While the plate theory adequately describes the rate of solute migration and can approximate the width of eluted peaks, the theory fails to consider the effects of such experimental variables as particle size, molecular structure, and temperature on the shape of the eluted peak or on the parameters H and N . Plate theory makes several assumptions that are proven incorrect by experimentation. It assumes that the distribution coefficient of the solute is constant in all plates and independent of the solute concentration. In practice, however, the distribution coefficient is only independent of concentration over a very narrow range of concentrations. Another incorrect assumption is that the mobile phase is added in increments and moves in a discontinuous manner through the plates.
Plate theory assumes that equilibration is rapid compared with mobile phase flow.
Diffusion is never instantaneous, however, and a solute may be transferred from one plate to another before equilibrium is attained. This often occurs with high mobile phase flow rates and can result in a lowered efficiency. It also assumes that the broadening of zones through longitudinal diffusion from one plate to another does not occur. Chromatographic zones are commonly broadened through longitudinal diffusion effects when the mobile phase flow is low and there is adequate time for diffusion between plates. A narrow peak can widen significantly because of diffusion effects.