Adsorption and ion exchange chromatography

Adsorption and ion exchange chromatography are well-known methods of LC. In adsorption, one frequently selects either silica or alumina as stationary phase for separation of nonionic, moderately polar substances (e.g. alcohols, aromatic heterocycles, etc.). This mode works best in the fractionation of classes of compounds and the resolution of isomeric substances (J). Ion exchange, on the other hand, is applicable to the separation of ionic substances. As to be discussed later, this mode has been well developed as a tool for analysis of urine constituents (8). 

In the first step, the adsorption isotherms of the compounds should be determined under non-linear chromatographic conditions, which can be done in several ways [11]. Afterwards, models should be implemented and used to simulate the chromatographic behavior and to find the optimum system parameters for a given separation problem. Different approaches for finding the optimum parameter are described in the literature [12-16] mainly for adsorption and ion exchange chromatography. 

G. Wang, Isolation and purification of phycoeryhtrin from red alga Gracilaria verrucosa by expanded-bed-adsorption and ion-exchange chromatography. Chromatogaphia 56 (2002) 509-513. 

New templated polymer support materials have been developed for use as re versed-phase packing materials. Pore size and particle size have not usually been precisely controlled by conventional suspension polymerization. A templated polymerization is used to obtain controllable pore size and particle-size distribution. In this technique, hydrophilic monomers and divinylbenzene are formulated and filled into pores in templated silica material, at room temperature. After polymerization, the templated silica material is removed by base hydrolysis. The surface of the polymer may be modified in various ways to obtain the desired functionality. The particles are useful in chromatography, adsorption, and ion exchange and as polymeric supports of catalysts (39,40). 

Column chromatographic techniques were originally used as preparative tools but over the years major advances have taken place. HPLC is now a highly developed technique and a wide range of stationary phases are available. These enable partition, adsorption, gel permeation, affinity and ion-exchange chromatography to be performed. 

Ion-interaction chromatography is an intermediate between reversed-phase and ion-exchange chromatography. Introduction of amphiphilic and Uo-philic ions into the mobile phase causes their adsorption on the hydrophobic surface of packing material with subsequent transformation into a pseudo ion-exchange surface. Ionic interactions with charged analytes can occur in the mobile phase and with counterions that may be adsorbed on the stationary-phase surface. 

Figure 2.29 is based on low-viscosity mobile phases typical of adsorption chromatography (viscosity 17 = 0.44 mPa s, e.g. dichloromethane). Figure 2.30 relates to higher viscosity mobile phases which are generally aqueous in nature, as found in reversed-phase and ion-exchange chromatography (17 =1.2 mPa s e.g. methanol-water, 8 2). 

Ion-exchange resin methods, which are well known as useful preconcentration methods for trace ions, have some drawbacks slow adsorption and desorption rates, poor selectivity, and requirement for a concentrated solution of electrolyte such as acid, base, or neutral salts for recovery. Chromatographic methods in which solvent extraction procedures have been used in a continuous separation process using inert supports impregnated with the extractants combine many of the advantages of both liquid-liquid extraction and ion-exchange chromatography, which are the two of most im-... 

In other modes of liquid chromatography the basis of separation involves such phenomena as partitioning, adsorption, and ion exchange, all of which are energetic in nature because they involve intermolecular forces between the solute and stationary phase. In such cases the free energy can be approximated by the enthalpy term alone the entropy term is negligible, and the equilibrium constant is given by... 

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