Theory, chromatography retention factor

There are two fundamental chromatography theories that deal with solute retention and solute dispersion and these are the Plate Theory and the Rate Theory, respectively. It is essential to be familiar with both these theories in order to understand the chromatographic process, the function of the column, and column design. The first effective theory to be developed was the plate theory, which revealed those factors that controlled chromatographic retention and allowed the... 

Stahlberg [134, 135] introduced the application of the Gouy-Chapman double layer theory for the retention of small ionic analytes in ion-exchange and ion-pair chromatography (Figure 4-41). The resulting equation for the retention factor is... 

A() and m are experimental constants, Ao being the retention factor in pure strong solvent. Eq. (1.15) can be derived also on the basis of molecular statistical-mechanical theory of adsorption chromatography [,131. Eq. (1.15) applies in systems where the solute retention is very high in the pure non-polar solvent. If this is not the case, another retention equation was derived from the original Snyder model [34,351 ... 

There is considerable discussion in the literature regarding the adsorption mechanism of ions from aqueous solutions onto RPLC stationary phases [87-90]. It has been shown that, under certain conditions, organic ions are adsorbed as ion pairs [87,89,91], and that, under other conditions, they may be adsorbed as separate ions. In this case, the model derived by StMUberg [92] may be useful. In his theory of the retention mechanism in ion-pair chromatography, StMilberg focused on the derivation of the isotherm of the amphiphilic compoimd, that is, the counter-ion used in this technique to adjust the retention factors of the sample components and their separation factors e.g., the cation tetrabutulammonium). The counter-ion (Br, Cl , H2PO4 ) may not be strongly associated with the cation in a mobile phase that is a mere aqueous buffer. Other cations, rmder other experimental conditions may adsorb as true ion pairs, in which case the isotherm behavior is quite different. 

The peak recorded in a chromatogram represents the distribution of molecules in a band as it elutes from the column, the overall broadness being conveniently m sured in terms of the width of the peak. A number of independent factors such as sample-injector and detector characteristics, temperature and column retention processes, contribute to the dispersion of molecules in a band and band broadening. The cumulative effect of small variations in these factors is described in statistical terms as the variance, cr, in the elution process. Classical chromatography theory considers that the separation process takes place by a succession of equilibrium steps, the more steps in a column the greater the column efficiency with less band broadening (variance) occurring, therefore... 

The third theory treats the medianism of reversed-phase chromatography as another form of adsorption. However, a pure adsorption mechanism foils to explain the discontinuities observed in plots of the logarithm of the retention factor versus the diain length of the bonded phase. 

In this, we use the fundamental parameters of the kinetic and thermodynamic theory of chromatography plate number N, column hold-up volume Vj, and retention factor k. They all determine the peak volume, which is crucial for the concentration at the peak summit. Substituting Eq. 2.24 into 2.23, solving for and... 

It is possible to correct this ultra-simple approach with a Plate Theory model for the variation of peak width with retention time (volume), conveniently expressed via the capacity factor k. The derivation and final result are complex (Scott, http //www.chromatography-online.org/) and are not reproduced here. Instead, Figure 3.5 shows representative plots of Cp vs k for several values of N, calculated from this more realistic model. The values of N and k are of course those for the last-eluting peak, but this last peak will be different for different chromatographic detectors with different sensitivities. It is clear from Figure 3.5 that any chromatographic conditions that limit the k value for the last-detected peak will thus limit the peak capacity, particularly at lower values of k. ... 

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