Retention time ( t_R), also called the total  retention time. The time between injection  and the appearance of the peak maximum. The total retention volume (V_R) is determined  by multiplying the retention time  by the flow rate. The adjusted retention  time (t_R') adjusts for the column void volume;  t_R' =  t_R -  t_M. It usually is measured  from the point of injection to the apex of  the peak, but it should be measured to the center of gravity of the peak for asymmetric peaks..