eluants

An eluant, also called an eluent, is a solvent or solvent mixture used to carry analytes through a chromatographic system. It acts as the mobile phase in techniques such as thin-layer chromatography (TLC), high-performance liquid chromatography (HPLC), flash chromatography, and solid-phase extraction (SPE). The eluant interacts with both the analytes and the stationary phase, and different compounds are eluted at different rates, enabling separation.

Eluant selection depends on the chromatography mode and the properties of the sample. In normal-phase chromatography, eluants are typically nonpolar or only mildly polar, such as hexane or heptane, sometimes with small amounts of ether. In reversed-phase chromatography, eluants are aqueous-organic mixtures, such as water–acetonitrile or water–methanol. Gradient elution gradually changes solvent composition to improve separation, while isocratic elution uses a fixed composition. The eluant must be compatible with the stationary phase and with detection methods, and may need to be volatile for fraction collection or mass spectrometry compatibility.

Key properties influencing eluants include polarity and eluotropic strength, boiling point and viscosity, UV transparency, toxicity, and cost. These factors affect separation efficiency, analysis time, detector compatibility, and handling requirements. In SPE and column chromatography, the eluant also determines how effectively target compounds are desorbed from the sorbent.

Safety and environmental considerations are important, as many organic eluants are flammable or hazardous. Waste disposal and the possibility of greener solvent systems are common design factors in modern separations. See also eluent and mobile phase.