Thermodynamic properties, analyte retention

In Section 2.1 the main chromatographic descriptors generally used in routine HPLC work were briefly discussed. Retention factor and selectivity are the parameters related to the analyte interaction with the stationary phase and reflect the thermodynamic properties of chromatographic system. Retention factor is calculated using expression (2-1) from the analyte retention time or retention volume and the total volume of the Uquid in the column. Retention... 

To derive the relationship of the analyte retention with the thermodynamic properties of chromatographic system, the mechanism of the analyte behavior in the column should be determined. The mechanism and the theoretical description of the analyte retention in HPLC has been the subject of many publications, and different research groups are still in disagreement on what is the most reahstic retention mechanism and what is the best theory to describe the analyte retention and if possible predict its behavior [8,9]. 

A review appeared on the practice and theory of enantioselective CGC with optically active selectors, e.g. 3-(perfluorobutyryl)-(17 )-camphorate residues forming complexes on a functionalized polysiloxane stationary phase (e.g. Chirasil, 65) SEC operates at temperatures lower than those of CGC, thus allowing better resolution, especially of thermally unstable enantiomers (e.g. those based on restricted free rotation, as is the case of dimethyl l,l -binaphthyl-2,2 -dicarboxylate, 66 ). Various analytical problems were addressed, such as determination of enantiomeric excess, assignment of absolute configuration, the elusive separation of protio- and deuterio-substituted enantiomers and the semipreparative separation of enantiomers. The following chromatographic parameters are related to the chemical and thermodynamic properties enclosed in parentheses of the enantiomeric system (i) peak retention (chemoselectivity, —AG), (ii) peak separation... 

In chromatography, a series of signals from the detector output is registered as the chromatogram. The qualitative information is derived from the retention time, tms, which is determined by the chromatographic process and which depends on the thermodynamic properties of both the stationary phase and the solutes. The quantitative information stems from the area under the signal, and is determined by the measurement process and depends on the detector properties. In so doing, it is assumed that the sample component elutes quantitatively from the analytical column. 

Numerous studies have been made characterizing the performance of silica-based rod columns [69], demonstrating the high level of reproducibility of analytical retention data [68] and of isotherm data [70], showing that the thermodynamic properties of the interactions between various solutes and chemically bonded Cig silica were very similar whether the silica support was made of particulate or monolithic material [71]. It has also been shown that the mass transfer kinetics was very similar for particulate and monolithic columns [72-74]. Even the satu-... 

The use of a typical equilibrium constant K in chromatographic theory indicates that the system can be assumed to operate at equilibrium. As the analyte (X) proceeds through the system, it partitions between the two phases and is retained in proportion to its affinity for the stationary phase. At any given time, a particular analyte molecule is either in the mobile phase, moving at its velocity, or in the stationary phase and not moving at all. The individual properties of each analyte control its thermodynamic distribution and retention, and result in differential migration of the components in the mixture—the basis of the chromatographic separation. The effectiveness of the separation, however, is a function of both thermodynamics and kinetics. 

Retention in i-LC is thus directly related to the thermodynamic distribution coefficient. K can be expressed in terms of fundamental thermodynamic properties, that is, the partial molar free energy associated with the transfer of one mole of analyte from the mobile phase to the stationary phase (Ag), the corresponding partial molar enthalpy, and the corresponding entropy effect (As), according to Eq. (2), where R is the gas constant and T the absolute temperature. 

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