Physical meaning of tm
The analyte spends the same time in the mobile phase as it tooks for the mobile phase to pass through the column.


  An analyte's time in the different phases is dependent on it's affinity.


The time $t_\mathrm{m}$ is given.

The ratio of an analyte's time in the different phases is dependent on it's affinity and is equal:

$$\frac{t_\mathrm{s}}{t_\mathrm{m}}=K_\mathrm{D} \cdot \frac{V_\mathrm{s}}{ V_\mathrm{m}} $$.

where


$t_\mathrm{s}, t_\mathrm{m}$ are time is stationary and mobile phase
$K_\mathrm{D}$ is the phase distribution constant
$V_\mathrm{s}, V_\mathrm{m}$ are volumes of stationary and mobile phase.