Reversed-phase micellar

T. Okada, Simultaneous Separation of Ionic and Nonionic Compounds using Reversed-Phase Micellar Chromatography, Anal. Sciences, 9 59 (1993). 

J. V. Posluszny and R. Weinberger, Determination of dmg substances in biological fluids by direct injection multidimensional liquid cliromatography with a micellar cleanup and reversed-phase cliromatography , Awa/. Chem. 60 1953-1958(1988). 

The comprehensive review by Gocan et al. [25] focused specifically on lipophilic-ity measurements by liquid chromatography, including reversed phase, thin-layer, micellar, RP-ion-pair and countercurrent chromatography. 

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. 

Dolezalova, M., Capova, H., and Jobanek, R. (2003). Determination of the purity of phenoxymethylpenicillin by micellar electrokinetic chromatography and reversed phase liquid chromatography on a monolithic silica column.. Sep. Sci. 26, 701—708. 

Many different types of interaction can induce reversible phase transitions. For instance, weak flocculation has been observed in emulsions stabilized by nonionic surfactants by increasing the temperature. It is well known that many nonionic surfactants dissolved in water undergo aphase separation above a critical temperature, an initially homogeneous surfactant solution separates into two micellar phases of different composition. This demixtion is generally termed as cloud point transition. Identically, oil droplets covered by the same surfactants molecules become attractive within the same temperature range and undergo a reversible fluid-solid phase separation [9]. 

Besides the above differentiation, restricted-access media can be further subdivided on the basis of the topochemistry of the bonded phase. Packings with a uniform surface topochemistry show a homogenous ligand coverage, whereas packings with a dual topochemistry show a different chemical modification of the pore internal surface and the particle external surface (114). Restricted-access media of the former type are divided into mixed-mode and mixed-function phases, bonded-micellar phases, biomatrix, binary-layered phases, shielded hydrophobic phases, and polymer-coated mixed-function phases. Restricted-access media of the latter type include the Pinkerton s internal surface reversed-phase, Haginaka s internal surface reversed-phase diol, alkyl-diol silica, Kimata s restricted-access media, dual-zone phase, tris-modified Styrosorb, Svec s restricted-access media, diphil sorbents, Ultrabiosep phases. Bio Trap phases, and semipermeable surface phases. 

Hall et al. (127) compared free solution capillary electrophoresis (FSCE) and micellar elec-trokinetic capillary chromatography (MEKC) techniques with HPLC analysis. Four major food-grade antioxidants, propyl gallate (PG), BHA, BHT, and TBHQ, were separated. Resolution of the 4 antioxidants was not successful with FSCE, but was with MEKC. Separation was completed with excellent resolution and efficiency within 6 min and picomole amounts of the antioxidants were detectable using UV absorption. In contrast, reversed-phase HPLC separation was not as efficient and required larger sample amounts and longer separation time. 

Although a great variety of analytical techniques have been applied to the simultaneous determination of methylxanthines in various matrices, HPLC is the one most frequently used nowadays. Most of the methods are based on reversed-phase HPLC, using ACN, MeOH, or THF in acetate or phosphate buffer as mobile phase and UV spectrophotometric detection (256 -270). Some RP-HPLC methods were proposed in combination with solid-surface room-temperature phosphori-metric detection (271), mass spectrometry (272), or amperometric (273) detection. The separation can also be achieved by RP ion-pair or ion-interaction HPLC (274-277) or micellar HPLC (278). In contrast, in recent years few normal-phase HPLC methods (279) were reported (see Table 5). 

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... 

Micellar electrokinetic capillary chromatography (MECC) is a mode of CE similar to CZE, in which surfactants (micelles) are added to the buffer system. Micellar solutions can be used to solubilize hydrophobic compounds that would otherwise be insoluble in water. In MECC the micelles are used to provide a reversed-phase character to the separation mechanism. Although MECC was originally developed for the separation of neutral species by capillary electrophoresis, it has also been shown to enhance resolution in the analysis of a variety of charged species.16... 

Capillary electrochromatography (CEC) is a rapidly emerging technique that adds a new dimension to current separation science. The major "news" in this method is that the hydrodynamic flow of the eluting liquid, which is typical of HPLC, is replaced by a flow driven by electro-endoosmosis. This increases considerably the selection of available separation mechanisms. For example, combinations of traditional processes such as reversed-phase- or ion-exchange- separations with electromigration techniques are now possible. Also, CEC is opening new horizons in the separation of non-polar compounds, and thus represents an alternative to the widely used micellar electrokinetic chromatography. 

The separation scientist with experience gained from a LC background may tend to limit the modes of electrochromatography to reversed phase (RP), normal phase, ion-exchange and, maybe, size-exclusion. Analysts from an electrophoretic background typically use the term "CE" in a much broader sense to include the main modes of capillary zone electrophoresis, micellar electrokinetic chromatography, capillary gel electrophoresis, isoelectric focusing and isotachophoresis. 

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. 

Liquid chromatographic analyses with ICP-MS detection may be divided into categories according to the mode of chromatography used reversed-phase, ion-pair, micellar, ion-exchange, and size-exclusion chromatography. 

Micellar liquid chromatography (MLC) is another variation on reversed-phase and ion-pair chromatography. In this mode, the counter ion is a surfactant of high concentration and a long-chain hydrocarbon. Micelles form when the concentration of the surfactant is increased to the point at which aggregation occurs and spherical particles are formed. The hydrophilic parts of the long-chain hydrocarbon are oriented toward the outside of the sphere with the hydrophobic end in the center of the sphere. A mixture of compounds of varying polarity partitions between the... 

The basis for separation employing micellar mobile phases stems from their ability to differentially solubilize and bind structurally similar solutes. Skeptics view MLC as a fascinating example of the incorporation of secondary equilibria for control or adjustment of retention (101). However, it is the ultimate of secondary equilibria since the types of interactions possible with micellar aggregates cannot be duplicated by any single other equilibrium system, or for that matter, any one or mixture of traditional normal or reversed phase mobile phase systems. This is due to the fact that solutes can interact with the surfactant aggregates via hydrophobic, electrostatic, hydrogen bonding, and/or a combination of these factors. 

In addition to the factors listed in Table VIII, the nature of the surfactant-modified stationary phase affects P (partition coefficient for distribution of solute between bulk solvent and modified stationary phases) and thus will influence the retention observed. It should be realized that most of the normal and reversed-phase packing materials will adsorb/absorb surfactant molecules from the mobile phase solution and become coated to different degrees when surfactant mobile phases are passed through them. Numerous adsorption isotherms have been reported for various surfactant - stationary phase combinations illustrating this point (82,85,106,115-128,206). The presence of additives can mediate the amount of surfactant surface coverage obtained (110-129,175,206). It has been postulated that the architecture which adsorbed surfactant molecules can assume on conventional stationary phases can range from micellar, hemi-micellar, or admicellar to mono-,bi-, or multilayered, and/or other liquid crystalline-type structures (93,106,124,128,129,... 

TABLE VIII. Comparison of the General Effect of Variables on Retention in Reversed-Phase Ion-Pair (RP-IPC) and Micellar Liquid Chromatography (MLC)... 

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