van Deemter plot and zone broadening:
The effect of different factors discussed earlier is shown in Figure. This is known as van Deemter plot. Beginning the van Deemter plot depicting the effect of carrier gas velocity on the factors affecting the plate height, it is clear that A is not affected, B decreases and C increases with the carrier gas velocity. The combined effect on the plate height gives a minimum at a certain value of carrier gas velocity. The velocity at which HETP (H) min is obtained is known as optimum carrier gas velocity (Uopt.).
Figure: A depiction of van Deemter equation showing uopt for Hmin.
Now, from the above discussion on reducing the zone broadening we can summarize the important points as under:
1. The column should be packed with smaller particles. The packing should be compact.
2. The column diameter should be narrow.
3. The thickness of the liquid layer in the column should be minimized.
4. With gaseous mobile phase, the longitudinal diffusion can be lowered by reducing the temperature. That effect is not noticeable within liquid chromatography because longitudinal diffusion has little effect on plate height.