Selectivity and Resolution

Until now, retention and efficiency was discussed separately, but both of these parameters are affecting the separation of the mixture. Retention is developing the separation, and band broadening is destructing it.

Values measured from a chromatogram containing two peaks.

Selectivity is the ratio of the capacity factors of both peaks, or the ratio of its adjusted retention times. Selectivity represents the separation power of particular adsorbent to the mixture of this particular components.

         (4)
 

This parameter is independent of the column efficiency, it only depends on the nature of the components, eluent type, eluent composition, and adsorbent surface chemistry. In general, if the selectivity of two components is equal to 1, then there is no way to separate them by improving the column efficiency.

Resolution is the parameter describing the separation power of the complete chromatographic system relative to the particular components of the mixture.

By convention, resolution (R) is expressed as the ratio of the distance between two peak maxima to the mean value of the peak width at the base line:

        (5)
 

If we approximate peaks by symmetric triangles, then, if R is equal to or more than 1 then components are completely separated. If R is less than 1, then components are overlapped.

By using the expressions for capacity factor and column efficiency the equation for R could be transferred to the form:

          (6)
 

where the dependence of resolution on the column efficiency is represented by the square root of N, which means that increasing the efficiency is not so favourable for resolution improvement.

Resolution can also be expressed in form:

          (7)