Adsorption of the Eluent Components

This expression describes the analyte retention in binary system using only the total volume of the liquid phase in the column, Vq, and total adsorbent surface area S as parameters and the derivative of the excess adsorption by the analyte equihbrium concentration. It is important to note that the position of Gibbs dividing plane in the system has not been defined yet. 

In case of injection of a very small amount of analyte, its concentration is in the linear region of adsorption isotherm (Henry region of linear variation of adsorption with the equilibrium concentration of the analyte) and the derivative could be substituted with the slope of excess adsorption isotherm, also known as Henry constant, Kh, to get 

This equation is very similar to the expression obtained for partitioning retention model (2-43). 

It is essential that while setting the conditions for the differential mass-balance equation we did not define the function of the excess adsorption isotherm. We can now use the expression (2-46) for measurement of the model independent excess adsorption values. It is convenient to use it for the study of the adsorption behavior of binary eluents [22]. 

Equation (2-46) is only applicable for binary systems (analyte—single component mobile phase). Similar expression could be derived if we assume that the adsorption of the analyte does not disturb the equilibrium of the binary eluent system. 

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The adsorption isotherms of the eluent components have been studied extensively [22,25,26,28,29]. However, significant controversy still exists with regard to the interpretation of where the accumulation of the molecules of organic eluent modifier actually occurs. Horvats and further development by Dill [30, 31] stated that organic molecules penetrate between the bonded ligands of the stationary phase, and thus this monomolecular layer of bonded ligands can be considered as a stationary phase. Significant drawback of this... 

The binary eluent adsorption equilibrium is considered to be not disturbed by the injection of a small amount of the analyte (essentially the third component in the system). In an isocratic mode at a fixed eluent composition, the organic adsorbed layer is a stationary phase for the analyte to partition into. The analyte can partition into the adsorbed layer followed by consequent adsorption on the surface of the reversed-phase adsorbent. The overall retention is a superposition of two consecutive processes. Since the eluent component adsorption could be measured independently and adsorbed layer volume could be represented as a function of the mobile phase composition, the analyte retention also could be expressed as a function of the eluent composition. 

Vr(csi) is the analyte retention as a function of the eluent concentration, Vo is the total volume of the liquid phase in the column, y Cei) is the volume of adsorbed layer as a function of eluent composition, Kp(cei) is the distribution coefficient of the analyte between the eluent and adsorbed phase, S is the adsorbent surface area, and is the analyte Henry constant for its adsorption from pure organic eluent component (adsorbed layer) on the surface of the bonded phase. 

Y. V. Kazakevich, R. LoBrutto, F. Chan, and T. Patel, Interpretation of the excess adsorption isotherms of organic eluent components on the surface of reversed-phase adsorbents Effect on the analyte retention J. Chromatogr. A 913 (2001), 75-87. 

Zone IV between the raffinate and the eluent ports where adsorption of the less retained component takes place. 

The one-step procedures can be utilized for the separations of the sample components with similar dimensions but different adsorption characteristics in the given system gel-eluent. The non-adsorbed part of the sample is separated according to the steric exclusion mechanism, while the elution of the adsorbed part of the sample is retarded [26]. Another combined procedure utilizes the thermodynamic partition of the sample as the auxiliary separation mechanism the gel is either swollen by a solvent immiscible with eluent [27] or a mixed eluent is used, one component of which is preferentially sorbed within the gel matrix [28]. In both cases, the composition of the stagnant phase within the gel differs significantly from the composition of the mobile phase and thermodynamic partition takes place. The extent of the partition is controlled by the porous structure of the gel so that steric exclusion remains the principal separation mechanism. 

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