Recall that the master resolution equation, or Purnell equation, links column efficiency, selectivity between solutes , and retention affinity to resolution.  

The simplest way to optimize separation is to increase the number of theoretical plates, or N, in the column by lengthening it.

However, reducing the plate height, or H, using rate theory is often more efficient for increasing N.

Another way to optimize separation is by increasing the capacity factor, or k, for the slower solute in a pair by changing the temperature of gaseous mobile phases or the solvent composition of liquid mobile phases.

Notably, this is most effective when the original k value is small, because of the threshold beyond which an increase in k only slightly improves the resolution.

Optimizing N or the slower solute's k has little effect when the separation factor, or α, approaches its minimum value of unity.  

Increasing α drastically improves resolution and is possible by changing the mobile phase composition, column temperature, or stationary phase composition.