Plate Theory and Extensions - The Capacity Ratio of a Solute > Page 17
This calculation, assumes that the extra-column volume (VE) is negligible. There are, however, two dead volumes in chromatography and, thus, there will be two values for the capacity ratio of a solute. These two dead volumes are called the thermodynamic and the dynamic dead volumes and they are not equal. It follows, that the corresponding capacity ratios are also quite different. The expression given above is for the thermodynamic dead volume of a solute. The two dead volumes and capacity ratios are used for quite different purposes.
Great care must be taken when comparing (k') values for the same solute measured on different columns and for different solutes measured on the same column. Both (Vm) and (VS) will be different for different columns and may (due to the exclusion properties of solid stationary phases and supports) vary between different solutes. A parameter that is largely independent of (Vm) and (VS) is the separation ratio (a).