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The separation factor α (Eq. (1.31)) enables the comparison of two adjacent peaks 1 and 2 present in the same chromatogram (Figure 1.7). It is defined by Eqs. (1.31) and (1.32).

By definition α is greater than unity:

(1.31)

or

(1.32)

The expression, connecting α to the Nernst distribution coefficients of the two solutes, shows that selectivity is dependent only on the value of these constants (intensity of interactions, temperatures) and does not depend on the column’s geometry (length and diameter) or its packing (diameter of particles and quantity of stationary phase). For nonadjacent peaks, the relative retention factor r is calculated in a similar manner to α, and cannot be less than 1.