Kinetic plate height

Perhaps more important is the observation that the kinetics of binding can contribute significantly to band spreading. If the results presented in that work are general, the kinetic plate height contribution sets a lower limit on the size of particles used in RPC at about 5 m and no practical gain in column efficiency would be made by using smaller particles. 

The general approach for kinetic optiaization of open i tubular columns has been to adopt the familiar Golay equation T (equation 1.34) and to assuae that the aobile phase can be approximated by an incompressible fluid with ideal gas properties, (44-50). Circumstances that are approximate at best but serve adequately to demonstrate some of the fundamental characteristics of open tubular columns operated at low fluid densities. The column plate height equation can be written in the form given in M equation (6.1)... 

Desmet, G., Clicq, D., Gzil, P. (2005). Geometry-independent plate height representation methods for the direct comparison of the kinetic performance of LC supports with a different size or morphology. Anal. Chem. 77, 4058 1070. 

In pHPLC, there are numerous types of columns used. The comparison and characterization of these columns are often discussed in terms of thermodynamic properties and kinetic characteristics. The retention factor, k, selectivity, a, and the peak asymmetry are believed to be representative parameters for the thermodynamic properties, while the kinetic characteristics are often expressed in dimensionless magnitudes of reduced plate height, h, separation impedance, E, and flow resistance factor, ( ). 3... 

Z/mc plate height due to sorption/desorption micellar kinetics... 

As is evident from the preceding discussion, the retention behavior of a polypeptide or protein P- expressed in terms of the capacity factor k is governed by thermodynamic considerations. Peak dispersion, on the other hand, arises from time-dependent kinetic phenomena, which are most conveniently expressed in terms of the reduced plate height he, . When no secondary effects, i.e., when no temperature effects, conformational changes, slow chemical equilibrium, pH effects, etc. occur as part of the chromatographic distribution process, then the resolution Rs, that can be achieved between adjacent components separated under these equilibrium or nearequilibrium conditions can be expressed as... 

A high electroosmotic flow through the stationary-phase particles may be created when the appropriate conditions are provided. This pore flow has important consequences for the chromatographic efficiency that may be obtained in CEC. From plate height theories on (pressure-driven) techniques such as perfusion and membrane chromatography, it is known that perfusive transport may strongly enhance the stationary-phase mass transfer kinetics [30-34], It is emphasised... 

The pore connectivity r of two types of silica (highly porous beads, monolithic silicas) was calculated according to the pore network model proposed by Meyers and Liapis. Nt was proportional to the particle porosity in the case of highly porous beads. The differences in the pore connectivity for both types of silica were reflected in the mass transfer kinetics in liquid phase separation processes by measuring the theoretical plate height-linear velocity dependencies. In a future study, monolithic silicas possessing different macro- as well as mesopores will be investigated and compared with the presented results. 

The kinetic contributions to zone broadening are evaluated by fitting data for the column plate height, as a function of the mobile-phase velocity, to a mathematical model describing the relationship between the two parameters. Several models have been used in the above experiment, but those by de Ligny and Remijnsee and Knox and Pryde, and developed by Guiochon and Siouffi are most widely used and, at least for a first approximation, allow for comparison and determination of the differences between TLC and column chromatography... 

The contribution of a slow adsorption kinetics to the reduced plate height is given by [29]... 

Lee [33] derived another plate-height equation that should be valid for nonlinear chromatography. He used the same assumption as made previously by Rnox and Pyper [30] and by Shirazi and Guiochon [27]. He assumed that the column HETP is the sum of two independent contributions which are due to the nonlinear behavior of the isotherm and to the mass transfer kinetics, respectively ... 

Lee also extended the non-equilibrium theory developed originally by Gid-dings [10] to obtain H in/ the plate height contribution due to the mass transfer resistances and to axial dispersion, the non-equilibrium contribution. He started from the kinetic equation of the lumped rate constant kinetic model ... 

If the second term of the RHS of Eq. 14.45 accoxmts for a fourth of the plate height, then the limit error made with a high value of k and bCo = 1 is only 25% under the most unfavorable set of experimental conditions. This illustrates the fact that deviations between experimental results and the prediction of the equilibrium-dispersive model will occur preferably at high mobile phase flow velocities. We see from Eq. 14.48 that the error introduced by the use of the equilibrium-dispersive model instead of a kinetic model decreases with increasing values of both Dakf/u = St/Pe and k /(1 -I- The value of D is of the... 

Giddings [3,4] has shown that the overall plate height arising from these kinetic factors can be expressed mathematically as... 

For forced flow separations a constant plate height independent of the solvent-front migration distance is obtained. Figure 6.3. The minimum plate height for capillary flow is always greater than the minimum for forced flow. This is an indication that the limited range of capillary flow velocities is inadequate to realize the optimum kinetic performance for the layers. At the mobile phase optimum velocity, forced flow affords more compact zones and shorter separation times compared with capillary flow. As expected the intrinsic efficiency increases with a reduction of the average particle size for the layer. 

A major criticism of the stochastic probability approach is that relatively slow secondary reactions, for which the near-equilibrium assumption does not apply, cannot be accommodated. In this situation, it is necessary to derive and solve simultaneous partial differential equations for mass conservation and obtain expressions for the first and second moments of the elution profile and the concomitant plate height arising from slow kinetics of secondary equilibrium. If, once again, the process can be represented as involving the reversible binding of two forms, the resolution of the interconverting species can be given by [59]... 

In this chapter, the possible causes of the reduced efficiency in MLC will be surveyed and a remediation path will be proposed and discussed. The chromatographic process is rapidly exposed pointing out the thermodynamics (retention time) and the kinetics (peak efficiency). The differences between a classical hydro-organic reversed mobile phase and a micellar phase are recalled. The kinetics of the chromatographic process can be modeled using the Knox equation that relates the reduced plate height to the... 

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