Calculating Theoretical Plates The efficiency, or the number of theoretical plates in a column, can be calculated from measurements made on the elution curve (chromatogram) of a mixture solutes separated on the column. The efficiency may increase with solute retention as with most packed columns or decrease with solute retention as with most capillary columns. The relationship between efficiency (number of theoretical plates) and mobile phase velocity for any given solute is hyperbolic and shows a maximum efficiency (minimum HETP or minimum variance per unit length) at a certain velocity called the optimum velocity. The relationship between column efficiency and solute retention is predicted by all the rate theory equations. The number of theoretical plates is calculated as 16 times the square of the ratio of the retention distance (the distance between the injection point and the peak maximum) to the peak width at the points of inflection (the points of inflection occur at 0.6065 of the peak height). The data is usually provided by the software if a computer data acquisition system is employed or directly from measurements made on a chart record of the chromatogram. To compare the efficiency of two columns, the efficiencies must be measured at the same retention, for the same solute, at the same temperature and the same mobile phase velocity.