Concept of Theoretical Plates:

With all the concepts learnt up to this stage, we may be in a position to assess the performance of a column. While we say performance, it automatically implies competence within attaining a separation. The several factors contributing to efficiency are examined by a concept known as height equivalent to theoretical plate (HETP). This concept of theoretical plates is similar to one occurring in fractional distillation or countercurrent extraction. As per to this, chromatographic column can be assumed to be made up of a large number of theoretical plates where equilibrium is achieved. Ideally, a theoretical plate is a fictitious concept which does not correspond to any actual entity in the column. Therefore, it is a convenient parameter for evaluation. Thus there are two terms, plate height (H) and plate count or number of plates (N). The two are related as

N = L/ H

where L is the length of column packing (usually in cm). H is defined as that length of the column which will yield an effluent in equilibrium with mean concentration over that length in the stationary phase. Instead, at each plate equilibrium of the solute between the mobile and stationary phase is assumed to take place.