Chromatographic systems thermodynamic relationships

Many chromatographic systems show linear relationships between the logarithm of the capacity factor and the reciprocal of the column temperature (van t Hoff plots) [255,258-261]. In thermodynamic terms the interaction of the solute with the stationary phase can be described by... 

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]. 

It is obvious that the retention of individual analytes in a given chromatographic system depends on their chemical structure. Unfortunately, the relationships between retention parameters and the parameters related to the structure of analytes cannot be solved in strict thermodynamic terms. 

The driving force in chromatography for the. separation of an analyte is the equilibrium between the stationary and the mobile phases. As it was di.scus.sed in Chapter 11 in more detail, the chromatographic equilibrium can be related to the chemical potential of the compound. Unfortunately, the relationship between retention parameters and the quantities related to the chemical structure cannot be solved in. strictly thermodynamic terms. Therefore, the extra-thermodynamic approach is applied to reveal the relationships. During chromatography we do not achieve a proper equilibrium, the separation is still a result of the difference of equilibrium constants for the compounds in the stationaiy and the mobile phases. The.se equilibrium con.stants can be related to measured retention data as was discussed in the previous chapter. So whenever our chromatographic system (the stationary and the mobile phase) can be considered as two immiscible phases the retention data (equilibrium data) will provide a partition coefficient. 

Isotherm The relationship between the concentrations of a component in the stationary and the mobile phases at equilibrium. The isotherm gives preferably the stationary phase concentration of the component as an explicit function of its mobile phase concentration and the mobile phase concentration of the other components of the chromatographic system. Isotherms provide the thermodynamic information required to design a chromatographic separation. Single-component isotherms are discussed in Chapter 3, multicomponent ones in Chapter 4. 

Linear relationships between In k and the reciprocal value of the thermodynamic temperature T (in Kelvins), have been observed in many chromatographic systems, according to van t Hoff equation (Eq. 7) ... 

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