Partition versus Adsorption Chromatography

Chromatographic methods are divided into two types according to how solute molecules bind to or interact with the stationary phase. Partition chromatography is the distribution of a solute between two liquid phases. This may involve direct extraction using two liquids, or it may use a liquid immobilized on a solid support as in the case of paper, thin-layer, and gas-liquid chromatography. For partition chromatography, the stationary phase 

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Equations (10-2a) and (10-3a), with constant group partition factors Rm re widely applicable in partition chromatography (both liquid-liquid and gas-liquid) but only occasionally reliable in adsorption chromatography. A major reason for this difference between partition and adsorption systems is the fluidity of solutions versus the rigidity of solid surfaces. This is illustrated in Fig. 10-2 for the hypothetical compounds X-benzene, Y-benzene, and /)-X,Y-benzene. In solution (a) solvent molecules S are free to adjust their relative positions for optimum interaction with X or Y, regardless of the molecule into which X or Y is substituted. 

Isotherms plot of distribution ratios K, that is, a plot of concentration of the component in the stationary phase versus concentration in the mobile phase. Isotherms represent the relative attraction of a solute for the stationary and mobile phases, the plot is linear for a symmetrical Gaussian peak, non linear isotherms lead to unsymmetrical peaks, e.g. peak fronting and tailing. Langmuir isotherms describe the equilibrium process in adsorption chromatography, Nernst isotherms relate to partition chromatography. 

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... 

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