Chromatographic zone migration

HPLC theory could be subdivided in two distinct aspects kinetic and thermodynamic. Kinetic aspect of chromatographic zone migration is responsible for the band broadening, and the thermodynamic aspect is responsible for the analyte retention in the column. From the analytical point of view, kinetic factors determine the width of chromatographic peak whereas the thermodynamic factors determine peak position on the chromatogram. Both aspects are equally important, and successful separation could be achieved either by optimization of band broadening (efficiency) or by variation of the peak positions on the chromatogram (selectivity). From the practical point of view, separation efficiency in HPLC is more related to instrument optimization, column... 

For symmetrical chromatographic bands, this is the ratio of the distance between peaks maxima to the peak width. The distance between peak maxima is proportional to the distance of the chromatographic zone migration, and the peak width is proportional to the square root of this distance. Figure 2-4 illustrates this relationship. 

FLOW. The rate at which zones migrate down the column is dependent upon equilibrium conditions and mobile phase velocity on the other hand, how the zone broadens depends upon flow conditions in the column, longitudinal diffusion, and the rate of mass transfer. Since there are various types of columns used in gas chromatography, namely, open tubular columns, support coated open tubular columns, packed capillary columns, and analytical packed columns, we should look at the conditions of flow in a gas chromatographic column. Our discussion of flow will be restricted to Newtonian fluids, that is, those in which the viscosity remains constant at a given temperature. 

Migration in TLC and PC is not measured by the time of elution since component zones are generally not eluted from the chromatographic bed migration is measured instead by the Rf value. The factor Rf is the ratio of the distance X - X0 migrated by the zone to the distance Xf - X0 advanced by the liquid front in the same time interval... 

Shake-flask measurements are often employed to design a suitable LLPC system for a given sample mixture by assisting in the selection of the two phases in which the compound of interest shows a partition coefficient sufficiently different from those of the impurities. One of the two phases is then immobilized on a suitable support that is packed into the column, and the second phase is used as the mobile phase. In general, partition coefficients of solutes obtained from static experiments compare favorably with those obtained from chromatographic experiments [3]. A comprehensive thermodynamic treatment of LLPC can be found in Ref. 4, and the prediction and control of zone migration is discussed extensively in Ref. 5. 

This process governs the rate of deposition of the molecules of nonvolatile compounds on the surface of gas ducts, and contributes to broadening of the chromatographic zones. Being of the order of 0.1 pm at STP, the mean free path of molecules, which is inversely proportional to pressure, reaches 1 cm only at about 0.01 mmHg. In dense enough gas, in the absence of convective flow, the macroscopic picture of migration of molecules (as well as of aerosol particulates) is described by the equations of diffusion. The mean squared diffusional displacement z2D of molecules, the time of diffusion t and the mutual diffusion coefficient >i 2 are related by ... 

Problem of sorption equilibrium in a migrating chromatographic zone... 

Theoretically, the maximum value of the breakthrough volume is determined by the rate of migration of the solute chromatographic zone... 

During this period of time, the chromatographic prolile migrates in the same direction as the fluid inside the separator. The distribution of these points along the separator is selected according to the chromatographic profile. Desorbent is injected into a buffer zone. The mixture to be separated is injected into the column that has the richest mixture of an identical composition. The raffinate and the extract are collected at the outputs of the columns with the maximum purity and concentration (these last two will evolve during the sequence). 

Fig.3 A migrating zone of solute molecules (spots) interacting with lipid bilayers (rings) in a chromatographic or electrophoretic separation system. The free solute molecules move (arrows) relative to the liposomes or vesicles in a flow of eluent or in an electric field. The solute molecules may either partition into the membranes and diffuse between the external and internal aqueous compartments of the structures as depicted, or interact with the external surface of the membranes and stay outside.

Following the separation of GSL by thin-layer chromatography, the GSLs were stained with iodine vapour and gel zones that corresponded in chromatographic migration with authentic human GSL of known chemical structure were scraped, transferred into scintillation vials and counted in 10 ml of "Liquiscint"... 

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