Size Exclusion 
    Chromatography
            Introduction
            Introduction
 The separation of various constituents by taking advantage of 
 the difference in size and geometry of the molecule constitute 
 the basis of size exclusion chromatography.
 By virtue of a difference in size, certain particles move faster 
 while others move slower thereby creating differential migration 
 front in a column packed with gels.
 Types of exclusion chromatography
    1. Gel permeation or gel filtration technique
    2. Ion exclusion and retardation
    3. Inorganic molecular sieves or (molecular sieve
        filtration)
          Gel chromatography
 Gel chromatography is a technique in which separation is based 
 upon the molecular size and shape of the species in the sample.
 The  stationary  phase  consists  of  beads  of  porous  polymeric 
 material, a gel. 
 The  liquid  percolating  through  the  bed  is  called  the  mobile 
 phase.
              Theory
 The  technique  involves  inclusion  and  exclusion  of  solutes 
 through a stationary phase consisting of a heteroporous cross 
 linked polymeric gels. 
 The  process  of  separation  is  based  upon  the  different 
 permeation rates of each solute molecule into the interior of the 
 gel particles.
 By flow of liquid, molecules diffuse in all parts of the gel. Larger 
 molecules  appear  first  in  the  effluent  followed  by  entrapped 
 smaller molecules. 
  Mechanism of Gel permeation chromatography 
 Gel chromatography is essentially a case of liquid-liquid partition 
 chromatography  process  in  which  the  solute  molecules  are 
 distributed between two liquid phases, liquid in the gel pores and 
 the liquid outside the gel.
 Three  mechanisms  have  been  proposed  to  explain  the 
 technique of gas chromatography:
 1.  Steric exclusion mechanism
 This effect is more prominent where the major portion of a solute 
 is larger than many pores of the gel. It is presumed that,
      i)    Because the gel particles contain a distribution  of  pore 
            sizes, small molecules can enter large number of pores 
            while  the  large  molecules  can  enter  small  number  of 
            pores. 
      ii)   The total volume (V) of a column packed with a gel that 
                                            t
            has been swelled by the solvent is given by 
                              V = V  + V +V                                           -(1)
                                 t      g      I     0
      Where, 
            Vg  is the volume occupied by the solid matrix of the gel, 
            VI  is  the  volume  of  solvent  held  in  the  pores  or   
            interstices, and 
            V0 is the free volume outside the gel particles.