This is afforded by a pressure assist sample introduction. Although a detailed theory of band broadening is complicated, it can be presented in a simple form through the van Deemter equation, which expresses the plate height H or the reduced plate height h as a function of A, B, C, and the linear flow velocity of the mobile phase: The equation H=f(u), the van Deemter equation, is shown in Figure 6. Tony Edge, in Handbook of Analytical Separations, 2003. These properties include pathways within the column, diffusion, and mass transfer kinetics between stationary and mobile phases. Longitudinal diffusion is the result of concentration differences in the mobile phase. Justify your answer. Unless otherwise noted, LibreTexts content is licensed by CC BY-NC-SA 3.0. Explain the van Deemter equation and demonstrate how it explains the relative resolution in capillary electrophoresis, HPLC and GC. It takes the solute molecules some time to reach the stationary phase, to enter it, to remain there for a duration that is governed by statistics, and to leave the stationary phase and enter the mobile phase again. The same applies for the movement into and out of the stationary phase. Analyte molecules with large diffusion coefficients in the mobile phase will need less time to move to the mobile/stationary phase interface. In 1956 J.J. Van Deemter introduced the equation which combined all three sources and represented them as the dependence of the theoretical plate height (HETP) on the mobile phase linear velocity. Only with very strongly bound drugs modification of the TFC procedure is required, e.g. Van Deemter equation, A term, B term, C term, Mobile phase velocity, Column, Particle size, Efficiency, Height equivalent theoretical plate, HETP (H), Eddy diffusion, Longitudinal diffusion, Mass transfer, Particle shape, Wall effect, Porosity, Diffusion coefficient, Retention factor, Viscosity, Temperature, Optimal flow. Cu u B H A ... is the capillary diameter d … Efficiencies of ∼200000–500000 theoretical plates per meter are typical for CEC. (A) The B term leads to longitudinal diffusion (along the column) as shown by the superimposed distribution. The light is efficiently transferred in an internal reflectance mode. The final analyte solution should be prepared in the mobile phase: The final analyte solution, if possible, should be dissolved in the mobile phase or a solvent “weaker” in strength than the starting mobile phase. The contribution of the two C-terms to the overall peak broadening appears to increase with the linear velocity of the mobile phase. Molecules that in the same period are not engaged in mass transfer will be well ahead, along the chromatographic bed, thereby broadening the band. For more information contact us at info@libretexts.org or check out our status page at https://status.libretexts.org. If equilibrium is not maintained, the peaks broaden and the efficiency and separation quality decrease. However, the longitudinal term in LC is marginal and increasing the flow rate will significantly increase band broadening. Another important parameter affecting the Cm-term is particle diameter. Evolution of production as a function of bed length. The van Deemter curve, which describes the dependence of the height of a theoretical plate H on the linear flow velocity of the mobile phase u. Solutes now traverse a shorter route at a higher velocity, a win-win situation. Resolution is related to both the column efficiency (which depends on the column design) and the separation factor (which depends on the solutes and the stationary phase) through the two following expressions: where N, the number of theoretical plates, is given by: Eq. After some time there will be a reverse process as the analytes move from the stationary phase into the mobile phase. The plate number N (column factor) can be increased by lengthening the column, changing the packing particle diameter, or optimizing flow rate.
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