Dispersion in Chromatography Columns - The Van Deemter Equation > Page 52

            Consequently,            g = 0.747

Giddings (11) also determined that, for a well packed column, (g) should be about 0.6 so again the dispersion from longitudinal diffusion as measured. confirms that the column was reasonably well packed.

For a packed column, the particle size has a profound effect on the minimum value of the HETP of a column and thus the maximum efficiency attainable. It would also indicate that the highest column efficiency would be obtained from the smallest particles. However, this assumes that an unlimited pressure is available and the apparatus can tolerate such pressures. Within practical limits of pressure, the smaller the particle diameter, the smaller will be the minimum HETP and thus, the larger the number of plates per unit length obtainable from the column. However, the total number of theoretical plates that can be obtained, will depend on the length of the column which, in turn, must take into account the available inlet pressure.

The optimum mobile phase velocity can be obtained by differentiating equation (33) with respect to (u) and equating to zero, thus,