Band broadening rate theories

TtiA rate theory a Kes the following assueptions in its explanation of band broadening ... 

The efficiency of a column is a number that describes peak broadening as a function of retention, and it is described in terms of the number of theoretical plates, N. Two major theories have been developed to describe column efficiency, both of which are used in modern chromatography. The plate theory, proposed by Martin and Synge,31 provides a simple and convenient way to measure column performance and efficiency, whereas the rate theory developed by van Deemter et al.32 provides a means to measure the contributions to band broadening and thereby optimize the efficiency. 

The plate theory assumes that an instantaneous equilibrium is set up for the solute between the stationary and mobile phases, and it does not consider the effects of diffusional effects on column performance. The rate theory avoids the assumption of an instantaneous equilibrium and addresses the diffusional factors that contribute to band broadening in the column, namely, eddy diffusion, longitudinal diffusion, and resistance to mass transfer in the stationary phase and the mobile phase. The experimental conditions required to obtain the most efficient system can be determined by constructing a van Deemter plot. 

The rate theory of chromatography describes the shapes and breadths of elution bands in quantitative terms based on a random-walk mechanism for the migration of molecules through a column. A detailed discussion of the rate theory is beyond the scope of this text. We can, however, give a qualitative picture of why bands broaden and what variables improve column efficiency. ... 

The van Deemter rate theory identified three major factors that cause band or zone broadening during the chromatographic process the eddy diffusion or the multi-path effect (A-term), longitudinal diffusion or molecular diffusion of the analyte molecules (B-term), and resistance to mass transfer in the stationary phase (C-term). The broadening of a zone was expressed in terms of the plate height, H, and was described as a function of the average linear velocity of the mobile phase, u. 

Rate theory of band broadening. The van Deemter equation in planar chromatography can be reduced to... 

Rate theory describes the contribution of different band broadening processes as a function of mobile-phase flow rate, The original rate theory developed by van Deemter in 1956 [1], relates the plate height to the three major band broadening terms. This theory is used to minimize peak width in terms of plate height (//) and was further refined to the Hawkes equation, which is shown in Eq. 8. 

The rate theory of chromatography, also known as the van Deemter model, examines the factors affecting band broadening, which is the amount of dispersion of a sample as it migrates through a column. 

The earliest attempts to explain chromatographic band broadening were based on an equilibrium model which came to be known as the Plate Theory. While it was of some value, it did not deal with the nonequUibrium conditions that actually exist in the column and did not address the causes of band broadening. However, an alternative approach describing the kinetic factors was soon presented it became known as the Rate Theory. 

The rate theory, on the other hand, describes the migration of molecules in a column. This includes band shape, broadening, and the diffusion of a solute. Rate theory follows the van Deemter equation, which is the most appropriate for prediction of dispersion in liquid chromatography colunms. It does this by taking into account the various pathways that a sample must travel through a column. Using the... 

The theory of chromatography has been reasonably well established with a kinetic or rate theory that describes the broadening of the bands or zones of separated components on a stationary-phase bed, describes their time of appearance at any particular point, and provides details of the separation power or resolution of the particular system employed. Numerous equations describing zone broadening have been proposed, the simplest being the general form of the van Deemter equation as derived for gas-liquid chromatography,... 

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