Dimensionless parameters for HPLC column characterization

Information on the number of theoretical plates, N, or plate height, H, can be problematic, as outlined in Section 8.1. Dimensionless values (numbers without 

More details of this and the following section can he found in P.A. Bristow and J.H. Knox, Chromato-graphia, 10, 279 (19771. 

First dimensionless parameter, reduced plate height, h, replaces plate height, H. His compared with the mean particle diameter, dp.  

Second dimensionless parameter, reduced velocity, v, replaces linear flow velocity u. u is set in relation to the diffusion coefficient, D, of the analyte in the mobile phase and to the particle diameter dp. 

Porosity s is ca. 0.8 (silica) or 0.65 (bonded phase) for a packing with porous particles. The average diffusion coefficient (see Section 8.7) can be taken as 2.5 X 10 cm min for small molecules in low-viscosity eluents (normal phase) or as 6 X lO cm min for small molecules in high-viscosity eluents (reversed phase). Therefore the ready-to-use equations emerge  

Information on the number of theoretical plates, N, or plate height, H, can be problematic, as outlined in Section 8.1. Dimensionless values (numbers without units) are much more suitable for absolute comparison of columns, a whole range of different types of columns (long or short, thick and thin, normal or reversed phase, large or microparticles, with various types of packing) being covered. Dimensionless values are also easy to remember. 

Note that both equations are only valid for the conditions given above (small 

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