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Jetzt kostenlos anmeldenImagine one day you come across an unknown liquid, and you want to know what it is. Luckily, you are a scientist who has access to lab facilities. You know that in the lab you have a database of known Rf values for different substances, a pencil, a thin layer of paper and some solvent. So, what can you do with all of this? All of these materials alongside the situation points to carrying out Thin-Layer Chromatography, and that is what we will be exploring in this article.
Fig. 1: Thin-Layer Cromatography. Taken from: Wikimedia Commons.
So, what exactly does a thin-layer chromatogram look like? If we look at the image on the right, we can see what is known as a chromatography tank. Inside the tank we can see the solvent, which is the mobile phase, the TLC plate which is the stationary phase and finally the solvent front, which shows how far the mobile phase has travelled.
Now, you may be thinking, what are all the coloured dots? These dots represent the different, separate components of a mixture. As the solvent front travels upwards, the mixture is separated and this is why they are all at different points on the plate. When two dots are on the same level, it shows that those two components from a mixture are identical.
After carrying out the practical, we need to figure out what each component is. This can be determined using Rf values. There is a large database that contains known Rf values for different components. We use the chromatogram to find the Rf value, then use the database to determine the unknown components. We use the distance travelled by the mobile phase to determine the Rf value. The longer the distance travelled, the less the unknown compound has adsorbed to the stationary phase and allows us to explore the relative Solubility of the unknown compound.
We use this equation to find the Rf value:
Rf = Distance travelled by the component ÷ Distance travelled by the solvent front
First, we measure how far the component has travelled. We do this by measuring the distance from the pencil line to the middle of the spot of the component. Next, we measure how far the solvent has travelled from the pencil line, which is also known as the solvent front. We then plug these figures into the equation above. The value we get is the Rf value of the component. We can then compare it to known values and determine what the component is.
| Component | Rf Value |
| β-carotene | 0.98 |
| Chlorophyll a | 0.59 |
| Chlorophyll b | 0.42 |
Let us say the solvent front has travelled 1.6 cm and the component has travelled 1.57 cm.
To calculate the Rf value, we just need to do the following:
$$\frac{1.57}{1.6}=0.98$$
Using the known values from the table above, we can determine that the unknown component is β-carotene.
We can also explore the different uses of thin-layer chromatography:
The principles of thin-layer chromatography are solubility and adsorption.
We first put the different components on a plate and put the plate in a solvent. Once the solvent has travelled near the top of the plate, we take it out and let it dry. We then analyse it to determine the unknown components.
It is an important technique that helps determine different components within a mixture.
It is mostly used in synthetic chemistry to identify different components within a mixture and determine their purity.
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SIGNUP SIGNUPFlashcards in Thin Layer Chromatography Practical15
Start learningWhat are some uses of thin-layer chromatography?
To identify different components in a mixture
Why do we use capillary tubes?
Small drops are required to prevent the different solutions from mixing
Why do we use gloves?
Prevents any contamination
Why do we use a pencil?
Used as it will not dissolve in the solvent
What does adsorbing mean?
Adhesion of a molecule to a surface
What is a type of chromatography?
Column chromatography
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