# Plate Theory and Extensions - Vacancy Chromatography > Page 72

It follows
that the concentration leaving the (n)th plate (X_{E}) will be

Employing the alternative Gaussian form of the elution equation,

(61)

Furthermore, at the peak maximum when v=n and w=0,

(62)

Equation (62)
shows that, when the charge is placed on the first plate, (X_{n}) can
never equal zero and pure mobile phase free of solute will never elute from the
column. However, in practice, it is impossible to place a sample exclusively on
the first plate, and there must be a finite volume of mobile phase that will
occupy a finite number of theoretical plates.