Chromatographic retention time ($t_R$)

The total chromatographic retention time $t_R$ is the elapsed time from sample injection to the maximum of a specified component's chromatographic peak. It includes the time spent moving through the system as well as the additional delay associated with retention. It is a property of the analyte under specified conditions, not a universal identifying number attached to the molecule.^[ref]

Let $t_M$ denote the hold-up time measured with a suitably unretained marker. The adjusted retention time is $t_R^{\prime }=t_R-t_M$. For example, a peak at $6$ minutes with $t_M=1.5$ minutes has $t_R^{\prime }=4.5$ minutes. Its total passage time and its delay relative to an unretained marker answer different questions. At constant measured volumetric flow $F$, the corresponding total retention volume is $V_R=F{t}_R$; $F=0.8$ milliliters per minute and $t_R=6$ minutes give $V_R=4.8$ milliliters.^[ref]

A sample peak matching a reference peak's chromatographic retention time under the same conditions supports an identity assignment, but another compound may coelute. Changing solvent composition or column conditions can change $t_R$ without changing molecular identity. A detector's peak area and chromatographic resolution supply different information from the time at which the maximum occurs.

The definition uses the peak maximum, rather than the first detectable signal or the mean arrival time of every molecule. For a broad or asymmetric peak, these alternatives need not coincide.