Ion Exchange Chromatography (IEC) –

1. Introduction

Ion exchange chromatography (IEC) is a separation technique based on the reversible exchange of ions between a charged stationary phase and ions in a solution (mobile phase). It is widely used to separate proteins, amino acids, nucleotides, and inorganic ions.

2. Principle

The technique works on electrostatic attraction between charged molecules and oppositely charged groups attached to the stationary phase.

• : Negatively charged resin
• : Initially bound ion
• : Ion from sample

Key idea:

• Positively charged ions (cations) bind to negatively charged resins.
• Negatively charged ions (anions) bind to positively charged resins.

3. Types of Ion Exchange Chromatography

(A) Cation Exchange Chromatography

• Stationary phase: Negatively charged (e.g., –COO⁻, –SO₃⁻ groups)
• Binds: Positively charged ions (cations like Na⁺, Ca²⁺, proteins with positive charge)

Examples of resins:

• Carboxymethyl cellulose (CM-cellulose)
• Sulfonated polystyrene resins

(B) Anion Exchange Chromatography

• Stationary phase: Positively charged (e.g., –NH₃⁺ groups)
• Binds: Negatively charged ions (anions like Cl⁻, PO₄³⁻, negatively charged proteins)

Examples of resins:

• DEAE-cellulose (Diethylaminoethyl cellulose)
• Q-sepharose

4. Components of the System

1. Stationary Phase (Ion Exchange Resin)

   • Insoluble matrix with charged functional groups
   • Can be organic polymers or cellulose-based materials

2. Mobile Phase (Eluent/Buffer)

   • Liquid that carries the sample
   • pH and ionic strength are crucial

3. Column

   • Glass or plastic tube packed with resin

5. Working Procedure

1. Column Equilibration

   • Column is equilibrated with a buffer of specific pH

2. Sample Application

   • Mixture of ions is introduced into the column

3. Binding

   • Oppositely charged ions bind to the resin

4. Washing

   • Unbound molecules are washed away

5. Elution

   • Bound ions are removed by:
     • Increasing salt concentration (ionic strength)
     • Changing pH

6. Detection

   • Collected fractions are analyzed

6. Factors Affecting Separation

• pH of buffer
  Determines charge on molecules (especially proteins)

• Ionic strength
  Higher salt concentration weakens electrostatic interactions

• Nature of resin
  Strong vs weak ion exchangers

• Flow rate
  Affects resolution and separation efficiency

7. Applications

• Separation and purification of proteins and enzymes
• Water softening (removal of Ca²⁺ and Mg²⁺ ions)
• Amino acid analysis
• Purification of nucleotides and nucleic acids
• Pharmaceutical and biochemical industries

8. Advantages

• High resolution and efficiency
• Selective separation based on charge
• Reusable resins
• Suitable for large-scale purification

9. Disadvantages

• Sensitive to pH and temperature changes
• Requires careful buffer preparation
• Some biomolecules may denature
• Cost of resins can be high

10. Summary

Ion exchange chromatography is a powerful and widely used analytical and preparative technique that separates molecules based on their charge. By adjusting pH and ionic strength, highly selective separations can be achieved, making it essential in biochemistry, environmental science, and industrial processes.