isocratic elution
/ eye-soh-KRAT-ik ih-LOO-shun /
Imagine running a tap at one steady setting throughout a job, never touching the handle. Isocratic elution is chromatography run that way — the mobile phase is mixed once and held at the same strength and composition from the start of the run to the finish.
Formally, isocratic elution is a separation in which the mobile-phase composition stays constant for the whole run. Every component travels under identical conditions; each emerges at a steady, reproducible time set purely by how it partitions between the unchanging phases. There is no programmed change in solvent strength, so the system stays at one equilibrium throughout.
It matters because constancy buys simplicity and reliability: isocratic methods are easy to set up, easy to reproduce on any instrument, and give a flat, stable baseline that makes small peaks easier to measure. Its honest limit is range — a single fixed strength cannot suit both weakly and strongly retained compounds at once, so mixtures with a wide span of stickiness give crowded early peaks and broad, late-eluting ones, which is exactly when gradient elution is needed instead.
For a routine assay of a single drug and its one main impurity, a lab uses an isocratic method with a fixed 60:40 methanol-water mobile phase, getting the same two peaks at the same times day after day with minimal fuss.
One unchanging mobile phase from start to finish — simple, steady, reproducible.
The name comes from Greek roots meaning 'equal strength'; it is the opposite of gradient elution, where the mobile phase is deliberately changed during the run.