Chromatography is a laboratory technique used to separate the components of a mixture. For example, if you want to isolate a specific protein of interest from a culture medium, chromatography is often the method of choice.
At its core, chromatography works by distributing components of a mixture between two phases:
- Mobile phase – the solvent that carries the mixture
- Stationary phase – the material that slows down components differently
Because different molecules interact differently with these two phases, they travel at different speeds, allowing separation.
Basic Principle of Chromatography
The mixture to be separated (called the analyte) is dissolved in a solvent known as the mobile phase. This mobile phase moves through or across a stationary phase, which is held in place by a column, plate, or paper.
As the analyte travels, individual components interact with the stationary phase to varying extents based on properties such as:
- Size
- Shape
- Charge
- Polarity
As a result, each component moves at a different rate. The time taken by a component to travel through the system is called its retention time.
Typically:
- Larger or more strongly interacting molecules move slower
- Smaller or weakly interacting molecules move faster
At the end of the process, the separated components appear as distinct bands or peaks, collectively called a chromatogram. Detection may involve UV light, mass spectrometry, or flame ionization detectors.
1. Paper Chromatography
Paper chromatography is one of the simplest and most widely used chromatographic techniques.
What can be separated?
- Organic compounds
- Inorganic compounds
Phases
- Stationary phase: Moisture trapped in cellulose fibers of the filter paper
- Mobile phase: Solvents or buffers, commonly:
- Isopropanol : Ammonia : Water (9:1:2)
- Methanol : Water (4:1)
- Butanol : Glacial acetic acid : Water (4:1:5)
Principle
The mobile phase carries the analyte upward through the paper by capillary action. Components separate based on their affinity for the stationary phase.
Chromatographic chamber
A closed glass or plastic container (usually a beaker).
Types
- Ascending chromatography – solvent moves upward against gravity
- Descending chromatography – solvent flows downward from a reservoir at the top
Rf (Retention factor) Value
The Rf value is characteristic for a compound under fixed conditions.
Rf = Distance travelled by solute / Distance travelled by solvent
Applications
- Checking purity of pharmaceutical products
- Detecting adulterants
- Detecting contaminants in food and beverages
- Cosmetic analysis
Limitations
- Not suitable for large sample quantities
- Poor separation of complex mixtures
2. Thin Layer Chromatography (TLC)
Thin layer chromatography is performed on glass plates, aluminum foil, or plastic sheets coated with an adsorbent.
Phases
- Stationary phase: Silica gel or aluminum oxide
- Mobile phase: Organic solvents
Analytes move at different rates depending on their interaction with the stationary phase, resulting in efficient separation.
Applications
- Assessing purity of compounds
- Analysis of ceramides and fatty acids
Advantages
- Cost-effective
- Faster than paper chromatography
- Easy comparison with purity standards
Limitations
- Cannot distinguish certain enantiomers or structural isomers
- Requires prior knowledge of Rf values
- Limited separation length due to short stationary phase
3. Column Chromatography
In column chromatography, the mixture passes through a packed column containing a stationary phase such as silica beads, while a mobile phase flows through it.
Key components
- Column (glass, plastic, or stainless steel)
- Stationary phase (packed or coated)
- Injector to introduce the sample
- Fraction collector to collect separated components
Separation occurs based on parameters such as size, charge, or density.
Major Types of Column Chromatography
- Ion Exchange Chromatography
- Size Exclusion Chromatography
3. a. Ion Exchange Chromatography
This technique separates molecules based on charge interactions between analytes and the stationary phase.
Types of ion exchangers
- Anion exchangers: Negatively charged; bind positively charged molecules
- Cation exchangers: Positively charged; bind negatively charged molecules
Principle
Charged groups are covalently attached to the matrix surface. These groups attract oppositely charged ions, forming an ion cloud. Ions can be reversibly exchanged without altering the matrix.
Applications
- Separation and analysis of amino acids
- Water purification
- Collection of trace metals from seawater using chelating resins
Limitations
- Only charged molecules can be separated
- Requires carefully controlled buffer systems
3. b. Size Exclusion Chromatography
Also known as gel filtration, this technique separates molecules based on molecular size.
Principle
The stationary phase consists of porous beads:
- Large molecules elute first (cannot enter pores)
- Medium-sized molecules elute next
- Small molecules elute last (enter pores and take longer paths)
Detection
Separated fractions can be analyzed using UV detectors, mass spectrometry, or flame ionization detectors.
4. Gas Chromatography (GC)
Gas chromatography differs from other techniques because the mobile phase is a gas, making it ideal for small, volatile compounds.
Key features
- Mobile phase: Inert carrier gas (commonly helium)
- Sample: Gas or vaporized liquid
- Stationary phase: Coated inside the column
Principle
Separation is based on retention time (Rt). Compounds with stronger interactions with the stationary phase elute later, while those favoring the mobile phase elute earlier.
As components reach the detector, signals are recorded as peaks. Peak height, width, and area provide quantitative information.
5. High Performance Liquid Chromatography (HPLC)
High Performance Liquid Chromatography is an advanced form of column chromatography designed for high resolution, speed, and sensitivity.
Phases
- Stationary phase: Granular material with precisely controlled pore sizes
- Mobile phase: Solvent or solvent mixtures forced through the column at high pressure
High pressure allows better separation of complex mixtures and accurate quantification.
Applications of HPLC
- Pharmaceutical quality control and drug analysis
- Clinical diagnostics (drug levels, metabolites)
- Protein, peptide, and nucleic acid analysis
- Food and beverage testing (additives, contaminants)
- Environmental monitoring
Limitations of HPLC
- High equipment and maintenance cost
- Requires skilled operation and method development
- Uses large volumes of organic solvents
- Not suitable for very volatile compounds
Chromatography remains one of the most powerful and versatile techniques in analytical science, with applications spanning research, industry, medicine, and environmental science.
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