Chromatographic separations offer a vast range of solutions to the analytical scientist for separation and quantification of components of organic mixtures. The separations are based on the physico – chemical interactions of the mixture components with the chromatographic stationary phases.
The basis of chromatographic separations is the resolution of sample components through distribution between the mobile phase and the stationary phase. Two separation mechanisms, namely, adsorption and partitioning play the main role in chromatographic separations. The objective of this article is to highlight the differences between the liquid – solid chromatography and liquid- liquid chromatography.
Liquid – Solid Chromatography (LSC)
The solid stationary phase serves as a selective adsorbent for the sample components. The mode of separation can be either two- dimensional as in thin layer chromatography or three-dimensional as on the stationary phase packed inside the chromatographic column.
Physical forces, typically, van der waal’s interactions are responsible for temporary retention of sample components. Due to the degree of interaction there is a steady equilibrium which replaces the retained molecules from others present in the mobile phase resulting in separation and selective extrusion of the adsorbed molecules. Separation factor α is a quantitative measure of degree of separation under a given set of conditions. It is the ratio of two components in the adsorbing phase divided by their ratio in the mobile phase.
Liquid – Liquid Chromatography (LLC)
The basics of liquid-liquid separation is to partition solutes between two immiscible liquid phases- one phase being the mobile carrier and the other being the thin liquid layer supported on the inert stationary phase packed inside the column. It is analogous to separation between two immiscible liquids with the help of a conventional separating funnel. The concurrent extraction process as the mobile phase travels down the column is much more efficient and faster than a single stage extraction using the separating funnel .It also results in clearly defined isolation bands which can be easily detected using a suitable detector.