retention time

Meaning: In gas or liquid chromatography, a mixture is injected into a mobile phase that carries it through a column of stationary phase. Retention time is how long a compound takes to reach the detector. Stronger attraction to the stationary phase means more time on the column and a longer retention time. Under fixed conditions a compound gives a reproducible value.

Equation: tR = tpeak − tinject, where tR is the retention time (s), tpeak is the time of the peak maximum (s), and tinject is the injection time (s).

On the MCAT: A passage shows a chromatogram, with detector signal plotted against time, and asks which peak is which compound. In normal-phase work with a polar stationary phase such as silica, polar compounds stick longer and elute later. In reverse-phase HPLC with a nonpolar stationary phase, nonpolar compounds elute later. In GC, compounds with higher boiling points generally elute later, and a hotter column shortens retention times.

Common confusion: Peak position gives retention time, while peak area relates to the amount of compound, not its time. Polar compounds are not always first or last; the order depends on whether the stationary phase is polar (normal phase) or nonpolar (reverse phase). Retention time is also not the Rf of TLC, which is a ratio of distances.