobile phase will result in the formation of lamellar liquid crystals at the surface of the reversed phase (i.e., Cjg) material... 

In conventional reversed phase HPLC, differences in the physicochemical interactions of the eluate with the mobile phase and the stationary phase determine their partition coefficients and, hence, their capacity factor, k. In reversed-phase systems containing cyclodextrins in the mobile phase, eluates may form complexes based not only on hydrophobicity but on size as well, making these systems more complex. If 1 1 stoichiometry is involved, the primary association equilibrium, generally recognized to be of considerable importance in micellar chromatography, can be applied (11-13). The formation constant, Kf, of the inclusion complex is defined as the ratio of the entrance and exit rate constants between the solute and the cyclodextrin. Addition of organic modifiers, such as methanol, into the cyclodextrin aqueous mobile phase should alter the kinetic and thermodynamic characteristics of the system. This would alter the Kf values by modifying the entrance and exit rate constants which determine the quality of the separation. 

The ftinction of the micellar pseudo-phase in MLC has been compared to that of the organic modifier in traditional RPLC, as for most solutes an increase in the concentration of surfactant in the mobile phase results in a decreased retention. This is in contrast to reversed-phase ion-interaction chromatography, where the surfectant concentration is below the cmc (z. e., no micelles exist), and the addition of an ionic surfactant will increase the retention of compounds that interact electrostatically with it. However, in MLC, the elution strength increases with micelle concentration only if the solute interacts with micelles. 

In the late 1970s, ionic surfactants were added for the first time to polar aqueous-organic mobile phases in reversed-phase liquid chromatography (RPLC) to form ion pairs (IPs). In IP chromatography, surfactant monomers adsorbed on a bonded silica-based stationary phase associate with neutral and ionic solutes, modifying their retention. The concentration of surfactant in the mobile phases is kept below the critical micellar concentration (CMC). In 1980, Daniel W. Armstrong reported the possibility of using... 

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