Retention Factor:

There is another term known as retention factor or capacity factor which is commonly used to describe the migration rates of the solutes. For a solute X, the retention factor k ' is expressed as

K' _X     = K _X V_S/V_M                                                                                                                 

K_X = distribution constant for X

Substituting Eq. 4.9 in Eq. 4.8, we found

v = u × 1/(1 + k _X)

Now, substituting Eqs. 4.3 and 4.2 into Eq. 4.10, we get

L/ t _R =   L/ t _M   × 1 /(1 + k _X)

Rearranging the above equation, we get

K_X= t _R  - t _M /t _M       

We can determine t_R and t_M from a chromatogram. If the retention factor is much less than unity, elution occurs so rapidly that the accurate determination of retention factor is hard. That the retention factor is larger, somewhere 20 or 30, the elution times are commonly long. The ideally separations of solutes are carried out for which the retention factors are between 2 and 10.