cavities were obtained by measuring the retention volumes of salts having different molar volumes. The salts were ionically excluded from the pores of the packing and, thus, only penetrated the interstitial cavities as they passed through the column. The results are shown as a curve relating retention volume against ion volume in Figure 6. Courtesy of the Analyst (ref.11)   Figure 6. Graph of Retention Volume of a Series of Ions against Their Ionic Volume   The retention volume decreases linearly as the ion volume increases. It should be pointed out that the retention i not related to the charge on the ion. The intercept of the curve on the retention volume axis gives a value for the total interstitial volume of the column, which differs only slightly from the retention volume of sodium nitrate. Thus, the retention volume of sodium nitrate would give a close approximation to the interstitial volume of the column. The slope of the curve shown in Figure 6 clearly

The Retention Volume of a Solute The retention volume of a solute is that volume of mobile phase that passes through the column between the injection point and the peak maximum. Consequently, by differentiating equation (10), equating to zero and solving for (v), an expression for the retention volume (Vr) can be obtained. Restating equation (12),                                                                          

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 smallestretention 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

Thus, Vo = Qto where Q is the flow rate in ml/min. The retention time (tr) is the time elapsed between the injection point and the peak maximum. Each solute has a characteristic retention time. The retention volume (Vr) is the volume of mobile phase passed through the column between the injection point and the peak maximum. Thus, Vr = Qtr where Q is the flow rate in ml/min. Each solute will also have a characteristic retention volume. The corrected retention time (t'r) is the time elapsed between the dead point and the peak maximum. The corrected retention volume (V'r) is the volume of mobile phase passed through the column between the dead point and the peak maximum. It will also be the retention volume minus the dead volume. Thus, V'r = Vr - Vo = Q(tr - to) where Q is the flow rate in ml/min

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

It follows that the retention of the solute will depend only on the volume or surface area of the bonded material. Thus, providing all the bonded phase is available for solute interaction, the retention volume will be proportional to the carbon content of the phase. Scott and Kucera (28) examined a series of commercially available reverse phases and determined the carbon content of each phase and the retention volume of a series of solutes on columns packed with each adsorbent. The retentive properties of the five reverse phase are shown in figure 37 where the corrected retention volume (V'r) of 2-ethyl anthraquinone is plotted against carbon content of the reverse phase. It is seen, somewhat surprisingly, that there is a linear relationship between retention volume and carbon content of the brush phases (R2, R8, R18). This relationship can only be expected to occur if all the stationary phase is