Embedded Files
Chromatography is a technique to separate mixtures based on the solubilities of the substances in a solvent and the affinity (attractions) the substances have for a stationary phase. The stationary phase is usually a solid component (e.g., paper or TLC plates) in which the substances move through. The solvent, known as the mobile phase, moves up the stationary phase via capillary action and moves the substances within the mixture at different rates.
The rate at which a substance moves is based upon two primary factors:
Solubility within the mobile phase: The substance that is most soluble in the mobile phase (the solvent) moves the furthest along the stationary phase.
Affinity with the stationary phase: The stubstance that has the highest affinity for the stationary phase moves the shortest distance.
On the right is an example of how chromatography is performed.
A sample is placed onto a TLC plate (the stationary phase) at the origin (starting point). The origin is often marked using a pencil as the graphite in the pencil will not move.
The plate is placed into the mobile phase (a solvent).
As the mobile phase moves up the plate, the substances within the mixture move up the TLC plate at different rates.
Once the solvent nears the top of the TLC plate, the plate is removed from the solvent and the distance the solvent front travels is determined.
The distance each substance travels from the origin is determined.
Compound A moved the furthest distance, suggesting that it was the most soluble in the solvent and/or had the lowest affinity for the TLC plate.
Compound C moved the shortest distance, suggesting that it was the least soluble in the solvent and/or had the highest affinity for the stationary phase.
Above: An example of how TLC is used to separate compounds within a mixture. https://microbenotes.com/wp-content/uploads/2023/09/Thin-Layer-Chromatography-TLC.jpeg
Once the components of the mixture have been separated from one another, the Rf value for each component can be calculated. The Rf value is a ratio of the distance the component traveled compared to the distance the solvent front traveled. The larger the Rf value, the farther the component traveled.
Rf = distance component traveled / distance solvent front traveled
On the right is a chromatograph showing the separation of three different components. The purple component traveled 6 cm from the origin. If the solvent front traveled a total distance of 10 cm relative to the origin, the Rf value for the purple component is 6 cm / 10 cm = 0.6.
Above: A sample chromatograph showing distances traveled. The distances allow for the Rf values to be calculated. From: https://cdn1.byjus.com/wp-content/uploads/2022/02/word-image104.png
Report abuse