elution
/ ee-LOO-shun /
Think of rinsing soap out of a sponge under the tap: you keep running water through until the trapped soap is washed free and carried away. Elution is that idea in chromatography — running fresh mobile phase through the column to coax the held-back compounds off the stationary phase and out the end.
Formally, elution is the process by which the mobile phase washes a retained component off the stationary phase and carries it through to the column outlet. Each compound elutes when the mobile phase is finally strong enough, or enough of it has passed, to overcome how tightly the stationary phase holds it; the order in which compounds come out is the elution order.
It matters because elution is the step that actually delivers the separated components to be detected and measured, and controlling how it happens — steadily, or with increasing strength over time — is central to method design. The honest caveat is that an elution that is too gentle leaves strongly held compounds stuck on the column for a damagingly long time, while one too aggressive rushes them out before they have separated.
After loading a mixture, the analyst pumps mobile phase through the column; the weakly held compound elutes first as an early peak, while the strongly held one elutes much later, after enough solvent has washed it free.
Washing held-back compounds off the column, in order of how tightly they cling.
Running the mobile phase at constant strength is isocratic elution; gradually strengthening it during the run is gradient elution, which speeds up stubbornly held compounds.