Dispersion in Chromatography Columns - The Van Deemter Equation > Page 62




Figure 12. Graph of Log. Maximum Efficiency against Particle Diameter

It is seen from figure 12 that changing the particle diameter from 1 to 20 micron results in an efficiency change from about 3500 theoretical plates to nearly 1.5 million theoretical plates and furthermore, this very high efficiency is achieved at an inlet pressure of only 3000 p.s.i.. It is also seen that the maximum available efficiency increases as the particle diameter increases. This is because, as already discussed, if the pressure is limited, in order to increase the column length to provide more theoretical plates, the permeability of the column must be increased to allow the optimum mobile phase velocity to be realized. It is possible to increase the inlet pressure to some extent, but ultimately the pressure will be limited and the effect of particle diameter will be the same but at higher efficiency levels.