Retention time (revision 6)

Retention time is the interval between injection and the appearance of a component's peak maximum at the detector. Under fixed conditions it is characteristic of a substance and reproducible, which makes it the basis of chromatographic identification.^[1]

It is not a property of the substance alone. Column chemistry and dimensions, mobile-phase composition, gradient profile, flow rate, temperature and even instrument dwell volume all change it, so a retention time is only meaningful alongside the conditions that produced it.^[1]

Relative retention time — the ratio of a peak's retention to that of a reference peak in the same run — is more robust, because system variations that shift both peaks cancel.^[2][3]

Matching a sample peak's retention to that of a reference standard run under identical conditions on the same day is the ordinary chromatographic identification. It is weak evidence on its own: many species can share a retention time on a given system.^[2]

Co-injection strengthens it substantially. Spiking the sample with the standard should enlarge the existing peak without producing a shoulder; a shoulder indicates two species.

The strongest form combines retention matching with an orthogonal determination — mass confirmation or a second chromatographic mode with different selectivity. This is what pharmacopoeial identification tests specify.^[1]