Selectivity Factor (Separation Factor):

The relative migration rates of the solutes are expressed in terms of selectivity factor. The selectivity factor/ separation factor, α, of a column for two solutes that are X and Y expressed as follows:

α =  K _Y/K _X                                                                                                                                

where, K_Y is the distribution constant for the more strongly retained species Y, and K_X is the distribution constant for more rapidly eluted species X. The selectivity factor should always be greater than unity. While substituting Eq. 4.9 and the analogous equation for Y in Eq. 4.13, the following equation is obtained for selectivity factor in terms of retention factor

α = k'_Y/k' _X                                                                                                                    

where, k´_y and k´_x are retention factors for Y and X, respectively. can be converted into the form of retention times as follows:

 α =  (t _R  )_Y  - t _M/(t _R  )_X   - t _M                                                                                      

Later, a selectivity factor and retention factor will be used to compute the resolving power of column.