# Topics - HETP Curve

HETP Curve The HETP curve is a graph relating the variance per unit length of a column to the linear velocity of the mobile phase. The variance per unit length of a column, for a specific solute, can be calculated as the ratio of the column length to the column efficiency. For an LC column the mobile phase velocity can be taken as the ratio of the column length to the retention time of an unretained solute of sufficient size so that it can not enter any pores of the support or stationary phase (e.g. a dispersion of silica smoke). The linear velocity in a GC column can be determined by dividing the column exit flow rate by the column cross-sectional area. However, in GC, the linear velocity calculated in this way must also be corrected for the compressibility of the gas. The relationship between the variance per unit length of the column and the linear mobile phase velocity is given by the HETP equation which takes the form of a hyperbolic function. This hyperbolic function may contain three or four individual expressions depending on whether it is for a packed column or for a capillary column. The HETP curve shows a minimum at a particular velocity, which is called the optimum velocity. It follows, that at the optimum mobile phase velocity, the column will provide a maximum number of theoretical plates.