Topics - Overload
In chromatography, the term ‘overload’ is given to a column condition where the sample size is so great that it impairs the performance of the column. Column overload can occur in two forms, either as volume overload
or mass overload
. Column overload can be by default, or be deliberate as in preparative chromatography. Volume overload causes peak broadening but the broadening is symmetrical and so the peak shape at the front and the rear of the peak is not distorted. As the volume of sample is increased (carrying a fixed concentration of solute) the peak broadens and the height increases until eventually the height remains constant and a rectangular type of peak is formed with a half Gaussian front and a half Gaussian tail. In volume overload the peak always spreads in one direction towards that of greater retention. In preparative chromatography the sample volume can be increased until the two solutes eluted closest to each other just touch which will then be the optimum sample volume. In analytical chromatography no type of overload should be tolerated except possibly for certain conditions that may arise in trace analysis. Mass overload results in the concentration range of solute in the stationary phase reaching a non-linear part of the adsorption isotherm and inevitably results in distorted peaks. If the isotherm is concave toward the axis representing the concentration in the mobile phase
, then at the higher solute concentrations of solute, the effective distribution coefficient will be smaller. Thus, the higher solute concentrations in the peak will move through the column more rapidly than the lower concentrations, and the peak will be distorted with a sharp front and a sloping tail. The overall retention of the solute will also be reduced. If, however, the isotherm is concave toward the axis representing the concentration in the stationary phase
, then at the higher solute concentrations of solute, the effective distribution coefficient will be larger. Thus, the high concentrations of solute in the peak will move through the column more slowly than the lower concentrations and the peak will be distorted with a sloping front and a sharp tail. In this case the overall retention of the solute will be decreased as the mass overload is increased..