phase, the distance traveled by the solute along
the column and time, respectively.
and Vr, l and tr refer to the retention volume, column length
and retention time, respectively.
Now, from the Plate Theory (see Plate Theory and Extensions) it has been shown that,
Thus,
and
Therefore,
The ratio, (
), (the column length divided by
the number of theoretical plates in the column) has, for obvious reasons,
become termed the Height Equivalent to the Theoretical
Plate (HETP) and has been given the symbol (H). However, it is seen that
(H) is numerically equal to, 
, which is, in fact, the variance per
unit length of the column. Thus, the function, 
, is the variance that
the Rate Theory will provide an explicit equation to define and can be
experimentally calculated for any column from its length and column efficiency.
It follows that the equations that give a value for, (H), the variance per unit
length of the column, have been termed HETP equations.
