108 



Diff erential Migration from a Narrow°Zone 



Chromatography, as shown by the studies with chloroplast pigments, 

 is a differential migration method of analysiso The separation of the 

 zones depends upon their relative rates of migration, upon the initial 

 width of the zone of the mixture, and upon the distance of the migra- 

 tiono For effective separations, the initial zone of the mixture should 

 be narrow, the zones should migrate at different rates, and the region 



for the migration should be much longer than the initial width of the 



TV 6 

 zone of mixt\ire * <> 



For the migration of the chromatographic zones, a driving force 

 is necessary,, This driving force is the flow of the wash liquid„ It 

 is a nonselective driving force, because it carries each nonsorbed 

 molecule along at the same rate,, 



The sorbent provides a selective resistive force <, It retards 

 the migration of the solutes as they come into contact with fresh por- 

 tions of the sorptive phaseo This retardation is proportional to the 

 degree of sorption * o 



Differential Migration Methods of Analysis 



By analogy with chromatography, any system that produces differ- 

 ential migration of molecules or ions may serve as a separatory tech- 

 niqueo The primary requirement is a driving foroOo This driving 

 force, the attendant resistive force, or both these forces must pro- 

 duce the differential migration^ »o 



These views led to the use of direct current (DC) electrical 

 potential as a driving force to produce differential migration of ions 

 from a narrow zone of the mixture in an electrically conducting 



