How do Column Variables Affect Efficiency Plate Height

HOW DO COLUMN VARIABLES AFFECT EFFICIENCY (PLATE HEIGHT)  

At the microscopic, molecular level, very complex theoretical equations are required to describe the chromatographic process. These include expressions for laminar or turbulent fluid flow random walk, diffusional broadening of analyte bands in both the mobile and stationary phases and the kinetics of near-equilibrium mass transfer between the phases. Such discussions are beyond the scope of this text. 

A generalized plot of the Van Deempter Equation is illustrated in Fig. 11.3. Each of the terms is plotted individually (A = 0 here, as this figure illustrates a plot for an open tubular column) and their sum is shown as the dashed line with a distorted U-shape. The important feature of the plot is that each chromatographic system will have a minimum value for 7/ as a 

The multipath term (A) This term applies to columns packed with support particles. It becomes zero for open tubular columns when the mobile phase velocity is slow enough for the flow to be laminar (i.e., without mrbulent eddies). In a packed column, the paths of individual analyte molecules will differ as they take different routes through the spaces between the particles. Thus they will travel varying distances before they exit the column, and the difference between these distances contributes to band broadening. The relative magnitude of the multipath term depends on the particle and column dimensions. If Fig. 11.3 depicted a packed column, A would be a constant value for all values of u, and would appear as a horizontal line raising the curve by a constant amount. The multipath process is illustrated in Fig. 11.4. 

The longitudinal diffusion term (B) This term accounts for the spreading of molecules in both directions from the band center along the length of the column as a result of random-walk diffusion. This occurs primarily in the mobile phase in GC, but significantly in both phases in LC (as the analytes 

Fortunately, Dutch scientists in the 1950s related the performance of columns in terms of H, the HETP, to a single variable, the linear mobile phase velocity, . This could be calculated from column dimensions and volume flow rates or more simply measured directly using the retention time of an unretained analyte, and the measured column length, L. Three constants, the ABCs of chromatographic column efficiency, combine in the Van Deemter equation (Equation 11.11), to describe how H varies with u for a particular geometry and construction. Three terms sum together, one independent of u, one inversely proportional to it, and one directly proportional to it  

Eddy diffusion B, Longitudinal diffusion C, Mass transfer Traditional chromatography 

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