Plate Theory and Extensions - Experimental Examination of Dead Volume Measurement > Page 36
The effect of solvent composition on the retention was also investigated using a series of solutes including a dispersion of silica smoke (mean particle diameter 0.002 mm). The silica smoke simulated a solute of very large molecular size and, thus, would be completely excluded in the interstitial volume. The mobile phases used were a series of methanol/water mixtures The results they obtained are shown in Figure 7.
Courtesy of the Analyst (ref.11)
Figure 7. Graph of Retention Volume of a Number of Different Solutes against Composition of the Mobile Phase
It is clear that the retention volumes of the solutes are virtually unaffected by the composition of the mobile phase. It should be pointed out, however, that methanol concentrations below 10%v/v were not examined and so the effect of methanol adsorption on the stationary phase surface was not disclosed. (At concentrations of methanol below 10%v/v the retention volume will be inversely proportional to the methanol concentration in accordance with the Langmuir adsorption isotherm.
The smallest retention volume was obtained for the silica 'dispersion'. (However, the authors reported that the silica dispersion required sonicating for 5 hours before the silica was sufficiently dispersed to be used as "pseudo-solute"). The retention volume of the silica dispersion gave a value for the kinetic dead volume, i.e., the volume of the moving portion of the mobile phase. The difference between the retention volume of sodium nitroprusside and that of the silica dispersion was very small, and so sodium nitroprusside could be used to measure the kinetic dead volume of a packed column. From such data, the mean kinetic linear velocity and the kinetic capacity ratio can be calculated and must be used with the Van Deemter equation  or the Golay equation .