Chromatography is a widely used analytical technique in IB Chemistry Topic 11 (Measurement & Data Processing) and Topic 21 (HL Organic Chemistry). It separates components of a mixture based on their interactions with a mobile phase and a stationary phase. This allows chemists to identify compounds, check purity, and analyze complex mixtures. Chromatography is fast, accurate, and essential across chemistry, biology, and environmental science.
What Is Chromatography Used For?
Chromatography is used to separate, identify, and sometimes quantify the components of a mixture.
It relies on the principle that different substances travel at different speeds depending on their attraction to a stationary phase and their solubility in a mobile phase.
In simpler terms:
- Components that are more soluble in the mobile phase move faster.
- Components that are more attracted to the stationary phase move slower.
This difference in movement creates separation.
Why Chromatography Works
Chromatography is based on:
- Differences in polarity
- Differences in solubility
- Differences in intermolecular forces
- Differences in volatility (for gas chromatography)
Each compound interacts differently with the two phases, leading to distinct movement patterns.
Major Uses of Chromatography
Chromatography is used in virtually every field of science.
1. Identifying components of a mixture
Chromatography can separate unknown mixtures, allowing chemists to determine what compounds are present.
2. Checking purity
A pure substance produces a single peak or spot.
Mixtures show multiple spots or peaks.
3. Monitoring reactions
Chemists can track:
- Whether a reaction has finished
- Which products formed
- Whether impurities remain
4. Separating very similar substances
Chromatography is sensitive enough to separate:
- Structural isomers
- Stereoisomers
- Compounds with similar boiling points
5. Quantifying components
Advanced methods like HPLC and GC can measure exact amounts.
Types of Chromatography Used in IB Chemistry
IB focuses on three main types:
1. Paper Chromatography
- Stationary phase: paper
- Mobile phase: solvent
- Used for inks, dyes, food colorings
Simple and visual, ideal for basic separation.
2. Thin-Layer Chromatography (TLC)
- Stationary phase: silica or alumina on a plate
- Mobile phase: solvent
- More accurate than paper chromatography
TLC is widely used for reaction monitoring.
3. Gas Chromatography (GC)
- Mobile phase: inert gas (He or N₂)
- Stationary phase: liquid coating inside a column
- Used for volatile compounds
GC produces precise separation and works with mass spectrometry (GC–MS) for full identification.
Interpreting Chromatography Results
Rf Value (for TLC and paper chromatography)
Rf = distance moved by substance ÷ distance moved by solvent
Rf values:
- Range from 0 to 1
- Are reproducible under identical conditions
- Help identify compounds by comparison to known standards
Peak Retention Time (in GC)
Retention time:
- Indicates how long a compound stays in the column
- Identifies compounds with high precision
- Depends on polarity, volatility, and temperature
Factors Affecting Chromatography
Several variables influence separation quality:
- Polarity of solvent — more polar solvents move polar compounds further.
- Strength of attraction to stationary phase — stronger attraction slows movement.
- Temperature (in GC) — higher temperature → faster movement.
- Molecular mass — heavier molecules often move slower.
Optimizing these factors improves separation resolution.
Real-World Applications
Chromatography is used in:
- Drug testing
- Food safety and contamination analysis
- Water and environmental monitoring
- Pharmaceutical purity checks
- Forensic science
- Biochemical research
Its versatility makes it one of the most important analytical techniques.
Common IB Misunderstandings
“Rf values are the same for every solvent.”
False—Rf values depend strongly on the solvent.
“Large spots on TLC mean high concentration.”
Spot size can be affected by how the sample was applied.
“GC can analyze solids.”
Only volatile compounds can be used in GC.
“Chromatography gives molecular structures.”
Chromatography separates mixtures; it does not directly reveal structure.
FAQs
Why do some spots move further in chromatography?
Because they are more soluble in the mobile phase or less attracted to the stationary phase.
Can chromatography separate isomers?
Yes—especially with GC and TLC, which can distinguish molecules with subtle differences.
Does chromatography destroy the sample?
TLC and paper chromatography do not; GC may due to heating.
Conclusion
Chromatography is a technique used to separate, identify, and analyze components of a mixture. By exploiting differences in polarity, solubility, and intermolecular interactions, chromatography can isolate compounds with precision. In IB Chemistry, understanding chromatography is essential for interpreting separations, analyzing mixtures, and solving real-world chemical problems.
