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On the basis of Craig’s model, each molecule is considered as passing alternatively from the mobile phase (in which it progresses down the column) to the stationary phase (in which it is immobilized). The average speed of the progression down the column is slowed if the time periods spent in the stationary phase are long. Extrapolate now to a case that supposes n molecules of this same compound (a sample of mass m_T). If we accept that, at each instant, the ratio of the n_S molecules fixed upon the stationary phase (mass m_S) and of the n_M molecules present in the mobile phase (mass m_M) is the same as that of the times (t_S and t_M) spent in each phase for a single molecule, the three ratios will therefore have the same value:

Take the case of a molecule that spends 75% of its time in the stationary phase. Its average speed will be four times slower than if it stayed permanently in the mobile phase. As a consequence, if 4 μg of such a compound has been introduced onto the column, there will be an average of 1 μg at all times in the mobile phase and 3 μg in the stationary phase.

Since the retention time of a compound t_R is such that t_R = t_M + t_S, the value of k is therefore accessible from the chromatogram (Figure 1.7):

(1.27)

This important relation can also be written:

(1.28)

In light of Eqs. (1.16) and (1.18), the retention volume V_R of a solute can be written:

(1.29)

or

(1.30)

This last expression linking the experimental parameters to the thermodynamic coefficient of distribution K is valid for ideal chromatography.