Plate Theory and Extensions - Experimental Examination of Dead Volume Measurement > Page 37
The retention volume of a small molecule that could enter all the pores but, at the same time, not be retained by differential interactive forces would provide a value for the thermodynamic dead volume. The maximum retention volume was obtained for methanol and water (viz. about 2.8 ml) which can be taken as the thermodynamic dead volume for small molecules (i.e., for concentrations of methanol above 10%v/v where the Langmuir adsorption isotherm become constant, see The Mechanism of Chromatographic Retention).
It should be noted that there is no significant difference between the retention volume of water and that of methanol over the complete range of solvent compositions examined, which confirms the validity of this method for measuring the thermodynamic dead volume. Again, however, the lower concentrations of methanol, where the surface area of the stationary phase was not completely covered with methanol and the Langmuir adsorption isotherm would apply, can not be used. It must also be stressed, that this method of measuring thermodynamic dead volume will only be valid for small molecules. Larger molecules will be partially excluded and, thus, their dead volumes will be commensurably smaller.
Alhedai et al. also examined the exclusion properties of the reversed phase material The stationary phase was a C8 hydrocarbon bonded to the silica, and the mobile phase chosen was n-octane. As the solutes, solvent and stationary phase were all dispersive in nature (hydrophobic in character see The Mechanism of Chromatographic Retention ) and the interacting moieties in both the stationary phase and the mobile phase was C8, the solute would experience the same interactions in both phases. Thus, any differential retention would be solely due to exclusion and not due to molecular interactions. This assumption was confirmed by carrying out the experiments at two different temperatures. If any interactive mechanism was present, then different retention volumes would be obtained for the same solute at different temperatures. Solutes ranging from n-hexane to n-hexatriacontane were chromatographed at 30˚C and 50˚C respectively. The results obtained are shown in Figure 8.