# Plate Theory and Extensions - The Capacity Ratio of a Solute > Page 17

This
calculation, assumes that the extra-column volume (V_{E}) 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
(V_{m}) and (V_{S}) 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 (V_{m}) and (V_{S}) is the *separation
ratio (**a)*.