Topics - Rate Theory
There are two basic theories applicable to chromatography, the Plate Theory and the Rate Theory. The Plate theory describes the mechanism of retention and gives an equation that allows the calculation of the retention volume of a solute and the column efficiency. The rate theory describes the process of peak dispersion (band spreading) and provides an equation that allows the calculation of the variance per unit length of a column (the height of the theoretical plate, HETP) in terms of the mobile phase velocity and other physical chemical properties of the solute and distribution system. In the development of the plate theory, a number of different peak dispersion processes are proposed and expressions are developed that describe the contribution of each process to the total variance of the eluted peak. The final equation gives an expression for the variance per unit length of the column. The processes proposed are eddy diffusion, longitudinal diffusion, resistance to mass transfer in the mobile phase and resistance to mass transfer in the stationary phase. The rate theory has been developed differently by a number of well established scientists in the field. This has resulted in a number of different equations; viz. The Van Deemter Equation, the Giddings Equation, the Huber Equation, the Horvath Equation and the Knox Equation. Each equation differs slightly from one another and are all developed from first principles except the Knox equation, which was developed from experimentally observed relationships and subsequently rationalized on a first principle basis. All the equations give a type of hyperbolic function that predicts a minimum plate height at an optimum velocity and, thus, a maximum efficiency. At normal operating velocities it has been demonstrated that the Van Deemter equation gives the best fit to experimental data.