The Efficiency of a Chromatographic Column Measured by its HETP

The efficiency of chromatographic columns is usually measured under conditions of linear adsorption equilibria from pulse tests. The Height Equivalent to a Theoretical Plate (HETP) is calculated from the first moment /ij and the variance of a chromatographic peak obtained in a column of length I by  

For conventional packings and linear isotherms Van Deemter et al. [10] developed the well-known equation for HETP including contributions from eddy dispersion (A-term), molecular diffusion (B-term) and intraparticle kinetics (C-term). 

For large-pore particles an extension of the Van Deemter equation was derived by Rodrigues [11, 12] since, % = Tj A) and therefore we get  

At low velocities /(A) = 1 and both equations lead to similar results. However, at high superficial velocities, /(A) = 3/A and so the last term in Rodrigues equation becomes a constant since the intraparticle convective velocity Vq is proportional to the superficial velocity u. The HETP reaches a plateau that does not depend on the value of the solute diffusivity but only on the particle permeability and pressure gradient (convection-controlled limit). 

Two important features result from the use of large-pore, permeable packings the column performance improves since HETP is reduced when compared with conventional packings (the C term in the Van Deemter equation is reduced) and the speed of separation can be increased (by increasing the superficial velocity) without losing column efficiency. 

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