The Concept of Theoretical Plates Van-Deemter Theory

The individual terms of the sum vary depending on the mobile phase velocity, v. 

Accordingly, term A is independent of the flow velocity and characterizes the peak dispersion caused by the Eddy diffusion. This effect considers the different pathways for solute molecules in the column packing. The longitudinal diffusion is described by the term Blv. The term C-v comprises the lateral diffusion and the resistance to mass transfer between mobile and stationary phase. These effects depend linearly on the flow velocity. 

With the peak width expressed in terms of length unit ti, it follows by taking Eq. (2.8)  

Various portions contribute to the broadening of a peak. The sum of their variances of gives the total band spreading  

The portion of the peak broadening that arises due to the irregularity of the column packing is called Eddy diffusion. It is approximated by Eq. (2.19)  

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What is H anyway The original interpretation, taken from distillation theory, was height equivalent to a theoretical plate, or HETP. We have seen that this concept was inadequate, and the preceding discussion of the van Deemter equation has presented it as a measure of the extent of spreading of an analyte zone as it passes through a column. Thus, a more appropriate term might be column dispersivity. In fact, another, independent approach to the theory of chromatography defines H as... 

The theoretical plate concept is useful in determining the efficiency of any given column, but it does not indicate the effects of various operational parameters. Here, however, the van Deemter rate theory proves valuable. The van Deemter equation is ... 

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