Size Exclusion Chromatography: Principle, Working, Applications, Advantages and Limitations
Size Exclusion Chromatography (SEC) is an important chromatographic technique used to separate molecules primarily according to their size and hydrodynamic volume.
It is widely used in biotechnology, biochemistry, molecular
biology, pharmaceutical research, and protein purification.
Unlike ion exchange chromatography, which separates
molecules based on electrical charge, size exclusion Chromatography separates
molecules according to their ability to enter pores within stationary-phase
particles.
Larger molecules are unable to enter some or all of the
pores and so travel through the column more quickly, while smaller molecules
enter more pores, follow a longer path through the column, and elute later.
Size exclusion chromatography is particularly useful because
of the usually low amount of binding between sample molecules and the
stationary phase, making it a relatively gentle technique for separating
sensitive biomolecules such as proteins, enzymes, nucleic acids, and protein
complexes.
It is commonly used for protein purification, desalting, buffer exchange, removal of aggregates, separation of biomolecules, and molecular size analysis.
📑 Table of Contents
- What is size exclusion chromatography?
- Principle of Size Exclusion Chromatography
- Role of Porous Beads in Size Exclusion Chromatography
- Components of Size Exclusion Chromatography
- Types of Size Exclusion Chromatography
- Exclusion Limit
- Fractionation Range
- Column and Pore Size Selection
- How Size Exclusion Chromatography Works
- Important Volumes in Size Exclusion Chromatography
- Calibration Curve in Size Exclusion Chromatography
- Factors Affecting Separation in SEC
- Applications of Size Exclusion Chromatography
- Advantages of Size Exclusion Chromatography
- Limitations of Size Exclusion Chromatography
- Size Exclusion Chromatography vs Ion Exchange Chromatography
- Size Exclusion Chromatography vs Affinity Chromatography
- References
What is size exclusion chromatography?
Size Exclusion Chromatography (SEC) is a chromatographic technique in which molecules are separated according to their size in solution as they pass through a column containing porous particles.
The stationary phase consists of beads containing pores of different sizes.
When a mixture enters the column:
- Large molecules cannot enter most pores and so travel relatively quickly through the column.
- Medium-sized molecules enter some pores and so have an intermediate retention time.
- Small molecules enter more pores and travel through a longer pathway, causing them to elute later.
Therefore, in size exclusion chromatography:
Large molecules elute first, while smaller molecules elute later.
This is one of the most important concepts to remember for examinations and laboratory viva questions.
Principle of Size Exclusion Chromatography:
Size exclusion chromatography works on the principle of differential access of molecules to pores present within the stationary-phase beads.
The separation depends primarily on the relationship between the size of the molecule and the size of the pores.
Large Molecules:
Large molecules are excluded from most or all pores, remaining mainly in the space between the beads.
As a result:
Large molecules → Shorter path → Elute first
Small Molecules:
Small molecules can enter many of the pores present inside the beads, increasing the path they travel through the column.
As a result:
Small molecules → Longer path → Elute later
Role of Porous Beads in Size Exclusion Chromatography:
The porous beads are the most important component of the stationary phase because they determine which molecules can enter the internal spaces of the matrix.
The beads act like a molecular sieve.
Different molecules experience different degrees of access to the pores depending on their size.
Molecules larger than the pores are excluded, molecules within the appropriate size range partially enter the pores, and very small molecules can access a large proportion of the pore volume.
The difference in pore access creates differences in the time needed by molecules to pass through the column.
❓ Why is size exclusion chromatography not based only on molecular weight?
Size exclusion chromatography separates molecules according to their hydrodynamic volume. Molecular shape and structure influence how a molecule moves through the pores, so molecules with similar molecular weights may sometimes elute differently.
Components of Size Exclusion Chromatography:
A Size Exclusion A chromatography system consists of several important components that work together to separate molecules according to size and hydrodynamic volume.
Proper selection of the column, gel matrix, pore size, and mobile phase is essential to achieving efficient separation.
1. Chromatographic Column:
The chromatographic column is the main part of the SEC system where molecular separation takes place.
It is packed with porous beads that form the stationary phase.
As the sample moves through the column, molecules interact differently with the pore spaces inside the beads.
- Large molecules move mainly between the beads, while small molecules enter the pores.
- Intermediate molecules partially enter the pores.
- The length and diameter of the column can influence separation efficiency and sample capacity.
2. Stationary Phase:
The stationary phase in size exclusion chromatography consists of porous particles or gel beads.
Unlike ion exchange chromatography, these beads ideally do not strongly bind the molecules being separated but instead act as a molecular sieve.
The pore size of the stationary phase determines which molecules:
- Are completely excluded from the pores
- Can partially enter the pores
- Can enter most of the available pore volume
Common Gel Matrices Used in SEC:
Various materials are used to manufacture porous beads for size exclusion chromatography.
Common matrices include:
- Agarose
- Dextran
- Polyacrylamide
- Porous silica
- Synthetic polymers
The selection depends on the type of sample and solvent system being used.
Biological molecules such as proteins and enzymes are commonly separated using hydrophilic gel matrices, whereas organic polymers are often separated using different solvent-compatible materials.
3. Mobile Phase:
The mobile phase carries the sample through the column.
For biological applications, the mobile phase is usually a buffer solution that maintains conditions suitable for the stability of proteins and other biomolecules.
The buffer should ideally:
- Maintain a suitable pH
- Preserve protein stability
- Minimise unwanted interactions with the gel
- Prevent aggregation of biomolecules
Unlike ion exchange chromatography, SEC generally does not require a salt or pH gradient for separation.
4. Sample:
The sample contains the molecules that need to be separated according to size.
Examples include:
- Proteins
- Enzymes
- Peptides
- Nucleic acids
- Protein complexes
- Polysaccharides
For best separation, the sample volume should be relatively small compared with the total column volume.
Large sample volumes can broaden the separated bands and reduce resolution.
5. Detector:
A detector is used to monitor the molecules as they leave the column.
Common detection methods include:
- UV absorbance
- Fluorescence detection
- Refractive index detection
- Conductivity detection
For protein purification, UV detection is commonly used because proteins often absorb ultraviolet light due to aromatic amino acid residues.
6. Fraction Collector:
The separated molecules leave the column at different times and are collected in individual fractions.
These fractions can then be analysed using techniques such as:
- SDS-PAGE
- Protein assays
- Enzyme activity assays
- Spectroscopy
Fractions containing the desired molecule can be combined for further purification or analysis.
❓ Why should the sample volume be relatively small in size exclusion chromatography?
A large sample volume can cause the molecular bands to broaden inside the column. Using a relatively small sample volume produces sharper peaks and improves the resolution between molecules of different sizes.
Types of Size Exclusion Chromatography:
Size exclusion chromatography can broadly be divided into two major types depending on the type of sample and solvent used.
1. Gel Filtration Chromatography:
Gel Filtration Chromatography (GFC) is mainly used for the separation of biological molecules in aqueous buffer systems.
It is widely used for:
- Protein purification
- Enzyme purification
- Separation or protein complexes
- Desalting
- Buffer exchange
The stationary phase usually consists of hydrophilic porous beads made from materials such as agarose, dextran, or polyacrylamide.
Because the separation conditions are generally mild, gel filtration chromatography is particularly suitable for sensitive biomolecules.
2. Gel Permeation Chromatography:
Gel Permeation Chromatography (GPC) is primarily used for the separation and analysis of synthetic polymers dissolved in organic solvents.
The technique is commonly used to:
- Determine polymer molecular size distribution
- Estimate molecular weight distribution
- Analyse polymer samples
- Separate polymer fractions
The stationary phase and solvent system used in GPC are selected to be compatible with organic solvents and polymer samples.
Gel Filtration vs Gel Permeation Chromatography:
| Feature | Gel Filtration Chromatography | Gel Permeation Chromatography |
|---|---|---|
| Common Abbreviation | GFC | GPC |
| Main Samples | Biological molecules | Synthetic polymers |
| Mobile Phase | Aqueous buffer | Organic solvent |
| Common Applications | Protein purification and desalting | Polymer analysis |
| Stationary Phase | Hydrophilic porous gels | Solvent-compatible porous polymers |
❓ Why is gel filtration chromatography considered a gentle technique?
Gel filtration separates molecules mainly according to their size without requiring strong binding or harsh elution conditions. This helps preserve the natural structure and biological activity of sensitive proteins and enzymes.
Exclusion Limit:
The exclusion limit is the approximate molecular size above which molecules are too large to enter the pores of a particular gel matrix.
Molecules larger than the exclusion limit are almost completely excluded from the pores.
Therefore, these molecules travel mainly through the spaces between the beads and elute close to the void volume.
Important Concept:
Molecule Larger Than Pores
↓
Cannot Enter the Beads
↓
Travels Through Spaces Between Beads
↓
Shorter Pathway
↓
Elutes Earlier
The exclusion limit is an important factor when selecting a gel matrix for a particular separation.
Fractionation Range:
The fractionation range is the range of molecular sizes that can be effectively separated by a particular size exclusion chromatography matrix.
Molecules within this range enter the pores to different extents.
As a result, differences in molecular size produce differences in elution volume.
For example:
- Molecules much larger than the fractionation range may elute together.
- Molecules much smaller than the fractionation range may also elute together.
- Best separation occurs when the molecular sizes of interest fall within the fractionation range.
❓ Why is the fractionation range important?
The fractionation range determines the molecular sizes that can be effectively separated by a gel matrix. Choosing a matrix with an appropriate range improves resolution between the molecules of interest.
Column and Pore Size Selection:
Selecting the correct column and gel pore size is essential to acquiring efficient separation.
Important factors include:
- Molecular size of the target molecule
- Size of impurities
- Sample volume
- Required resolution
- Sample quantity
- Type of solvent or buffer
The ideal matrix should allow the molecules of interest to fall within its fractionation range.
If the pores are too small, several molecules may be excluded simultaneously. If the pores are too large, smaller molecules may enter the beads to similar extents and may not separate efficiently.
How Size Exclusion Chromatography Works:
Size exclusion chromatography separates molecules according to their size and hydrodynamic volume as they move through a column packed with porous gel beads.
The separation process can be divided into several steps:
- Column equilibration
- Sample loading
- Separation through porous beads
- Elution
- Fraction collection and analysis
The key principle remains simple:
Large molecules travel around the pores and elute first, while smaller molecules enter the pores and elute later.
Step 1: Column Equilibration:
Before the sample is applied, the SEC column is equilibrated with a suitable buffer or solvent.
Equilibration ensures that the entire gel matrix is surrounded by the same mobile phase that will be used during separation.
It also helps maintain suitable conditions for sensitive biomolecules such as proteins and enzymes.
The buffer should be selected to:
- Maintain sample stability
- Maintain suitable pH
- Prevent unwanted interactions
- Prevent protein aggregation
Step 2: Sample Loading:
The sample mixture is carefully applied to the top of the equilibrated column.
For efficient separation, the sample should form a narrow and concentrated band at the top of the stationary phase.
A large sample volume can cause band broadening, reducing the separation between molecules of different sizes.
Once the mobile phase begins flowing, the sample moves downward through the porous gel matrix.
Step 3: Separation Inside the Column:
As the sample travels through the column, molecules experience different pathways depending on their size.
- Large molecules are unable to enter most of the pores inside the gel beads.
- Therefore, they travel mainly through the spaces between the beads.
- This gives them a relatively short path through the column, allowing them to elute first.
Medium-Sized Molecules:
- Medium-sized molecules can enter some of the available pores.
- Their pathway is longer than that of large molecules but shorter than that of very small molecules.
- Therefore, they usually elute between the largest and smallest molecules.
- Small molecules can enter a larger number of pores inside the gel beads.
- Because they travel through both the spaces between beads and the internal pore spaces, they travel through a longer effective pathway.
- Therefore, they elute last.
Molecular Separation Pattern:
Large Molecules
Cannot Enter Most Pores
↓
Shortest Path
↓
Elute First
Medium Molecules
Enter Some Pores
↓
Intermediate Path
↓
Elute Next
Small Molecules
Enter Many Pores
↓
Longest Path
↓
Elute Last
❓ Why do large molecules elute first in size exclusion chromatography?
Large molecules are excluded from most of the pores inside the gel beads. They travel mainly through the spaces between the beads, following a shorter pathway and therefore leaving the column before smaller molecules.
Step 4: Elution:
Unlike techniques such as ion exchange chromatography, size exclusion chromatography usually does not require a salt gradient or pH gradient to release molecules.
The molecules continuously move through the column with the mobile phase.
Separation occurs because different molecules take different amounts of time to pass through the porous stationary phase.
The molecules therefore elute according to their size:
Large → Medium → Small
The same buffer or solvent can generally be used throughout the separation process.
Step 5: Fraction Collection:
As molecules leave the column, the eluate is collected in separate fractions.
For example:
Fraction 1 → Large molecules
Fraction 2 → Medium-sized molecules
Fraction 3 → Small molecules
In practice, each fraction may contain different concentrations of the separated molecules.
The fractions are usually analysed using appropriate techniques to identify the target molecule.
Common analysis methods include:
- UV absorbance
- SDS-PAGE
- Protein assays
- Enzyme activity assays
- Fluorescence detection
Fractions containing the desired molecule can then be pooled together.
Important Volumes in Size Exclusion Chromatography:
Several important volume parameters are used to understand and analyse SEC separations.
1. Void Volume (V₀)
The void volume, represented by V₀, is the volume of mobile phase present in the spaces between the gel beads.
Molecules that are too large to enter the pores are completely excluded from the stationary phase.
These molecules travel only through the spaces between the beads and so elute approximately at the void volume.
2. Total Column Volume (Vt):
The total column volume, represented by Vt, refers to the total volume occupied by the column bed.
It includes:
- The volume outside the beads
- The volume inside the pores
- The volume associated with the packed gel bed
Very small molecules that can access almost all available pore spaces may elute closer to the total accessible volume.
3. Elution Volume (Ve):
The elution volume, represented by Ve, is the volume of mobile phase required to elute a particular molecule from the column.
Different molecules have different elution volumes.
- Large molecules → Lower Ve
- Medium molecules → Intermediate Ve
- Small molecules → Higher Ve
Therefore, elution volume provides useful information about the relative size of molecules.
Relationship Between Molecular Size and Elution Volume:
Large Molecule
↓
Low Elution Volume (Ve)
↓
Elutes First
Medium Molecule
↓
Intermediate Ve
↓
Elutes Next
Small Molecule
↓
High Elution Volume (Ve)
↓
Elutes Last
❓ Why is elution volume important in size exclusion chromatography?
Elution volume indicates how long a molecule interacts with the porous stationary phase. Larger molecules usually have lower elution volumes, while smaller molecules have higher elution volumes because they enter more pores.
Calibration Curve in Size Exclusion Chromatography:
A calibration curve can be used to estimate the approximate molecular size of an unknown molecule.
The column is first run using standard molecules with known molecular weights or hydrodynamic sizes.
The elution volume of each standard is measured and plotted against the logarithm of molecular weight.
The unknown sample is then analysed under similar conditions.
Its elution volume can be compared with the calibration curve to estimate its approximate molecular size.
Basic Calibration Concept:
Known Molecular Standards
↓
Run Through SEC Column
↓
Measure Elution Volume (Ve)
↓
Create Calibration Curve
↓
Run Unknown Sample
↓
Measure Ve
↓
Estimate Approximate Molecular Size
Factors Affecting Separation in SEC:
Several experimental factors influence the resolution and efficiency of size exclusion chromatography.
1. Pore Size of the Gel:
The pore size must be suitable for the molecular sizes being separated.
If molecules are all excluded or all enter the pores completely, efficient separation may not occur.
2. Column Length:
Longer columns generally provide more opportunity for separation between molecules.
However, increasing column length can also increase separation time and may contribute to band broadening.
3. Particle Size:
Smaller gel particles can improve separation resolution because they provide more uniform pathways.
However, smaller particles may also increase flow resistance and pressure requirements.
4. Flow Rate:
An excessively high flow rate can reduce separation efficiency in some systems.
A properly optimised flow rate helps maintain good resolution while keeping the separation time practical.
5. Sample Volume:
Large sample volumes can broaden molecular bands and cause overlapping peaks.
Using an appropriately small sample volume generally improves separation resolution.
6. Sample Concentration:
Highly concentrated samples may lead to poor separation, aggregation, or increased solution viscosity.
Proper sample concentration helps produce sharper and more reproducible peaks.
7. Buffer or Solvent Conditions:
The mobile phase should maintain sample stability and minimise unwanted interactions between the molecules and the stationary phase.
Changes in pH, ionic strength, or solvent composition can influence molecular shape, aggregation, and separation behaviour.
Applications of Size Exclusion Chromatography:
Size exclusion chromatography has numerous applications in biotechnology, biochemistry, pharmaceutical science, and polymer analysis.
Major Applications:
- Protein purification
- Separation of protein complexes
- Removal of protein aggregates
- Desalting of protein samples
- Buffer exchange
- Separation of enzymes
- Purification of antibodies
- Analysis of nucleic acids
- Molecular size estimation
- Polymer molecular-weight distribution analysis
SEC in Protein Purification:
SEC is often used as a final polishing step during protein purification.
For example, after techniques such as ion exchange chromatography or affinity chromatography, SEC can help separate:
- Monomeric protein
- Protein aggregates
- Protein complexes
- Remaining contaminants
Because SEC generally operates under gentle conditions, it is particularly useful for maintaining the native structure and activity of proteins.
🔬 Why is SEC commonly used as a final polishing step?
Size exclusion chromatography can separate target proteins from aggregates, protein complexes, and remaining contaminants under gentle conditions. It is therefore commonly used after other purification methods for final purification and quality improvement.
Advantages of Size Exclusion Chromatography:
- Separates molecules under relatively gentle conditions.
- Usually does not require harsh salt or pH gradients.
- Helps preserve the natural structure and biological activity of proteins.
- Useful for protein purification and final polishing.
- Effective for desalting and buffer exchange.
- Can separate protein aggregates from monomeric proteins.
- Requires relatively simple mobile-phase conditions.
- Useful for molecular size estimation.
- Can be applied to proteins, enzymes, nucleic acids, and polymers.
- Easily combined with other purification techniques.
Limitations of Size Exclusion Chromatography:
- Lower sample capacity compared with some other chromatography techniques.
- Large sample volumes can reduce separation resolution.
- Molecules with very similar hydrodynamic sizes may be difficult to separate.
- Separation can take longer when high resolution is required.
- Improper matrix selection can significantly reduce efficiency.
- Samples may become diluted during separation.
- SEC is generally less suitable as the first purification step for highly complex mixtures.
- Molecular weight estimation may be affected by molecular shape.
⚠️ Important Limitation
Size exclusion chromatography separates molecules according to their hydrodynamic size, not molecular weight alone. Molecules with similar molecular weights but different shapes may therefore elute differently.
Size Exclusion Chromatography vs Ion Exchange Chromatography:
Size exclusion chromatography and ion exchange chromatography are both widely used in biotechnology, especially for protein purification. However, they separate molecules using completely different principles.
SEC separates molecules according to size and hydrodynamic volume, whereas ion exchange chromatography separates molecules according to electrical charge.
SEC vs IEC:
| Feature | Size Exclusion Chromatography | Ion Exchange Chromatography |
|---|---|---|
| Separation Basis | Molecular size and hydrodynamic volume | Electrical charge |
| Stationary Phase | Porous gel beads | Charged resin |
| Molecular Binding | Ideally minimal | Molecules bind to resin. |
| Elution | Continuous buffer flow | Salt or pH gradient |
| Large Molecules | Elute first. | Depends on molecular charge |
| Protein Concentration | Usually causes dilution | Can concentrate proteins |
| Major Use | Desalting, buffer exchange and polishing | Protein purification and separation |
Size Exclusion Chromatography vs Affinity Chromatography:
SEC and affinity chromatography are also commonly used together during protein purification.
While SEC separates molecules according to their physical size, affinity chromatography separates molecules based on a specific biological interaction between the target molecule and a ligand attached to the stationary phase.
For example, an antibody can bind specifically to a particular antigen, or a His-tagged protein can bind to a metal-containing affinity matrix.
Main Difference:
SEC = Separation based on molecular sizeAffinity Chromatography = Separation based on specific biological recognition
Affinity chromatography generally provides very high selectivity, whereas SEC is especially useful for removing aggregates, exchanging buffers, and final polishing.
🔬 SEC and Affinity Chromatography in Protein Purification
Affinity chromatography is often used to capture a specific target protein from a complex mixture, while size exclusion chromatography may be used later to remove aggregates, exchange the buffer, and improve the final purity of the protein.
References:
- Wilson K, Walker J. Principles and Techniques of Biochemistry and Molecular Biology.
- Scopes RK. Protein Purification: Principles and Practice.
- Janson JC. Protein Purification: Principles, High-Resolution Methods, and Applications.
- GE Healthcare. Gel Filtration Principles and Methods.
- Cytiva. Size Exclusion Chromatography: Principles and Methods.
- Skoog DA, Holler FJ, Crouch SR. Principles of Instrumental Analysis.
- Harris DC. Quantitative Chemical Analysis.
- Nelson DL, Cox MM. Lehninger Principles of Biochemistry.
- Berg JM, Tymoczko JL, Gatto GJ, Stryer L. Biochemistry.
- Poole CF. Chromatography Today.
- IUPAC. Compendium of Chemical Terminology (Gold Book).
- Pharmacopeial Forum. Chromatographic Separation Methods.
- USP General Chapter : Chromatography.
- European Pharmacopoeia. Chromatographic Methods.
- Cytiva Life Sciences. Protein Purification Handbook.
About the Author
Aftab Baig is a biotechnology student, science writer, and founder of BioTechyEdu. He is passionate about biotechnology, genetics, molecular biology, microbiology, and biomedical sciences. Through BioTechyEdu, he publishes research-based and student-friendly articles that simplify complex scientific concepts for students, educators, and science enthusiasts.
Comments
Post a Comment