Dispersion mobile phase,HETP equation

The Van Deemter equation remained the established equation for describing the peak dispersion that took place in a packed column until about 1961. However, when experimental data that was measured at high linear mobile phase velocities was fitted to the Van Deemter equation it was found that there was often very poor agreement. In retrospect, this poor agreement between theory and experiment was probably due more to the presence of experimental artifacts, such as those caused by extra column dispersion, large detector sensor and detector electronic time constants etc. than the inadequacies of th Van Deemter equation. Nevertheless, it was this poor agreement between theory and experiment, that provoked a number of workers in the field to develop alternative HETP equations in the hope that a more exact relationship between HETP and linear mobile phase velocity could be obtained that would be compatible with experimental data. 

In 1961, Giddings (1) developed an HETP equation of which the Van Deemter equation appeared to be a special case. Giddings was dissatisfied with the Van Deemter equation insomuch that it predicted a finite contribution to dispersion independent of the solute diffusivity in the limit of zero mobile phase velocity. This concept, not surprisingly, appeared to him unreasonable and unacceptable. Giddings developed the following equation to avoid this irregularity. 

In the previous relationships used to express the characteristics of a separation, the speed of the mobile phase in the column does not appear. Obviously, the speed has to affect the progression of the solutes, hence their dispersion within the column, and must have an effect on the quality of the analysis. These kinetic considerations are collected in a famous equation proposed by van Deemter. First used in gas chromatography, this equation has been expanded to liquid chromatography and relates H (HETP) to the mean linear velocity of the mobile phase in the column, it (see Fig. 1.9). The simplified form of this equation is given below ... 

The rate theory examines the kinetics of exchange that takes place in a chromatographic system and identifies the factors that control band dispersion. The first explicit height equivalent to a theoretical plate (HETP) equation was developed by Van Deemter et al. in 1956 [1] for a packed gas chromatography (GC) column. Van Deemter et al. considered that four spreading processes were responsible for peak dispersion, namely multi-path dispersion, longitudinal diffusion, resistance to mass transfer in the mobile phase, and resistance to mass transfer in the stationary phase. 

---