Dispersion in Chromatography Columns - The Van Deemter Equation > Page 61

It would be of interest to use the equations to calculate the maximum efficiency, column length and analysis time that would be obtained from columns packed with particles of different diameter under conditions that commonly occur in LC analyses.

The diffusivity of solutes having molecular weights ranging from about 60 to 600 vary from about 1x10-5 to 4x10-5 cm2/sec.(13) Thus, an average value of 2x10-5 cm2/sec will be taken in the following calculations. In a similar way a typical viscosity value for the mobile phase will be assumed as that for acetonitrile (14), i.e. 0.375x10-2 poises. The inlet pressure taken will be 3,000 p.s.i. or about 200 atmospheres.

Many LC pumps available today will operate at 6,000 p.s.i. or even 10,000 p.s.i., however, it is not the pump that determines the average operating pressure of the chromatograph, but the sample valve. Sample valves can, for a limited time, operate very satisfactorily at very high pressures but their lifetime at maximum pressure is significantly reduced. Most valves, designed for high pressure operation will have extensive lifetimes when operated at 3,000 p.s.i. and so this is the pressure that will be employed in the using the above equations. The value assumed for the d'Arcy constant,(j) is 35 when (P) is measured in p.s.i. and has been determined experimentally (14). The capacity factor of the last eluted peak was taken as 5 for calculating the elution time. Employing the above values, equation (16) was used to calculate maximum efficiency attainable for columns packed with particles of different diameters. The results obtained are shown in figure 12.