Chromatography is one of the most widely used separation techniques in chemistry. It appears in IB Chemistry Topic 11 (Analytical Techniques) and Paper 3 practical questions. Whether you're working with pigments, amino acids, organic mixtures, or environmental samples, chromatography provides a reliable way to separate components based on their interactions with different phases. This article clearly explains the fundamental principle behind chromatography and how it achieves separation.
What Is Chromatography?
Chromatography is a separation technique based on the different affinities of substances for a stationary phase and a mobile phase.
The key idea is that each component in a mixture interacts differently with these phases.
Because of these differences, components move at different speeds and eventually separate.
Chromatography works for:
- Solids
- Liquids
- Gases
- Complex mixtures
It is widely used in laboratories, industry, and forensic science.
The Two Phases in Chromatography
Every chromatography technique involves two phases:
1. Stationary Phase
This phase does not move.
It could be:
- Paper
- Silica gel
- Resin beads
- A metal column coating
The stationary phase attracts some components more strongly than others.
2. Mobile Phase
This phase moves through or across the stationary phase.
It could be:
- A solvent (liquid chromatography)
- A gas (gas chromatography)
- A buffer solution (ion-exchange chromatography)
The mobile phase carries components along at different rates.
The Core Principle: Differential Adsorption or Solubility
Different substances spend different amounts of time in each phase.
If a component is more attracted to the stationary phase:
- It moves slowly
- It travels a short distance
- It appears lower on the chromatogram
If a component is more attracted to the mobile phase:
- It moves quickly
- It travels further
- It appears higher or elutes sooner
This difference in movement leads to separation of the mixture.
Chromatography Techniques in IB Chemistry
1. Paper Chromatography
- Stationary phase: cellulose fibers in paper
- Mobile phase: solvent
Used to separate pigments, dyes, and amino acids.
2. Thin-Layer Chromatography (TLC)
- Stationary phase: silica or alumina on a plate
- Mobile phase: solvent
More accurate than paper chromatography.
3. Column Chromatography
- Stationary phase: silica packed in a column
- Mobile phase: solvent
Common in organic chemistry labs.
4. Gas Chromatography (GC)
- Mobile phase: inert gas
- Stationary phase: coating in a long tube
Used to separate volatile compounds.
These techniques all rely on the same underlying principle.
Retention Factor (Rf Value)
Rf values are commonly used in paper and TLC chromatography.
Rf = distance traveled by substance ÷ distance traveled by solvent
- Rf ranges from 0 to 1
- Larger Rf → greater affinity for mobile phase
- Smaller Rf → greater affinity for stationary phase
Rf values help identify substances by comparison to known values.
Why Chromatography Is a Powerful Technique
Chromatography is valuable because it:
- Requires small sample sizes
- Produces clear separation
- Works for complex mixtures
- Provides qualitative and quantitative information
- Can identify unknown substances
- Connects directly to real-world chemical analysis
It is one of the most versatile analytical tools in modern chemistry.
Common IB Questions About Chromatography
- Explain why certain components move faster than others
- Calculate and compare Rf values
- Predict separation based on polarity
- Interpret chromatograms (spots, peaks, retention times)
- Suggest suitable chromatographic methods for a mixture
These skills appear frequently in assessments.
FAQs
Why does polarity matter in chromatography?
Polar stationary phases attract polar molecules more strongly, causing them to move more slowly. Non-polar molecules move faster. Polarity determines affinity.
Why is the solvent front important?
It defines the maximum distance traveled and is used to calculate Rf values. All measurements must be taken before it dries.
Why must the initial sample spot be above the solvent level?
If submerged, the sample dissolves directly into the solvent instead of traveling upward with the mobile phase.
Conclusion
The principle of chromatography is that components in a mixture separate based on different affinities for the stationary and mobile phases. Substances that prefer the stationary phase move slowly, while those attracted to the mobile phase move quickly. This simple but powerful concept underlies paper chromatography, TLC, GC, and many other analytical techniques essential in IB Chemistry and beyond.
