Dispersion in Chromatography Columns - The Van Deemter Equation > Page 64

The time taken to achieve these efficiencies when eluting the last peak at a k' value of 10 can be calculated employing equation (45). The results obtained are shown in figure 4 where the elution time obtained from columns of maximum efficiency are plotted against particle diameter.


Figure 14. Graph of Log. Elution Time against Particle Diameter

On examination of the curve in figure 14, the problem associated with high efficiencies becomes apparent. The elution time for the short column having about 3500 theoretical plates is only just about one half minute. The elution time from the 1.5 million plate column, however, is about 54 days, a rather long time to wait for a chromatographic separation. It is also seen that the higher the efficiencies that are required (the more difficult the separation problem) the longer the separation time and this is inevitable as a result of practical limits to the column inlet pressure.