Chromatographic migration, mechanism

A review on TLC and PLC of amino adds, peptides, and proteins is presented in the works by Bhushan [24,25]. Chromatographic behavior of 24 amino acids on silica gel layers impregnated tiraryl phosphate and tri-n-butylamine in a two-component mobile phase (propanol water) of varying ratios has been studied by Sharma and coworkers [26], The effect of impregnation, mobile phase composition, and the effect of solubility on hRf of amino acids were discussed. The mechanism of migration was explained in terms of adsorption on impregnated silica gel G and the polarity of the mobile phase used. 

Migration of fluids in a porous matrix with solid-liquid fractionation results in a process much similar to the chromatographic separation of elements (DeVault, 1943 Korzhinskii, 1970, Hofmann, 1972). This mechanism has recently been revived in the context of mantle metasomatism by Navon and Stolper (1987), Bodinier et al. (1990), Vasseur et al. (1991), in the context of hydrothermal systems by Lichtner (1985) and, for stable isotopes, by Baumgartner and Rumble (1988). Only a simplified account of this model will be given here. Let

solid matrix and melt, respectively, and vHq the fluid velocity relative... 

A variety of microscale separation methods, performed in capillary format, employ a pool of techniqnes based on the differential migration velocities of analytes under the action of an electric field, which is referred to as capillary electromigration techniques. These separation techniques may depend on electrophoresis, the transport of charged species through a medium by an applied electric field, or may rely on electrically driven mobile phases to provide a true chromatographic separation system. Therefore, the electric field may either cause the separation mechanism or just promote the flow of a solution throughout the capillary tube, in which the separation takes place, or both. 

Apparently the above results indicate that the chromatographic behavior of copolymers is different for different chain architecture. When the Rf values are compared at equimolar composition, the sample migration occurs first for the statistical copolymer and belatedly for the alternating, whereas the block copolymer remains immobile on the starting point. Such observations allow us to conclude that the mechanism of chromatographic separation of copolymers is related not only to the chemical composition but also to the chain architecture. 

Although there is evidence that complexation with silver ions is the governing interaction in Ag-TLC, other factors should also be considered. Thus sihca gel, which is the most widely used supporting material, possesses appreciable polarity and adsorption activity. Therefore, in many cases, an impact of mixed retention mechanism on migration, geometry of spots, and selectivity of resolution is to be expected. Also, the mobile-phase solvents are active elements of the chromatographic system and interactions both with the supporting material and FA is possible this may also have a serious effect on the whole separation process. 

The forced migration of a liquid phase in continuous contact with a stationary phase is encountered in several chromatographic techniques. One of the aspects particular to HPLC is that of the partition mechanisms between analyte, mobile phase and stationary phase. They are based on coefficients of adsorption or partition. 

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