THE THICKNESS OF SHOCK LAYERS IN LIQUID-CHROMATOGRAPHY

At high concentrations, one boundary (the front if the isotherm is con vex upward) of the elution band of a single component becomes very ste ep. Similarly, in frontal analysis, the same boundary is self-sharpeni ng. These steep boundaries are called shock layers. They result from t he steady-state equilibrium between a nonlinear thermodynamics of phas e equilibrium on the one hand, which tends to create a concentration d iscontinuity, and axial dispersion and a finite rate of mass transfer on the other hand, which tend to relax the concentration gradient. We show that the width of experimental breakthrough curves recorded in fr ontal analysis agrees well with the prediction of the shock layer theo ry of Rhee and Amundson in the high concentration range. It is a funct ion of the coefficients of the column HETP equation and the height of the concentration step and is independent of the column length, after the steady state is achieved. In the low concentration range, a discre pancy is observed when the column length is short and the migration di stance is insufficient to achieve constant pattern behavior.