Pseudo-stationary phase” forms

The thick acetonitrile layer adsorbed on the bonded phase surface acts as a pseudo-stationary phase, thus making retention in acetonitrUe/water systems a superposition of two processes partitioning into the acetonitrile layer and adsorption on the surface of the bonded phase. Based on the model described in reference 166, analyte retention could be represented in the following form ... 

The use of a high-molecular-mass surfactant (HMMS) or polymerized surfactant has been recently investigated as a pseudo-stationary phase in MEKC. Because a HMMS forms a micelle with one molecule, enhanced stability and rigidity of the micelle can be obtained. 

In MEKC, an ionic surfactant is used as a pseudo-stationary phase, and the Krafft point is also an important temperature. At a temperatures lower than the Krafft point, C f does not exceed the CMC, due to reduced solubility and, therefore, no micelle is formed. At the Krafft point, C f reaches the CMC and then the formation of the micelle is begun. The Krafft point of SDS is 16°C in a pure water, whereas it is 31°C for potassium dodecyl sulfate in pure water. Thus, a potassium salt is not an adequate buffer component for the SDS-MEKC system. 

In this variant of CZE, adapted to the separation of neutral or polar molecules, a cationic or anionic surfactant, e.g. sodium dodecylsulfate, is added in excess to the background electrolyte to form charged micelles. These small spherical species, immiscible in the solution, form a pseudo-stationary phase analogous to the stationary phase in HPLC. They trap neutral compounds efficiently through hydrophilic/hydrophobic affinity interactions (Figure 8.9). Neutral molecules as well as ionic species can then be conveniently separated as a direct result of their solubilization within the micelles. 

Here, electrophoresis often employs principles used in chromatography - interaction of the separands with another phase, which is called the pseudo-stationary phase. The pseudostationary phase is a substance that is added to the separation system in the column, and can interact with the species to be separated. The substance can be neutral, then it does not have its own electrophoretic movement or it can be charged, then it can move in the column with certain mobility. In both cases, the analytes with their own electrophoretic movements encounter on their way the molecules of the pseudostationary phase and interact with them by forming a temporary complex or by association. During the time when the separands are bound to the pseudostationary phase its mobility is different however, when it is free it moves with its own mobility. As the rate constants of the interaction are mostly very high, in analogy with weak electrolytes, the analyte then moves with a certain mean mobility that lies somewhere between the bound and free mobilities. The mean mobility is in this way dependent on the interaction (complex-ation) constant. Many substances can used for this purpose, such as 2-hydroxyisobutyric acid, which forms complexes with many ions, especially with lanthanides, and enables their electrophoretic separation when added to the separation systems. 

The use of a high-molecular-mass surfactant (HMMS) or polymerized surfactant has been recently investigated as a pseudo-stationary phase in MEKC. Because a HMMS forms a micelle with one molecule, enhanced stability and rigidity of the micelle can be obtained. Also, it is expected that the micellar size is controlled easier than with a conventional low-molecular-mass surfactant (EMMS). The first report on enantiomer separation by MEKC using a chiral HMMS appeared in 1994, where poly(sodium N-undecylenyl-L-valinate) [poly(L-SUV)] was used as a chiral micelle and binaphthol and laudano-sine were enantioseparated. The optical resolution of 3,5-dinitrobenzoylated amino acid isopropyl esters by MEKC with poly(sodium (lO-undecenoyl)-L-valinate) as well as with SDVal, SDAla, and SDThr was also reported. 

The use of bilayer coatings was reported from Kapnissi et al. [31], where a permanently adsorbed coating of a cationic polymer salt [poly(diaIlymethylammo-nium chloride)] was covered with a dynamically adsorbed polymeric surfactant [poly (sodium undecylenic sulfate)]. In contrast to the stable coatings, the adsorbed layers can be easily prepared. Traditionally, polymeric surfactants have been used in MEKC [38] and the separation principle can therefore be transferred to o-CEC. However, several other types of dynamically attached pseudo-stationary phases (PSPs) exist, such as cyclodextrins [39], dendrimers [40], proteins [41], liposomes [42], ionenes [43], siloxanes [44] micelles [3, 38] and microemulsions [45]. Comparisons between MEEKC and MEKC are often made, as their separation basis is similar [46-48]. In MEKC, surfactant molecules form micelles and solutes dissolve in them, which facilitates separation. Solutes can penetrate a microemulsion droplet more easily than a more rigid micelle and the loadability of a droplet compared with a micelle is much higher. 

Using the indirect approach to achieve a chiral separation diminishes the need for a CS in the running buffer. Prior to analysis with CE, the enantiomers are derivatized with an optically pure agent to form diastereomers. An achiral environment, in which a pseudo-stationary phase is present, is sufficient to separate these diastereomers because they possess different physicochemical properties. The indirect separation is an efficient and versatile approach mainly because of the availability of numerous chiral derivatization reagents. These derivatizing reagents can contain chromo-phore, fluorophore, or electrochemical groups, which improves the detection of hardly detectable compounds. 

Although mentioned in theories of pseudo-phase chromatography, the possibility of a direct transfer of an insoluble or a sparingly water-soluble solute, from the micelle in the mobile phase to die surfactant-coated stationary phase, was largely ignored. For this situation, a modified form of the equation of Armstrong and Nome (eq. 5.1) was derived [29], which successfully accounts for the dependence between k and [M], observed in the elution of such hydrophobic solutes ... 

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