The Principle of Adsorption Chromatography

The principle of adsorption chromatography (normal-phase chromatography) is known from classical column and thin-layer chromatography. A relatively polar material with a high specific surface area is used as the stationary phase, silica being the most popular, but alumina and magnesium oxide are also often used. The mobile phase is relatively nonpolar (heptane to tetrahydrofuran). The different extents to which the various types of molecules in the mixture are adsorbed on the stationary phase provide the separation effect. A nonpolar solvent such as hexane elutes more slowly than a medium-polar solvent such as ether. 

The principle of adsorption chromatography is the same as for gas chromatography, which was discussed in some detail in a previous section. Another description is not very essential at this point. The most popular filling or packing for... 

The Independent Site Model Describes the Principle of Adsorption Chromatography... 

Principles of Adsorption Chromatography The Separation of Nonionic Organic Compounds, Lloyd R. Snyder Multicomponent Chromatography Theory of Interference,... 

Readers interested in a definitive treatment of linear adsorption chromatography should consult the book Principles of Adsorption Chromatography by L. R. Snyder (2). Although written a decade ago, the book contains an extensive and still entirely valid presentation of the subject and forms one of the main sources used in writing this article. 

Adsorption energies of numerous solvents on alumina (8° values, see below) have been measured and this scale can be used as a good guide to the elution strengths of eluents on silica as well as alumina (L. R. Snyder, Principles of Adsorption Chromatography, New York, Marcel Dekker, 1968, pp.194-195). 

L. R. Snyder, Principles of Adsorption Chromatography the Separation of Nonionic Organic Compounds, Marcel Dekker, New York, 1968. 

Gas chromatography is a very sensitive technique requiring only very small amounts of sample (lO g). A solution of about 1% is sufficient and a few microlitres of this is injected into a heated injector block. A stream of carrier gas, usually helium, passes through the injector and sweeps the vapours produced onto the column, which is contained in an oven. The temperature of the oven can be accurately controlled and can either be kept constant or increased at a specified rate. Separation of the components in gc is not based on the principle of adsorption, as it is in liquid chromatography, but on partition. A gc column is rather like an extremely effective distillation column with the relative volatility of the components being the main factor which determines how quickly they travel through the column. 

The first reported use of a vapor as the mobile phase is attributed to Martin and Synge in 1941. They used the principles of partition chromatography, whereas James and Martin, in 1952, described the first application of this method, gas-liquid chromatography (GLC), for the analysis of fatty acids and amines. Gas adsorption chromatography (GSC), on the other hand, involves the use of a solid stationary phase and separation is based on an adsorptive mechanism. This technique was first described in 1947 in a doctoral thesis by Prior,under the supervision of Professor Cremer, and subsequently in their 1951 publication.f ... 

PRINCIPLES OF ADSORPTION CHROMATOGRAPHY The Separation of Nonionic Organic Compounds... 

The principle of liquid chromatography under critical conditions of enthalpic interactions (LC CC) was elucidated in section 11.5.2.3. Mutiral compensation of the exclusion - entropy, and the interaction - enthalpy based retention of mac-romolecitles (see Figitre 3(d)) can be attained when the interactions that lead to either adsorption or enthalpic partition of sample are in the controlled way added... 

We have already observed in Figures 4.1.28(b) and (c) how polymeric chains anchored on the surfaces of pores of a membrane can bind metals, etc., present in the solution flowing through the membrane pores. This concept essentially applied the principle of membrane chromatography developed first with affinity adsorption of proteins to solve the problem of removal of metals from a solution (Brandt et al., 1988). AfBnity ligands attached to the membrane pore surfaces pick up proteins/antibodies/enzymes from the solution in a highly selective fashion. The membranes may be in the form of flat films (or hollow fibers, see Section 7.2). However, the amount of pore surface area available in one flat membrane (which is usually quite thin) is not substantial. Therefore a stack of membranes is used (Figure 7.1.27(b)). 

Adsorption Chromatography. The principle of gas-sohd or Hquid-sohd chromatography may be easily understood from equation 35. In a linear multicomponent system (several sorbates at low concentration in an inert carrier) the wave velocity for each component depends on its adsorption equihbrium constant. Thus, if a pulse of the mixed sorbate is injected at the column inlet, the different species separate into bands which travel through the column at their characteristic velocities, and at the oudet of the column a sequence of peaks corresponding to the different species is detected. 

Fig. 4 Coupling the separation principles of adsorption and partition chromatography.

Tswett s initial column liquid chromatography method was developed, tested, and applied in two parallel modes, liquid-solid adsorption and liquid-liquid partition. Adsorption ehromatography, based on a purely physical principle of adsorption, eonsiderably outperformed its partition counterpart with mechanically coated stationary phases to become the most important liquid chromatographic method. This remains true today in thin-layer chromatography (TLC), for which silica gel is by far the major stationary phase. In column chromatography, however, reversed-phase liquid ehromatography using chemically bonded stationary phases is the most popular method. 

The most popular and versatile bonded phase is octadecylsilane (ODS), n-C18H37, a grouping that is non-polar and used for reverse phase separations. Octylsilane, with its shorter chain length, permits faster diffusion of solutes and this results in improved peak symmetry. Other groups are attached to provide polar phases and hence perform normal phase separations. These include cyano, ether, amine and diol groups, which offer a wide range of polarities. When bonded stationary phases are used, the clear distinction between adsorption and partition chromatography is lost and the principles of separation are far more complex. 

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