Paper chromatography is a laboratory technique used to separate components within a mixture. This method relies on the differing rates at which substances migrate across a specialized paper. It is an inexpensive analytical tool that requires only small quantities of material. The technique involves placing a sample spot near one end of a filter paper strip, which then interacts with a moving liquid to separate its constituents.
What the Mobile Phase Is
In paper chromatography, the mobile phase is the moving component that carries the sample through the paper. It is typically a liquid solvent or a mixture of solvents. The mobile phase interacts with the sample, dissolving its various molecules. Different components will dissolve to varying degrees, which is key to the separation process.
Movement of the Mobile Phase
The mobile phase moves through the paper by a process called capillary action. This phenomenon occurs when the liquid solvent is drawn up the paper’s cellulose fibers through narrow spaces, defying gravity. The leading edge of the mobile phase as it travels up the paper is known as the solvent front. Marking the solvent front is important for later analysis when calculating retention factors.
How Separation Occurs
Separation in paper chromatography relies on the differential interactions between the sample components, the mobile phase, and the stationary phase (the paper). The paper itself contains water trapped within its cellulose fibers, which acts as the stationary phase. Components in the mixture separate based on their varying affinities for the stationary phase versus their solubility in the mobile phase.
Substances that are more soluble in the mobile phase and have less attraction to the stationary phase will travel further up the paper. Conversely, components with a stronger attraction to the stationary phase will move more slowly. This differential partitioning causes the mixture’s components to separate into distinct spots or bands.
The distance a component travels relative to the solvent front is quantified by its retention factor (Rf value). The Rf value is a ratio calculated by dividing the distance traveled by the substance by the distance traveled by the solvent front. This value is characteristic for a given compound under specific conditions and helps in identifying substances.
Selecting Mobile Phases
The selection of an appropriate mobile phase is important for effective separation. The choice depends on the polarity of the components being separated and the nature of the stationary phase. For instance, if the stationary phase is polar, a non-polar solvent or a less polar mixture is often used as the mobile phase. Common mobile phases include water, ethanol, acetone, or various mixtures of these solvents. Adjusting the polarity and strength of the mobile phase allows for optimization of the separation for different types of mixtures.