Reversed-phase retention process models

In the past, several theoretical models were proposed for the description of the reversed-phase retention process. Some theories based on the detailed consideration of the analyte retention mechanism give a realistic physicochemical description of the chromatographic system, but are practically inapplicable for routine computer-assisted optimization or prediction due to then-complexity [9,10]. Others allow retention optimization and prediction within a narrow range of conditions and require extensive experimental data for the retention of model compounds at specified conditions [11]. 

Solute retention in reversed-phase HPLC is dependent on the different distribution coefficients established between a polar mobile and a nonpolar stationary phase by the peptidic components of a mixture. Although there are many similarities between reversed-phase HPLC separations of peptides and the classical liquid-liquid partition chromatographic methods, it is debatable whether the sorption process in reversed-phase HPLC arises due to partition or adsorption events, i.e., whether the nonpolar stationary phase functions as a bulk liquid or as an adsorptive monolayer. These aspects and the theoretical models for reversed-phase HPLC are discussed in a subsequent section. 

Formation of a thick adsorbed layer of acetonitrile on the surface of reversed-phase adsorbent allows the introduction of a two-stage model of the analyte retention process. The first process is the partitioning of the analyte molecules from the bulk eluent into the adsorbed acetonitrile layer, and the second process is the analyte adsorption on the surface of the packing material. 

The suggested phenomenological model describes the retention of PFe ions on different reversed-phase columns very well. Average deviation of calculated values from experimentally measured values is on the level of 1%, which confirms that indeed a superposition of several processes govern the retention of liophilic ions in acetonitrile/water systems. Experimental values along with the theoretical curves are shown in Figure 4-53. 

Reversed phase silicas are called brush type, because of their structure. Numerous alkyl groups point from the surface into the mobile phase and thus the surface is similar to a brush, with bristles of alkyl groups. To describe the adsorption process a simple two layer model of the system has been proposed (Galushko, 1991), which means that the surface layer of the alkyl groups is assumed to be quasi-liquid. The retained solutes penetrate into the surface layer and retention can be regarded as partitioning between hydrophobic stationary phase and mobile phase, similar to liquid-liquid chromatography. 

The cavity model of solvation provides the basis for a number of additional models used to explain retention in reversed-phase chromatography. The main approaches are represented by solvophobic theory [282-286] and lattice theories based on statistical thermodynamics [287-291]. To a lesser extent classical thermodynamics combining partition and displacement models [292] and the phenomenological model of solvent effects [293] have also been used. Compared with the solvation parameter model all these models are mathematically complex, and often require the input of system variables that are either unknown or difficult to calculate, particularly for polar compounds. For this reason, and because of a failure to provide a simple conceptual picture of the retention process in familiar chromatographic terms, these models have largely remained the province of the physical chemist. 

Despite reversed phase chromatography being more than 50 years mature, there still exists considerable debate regarding the retention processes involved. Much of this debate has discussed processes of adsorption versus those of partitioning. Here the basic difference between these models lies in the solute association with the stationary phase. In partitioning, the solute is embedded within the stationary phase, as distinct from adsorption where the solute is... 

In its simplest form the competition model assumes the entire adsorbent surface is covered by a monolayer of solute and mobile phase molecules. Under normal chromatographic conditions, the concentration of sample molecules will be small and the adsorbed monolayer will consist mainly of mobile phase molecules. Retention of a solute molecule occurs by displacing a roughly equivalent volume of mobile phase molecules from the monolayer to make the surface accessible to the adsorbed solute aiolecule. For elution of the solute to occur -the above process must be reversible, and can be represented by the equilibrium depicted by equation (4.6)... 

Irrespective of the sources of phenolic compounds in soil, adsorption and desorption from soil colloids will determine their solution-phase concentration. Both processes are described by the same mathematical models, but they are not necessarily completely reversible. Complete reversibility refers to singular adsorption-desorption, an equilibrium in which the adsorbate is fully desorbed, with release as easy as retention. In non-singular adsorption-desorption equilibria, the release of the adsorbate may involve a different mechanism requiring a higher activation energy, resulting in different reaction kinetics and desorption coefficients. This phenomenon is commonly observed with pesticides (41, 42). An acute need exists for experimental data on the adsorption, desorption, and equilibria for phenolic compounds to properly assess their environmental chemistry in soil. 

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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