Plate Theory and Extensions - Experimental Examination of Dead Volume Measurement > Page 38

 

 Courtesy of the Analyst (ref.11)

 

Figure 8. Graph of Retention Volume of n-Alkanes against Carbon Number   

In figure 8, all the points lie on the same straight line, irrespective of the operating temperature and, thus, the enthalpy term (see The Thermodynamics of Chromatography ) is close to zero and the solutes are not retained by differential molecular forces. Consequently, the retention of the hydrocarbon solutes is solely due to exclusion.. It is clear that the exclusion effect is significant and, by extrapolating to a carbon number of zero, the total volume of mobile phase in the column is obtained. In this case,

 the total interstitial volume plus the total pore volume is about 2.9 ml.

This value agrees well with the data for the retention of water and methanol.

 

The problems associated with exclusion and dead volume measurement cannot be avoided but they can be minimized.

Probably the best compromise for silica based stationary phases is to use corrected retention volume data for solutes eluted at a (k') of greater than 5 and only compare chromatographic data for solutes of approximately the same molecular size.