Eluents

An eluent, or eluant, is the solvent or solvent mixture used to move samples through a chromatographic system. It is the mobile phase that carries analytes from the inlet toward the detector or collection point. The liquid that exits the column and contains the eluted compounds is called the eluate or eluate fraction; elution refers to the process of washing substances from the stationary phase.

Eluents are central to many chromatography techniques. In liquid chromatography, including high-performance liquid chromatography (HPLC) and thin-layer chromatography (TLC), the eluent’s composition, polarity, and pH determine the separation quality. In TLC, the chosen solvent system migrates with capillary action to separate components by differential affinities to the stationary phase. In ion-exchange chromatography, eluents are often solutions with specific ionic strength or pH that displace bound ions, with common eluents including salt solutions or buffered salts. In affinity and size-exclusion chromatography, eluents are buffers that maintain stability and control elution behavior; in some cases, competitive inhibitors or changing pH or salt concentration achieve elution.

Selection of an eluent depends on factors such as compatibility with the stationary phase, detector requirements, and sample stability. Important properties include polarity, ionic strength, pH, viscosity, and UV transparency if UV detection is used. Eluents should minimize side reactions, prevent precipitation, and maintain column integrity. In practice, eluents may be used isocratically or as gradients (gradual change in solvent composition) to improve resolution and separation efficiency.