BAND BROADENING AND SEPARATION EFFICIENCY

The band broadening may be characterised by a plate height, and its causes provide a basis for understanding why modem chromatography is such an efficient separation technique. 

In a hypothetical ideal column, a solute band would retain its initial profile unaltered as it migrated along the column. In a real, non-ideal, column an initially narrow band broadens by dispersion as it migrates. The band width is proportional to the square root of the distance travelled along the column. 

The greater the ratio tR/tw, then the greater the number of theoretical plates N in the column. 

To maximise separation efficiency requires low H and high N values. In general terms this requires that the process of repeated partitioning and equilibration of the migrating solute is accomplished rapidly. The mobile and stationary phases must be mutually well-dispersed. This is achieved by packing the column with fine, porous particles providing a large surface area between the phases (0.5-4 m2/g in GC, 200-800 m2/g in LC). Liquid stationary phases are either coated as a very thin film (0.05-1 pm) on the surface of a porous solid support (GC) or chemically bonded to the support surface as a mono-molecular layer (LC). 

Band broadening processes and particle size of packing 

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