Dispersion in Chromatography Columns - The Van Deemter Equation > Page 50

 

The HETP curve is the result of a curve fitting procedure to the experimental points shown. The column was 25 cm long, 9 mm in diameter and packed with 8.5 micron (nominal 10 micron) Partisil silica gel. The mobile phase was a solution of 4.8%w/v ethyl acetate in n-decane. The hyperbolic form of the curve is confirmed, the minimum is clearly indicated and the fit of the points to the curve is good. From the curve fitting procedure the values of the Van Deemter constants could be determined and the separate contributions to the curve from the multipath dispersion, longitudinal dispersion and the resistance to mass transfer calculated and included in the figure.

It is seen that the major contribution to dispersion at the optimum velocity (where the value of (H) is a minimum) is the multipath effect. Only at much lower velocities, does the longitudinal diffusion effect become significant. Conversely, the mobile phase velocity must be increased to about 0.2 cm/sec before the dispersion due to the resistance to mass transfer begins to become relatively significant (compared to that of the multipath effect).

 

The values for the Van Deemter constants were found to be,

                                         A=0.00117

                                         B=0.0000175 cm2/sec