Thin-layer Chromatography
Quick Notes
- Thin Layer Chromatography (TLC) is used to separate and identify components in a mixture.
- Separation depends on:
- Solubility in the moving phase (mobile phase).
- Retention by the stationary phase.
- Stationary phase: solid layer (e.g. aluminium oxide or silica on a glass/plastic plate).
- Mobile phase: solvent that moves up the plate (can be polar or non-polar).
- Rf value: distance moved by a compound ÷ distance moved by solvent front.
- Rf values depend on solubility in the mobile phase and interaction with the stationary phase.
- Baseline: starting line where samples are spotted.
- Solvent front: the furthest point reached by the solvent during chromatography.
Full Notes
Chromatography has been covered with more background theory and detail
here.
This page is just what you need to know for TLC for CIE A-Level Chemistry :)
Chromatography separates components of a mixture based on differences in their movement between two phases:
- Stationary phase – Solid or liquid that ‘holds on’ to components.
- Mobile phase – Liquid or gas that carries and moves components.
Substances that are more soluble in the solvent will spend more time in the mobile phase compared to less soluble substances and move further in the same time, while those with stronger interactions with the stationary phase move less.
Thin Layer Chromatography (TLC)
Thin-layer chromatography (TLC) is a quick and simple way to separate and identify small amounts of substances in a mixture.
A sample or mixture is spotted on the baseline of a TLC plate. When the plate is placed in a small volume of solvent (with the solvent below the baseline), the solvent begins to rise up the plate As it travels, it carries the components of the mixture with it.
Each component moves at a different rate depending on how soluble it is in the mobile phase and how strongly it adsorbs (sticks) to the stationary phase.
Key Terms
- Stationary phase: This is the solid material coated on the TLC plate (commonly aluminium oxide or silica gel). It stays fixed in place and interacts with the substances in the mixture.
- Mobile phase: A solvent (polar or non-polar) that moves up the plate via capillary action, carrying the sample with it.
- Baseline: A pencil line drawn near the bottom of the TLC plate where the sample is applied.
- Solvent front: The point the solvent reaches as it moves up the plate. It must be marked before the plate dries out.
Rf Value
Rf stands for retention factor and is calculated for each spot:
Rf = (distance moved by compound) ÷ (distance moved by solvent front)
- Rf values are always less than 1.
- They help identify compounds by comparing to known reference values under the same conditions.
Explaining Differences in Rf Values
Rf values vary based on two main factors:
- Solubility in the mobile phase: A compound that is more soluble in the solvent will travel further up the plate giving a higher Rf.
- Interaction with the stationary phase: If a compound forms strong interactions (e.g. hydrogen bonds) with the stationary phase, it moves more slowly giving a lower Rf.
For Example: In a polar solvent on a silica plate (which is polar), polar compounds often have lower Rf because they are more strongly attracted to the stationary phase.
If you run a known standard alongside unknown samples you can compare Rf values under identical conditions. This makes TLC a powerful tool for checking purity or identifying unknown substances in organic synthesis.
Summary
- TLC separates mixtures using a stationary phase (e.g. silica) and a mobile phase (solvent).
- Different compounds move at different rates based on solubility in the solvent and attraction to the stationary phase.
- Rf = distance travelled by compound ÷ distance travelled by solvent front (always < 1).
- Comparing Rf values with standards allows identification of compounds and assessment of purity.