Chromatographic processes liquid-solid adsorption based

In adsorption chromatography the mobile phase is usually a liquid and the stationary phase is a finely-divided solid adsorbent (liquid-solid chromatography). Separation here depends on the selective adsorption of the components of a mixture on the surface of the solid. Separations based on gas-solid chromatographic processes are of limited application to organic mixtures. The use of ion-exchange resins as the solid phase constitutes a special example of liquid-solid chromatography in which electrostatic forces augment the relatively weak adsorption forces. 

Some of the variety of techniques described In the literature have resulted in the commercialization of modules independent of the chromatograph and providing it with different degrees of automation. Such modules are based on extraction (both liquid-liquid and solid-liquid), sorption (adsorption, ion exchange), vaporization, filtration (simple or through molecular sieves) or dialysis processes, or on chemical derivatization techniques. Some of these preliminary operations are better suited to HPLC, others to GC and the remainder equally to both. Only those involving the reduction of human Intervention to some extent are described here. This is a wide topic, so a comprehensive treatment is beyond the scope of this book. Below are described some representative examples of both HPLC and GC. Many of the systems described are based on the continuous separation systems dealt with in Chapter 4, devoted to the automation of sample treatment. The foundation of continuous and segmented flow analysers plays a major role in this context. 

Distribution of solutes between phases is the result of partitioning or adsorption phenomena. Partitioning involves the difference in solubilities of a substance in two immiscible solvents—in other words, selective dissolution. Adsorption, on the other hand, is based on the selective attraction of a substance in a liquid or gaseous mixture to the surface of a solid phase. The various chromatographic techniques depend on both of these processes, whereas the process of extraction relies only on partitioning. 

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