Topics - Theoretical Plates

Theoretical Plates The concept of the theoretical plate arose from distillation column theory and was borrowed by A J. P. Martin to develop the first elution curve theory for the chromatography column. Neither in a distillation column, nor in a chromatographic column, does equilibrium exist in any part of the distribution system. This non-equilibrium condition arises from the dynamic nature of the distribution systems as the mobile phase is continually flowing past the stationary phase. The solute does not spend sufficient time at any point in the column for equilibrium to be achieved. To avoid this difficulty in the theoretical treatment of retention, the column is considered to be divided into a number of theoretical plates or cells and each is allotted a finite height (or length) that will allow the solute sufficient theoretical ‘dwell-time’ for equilibrium to take place. It is clear that the faster the equilibrium, the smaller the theoretical plate, and the more theoretical plates there will be in the column. Thus, the more efficient column has more theoretical plates. The plate theory was criticized when first developed, as it was claimed that equilibrium was not achieved in the column and, thus, the theoretical approach was invalid. However, the theoretical plate concept was introduced specifically to attend to this non- equilibrium problem The theory gave rise to the elution equation, the equation for resolution, and the equation used for calculating the column efficiency, all of which, have been exhaustively validated experimentally over many years.