Separation Techniques – Complete Notes

CSIR-NET Life Science | GATE Biotechnology | DBT BET | ICAR JRF | ICMR JRF

Separation techniques are fundamental methods used in biology and biotechnology to separate cells, organelles, nucleic acids, proteins and other biomolecules from complex mixtures. This chapter covers Sheet Chromatography, Column Chromatography, Agarose Gel Electrophoresis, Polyacrylamide Gel Electrophoresis (PAGE), and Centrifugation in detail.

The emphasis is on the principle, mechanism, important terms, applications, factors affecting separation, advantages, limitations and examination-oriented concepts.

Topics Covered

Introduction to separation techniques
Basic principles of separation
Sheet chromatography
Paper chromatography
Thin-layer chromatography
Retention factor (Rf)
Column chromatography
Adsorption and partition chromatography
Elution and fraction collection
Ion-exchange chromatography
Size-exclusion chromatography
Affinity chromatography
Agarose gel electrophoresis
DNA and RNA separation by agarose gel
Factors affecting agarose electrophoresis
Polyacrylamide gel electrophoresis
SDS-PAGE
Native PAGE
Stacking and resolving gels
Protein molecular-weight estimation
Two-dimensional electrophoresis
Centrifugation
Relative centrifugal force
Differential centrifugation
Density-gradient centrifugation
Rate-zonal centrifugation
Isopycnic centrifugation
Comparison of separation techniques
Important CSIR-NET/GATE points
10 interactive MCQs

1. Introduction to Separation Techniques

Biological samples are highly complex mixtures. A cell contains proteins, DNA, RNA, lipids, carbohydrates, metabolites, salts and numerous other molecules. Many biological experiments require one particular component to be isolated from this complex mixture.

For example, a molecular biology researcher may need to separate DNA fragments according to their size, a biochemist may need to separate proteins according to their charge, and a cell biologist may need to separate organelles according to their size and density.

Separation techniques exploit differences in one or more physical or chemical properties of the components of a mixture.

Size: Molecules or particles can be separated according to molecular size or hydrodynamic volume.
Charge: Charged molecules behave differently in an electric field or interact differently with ion-exchange matrices.
Density: Particles with different densities sediment at different rates.
Solubility: Different molecules have different solubilities in solvents.
Polarity: Molecules interact differently with polar and non-polar phases.
Hydrophobicity: Hydrophobic molecules interact differently with hydrophobic surfaces.
Affinity: Specific biological interactions can be used for highly selective purification.
Adsorption: Molecules may bind to the surface of a stationary phase.

Central concept:

Every separation method depends on a measurable difference between the components being separated.

2. Classification of Separation Techniques

Technique	Main Separation Principle	Common Application
Paper chromatography	Partition	Amino acids, pigments and small molecules
TLC	Adsorption/partition	Small organic molecules
Column chromatography	Adsorption, charge, size or affinity	Biomolecule purification
Agarose electrophoresis	Charge and size	DNA/RNA separation
PAGE	Charge, size and shape	Protein separation
SDS-PAGE	Primarily molecular size	Protein molecular-weight analysis
Centrifugation	Size, density and mass	Cells, organelles and macromolecules

3. Sheet Chromatography

Sheet chromatography refers to chromatographic techniques in which the stationary phase is present as a flat sheet or planar surface. Important examples include paper chromatography and thin-layer chromatography (TLC).

3.1 Basic Principle

A mixture is applied near one end of the chromatographic sheet. A solvent, called the mobile phase, moves through the stationary phase by capillary action. Different compounds migrate at different rates because of differences in their interactions with the stationary and mobile phases.

The stationary phase remains fixed.
The mobile phase moves through the stationary phase.
The sample is applied as a small spot.
The solvent carries components away from the original spot.
Different compounds migrate different distances.
Separated spots can be visualized using suitable detection methods.

Figure: Basic principle of planar/sheet chromatography.

4. Paper Chromatography

Paper chromatography is a planar chromatographic technique in which cellulose paper acts as the support for the stationary phase. It has been widely used for separating amino acids, sugars, pigments and other small molecules.

Cellulose paper provides the stationary support.
The solvent acts as the mobile phase.
The sample is spotted close to the baseline.
The solvent rises by capillary action.
Components partition differently between the stationary and mobile phases.
Separated components appear at different positions.

4.1 Applications

Separation of amino acids
Separation of sugars
Plant pigment analysis
Identification of small molecules
Teaching and demonstration of chromatographic principles

5. Thin-Layer Chromatography (TLC)

Thin-layer chromatography uses a thin layer of an adsorbent material such as silica gel or alumina coated onto a solid support.

Stationary phase: Often silica gel or alumina.
Mobile phase: Appropriate solvent or solvent mixture.
Sample: Applied as a small spot.
Development: Solvent moves through the plate.
Detection: UV light, iodine vapor or chemical staining may be used.

Paper chromatography vs TLC:

Paper chromatography commonly uses cellulose paper, whereas TLC commonly uses an adsorbent layer such as silica gel or alumina.

6. Retention Factor (Rf)

The retention factor is one of the most important concepts in planar chromatography.

Rf = Distance travelled by solute / Distance travelled by solvent front

Rf is a dimensionless value.
It is generally between 0 and 1 under normal conditions.
A compound moving farther relative to the solvent front has a larger Rf.
Rf depends on the stationary phase, mobile phase, temperature and experimental conditions.
Rf values can help compare compounds under identical conditions.

Example:

If a compound travels 4 cm and the solvent front travels 8 cm:

Rf = 4 / 8 = 0.5

7. Column Chromatography

Column chromatography is a preparative separation method in which the stationary phase is packed into a column and the sample is loaded onto the top of the column. A mobile phase passes through the stationary phase and different components elute at different rates.

Column contains the stationary phase.
Sample is loaded onto the column.
Mobile phase flows through the column.
Different molecules interact differently with the stationary phase.
Components leave the column at different times.
Fractions are collected separately.

Figure: Basic column chromatography showing differential migration and fraction collection.

8. Types of Column Chromatography

Adsorption chromatography: Separation based on interaction with a solid adsorbent.
Partition chromatography: Separation based on differential partition between phases.
Ion-exchange chromatography: Separation according to charge.
Size-exclusion chromatography: Separation according to hydrodynamic size.
Affinity chromatography: Separation according to specific molecular interactions.
Hydrophobic interaction chromatography: Separation according to surface hydrophobicity.

9. Ion-Exchange Chromatography

Ion-exchange chromatography separates molecules according to their net electrical charge at a particular pH.

Cation Exchange

Stationary phase carries negative charges.
It binds positively charged molecules.
Proteins with positive net charge can bind.

Anion Exchange

Stationary phase carries positive charges.
It binds negatively charged molecules.
DNA and RNA can interact with positively charged matrices because their phosphate backbone is negatively charged.

Memory trick:

Cation exchanger → binds cations → negative stationary phase.

Anion exchanger → binds anions → positive stationary phase.

10. Size-Exclusion Chromatography

Size-exclusion chromatography separates molecules based primarily on their hydrodynamic size.

The stationary phase contains porous beads.
Large molecules are excluded from many pores.
Large molecules therefore travel through a shorter effective path.
Small molecules enter the pores.
Small molecules therefore take a longer path.
Large molecules generally elute before small molecules.

CSIR-NET GOLDEN RULE:

In size-exclusion chromatography:

LARGE → FIRST

SMALL → LATER

11. Affinity Chromatography

Affinity chromatography is one of the most selective forms of chromatography. It uses a highly specific interaction between the target molecule and an immobilized ligand.

Target molecule binds specifically to the stationary phase.
Unwanted molecules are washed away.
Target molecule is subsequently eluted.
Competitive ligands can be used for elution.
Changes in pH or ionic conditions can also disrupt binding.

His-Tagged Protein Purification

Recombinant proteins may contain a polyhistidine tag.
The histidine residues interact with immobilized metal ions.
Nickel and cobalt matrices are commonly used.
Imidazole is commonly used to compete with histidine during elution.

12. Agarose Gel Electrophoresis

Agarose gel electrophoresis is one of the most important techniques in molecular biology for separating DNA and RNA fragments according to their electrophoretic mobility.

DNA contains a negatively charged phosphate backbone. Therefore, when an electric field is applied, DNA migrates toward the positive electrode (anode).

DNA direction:

Negative DNA → moves toward positive electrode.

Figure: Schematic representation of DNA separation by agarose gel electrophoresis.

12.1 Principle of Agarose Electrophoresis

Agarose forms a porous three-dimensional matrix.
DNA is negatively charged.
An electric field causes DNA to migrate toward the positive electrode.
The agarose matrix provides resistance to migration.
Smaller DNA fragments move more easily through the pores.
Larger fragments migrate more slowly.
DNA fragments are separated according to size.

12.2 Factors Affecting DNA Migration

DNA size: Smaller fragments generally migrate faster.
Agarose concentration: Higher agarose concentration produces smaller pores.
Voltage: Higher voltage increases migration but excessive voltage can reduce resolution.
DNA conformation: Linear, circular and supercoiled DNA can migrate differently.
Buffer: Buffer composition and concentration influence conductivity and migration.
Gel thickness: Can affect heat dissipation and resolution.
Electrophoresis time: Longer runs can improve separation within an appropriate range.

13. Agarose Concentration and DNA Size

The concentration of agarose determines the pore size of the gel. Therefore, different agarose concentrations are useful for different ranges of DNA fragment sizes.

Agarose Concentration	General Use
Low concentration	Better suited for relatively large DNA fragments.
Medium concentration	Commonly used for routine DNA fragment analysis.
High concentration	Useful for relatively small DNA fragments.

Important:

Increasing agarose concentration generally decreases pore size. Therefore, small DNA fragments are better resolved in higher-percentage agarose gels, whereas large fragments are generally better handled with lower-percentage gels.

14. DNA Visualization

DNA is colorless and therefore requires visualization after electrophoresis. Fluorescent DNA-binding dyes or stains can be used.

DNA-binding dyes interact with nucleic acids.
Fluorescence can be detected using suitable illumination.
DNA ladders are used to estimate fragment sizes.
A DNA ladder contains fragments of known sizes.

15. Polyacrylamide Gel Electrophoresis (PAGE)

Polyacrylamide gel electrophoresis is widely used for separating proteins and can also be used for small nucleic acids. Polyacrylamide gels generally have smaller and more uniform pores than agarose gels.

Polyacrylamide is formed by polymerization of acrylamide and a cross-linker.
The pore size can be controlled by changing gel composition.
PAGE provides high resolution.
It is particularly useful for proteins and small nucleic acids.

16. SDS-PAGE

SDS-PAGE is one of the most important protein separation techniques in biochemistry and molecular biology.

Sodium dodecyl sulfate (SDS) is an anionic detergent. It denatures many proteins and gives them a large negative charge approximately proportional to their length.

SDS disrupts many non-covalent interactions.
Proteins become unfolded or substantially denatured.
SDS binds along the polypeptide chain.
The native differences in charge are largely masked.
Protein migration becomes primarily related to molecular size.
Smaller proteins migrate faster through the gel.

Key point:

SDS-PAGE separates proteins primarily according to molecular mass because SDS gives proteins a relatively similar charge-to-mass relationship.

Figure: Schematic SDS-PAGE separation of proteins according to molecular size.

17. Native PAGE

Native PAGE differs from SDS-PAGE because proteins are not deliberately denatured by SDS. Therefore, migration depends on a combination of properties.

Protein structure is generally retained better than in SDS-PAGE.
Protein charge affects migration.
Protein size affects migration.
Protein shape affects migration.
Protein complexes may remain intact.
Enzyme activity may sometimes be retained.

Feature	SDS-PAGE	Native PAGE
Denaturation	Usually denaturing	Usually non-denaturing
SDS	Present	Absent
Main separation property	Primarily molecular size	Size + charge + shape
Protein structure	Mostly disrupted	More likely retained
Protein complexes	Usually disrupted	May remain intact

18. Stacking and Resolving Gels in SDS-PAGE

Many SDS-PAGE systems use a discontinuous buffer system containing a stacking gel and resolving gel.

Stacking Gel

Usually has lower acrylamide concentration.
Has larger pores.
Concentrates proteins into a narrow band.
Improves resolution in the resolving gel.

Resolving Gel

Usually has higher acrylamide concentration.
Has smaller pores.
Provides the major separation of proteins.
Protein migration depends strongly on molecular size.

Remember:

Stacking gel = concentrates the sample.

Resolving gel = separates the proteins.

19. Estimation of Protein Molecular Weight

SDS-PAGE can be used to estimate the molecular mass of an unknown protein. A molecular-weight marker containing proteins of known molecular masses is run alongside the sample.

Run the molecular-weight ladder with the unknown sample.
Measure migration distances.
Calculate relative migration.
Compare the unknown band with standards.
A calibration plot can be prepared using log molecular mass.

Relative mobility (Rf) = Distance migrated by protein / Distance migrated by tracking front

20. Two-Dimensional Gel Electrophoresis

Two-dimensional electrophoresis separates proteins using two independent properties.

First dimension: Isoelectric focusing separates proteins according to isoelectric point (pI).
Second dimension: SDS-PAGE separates proteins according to molecular mass.
This provides very high resolving power.
It is useful in proteomics and comparative protein analysis.

2D-PAGE:

First dimension → pI

Second dimension → Molecular mass

21. Centrifugation

Centrifugation is a separation technique that uses centrifugal force to separate particles suspended in a liquid.

When a sample rotates rapidly around an axis, particles experience an outward-directed effective force. Particles with different sizes, masses, densities and shapes sediment at different rates.

Cells can be separated from culture medium.
Cell debris can be removed from lysates.
Organelles can be separated.
Proteins and nucleic acids can sometimes be concentrated.
Density-gradient centrifugation can separate particles according to density.

Figure: Simplified representation of centrifugal separation.

22. Relative Centrifugal Force (RCF)

Centrifugation is often described using relative centrifugal force (RCF), expressed in units of ×g.

RCF depends on the rotational speed and the radius of rotation. Therefore, RPM alone does not completely describe the centrifugal force.

RCF ≈ 1.118 × 10^-5 × r × RPM²

RCF: Relative centrifugal force.
r: Radius in centimeters.
RPM: Revolutions per minute.

Exam Point:

Two centrifuges operating at the same RPM can generate different RCF values if their rotor radii are different.

23. Differential Centrifugation

Differential centrifugation separates cellular components based mainly on differences in sedimentation rate.

A cell homogenate is centrifuged sequentially at increasing centrifugal forces. Larger and denser components generally sediment at lower speeds, while smaller components require higher speeds.

General Sequence

Homogenize the cells gently.
Perform a low-speed centrifugation.
Large debris and unbroken cells form the pellet.
Transfer the supernatant to another tube.
Increase the centrifugal force.
Progressively smaller components sediment.

24. Density-Gradient Centrifugation

Density-gradient centrifugation uses a gradient of increasing density in the centrifuge tube. Particles can separate based on sedimentation behavior and density.

Common gradient materials include sucrose and other suitable density-gradient media.

Major Types

Rate-zonal centrifugation
Isopycnic centrifugation

25. Rate-Zonal Centrifugation

In rate-zonal centrifugation, particles are separated primarily according to their sedimentation rate.

Sample is layered over a preformed density gradient.
Centrifugation causes particles to move through the gradient.
Larger or faster-sedimenting particles move more rapidly.
Centrifugation time must be carefully controlled.
Excessive centrifugation can cause particles to reach the bottom of the tube.

Rate-zonal:

Mainly separates particles according to sedimentation rate.

26. Isopycnic Centrifugation

Isopycnic centrifugation separates particles according to their buoyant density. Particles migrate until they reach a position where the density of the surrounding medium matches their own buoyant density.

Separation is based on buoyant density.
Particles form bands at their equilibrium positions.
The final position is determined by density rather than simply sedimentation rate.
Longer centrifugation can allow particles to reach equilibrium.

Difference:

Rate-zonal → sedimentation rate.

Isopycnic → buoyant density.

27. Comparison of Centrifugation Methods

Method	Main Basis	Major Application
Differential centrifugation	Different sedimentation rates	Cell fractionation
Rate-zonal centrifugation	Sedimentation rate	Macromolecule/particle separation
Isopycnic centrifugation	Buoyant density	Density-based particle separation

28. Agarose vs Polyacrylamide Gel

Feature	Agarose	Polyacrylamide
Common application	DNA/RNA	Proteins and small nucleic acids
Pore characteristics	Relatively large and variable	Small and controllable
Resolution	Good for many DNA applications	Very high for proteins
Typical protein use	Limited	Extensive
DNA fragment analysis	Very common	Useful for small fragments

29. Sheet Chromatography vs Column Chromatography

Feature	Sheet Chromatography	Column Chromatography
Format	Planar	Column
Sample capacity	Usually smaller	Can be larger
Purpose	Analytical/separation	Often preparative purification
Detection	Spots	Fractions
Common measurement	Rf	Elution volume/time

30. Major Factors Affecting Separation

Chromatography

Stationary phase
Mobile phase
pH
Ionic strength
Temperature
Flow rate
Sample volume
Column dimensions
Particle size of stationary phase

Electrophoresis

Voltage
Buffer composition
Buffer pH
Gel concentration
Sample concentration
Molecular size
Molecular charge
Molecular shape
Temperature

Centrifugation

RPM
RCF
Rotor radius
Particle size
Particle density
Particle shape
Viscosity of medium
Centrifugation time
Density gradient

31. Important CSIR-NET and GATE Concepts

Rf is the ratio of distance travelled by solute to distance travelled by solvent front.
DNA is negatively charged because of its phosphate backbone.
DNA migrates toward the positive electrode during electrophoresis.
Smaller DNA fragments generally migrate faster through agarose.
Higher agarose concentration generally produces smaller pores.
SDS is an anionic detergent.
SDS denatures many proteins and provides them with a relatively uniform negative charge-to-mass relationship.
SDS-PAGE primarily separates proteins according to molecular mass.
Native PAGE depends on size, charge and shape.
The stacking gel concentrates proteins into narrow zones.
The resolving gel performs the major separation.
Ion-exchange chromatography separates according to charge.
Size-exclusion chromatography separates according to hydrodynamic size.
Large molecules usually elute before smaller molecules in size-exclusion chromatography.
Affinity chromatography uses specific molecular interactions.
Cation exchangers have negatively charged functional groups.
Anion exchangers have positively charged functional groups.
Ammonium sulfate is commonly used for protein precipitation by salting out.
Differential centrifugation separates cell components according to sedimentation behavior.
Rate-zonal centrifugation depends mainly on sedimentation rate.
Isopycnic centrifugation depends on buoyant density.
RPM and RCF are not identical.
RCF depends on both RPM and rotor radius.

32. Quick Revision Table

Concept	Remember
Rf	Distance solute / Distance solvent front
DNA charge	Negative
DNA movement	Toward positive electrode
Agarose	Commonly DNA/RNA separation
PAGE	High-resolution protein separation
SDS-PAGE	Primarily molecular mass
Native PAGE	Size + charge + shape
Ion exchange	Charge
Size exclusion	Size
Affinity	Specific interaction
Large molecule in SEC	Elutes first
Ammonium sulfate	Salting out
Differential centrifugation	Sedimentation behavior
Rate-zonal	Sedimentation rate
Isopycnic	Buoyant density
RCF	Depends on RPM and radius

33. 10 Interactive MCQs – Separation Techniques

34. MCQ Answer Key

Question	Answer	Concept
1	B	Rf calculation
2	B	DNA electrophoretic movement
3	B	SDS-PAGE
4	C	Ion exchange
5	B	Size exclusion
6	B	SDS chemistry
7	A	Isopycnic centrifugation
8	B	Polyacrylamide gel
9	A	Differential centrifugation
10	B	Stacking gel

35. Final One-Minute Revision

Paper/TLC → Planar chromatography
Rf → Solute distance / Solvent-front distance
Column chromatography → Differential interaction with stationary phase
Ion exchange → Charge
Size exclusion → Size
Affinity → Specific interaction
DNA → Negative charge
DNA → Positive electrode
Agarose → DNA/RNA
PAGE → High-resolution protein separation
SDS-PAGE → Molecular mass
Native PAGE → Size + charge + shape
Stacking gel → Concentrates protein bands
Resolving gel → Separates proteins
Differential centrifugation → Sequential sedimentation
Rate-zonal → Sedimentation rate
Isopycnic → Buoyant density
RCF → RPM + rotor radius

36. Conclusion

Separation techniques form one of the most important practical components of modern biological research. Chromatography separates molecules by exploiting differences in their interactions with stationary and mobile phases, whereas electrophoresis uses an electric field to separate charged molecules. Centrifugation uses differences in sedimentation behavior and density.

For CSIR-NET, GATE Biotechnology, DBT BET and other competitive examinations, it is essential to understand the underlying principle of each technique rather than memorizing only names.

The most important relationships to remember are: chromatography → interaction, electrophoresis → charge/size, and centrifugation → sedimentation/density.

CSIR NET Life Science GATE Biotechnology DBT BET Separation Techniques Chromatography Agarose Gel Electrophoresis PAGE SDS PAGE Centrifugation Biotechnology Notes

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