Plate Height in Elution Systems

An extended discussion of zone spreading and plate height in nonuniform separation systems is beyond the scope of the present chapter. However, later we account for flow nonuniformities due to gas compressibility in gas chromatography. More generally, the treatment developed by the author [8,16] for chromatographic columns can be expanded to describe most other zonal systems. 

One object of separation theory is to predict H in terms of controllable experimental parameters such as flow velocity, field strength, and support particle diameter. This theory is important in optimizing separations. Details will be provided in later chapters. 

While H and the related parameter N generally characterize the efficiency of a separation system, their values vary somewhat from one component to another and they therefore cannot be considered true constants of the system. The variation with component identity occurs because the ratio Dt/W of Eq. 5.38 depends to some degree on diffusion coefficients and other component-specific properties. 

The elution time tr, by analogy, is the time needed to carry the zone through the entire length L of the column at velocity W 

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This is the basic equation for measuring plate height in elution systems. In practical use, H must be modified by a numerical constant if one chooses to use width at half-height (2t1/2, see Figure 5.6) in place of r as a measure of peak broadening. Thus... 

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