# Concept of Adjusted Retention Time in Chromatography

Why is the relation between the adjusted retention time, retention time and dead time as follows :

tR' = tR - tM ?

To find the time that the sample spends in stationary phase (adjusted retention time), we just need to subtract the time, in which it is in the column but not in stationary phase. Right ?

But dead time is different from this idea. How does it help in finding the time spent in stationary phase ?