Describe the process of chromatographic separation. You need to include both the equilibrium model to predict retention times and the kinetic model that describes band broadening. The following terms should be used in your response: elution, mobile phase, stationary phase, distribution coefficient, retention time, selectivity factor, a plate, plate height, eddy and longitudinal diffusion and the resistant to mass transfer.

Chromatography is actually a way of separating out a mixture of chemicals, which are in liquid or gas form, by letting them flow slowly past another substance, which is either a solid or a liquid. It consist of a stationary phase and a mobile phase.

Explanation: All chromatographic systems rely on the fact that a substance placed in contact with two unmixable phases, one movable phase and one stationary phase, will equilibrate between them. A selectivity (or separation) factor (α) is used to 'chemically' distinguish between sample components. It is usually measured as a ratio of the retention (capacity) factors (k) of the two peaks in question and can be visualized as the distance between the apices of the two peaks. reproducible fraction will partition into each phase, depending on the relative affinity of the substance for each phase. A substance which has affinity for the moving or mobile phase will be moved rapidly through the system. A material which has a stronger affinity for the stationary phase, on the other hand, will spend more time immobilized in that phase, and will take a longer time to pass through the system. Therefore, it will be separated from the first substance. By definition, chromatography is a separation technique in which a sample is equilibrated between a mobile and a stationary phase. A theoritical plate or tray is used to produces the best possible difference between the liquid and vapour phases in equilibrium with it

Chromatographic separations are best done with a small amount of analyte (substance to be separated during analysis), which keeps either phase from becoming saturated with analyte, so that the concentrations in the two phases are directly proportional. Overloading the column with sample causes one of the phases to become saturated, leading to a loss of column efficiency, and poorly shaped peak profiles.

The retention volume in chromatographic separation (Vr) is the volume of the mobile phase required to carry the solute through the column to elution, is related to the column flow (Fc) and the retention time (tr). Likewise, the volume of the mobile phase(Vm), is related to the flow and the time the void volume takes to pass through the column.

Band broadening using the kinetic model is a phenomenon that reduces the efficiency of the separation being carried out, leading to poor resolution and chromatographic performance. This is problematical in terms of both the quality of the separation obtained and the accuracy with which sample components can be quantified.The wider band results in a dilution effect that produces a decrease in peak height accompanied by a loss in sensitivity and resolution. The eddy dispersion, accounts for the source of band broadening related to any flow unevenness in the column.

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