Geometry-Independent Plate Height Representation Methods for the Direct Comparison of the Kinetic Performance of LC Supports with a Different Size or Morphology

Abstract

The advantages of representing experimental plate height data as a plot of K_v/u₀² or H²/K_v versus K_v/(Hu₀) instead of as H versus u₀ are discussed (K_v=column permeability). Multiplying the values on both axes by the ratio of a reference pressure drop and mobile-phase viscosity, the obtained plots directly yield the kinetic performance limits of the tested support structure, without any need for further numerical optimization. Directly showing the range of plate numbers or analysis times wherein the tested support geometry can yield faster separations or produce more plates than another support type, such kinetic plots are ideally suited to compare the performance of differently shaped or sized LC supports. The approach hence obviates the need for a common reference length, which is a clear problem if it is attempted to compare differently shaped supports on the basis of their flow resistance φ and reduced plate height h. It is also shown how an MS Excel template file, only requiring the user to paste the column permeability K_v and a series of experimental (u₀, H) data, can be used to automatically establish a series of so-called kinetic performance (KP) numbers, which can be used to completely describe the performance characteristics of the considered support. The advantages of the proposed data representation methods are demonstrated by applying them to several recent literature plate height data sets, showing that the obtained kinetic plots directly visualize the range of plate numbers where new approaches such as ultra-high-pressure HPLC or the use of open-porous silica monoliths can be expected to provide a substantial gain and where not. The data analysis also showed that the most generally relevant KP numbers are N_opt (the plate number for which the support achieves its best analysis time/pressure cost ratio), t_opt (the time needed to obtain N_opt plates), and t_1K (the time needed to generate 1000 or 1 kilo of theoretical plates). These KP numbers are much more informative than the H_min, u_0,opt, and K_v data traditionally employed to quantify the performance of LC supports.