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To compare the performance of columns of different design for a given compound – or to compare, in gas chromatography, the performances between a capillary column and a packed column – more realistic values are obtained by replacing the total retention time t_R, which appears in Eqs. (1.13)–(1.15), with the adjusted retention time (Figure 1.7), which does not take into account the hold‐up time t_M spent by the compound in the mobile phase. The three equations then become:

(1.17)

(1.18)

Figure 1.7 Retention factors and separation factor (or selectivity factor) between two adjacent compounds. Each compound has its own retention factor. On this figure, the separation factor is around 1.3. α alone is not enough to determine whether separation is really possible.

(1.19)

Currently, it is considered that these last three equations are not very useful.