# Plate Theory and Extensions - Resolving Power of a Column > Page 61

_{ } Then,

and (55)

Equation (55),
developed by Purnell (13) in 1959 can be used to calculate the efficiency
needed to separate a given solute pair from the capacity factor of the first
eluted peak and their separation ratio. It is very useful in column design. This
derivation utilizes, (k_{A}), the
capacity ratio of the *first* eluted peak but A similar equation which
takes a slightly different form, can be derived using (k_{B}), the capacity ratio of the second
peak. Both equations give the same numerical result.

** **

**Figure 18.
Graph of Log Efficiency against Capacity Factor for Solute Pairs Having
Different Separation Ratios**